|Wells Construction: Hand Dug and Hand Drilled (Peace Corps, 1980, 282 p.)|
|Section three: Drilled wells|
A. Overview of Small Diameter Wells
1. Basic Features
Small diameter wells usually have diameters of less than 50 cm and can be as small as 2.5 cm. For the purposes of this manual, all wells that are sunk by using tools from the ground surface, and therefore do not require that people go down and work in the well to sink them, are small diameter wells. They are also called "drilled" wells, "tube" wells, or "boreholes."
Small diameter wells normally require a pump to supply water on demand at the surface. These wells are usually small enough so that a bucket used to lift water from a large diameter well will not fit into the well. A special water lifting device must then be built and installed in the well to allow people to draw water.
There are many different kinds of water lifting devices that may be used, ranging from a modified bucket arrangement that will fit in the well to a motor-powered turbine pump capable of delivering thousands of liters per minute. This manual emphasizes pumps that will comfortably supply enough water to meet the minimal needs of the local population.
Every small diameter well is sunk by using the same arrangement of equipment. At the ground surface is a power unit which supplies the necessary motion to sink the hole.
At the bottom of the hole is a cutting tool (drill bit) which, when moved in a certain way, loosens whatever soil or rock is beneath it in the hole. (See Fig.10-1.)
Between the power unit and the bit is a connecting mechanism that transmits the motion of the power unit to the cutting tool.
FIG. 10-1. DRILL BITS
2. Drilling Motions
Because of the size and shape of small diameter wells and the equipment required to sink them, there are only two different kinds of drilling motion that are used (Fig. 10-2 ):
a. up and down - called percussion
b. around and around - called rotary
FIG. 10-2. DRILLING MOTIONS
All well sinking equipment is intended to use one of these two different motions. However, all such equipment is designed to use one or the motions predominantly and often the other motion in some secondary capacity. Thus, tools that use an up-and-down motion are referred to as "percussion" tools because they rely on their downward fall to stike and loosen ground materials. Tools that must be turned to gouge and scrape ground materials are referred to as "rotary" tools for obvious reasons.
Both of the different types of equipment can be adapted for use in most ground conditions. However, when using simple hand powered variations of either of these techniques there are certain limitations which will be noted later.
3. Removal of Drill Cuttings from the Hole
In order for the cutting tool or drill bit to effectively do its job of loosening the soil at the bottom of the hole, the soil and rock that it has already loosened or chipped away must be removed so as not to hinder the bit's continued operation. There are several different techniques and tools that can be used.
· A special tool can be lowered into the well to remove the drill cuttings. For example, a bailer, which is essentially a hollow tube with a valve at the bottom end, can be lowered to the bottom of the hole and then dropped, to pick up the drill cuttings. When the bailer is dropped onto a pile of drill cuttings, the cuttings force the valve open as the bailer falls down around the cuttings. When the bailer is lifted, the valve closes, allowing the bailer with the cuttings inside it to be lifted to the surface and emptied. (Fig. 10-3)
· The cutting tool itself can be used to remove drill cuttings from the hole. For example, auger bits which arc screwed into the ground remove soil by either accumulating the loosened soil inside the bit (see Fig. 10-4a) which will have to be emptied regularly, or by forcing the soil upwards as the bit is rotated. (See Fig. 10-4b.)
A fluid can be used to continually pick up drill cuttings and carry them to the surface. Fluid is normally pumped down through the drill rod and out of holes in the bit where it picks up drill cuttings and carries them back up through the hole.
FIG. 10-3. BAILER
FIG. 10-4. AUGER BITS.
4. Equipment and Materials
Here are the equipment and materials that are usually employed (see Fig. B for illustration of Materials):
· hole sinking equipment: Choose equipment which is appropriate to the particular ground conditions.
· hand tools: Each sinking method will require a specific set of hand tools. Example of tools that are often needed include pipe wrenches, screwdrivers, tape measures, and hack saws.
· lifting/lowering equipment: This equipment is always useful but the particular sinking technique chosen and the depth of the well will determine whether it is necessary.
· casing: This pipe is used to hold the hole permanently open to allow the use of pumps.
· well screen: This allows water entry and prevents the entrance of aquifer material.
· water lifting device (see Pumps!: This pulls water out of the narrow casing pipe.
· material to build platform around well head (cement preferred).
A pipe is installed in a well to prevent the hole walls from caving in and provide a conduit through which water can be brought to the surface.
If a well is sunk through anything other than consolidated rock, it will need to be cased to assure a permanent hole.
The casing extends from just above the ground surface to the top of the well screen which is at the bottom of the well.
Commercially drilled wells are usually cased with steel pipe which is either welded together or coupled, using specially threaded couplings. Although plastics are being used increasingly because they will not rust or corrode, nevertheless they cannot be driven into the hole as required by some drilling techniques and ground conditions. Basically, any cylindrical product which can be installed in the hole to prevent its collapse will work. For small diameter shallow wells, galvanized iron pipe, clay tile, bamboo and even hollowed logs have been used, although the last two are, for obvious reasons, not recommended.
NOTE: Before using steel or even galvanized iron casing, test the water to obtain some kind of measure of its corrosive properties. In Liberia, wells with steel casing had life spans of as little as six months due to corrosion.
6. Well Screen
This is the water intake section at the bottom of the well. It is about the same diameter as the casing and is made as long as is necessary a) for the depth of aquifer and 2) to produce the amount of water needed.
The screen itself acts both as a filter and as a medium through which the soil particles immediately surrounding the screen can be rearranged to permit easier and better water flow into the well, a process known as well development. Wells are usually developed by rapidly forcing water in and out of the well screen. This removes the fine soil particles from right next to the screen leaving only larger soil particles with larger spaces between them,thus permitting more and easier water movement into the well. (Fig. 10-5)
The different kinds of screens are discussed in detail in Chapter 15.
FIG. 10-5. CONTINUOUS SLOT WELL SCREEN IN DEVELOPED GROUND FORMATION
B. Design of Drilled Wells
The basic objective of well design is to achieve the best combination of materials, techniques, and cost to produce a well which is useful to a local community. Against a background of scarce material and financial resources, the design of wells for small rural communities in the developing countries requires a flexible approach to individual water supply needs that involves compromises on the allocations of community resources.
1. Design Problems
Two major problems in all water improvement projects: a) system maintenance and b) water testing and treatment should be considered in the design and planning of the well.
· Maintenance is the major problem in water development. A system cannot last anywhere nearly as long as expected unless regular maintenance is performed on the system and the usual minor problems are fixed as they happen.
· Water testing and treatment:
Water is rarely tested to determine its chemical and bacteriological characteristics. Chemical testing is necessary to determine possible appropriate materials for intake and pumping equipment, as well as potability. Bacteriological testing is needed to detect possible disease-carrying organisms. Many simple, inexpensive, and relatively effective methods of treatment are available and could easily be used to help ensure water potability and to lower disease rates. (See Appendix VIII.)
Where large supplies of water are needed, the normal procedure for determining the well diameter is to:
· determine the quantity of water needed;
· find the size of the pump needed to deliver a certain quantity of water per unit of time which is lifted a certain distance (head);
· choose casing that is slightly larger than the pumping equipment;
· excavate a hole whose diameter can accommodate the required casing. (See p. 145.) This presumes finding one or more aquifers that can deliver the quantity of water needed.
For small wells, the same general considerations apply, although based on the selection of the most appropriate equipment available. The casings in most of these wells will be between 5 cm and 10 cm in diameter.
· Where it is possible to get any idea of what the depth of the well will be, a sinking method must be chosen that is capable of reaching that depth.
· Where small water supplies are being developed, the usual Practice is to develop the first major aquifer, reached by sinking the intake section as far into the aquifer as possible with the given equipment, until a sufficient supply to meet local needs is assured.
· The possible depth of the well sunk with handpowered drilling techniques is usually about 20 meters except where a fluid is used to remove drill cuttings, in which case wells can often be sunk deeper.
4. Choice of Pump (Also see Pump Appendix.)
· The largest commonly manufactured hand pump cylinder will fit in a 14 cm hole.
· The deeper a well is, the smaller the diameter of the pump cylinder used.
· It may be difficult to introduce hand pumps on previously open wells where the depth is greater than 30 meters. The reason is that people who probably had been using animals to draw their water will be required to do the pumping themselves.
· If the well is properly developed, most small diameter wells cannot be pumped dry with a hand pump.
Here is an overview of planning which presents major choices and their determining factors. Use it as a checklist in your planning activities.
a. How you sink the hole depends on:
· ground conditions: What ground conditions do you expect? Are you prepared to go through rock? How much rock must be penetrated?
· sinking methods: What sinking methods can you use in these ground conditions?
· equipment: What equipment do you have? How accessible is it? Which equipment is suitable for the ground conditions you expect? (See Small Equipment and Drilling Techniques, p. 137.)
b. How you case the well (reinforce the hole walls) will depend on:
· material: What material will the casing be made of? What casing material would be suitable for the chemical and bacteriological characteristics of the water to be tapped?
· ground conditions: If loose, caving formations are encountered, it may be necessary to sink the casing along with the excavation to keep the hole open and thus allow continued sinking. It is not necessary to case a hole that is sunk into solid rock, although if the hole continues on through the rock into another unconsolidated formation, the entire hole should be cased.
· sinking methods: The casing can be lowered into the hole after the hole is excavated or driven along with the excavation process.
c. How the intake section is constructed will depend on:
· materials: What kinds of screens can be built locally or are commercially available? What effect will different possible choices of screens have on well production?
· ground conditions: The aquifer depth and particle size will determine the optimum length and opening size of the intake section.
· sinking methods: The method used to sink the intake section can determine the type of well screen that will be used.
· casing: Whether the casing is sunk along with drilling or lowered into the completed hole will determine the type of screen connection to the casing.
d. How the top section is constructed will depend on: 1) the degree of sanitary protection that is felt necessary and 2) users' preferences for a particular method of water delivery.
e. The maintenance performed on the well depends on:
· the arrangements made for regular maintenance.
· the community involvement and interest in the well throughout the construction process.
· the understanding of the connection between improved water supplies and better health.
D. Work Outline
Here is an outline of the essential construction activities, in working order:
· Community awareness and education activities should be in progress so that actual construction can serve to illustrate ideas that have already been presented.
· Gather and arrange supplies at the well site. Planning the placement and use of supplies around the well can make work more efficient and easier.
· Excavate hole (sink hole, dig or drill hole). The hole is sunk as near to the desired depth as possible. This will often include sinking the hole as far as necessary into the aquifer, as ground conditions permit.
· Case hole (line and reinforce walls). (See p. 145.) In all wells except those sunk through hard rock, a casing must be installed to prevent possible collapse of hole walls. For small diameter wells, the casing is usually pipe.
· Excavate bottom section. If the bottom section cannot be sunk by whatever method was used to excavate the middle section, then another sinking method which is more appropriate to the ground conditions will have to be used.
· Install a well screen. (See p. 122.) A well screen is a filter which enables the development of the surrounding aquifer to allow as much water and as few aquifer particles as possible to enter the well. The well screen can be attached to the casing and installed with it. Or it can be installed separately after the casing is in place.
· Seal top 3 m of casing. Grout around the casing clown to at least 3 m below the ground surface, to help prevent surface contamination from entering the well.
· Construct a platform around the hole which, by gently sloping away from the hole (1:40 slope), allows water to run off, preventing accumulation and possible well contamination from surface water seeping in around the pump.
· Install a pump or water lifting device which conforms as much as possible with local water gathering customs and practice.
Small diameter well sinking techniques include:
· techniques that require large, expensive equipment;
· techniques that need only small equipment.
Large scale techniques are covered here in enough detail to understand their basic operating principles. These methods use large, expensive, complicated machinery whose detailed operation can be adequately understood only with the benefits of hands-on training. However, the basic drilling techniques used with these larger rigs are, in many cases, the same as those which can be used with smaller equipment and may serve to better illustrate the possible variations of each technique.
Small scale techniques are covered later in enough detail to enable interested persons to perform them.
B. Drilling Techniques with Large Scale Equipment
1. Types of Rigs
These techniques all use a specially made unit known as a "rig" which includes all of the different power systems needed to operate one or a variety of types of drilling tools. The cost of these rigs ranges from about $20,000 to $500,000 and up. Depending on the type of rig and the drilling method, holes can be sunk with diameters ranging from 4 cm to 1.4 m. (See Fig. 11-1.)
There are two basic types of large drilling rigs, percussion rigs which give an up-and-down motion to the tools and rotary rigs which turn the drilling tools.
By far the most common type of percussion rig is the cable tool. Drilling tools are suspended on a cable which is alternately pulled and released to create the up-anddown motion of the tools. When drill cuttings have accumulated to the point that they impede the action of the bit, it is removed from the hole and a bailer is lowered, usually on a second cable, to pick up the cuttings. This is one of the two most common methods of well drilling and is known as cable tool percussion. Cable tool rigs are also adaptable to other forms of percussion drilling such as hydraulic percussion and jet percussion. These other percussion techniques are not, however, commonly practiced.
FIG. 11-1. CABLE TOOL PERCUSSION DRILLING RIG
Unlike percussion,there are several different relatively common types of rotary rigs. The biggest of these, and perhaps the most complicated of all well drilling machinery, is the rig capable of hydraulic rotary drilling and its variants. This and the cable tool are the two most commonly used water well drilling rigs.
Other types of rotary rigs are those that use augers and core drills. Auger rigs are designed to drill relatively large diameter holes to a relatively shallow depth in non-caving formations. Core drills are designed to drill very small diameter holes to great depths and recover samples of all the materials drilled through. Both of these kinds of rotary drills have been used to drill water wells, although both have limitations for such use.
NOTE: When considering using this equipment, carefully check the manufacturer's size specifications. There has been much confusion and delay resulting from the fact that tools and equipment are commonly manufactured in both English and Metric sizes, which are not generally compatible. To compound the problem there are many different kinds of thread patterns in use which can be cut on either English or Metric pipe sizes.
2. Drilling Variations
There are a number of possible drilling variations given the two basic methods, percussion and rotary, depending on whether or not a fluid is used and which way it is circulated.
· percussion only - cable tool percussion.( See Fig. 11-2a.) This is commonly used to sink wells through all kinds of formations.
· percussion and regular fluid circulation - jet percussion. (See Fig. 11-2b.) This method is no longer frequently used but where wells are jetted from the surface it is usually a form of jet percussion.
· percussion and reverse fluid circulation - hydraulic percussion. (See Fig. 11-2c.) This method is not commonly used with large machines, although the small scale version, known as the sludger technique, has been extensively used in some areas.
· rotary only - auger.(See Fig. 11-2d.) Augering or boring is commonly used to drill relatively shallow holes for wells and many other uses.
· rotary and regular fluid circulation - hydraulic rotary. (See Fig. 11-2e.) This method is commonly used to drill wells through all kinds of formations.
· rotary and reverse fluid circulation - reverse circulation. (See Fig. 11-2f.) Reverse circulation drilling has the same uses as hydraulic rotary and is sometimes perferred.
· rotary and air as drilling fluid - air rotary or downthe-hole hammer. This is a relatively new technique which uses compressed air as the drilling fluid and is most suitable for drilling through rock.
The following two techniques are not really primary drilling techniques since they are not often used with large drilling equipment to sink wells from the ground surface. They are, however, frequently used in well drilling operations to sink the well screen and casing into place in the aquifer after a hole has been drilled down to the water table by any of the above techniques. For more information, see Chapter 14.
· regular fluid circulation only - jetting or washing. (See Fig. 11-2g.) The fluid itself loosens the soil at the bottom of the hole and also carries the loosened soil to the surface.
· no fluid circulation - driven. (See Fig. 11-2h,) With this technique the casing and well screen are simply pounded into the ground.
FIG. 11-2. DRILLING METHODS. THE CHOISE OF DRILLING METHOD DEPENDS MAINLY ON WHAT MATERIALS ARE AVAILABLE.
3. Drilling Depth
Assuming that the rig and tools are appropriate for the particular ground formation being penetrated, the depth that can be drilled is limited only by the ability of the rig to lift the tools from the hole.
For example, one particular manufacturer provides the following suggested depth capabilities for their topofthe-line hydraulic rotary drilling rig fitted out with various drilling tools.
continuous flight auger
The same company's core drilling rig is capable of drilling a 48 mm diameter hole to 1400 meters.
C. Overview of Small-Scale Techniques
The small-scale techniques given here have been adapted for use with a minimum amount of equipment and expense. These techniques may require only hand tools, pipe, and a great deal of labor, or they may require a tripod, a motor pump, drilling pipe, and drill bit plus whatever material will be permanently installed in the well. Wells workers may select from a variety of equipment varying in cost and technical difficulty, all of which are less expensive and less complicated than a drilling rig.
These sinking methods may be used in largely consolidated ground formations (see Glossary). The suitability of the different methods varies with the degree of caving expected and whether rock will be encountered.
It is common practice to use more than one method to sink the complete hole. With adequate finances and equipment, it is helpful to use the particular technique most suited to the (1) ground conditions, (2) expected depth of water, and (3) the section of well being worked on.
Each individual small equipment sinking technique is generally limited to use in a certain type of soil. Where the soil layers to be penetrated are similar, using one appropriate sinking technique will be most efficient. However, it is much more likely that you will encounter several different soils, requiring the use of variations on a single sinking technique, or even different techniques. When evaluating the various sinking techniques, choose the one most appropriate to a particular situation. You may also want to consider what other equipment might be needed in different ground conditions.
In many cases, ground conditions will vary between the middle and bottom sections of the well. The same type of soil may surround the complete well, but where that soil is saturated with water (bottom section), its drilling characteristics will usually be different. For example, a sand or combination of sand and gravel aquifer will usually be subject to caving, making it impossible to excavate a hole down into the aquifer without sinking a casing with the drilling tools. Sand that is only damp, on the other hand, will tend to stick together and not cave in.
However, if rock is struck, the well must be moved to a different site where rock might not be encountered, or the workers must switch to drilling tools appropriate for rock. Hand-powered tools commonly used in unconsolidated formations generally gouge and slice through soil to loosen it. This is not suitable for rock, which must be smashed and chipped or broken into smaller pieces in order to be removed from the hole.
2. Sinking Methods and Ground Conditions
There are then three basic types of ground conditions that can be encountered while drilling.
Suitable Sinking Technique
rotary, percussion or sludger
Where all three ground conditions are likely to be encountered you should be equipped with the types of tools appropriate to each.
3. Level of Complexitv and Common Uses of Small
a. techniques that require the least equipment and expertise
· driven - Variations of this technique are used in almost all methods. By itself it can only reach a limited depth in relatively loose soil.
· sludger - This is a specific adaptation of a percussion technique which uses a drilling fluid but does not require a pump during drilling. The technique is quick and there are good possibilities where the ground permits.
b. techniques that require a minimum of manufactured equipment but require that materials and semiskilled labor be available to produce the necessary equipment.
· hand percussion - This excavation technique works in the widest range of soil conditions. However, it is slow and requires considerable hard labor. Although it is simple, there are many possible minor variations which could affect drilling but which require some experience to use effectively.
· hand auger - This is one of the simplest and most easily understood methods for simple field work. It is limited to non-caving formation without rocks bigger than the auger. It is also slow and can only reach a limited depth.
c. techniques that require at least local manufacture of the necessary equipment and pumps.
· jetted - This technique will only work in the same places that the sludger method will and it requires more equipment and expertise. It will, however, usually be faster and require less work than sludger.
D. Small Equipment and Drilling Techniques
Here is basic information on small equipment drilling techniques, which provides the following information on each method.
· materials required;
· quick description of method;
· suitability of ground conditions;
1. Rotary hand auger
· Materials required - auger, drill stems to connect the auger bit with the handle, and a handle.
· The particular operation will depend on the exact bit being used. Most often the bit is turned in the hole until it is full of drill cuttings. It is then brought to the surface, emptied, and returned to the hole. This process is continued until the desired depth is reached or ground conditions change causing this method to be unusable. (See Fig. 11-3.)
FIG. 11-3. HAND AUGERING
· Suitability to ground conditions - This method can be used in clay, silt, and sand formations not subject to caving. In caving formations a casing slightly larger than the bit can be driven down as the hole is sunk. Where rock is reached this sinking method will have to be discontinued.
· Cost - The U.S. cost of a 10 cm diameter auger, drill stem, and handle in the spring of 1978 was $16.50.
· Work - Unskilled workers can easily be taught to use this method to sink and finish wells up to 20 m deep in five days.
· Depth - 20 m is the usual maximum depth.
2. Hand percussion
· Materials required - You will need a chopping bit (see Fig. 11-4), bailer (see Fig. 11-5), either drill shafts or rope, and a tripod and pulley to assist in lifting and dropping the drilling tools.
· The bit is lifted and dropped until it has loosened so much material in the bottom of the hole that its progress is stopped. The bit is then removed and the bailer is dropped into the hole to pick up the cuttings. The bailer is worked to pick up as much as it can and then removed from the well and emptied. The bit can then be lowered back into the hole and worked as before. This sequence is continued until water is reached or the ground conditions change to necessitate a change in sinking technique.
· Suitability of ground conditions - This sinking technique can be used to sink wells in all kinds of ground conditions although it can be very slow in hard rock.
· Cost - The cost of hand percussion tools will depend on the sophistication and quality desired but, compared to most other methods, these tools are cheap and easily made.
· Work - Unskilled workers can easily be taught to use this method although semi-skilled supervision will be necessary for optimum performance.
FIG. 11-4. CHOPPING BIT
· Depth - Great depths are possible in optimum ground conditions but as the hole gets deeper more and more time will be spent switching back and forth between the bit and the bailer because of the greater distance they must be raised and lowered.
This method is best suited for drilling through soft rock and well packed, non-caving soils. It can be used in hard rock although progress will be slow and drilling bits will need to be sharpened frequently. It can also be used in caving soils if the casing is driven in as the hole is sunk.
3. Hand percussion and fluid (sludger)
· Materials required - You will need metal pipe and couplings, a cutting bit which can be made from a coupling, pipe wrenches, and a tripod or lever structure with which to lift and drop the metal pipe.
FIG. 11-5. BAILER
FIG. 11-6. SLUDGER TECHNIQUE
· The pipe string equipped with a one-way valve is lifted and dropped into the hole which is full of drilling fluid (usually water). The one-way valve permits fluid and cuttings to flow up through the pipe but not down. The upand-down action of the pipe, acting with the valve, allows the tool string to act as a pump removing fluid and cuttings from the well. The fluid can be recirculated if cuttings are allowed to settle out. (See Fig. 11-6.)
· Suitability to ground conditions - This may be used in fine or sandy soils as long as rocks are no larger than mediumsized gravel.
· Cost - Obviously the cost will vary from place to place, but to give an example, wells using this technique drilled with 1.5" G.I. and finished with plastic casing cost 12¢ to 16¢ per ft. in Nepal in 1976.
· Work - Six relatively unskilled men can build a 4 cm diameter 60 m deep well in four days.
· Depth - Maximum 80 m.
· Materials required include a drive point, a casing pipe, a drive cap to protect casing pipe when driven, and a heavy weight to strike the cap.
· Driven wells are sunk by pounding a special type of well screen called a drive point (well point) into the ground. The drive point is attached to the casing pipe and more casing pipe is added as necessary as the whole string of casing pipe and drive point is driven down. (See Fig. 11-7.)
· Suitability to ground conditions - This method is generally used and easiest to drive in loose, caving formations. It will not go through rock and sinks through clay only with difficulty.
FIG. 11-7. DRIVEN WELL
· Cost - As of August 1976,a 91 cm long, 5 cm diameter, stainless steel, continuous slot drive point cost $80.14 and a 305 cm length of 5 cm diameter seemless, ungalvanized (black), steel casing pipe cost $43.65 in the U.S. These are manufacturers' prices and are given here merely as examples. Acceptable substitutes for both of these can usually be locally manufactured.
· Work - The majority of the work involved in sinking a well with this method is lifting and dropping a heavy weight to drive the pipe into the ground.
· Depth - Wells can usually not be driven more than 10 m by hand or 20 m be machine because of the friction between the casing pipe and the soil.
· Materials required include a pump, hoses, a watertight connection of the hose to the top of the drilling string, a hollow drill pipe, a hoist, and a jetting bit.
· Jetted wells are sunk by pumping drilling fluid (usually water) down through the drill pipe and out a special jetting bit. This works on the -same principle as a flowing garden hose that can be pushed into the ground. The washing action of a stream of water alone can be used to sink small diameter pipe and well screens in sandy formations. (See Fig. 11-8.)
FIG. 11-8. JETTED WELL
· Suitability to ground conditions - Jetting with percussion can be used to go through all but hard rock formations. It is commonly used to sink small diameter well screens in water bearing sand.
· Cost - Because of the equipment necessary, this sinking method is more expensive than the other techniques covered here. Its actual cost is difficult to estimate because of the wide range of equipment possibilities that can be used. A simple hand-operated diaphragm pump can be used to sink one long pre-assembled length of casing into place at a cost comparable to other methods discussed.
· Depth - Jet percussion builds wells 8 to 10 cm in diameter up to 60 m. With simple equipment, a well can be jetted to a depth of about 20 m.
· Work - This method requires some technical skill, experience, and judgment, although all of these are straightforward and easily understood by people with modest technical backgrounds.
E. Casing Installation
1. Overview of Casing Techniques
How the casing is installed in the well depends on such factors as casing materials, and ground conditions.
Before the casing is set into place it is very important that you also consider how the well screen will be attached to it. (See p. 192.) If the casing above is set in place the well screen must later be telescoped into place. Special tools and materials are then required to seal the well screen to the casing.
The same three methods of installing the casing are possible whether or not the well screen is attached.
· Lowering the casing into the hole - Where the drilled hole will remain open without caving for a long enough period of time, the casing can simply be lowered into the completed hole. Normally, the casing is lowered as far as possible into the hole, after which it can be sunk further by one of the following methods.
· Driving the casing - This method can only be used where heavy metal pipe, which will not deform as it is driven, is used as the casing. It can be used in all but very hard formations although it is most often used in loose caving formations. (See p. 163.)
The casing can be driven into place either after the hole has been sunk as far as possible or as the drilling proceeds. The latter is used primarily with percussion techniques where a bailer can be worked inside the casing to remove loose, caving types of soil as the casing is driven in to reinforce and maintain the hole. In this instance special drive clamps must be attached around the casing pipe so that it can be driven while the bailer is inside it.
When the casing will be driven into place before the well screen is attached, a sharpened coupling can be screwed on to the bottom of the casing to facilitate sinking. (See Fig. 11-9.)
FIG. 11-9. SHARPENED COUPLING
· Washing the casing into place - This method uses the process of pumping a fluid down through the pipe to remove loosened soil particles from the hole and allow the casing to sink. The only difference between this and the jetting process described on page 176 is that a special jetting bit which directs a high velocity fluid stream at the bottom of the hole is not used. Because this method requires that the pipe joints be watertight, it generally is used only with manufactured pipe. It also requires the use of a pump, to force water down through the casing, and a watertight connection at the top of the casing through which the water can be pumped. (See p. 239.)
2. Sealing the Casing
Once the casing is set in its final position in the well any space around the casing should be filled in. The top three meters of this space should be filled with mortar or concrete to seal the casing to the ground formation and thus prevent possible contaminants from easily flowing down along the outside of the casing pipe .
A. The Hand Rotary Method
1. Overview of Method
This method of well sinking has been commonly referred to as boring. Sometimes percussion techniques are included under the general title of bored wells. Because of the limited soil conditions in which simple rotary methods are effective, it is often useful to have percussion tools available. However, for clear explanation, simple rotary and percussion techniques are discussed separately here.
Where sophisticated drilling methods are available for well sinking, hand augering is used only for taking soil samples at relatively shallow depths. It is cheap and provides very accurate soil samples, but requires time and effort and is limited to unconsolidated noncaving formations.
An auger, which functions as a drill bit, is attached to the bottom of a length of drilling rod and turned with the handle, which is attached to the top of the drilling rod. (Fig. 12-1.)
The auger serves first to loosen soil at the bottom of the hole and second to remove it. As the auger is turned, the loosened soil accumulates in or on the auger.
When the auger is full, it is lifted to the surface and emptied and then returned to the hole to continue sinking. Lengths of drilling rod are added to the tool string as the hole is deepened. It has been estimated that 70 to 80% of the time required for this sinking method is taken up by raising and lowering the tool string.
2. Advantages and Disadvantages
There are several advantages to this method.
· The equipment is simple, light, portable and easy to make from available materials.
FIG. 12-1. HAND AUGERING
· It provides excellent samples of soil layers penetrated for future reference.
· The technique is simple and easily taught.
· It is easily combined with simple percussion techniques to make the combination method suitable even where some rock exists.
It has, however, certain disadvantages.
· It is limited to unconsolidated, non-caving ground formations.
· Well depth is limited to 15 or 20 meters maximum because of the physical difficulty of operating a tool string any longer than that.
Excavating equipment includes:
· auger - An auger works at the bottom end of a tool string to excavate soil. There are two general types of augers each of which has variations which are suitable for use in different ground conditions:
a. cylindrical bucket type - different variations used for hand or power drive. (See Fig.12-2.)
b. open blade type - used mainly with power equipment (continuous spiral) although some are available with hand drive. (See Fig. 123.)
FIG. 12-2. BUCKET AUGERS
FIG. 12-3. OPEN BLADE AUGERS
· drill shaft (drill stems, drill rod) - The drill shaft is made up of 1 to 5 meter sections which are connected together between the auger and the handle. It is most often fairly small diameter hollow metal pipe. This size and shape help to reduce the weight of the tool string as well as provide the appropriate motion and strength necessary to transmit the drilling motion from the handle to the bit. Because so much time is spent removing and lowering the bit, the shaft connections should be quick, easy, and sure. It is necessary that all of the equipment use the same type of drill shaft connection.
The following are examples of types of connections that have been used.
- Threaded pipe and coupling is probably the most frequently used connection because it is so readily available.
- Another connection involves slipping the two connecting ends of the drill shaft into a slightly larger piece of pipe and bolting them in place. Only one bolt needs to be removed to disconnect the two shaft sections. That can usually be easily done because it has been found that the bolts need only be finger tight. An alternative to using a regular nut and bolt could be to use a toggle type bolt.
· Rock hammer - This is a percussion tool which can be dropped or thrown down the hole to chip or break rocks which are too large to be removed by the auger.
· Sand bailer - This can be used to remove sand from loose caving formations.
· Casing - Some type of casing must be installed in the well to reinforce the walls. In caving formations, a sand bailer can be operated inside the casing to sink the hold and the casing. The inside diameter of the casing will need to be slightly larger than the outside diameter of the sand bailer.
· Tripod - This can hold a pulley to aid in lifting the tool string and also support the upper end of the drill stems when drilling through harder layers, such as laterite (see Glossary). The support is to hold drill stems plumb while drilling with extra weight on the stems to enable penetration of the hard formations.
· Fishing tool - This is used for retrieving tools and equipment that have come disconnected from working pipe string or have dropped into the hole. (See Fig. 12-4.)
NOTE: A magnet can be a fishing too] for small items dropped into the hole.
· Plumb bob and level - These may be used to check whether a hole is perfectly straight and vertical.
· Handle - The handle attaches to the drill shaft enabling the auger to be turned by people at the ground surface. It can either be attached to the top end of drill stems to form a T or be a cross piece that clamps to the drill stem wherever it is needed.
4 The Sinking Process
· Arrange and set up all tools, equipment and supplies; clear the area immediately around well site of all unnecessary tools. Set up and locate a tripod or headframe if it is necessary to provide support for the upper end of the drill stem. Locate it directly over the hole so that the drill stem will be plumb.
· Assemble the auger, drill stems and handle.
· Begin boring by forcing the auger blades down into the soil while turning the tool. Digging a 30 to 50 cm deep hole of sufficient diameter to allow introduction of the auger usually helps to get the drilling started.
· Turn the bit until it is full of loosened soil. It is only necessary to turn the bit just enough to fill it. The number of turns required to fill the bit is determined by the soil hardness, and may be only two or three turns. Be careful with some augers not to screw them too tightly into the ground. In hard formations, it may be necessary to push down as well as turn the tool. (See Fig. 12-5.)
FIG. 12-5. ADDED WEIGHT NECESSARY TO PENETRATE HARD GROUND
· Remove loosened soil from the hole. Lift the auger from the hole and empty the soil accumulated on or in it. By systematically depositing the components of the auger in short ridges, each containing the soil from 1/2 or 1 meter of well depth, you can keep a record of the soil layers penetrated. Keep the samples far enough away from the well so as not to hinder the well sinking operations.
· Return the auger into the hole and continue sinking .
· Continue turning the auger, removing the auger from hole when full, emptying and returning the auger to the hole to be turned more. As the hole is sunk deeper and deeper, more and more drill stem sections will need to be added.
· Continue sinking the hole by the same process until a) you have reached a sufficient depth into the water table or b) you can no longer sink the hole any deeper by this process. If (a) is the case, see Chapter 15, p, 191. If (b) is the case, you were probably stopped for one of the following reasons: the auger encountered rock, the hole continues to cave in permitting you to go no deeper, or the tool string is not long enough to go further.
If the auger encounters rock you still have several options. Try to remove the entire rock from the hole with a ram's horn (spiral auger). (See Fig. 12-6.) This will work only if the rock is smaller than the hole.
Although a spiral auger is a piece of equipment listed in the literature for removing rocks from augered holes, field experience has found that percussion techniques work better. (See Hand Percussion, p. 155.) A percussion rock bit would be attached to a rope and dropped in the hole or attached to the end of the drill shafts just as the auger is. Abandon the hole and try some other place if you have no equipment to penetrate a rock layer or if it is too thick or hard to be penetrated by whatever tools you have.
FIG. 12-6. SPIRAL AUGER
If the hole continues to cave in permitting you to go no deeper, you have two options. If you have hit water at the same time, see "Bottom Section." If the hole is still dry, however, you must evaluate your situation to decide whether you want to continue sinking this hole. The hole will usually be caving because the earth is loose sand. To go deeper you will need to sink a casing to hold the walls in place. Determine how much deeper you can go and whether it is worthwhile.
If the tool string is not long enough to go further, abandon this hole and try another location.
5. Time Saving Suggestions
· Disconnect as few of the drill shaft connections as possible. Simply remove and replace shaft sections in lengths as long as you can comfortably handle.
· Leave disconnected shaft lengths standing upright right next to the well. A scaffold could be built against which long shaft sections could be leaned while waiting to be reconnected and go back into the hole.
· Use an auger with a long central cavity in which to accumulate drill cuttings.
· Leave an opening in the auger which will enable the cuttings to be quickly and easily poked out.
B. The Hand Percussion Method
This method has usually been classified as a variation or useful addition to hand augering. It makes use of the same methods that are used on a larger scale in cable-tool percussion drilling with large rigs.
The basis of this method is the up and down motion used to sink a hole. The tool string is lifted by an appropriate means and dropped, causing the bit at the bottom of the string to come into sudden, forceful contact with the bottom of the hole. The heavier the tool string, the harder it will strike the bottom of the hole.
It is usually useful to have several different bits suitable for varying ground formations. All of these bits are operated by dropping them onto the bottom of the hole. It will often be useful to turn the bit in the hole, either with a wrench or with a handle, that can be attached similar to an auger handle to make drilling easier and help ensure a round hole.
Cuttings can be removed from the hole in several different ways depending on the particular bit being used. Hard rock cuttings and very loose, caving material is usually removed with a bailer. (See Fig. 1210.) Non-caving soil can usually be packed into a hollow bit. (See Fig. 12-9.)
2. Advantages and Disadvantages
The hand percussion method is suitable for use in a wide range of ground conditions, and may be effective where an auger is not.
This method can be slow, especially in hard formations.
3. Equipment Overview
· Cutting bits. There are several different kinds of bits that are commonly used depending on the characteristics of the formation being penetrated.
- A cutting bit is needed for hard formations. (See Figs. 12-7 and 12-8.) Heavy bits with sharp hard edges are used to smash and chip rock. The bit action cuts and mixes the drill cuttings with a small amount of water added to hole to form a paste which can be easily removed with the bailer (too much water will slow drilling). While a solid piece of regular steel can be used to make the rock bit, it is a good idea to face or fit the cutting edges of hard steel. This can be done by building up the tip with welding steel and grinding it down to the desired shape. The bit will require less frequent sharpening and will last longer. The bit can be worked either hanging by rope or cable, or connected to drill shafts.
Rope or cable will tend to wear and may break during the drilling process. If this happens, the bit will have to be "fished" from the hold. (p. 163.) Although rope wears faster, it does have one advantage over cable. When rope suddenly reaches the end of its fall, it gives a quick little turning motion to the bit which helps prevent the bit sticking in the hole. For the very heavy (80 kg) bits which are more effective in harder rock, rope or cable seems far more suitable than drill shafts. With an 80 kg bit, five to seven people are needed and they require frequent rests (every 50 to 100 strokes). The best action with a rock bit can be achieved with short (50 cm) rapid strokes.
FIG. 12-7. PERCUSSION BIT
FIG. 12-9. DETAIL OF PERCUSSION BIT
- A hollow rod bit can be easily made locally from a piece of heavy metal pipe. (See Fig.129.) Galvanized iron pipe is sufficient. It can be made with either a sharpened straight bottom edge as in the figure or a jagged bottom edge. The straight edge is simply sharpened with a file while the jagged edge can be Cut with a hack saw. The opposite end of the bit should be fitted so that it can connect to a drill shaft or rope. It will probably be better with this type of bit to attach it to a drill shaft so that the bit can be forcibly turned or pushed down as necessary. It will be very useful to leave or cut a narrow slot almost the entire length of the bit so that drill cuttings that have been packed up into the bit can be removed by prying with a piece of re-rod or something similar.
FIG. 12-9. HOLLOW ROD BIT
NOTE: If you reach rock, it is usually advisable to continue drilling for three to five days to get an idea of whether this is an isolated boulder; a thin, easily penetrable rock layer; or a virtually impenetrable thick rock layer. After about 50 cm penetration into rock, it is normally possible to identify a boulder, because a boulder will usually break up.
FIG. 12-10. BAILER
- A bailer is the most commonly used tool in loose soils. It is a long cylindrical tube with a valve in the bottom end which permits material to be forced up into the tube but will not permit it to fall back out (See Fig.12-10.) Bailers used with hand equipment are most often equipped with flap valves. The bailer is lowered to the bottom of the hole. Lift it 1 to 2 meters and drop it. The impact of the bailer on the bottom of the hole will force some of the loose soil up into the hollow core. Continue lifting and dropping until the bailer is full or until it has picked up as much of the loose material as it can. Experience will show how long to continue lifting and dropping the bailer to get the maximum usage. When the bailer is full, pull it up to the surface and empty it away from the well.
A bailer has several uses in different situations. It removes rock pieces loosened by rock bit; it removes sand in caving formations from inside the casing; it functions to remove loose material that cannot be packed or retain its shape.
· Drill shaft. The shaft lengths connect the bit to the lifting apparatus. Connections between the rods need to be strong, to withstand constant lifting and dropping, and quick because of the frequent necessity to disconnect and reconnect rods when raising and lowering bit to empty it (see details on drill rod).
· Turning handle. This is not necessary, but it can be useful to turn the tool string to facilitate sinking and help ensure a round hole.
· Lifting apparatus. A lifting apparatus can be made from whatever materials are locally available.
· "Fishing" tool. A fishing tool is useful for retrieving tools that have become disconnected in the hole.
· Casing. Casing is necessary for permanent well construction and can be used to reinforce the hole when sinking through caving formations.
· A plumb-bob or level is needed to check the verticality of the hole especially when the hole is started.
· Hand tools. Very few hand tools are absolutely necessary although many will be found useful by those who know how to use them.
· Rope is always useful for a variety of purposes.
FIG. 12-11. FISHING TOOL - WHICH CAN BE USED TO PICK UP BROKEN ROPE OR CABLE
4. The Sinking Process
· Arrange and set up all tools, equipment and supplies. Clear the area immediately around the well. Locate a tripod or other lifting apparatus directly above hole site. It should be anchored well to prevent the legs moving when a heavy weight is supported by the tripod. The exact hole location can be found by hanging a plumb-bob from the drill shaft guide or from the lifting point of the tripod or frame. This will help ensure a vertical hole.
· Dig a shallow starting hole before beginning the sinking process with tool string.
· Assemble the tool string and set it in place. It is very useful at this point and also during the sinking process to have a guide for that part of the drill shaft which will extend above ground level. This will help to ensure that the hole is perfectly vertical, especially when starting the hole. However, do not depend on the shaft guide alone to ensure a plumb hole.
· Frequently and carefully check to see that the tool string continues to be plumb, especially when the hole begins to guide the tools. At a depth of about 50 cm, the hole itself will probably have more of a guiding effect than any above-ground tool string guide.
· Begin the sinking process by lifting and dropping the tool string; the lifting and dropping tools may be connected to a rope. The rope comes from the bit or tool string over a pulley and back to a solid post or tree around which it can be wrapped or tied at a convenient height so that workers can easily reach it. Workers can then raise the bit by lining up along the rope and pressing down. From there it is quickly released to allow the bit to fall in the hole. You can also use a rear car wheel as shown in Figs. 5-7 and 58.
5. Sinking Variations
There are three variations of this sinking process that can be used depending on the ground conditions encountered.
· In hard, compacted formations, a club type bit has been found most effective. (See Fig.12-7.) When dropped, the bit smashes and chips the hard formation to break away small pieces at a time. As the bit continues to be lifted and dropped, the small drill cuttings will gradually accumulate in the hole until they prevent the bit from coming in contact with the bottom of the hole. The driller should continually watch the progress of the bit so that he can stop the drilling and remove the accumulated cuttings whenever the progress of the bit is slowed significantly. Determining when to stop and remove the cuttings is a matter of judgment and experience. After the bit has been lifted from the hole, the cuttings are removed by lowering a bailer into the hole and lifting and dropping it until it has accumulated as much of the cuttings as possible. Adding a small amount of water to the hole sometimes aids the bit in pulverizing rock and producing a slurry that can easily be removed by a bailer.
· In loose, non-caving formations, a hollow rod bit can sometimes be used effectively. (See Fig. 12-9.)when this bit is dropped in this type of ground, it does two things: 1) the circular bottom edge of the bit cuts out a plug of material that it has sunk into and 2) this plug forces material already picked up in the hollow inner core of the bit farther up in the bit and packs it in so that it cannot fall out when the bit is lifted again. It is this packing of the loosened, cut-away material into the bit that allows the material to be removed from the hole by lifting the bit to the surface and prying out the packed in material.
Here are some comments and suggestions that may help. It may be necessary to add water to the hole to facilitate sinking because either too much or not enough soil is sticking to the bit. Rotating the bit in the hole is sometimes useful to loosen the bit after it has been dropped and to help maintain a round hole. Leaning the tool string against the rod support or tripod to empty the bit, as opposed to laying it down, saves time and energy. Where the handle is needed to turn the bit, it will be most useful if it can simply be clamped into the tool string wherever desired. This can be long hard work but a system of alternating work and rest periods for teams of workers has proved efficient.
As the bit fills with the loosened soil, it picks up less and less with each stroke until it becomes so packed that it can no longer loosen and pick up any soil. Because different soils pack differently, the bit may be as little as one third full when it must be removed and emptied. With experience, one quickly gains a feel for when the bit needs to be emptied.
In loose, caving formations like saturated sand, a bailer can be operated inside the casing to sink the hole as the casing is sunk or driven. (See Fig. 12-10.) The outside diameter of the bailer should be slightly less than the inside diameter of the casing being installed.
If the casing will sink by itself, the bailer is simply worked up and down inside the casing pipe to pick up loose ground material. As the bailer picks up the material from the bottom of the hole, the casing should sink under its own weight. (If the casing does not sink, it can be driven.) When the bailer fills up, it will need to be removed from the hole and emptied.
The casing can be driven at the same time that the bailer is being worked or it can be driven down about .5 m and then bailed. This particular process of driving and bailing is very commonly used to penetrate loose caving formations.
6. Problems in Sinking
· If the bit becomes stuck in the bottom of the well, it may be freed by attaching to the rope or shaft a long pole which pivots over a rock or log next to the hole to lever up the bit. (See Fig. 14-10.)
· Fishing - When a rope becomes worn, it may break, especially under the added strain of trying to retrieve a stuck bit. It may be possible to fit a hook onto the end of the drill shaft, hook the bit, and pull it up. A solid drill shaft is much easier to control and manuever in the hole than rope or cable.
A. Hand Percussion and Fluid (Sludger)
1. The Method
A sinking method that has been used quite successfully in India and Pakistan is called the "sludger" method. It is an adaptation of the "hollow rod" technique which has been used with large drill rigs to sink wells. It is more formally referred to as the "hydraulic percussion" method; "hydraulic" because drilling fluid is used and "percussion" because the motion of the tools is up-anddown.
The tool string consists simply of a bit, a check valve, and lengths of hollow drill pipe. The tool string is lifted and dropped in the hole which is full of drilling fluid (usually water). The check valve in the tool string allows drilling fluid and the drill cuttings which are suspended in the fluid to pass through the valve on the downstroke of the pipe. These are not permitted to flow back out on the upstroke of the pipe. With the next downstroke more fluid and cuttings are forced up through the valve, thereby forcing the first mass further up the pipe. This continued up-and-down motion of the tool string then causes it to function as an inertia pump which acts to remove the drill cuttings from the hole. (See Fig. 13-1.)
FIG. 13-1. SLUDGER DETAIL
(a)As the pipe is lifted, there is room for more fluid to flow down into the hole,
(b) Dropping or pushing the pipe down forces fluid and cuttings to come up through the valve into the pipe and, after several strokes, out the top.
(c.) When the pipe strikes the bottom of the hole as you drill, the valve closes, preventing the fluid in the pipe from flowing back down into the hole.
The cutting action is performed by a bit attached to the lower end of the tool string. This can be simply a sharpened or jagged edged coupling that is screwed on to the bottom length of pipe. The bit strikes the bottom of the hole at the end of the downstroke and acts to loosen the material which can then be picked up by the drilling fluid to be removed from the hole. (See Fig. 13-2.)
Several variations of this technique have been used effectively. Wells have been sunk using standard pipe and coupling as the bit while a person uses his/her hand to act as the check valve by covering and uncovering the top end of the pipe. Hollow drill pipe with a swivel joint attached on the top has also been used. (See Fig. 13-3.)
Where a motorized lifting apparatus is not available, more and more labor will be required to lift and drop the drill string as it gets longer and heavier.
This particular sinking method is the only one that uses drilling fluid to remove drill cuttings from the hole but does not need a special pump. Normally the pump is required to move the fluid so that it will pick up the drill cuttings but with this technique the drill string itself acts as the fluid pump.
FIG. 13-2. SHARPENED COUPLING
FIG. 13-3. SLUDGER METHOD USING SWIVEL
2. Advantages and Disadvantages
Here are the major advantages of using this technique:
· very few tools are required;
· the work is completed quickly;
· the subsurface conditions are easily determined from 1) cutting samples taken from the drilling fluid issuing from the top of the drill pipe and 2) the rate of descent of the tools.
The major disadvantages are:
· the method requires that water be available;
· rocks larger than medium sized gravel cannot be penetrated.
· Cutting bit: A regular pipe coupling can work as a cutting bit if the edges are sharpened. File or grind only the inside edge of the coupling to avoid narrowing the diameter. (See Figure 13-2.)
· Check valve: A valve prevents the downward flow of liquid in the drill pipe. Ball valves are commonly used. Where materials are scarce a well worker could use his/her hand to cover the top of the pipe when it is lifted, lifting the hand as the pipe is dropped. (See Fig. 11-6.)
· Drill pipe: Metal pipe which can be coupled together in suitable lengths as needed to provide necessary drill string height is usually used.
· Swivel hose connection to top of drill pipe: A water discharge piece is not absolutely necessary, although it is useful in helping to channel the water to a specific location rather than having it come directly out of the top of the drill pipe. (See Fig.13-3.) Depending on the lifting apparatus used, a flexible hose could even be connected directly to the top of the drill pipe although it may become damaged with the continual up and down motion adding stress to it.
· Lifting mechanism: This can be made of local materials and be whatever you think appropriate. Some options are:
- a tripod with pulley and rope and some way to pull and release the rope;
- a springpole;
- a capstan made of an empty rear car wheel;
- a lever type assembly attached directly to the pipe by means of a rope or chain. (See Fig. 116.)
· Hand tools
An assortment of hand tools is always handy at a well site. When using metal pipe,pipe wrenches will be almost indispensable.
4. Sinking Process
· Arrange and set up all tools, equipment and supplies. Leave the area immediately around the well clear of materials that do not have to be there. Set up the lifting device and locate it appropriately. A tripod should have the pulley centered over the hole. A lever type arrangement should have the end of the lever adjacent to the hole.
· Start the hole in non-caving formations by sinking a hole as far as possible with a post hole digger or spade. In caving formations drive a section of pipe down so that the top end is slightly below ground level.
· Dig out a settling pit appropriate to the water discharge piece. With a swivel type discharge head or where water can be channeled to one location, dig a settling pit on one side of well with a small channel feed for water from the pit to the hole. With no discharge piece,the only alternative is to make the settling pit a circular shape around the hole. The problem with that arrangement is that it allows too many cuttings to fall back into the hole.
· Fill the settling pit and hole with water. (See Fig. 13-4.)
FIG. 13-4. MORE FLUID WILL NEED TO BE ADDED TO ALLOW CONTINUOUS CIRCULATION THROUGH HOLE AND SETTLING PIT.
· Assemble the tool string,which consists of a bit, check valve, drill pipe and discharge piece. Set the tool string upright in the hole, and attach it to the lifting device.
· Begin the drilling operation by raising and dropping the tool string. The harder the material being drilled, the more rapid and shorter the strokes should be for the best results. Up to 120 strokes per minute may be desirable for shales, sandstone and limestone. The softer the material being drilled, the slower the stroke should be, from 30 to 60 strokes per minute in clays, loose sand, and gravel. In extremely soft material take care not to let the bit sink too far into the formation, thus plugging the bit or valve and making removal difficult. The best and safest method is to allow the bit to cut and mix the cuttings so that no chunks large enough to clog it are passed up the pipe. When threaded pipe is used in the tool string, it should be continually rotated in a clockwise direction to help ensure that the connections stay tight and a straight round hole is being drilled.
· Add more drill pipe when necessary as the hole is deepened. As the string becomes heavier it will require more and more power to lift it.
· It is also necessary to add water as the hole is being sunk to keep the hole full. The settling pit may need to be cleaned out to prevent excavated material from flowing back into the hole with the drilling fluid.
· When a good water-bearing layer is reached there is usually a noticable drop in drilling fluid level. Continued pumping action will produce water and if sufficient quantity of water is produced drilling can be stopped. Drilling fluid will also noticably drop when some dry sand, or gravel formations are reached. In this case it will be necessary to increase the weight (thickness) of the drilling fluid with additional clay, to help seal the hole and prevent fluid loss. (In this situation the casing could also be driven in to reinforce the walls.) Rice hulls, wheat chaff, cow dung or other material can also be added to the fluid but the more and coarser the sealing material used the more difficult it will be to develop and clear up the well later. Excessive use of hulls or straw-loaded material may later plug the well screen and render the well useless.
· Casing - As in most other sinking methods, the casing can either be installed after the hole is sunk or driven down as the hole is sunk.
- Casing the hole after it is completed is usually possible because drilling mud will tend to reinforce hole walls to prevent them from caving. Remove the drilling tool string and start the first section of casing pipe. It will usually have to be driven. Use a drive shoe (sharpened coupling) on the bottom of the casing to protect it. Add more sections of casing pipe as necessary. After the casing is driven it will be necessary to remove the material that will have accumulated inside the casing. This will not be difficult because the accumulated cuttings will be loose.
- Casing the hole while drilling proceeds is only necessary where the hole caves in, hampering drilling efforts. The casing is driven down as drilling proceeds so that the bottom of the casing is at the same level as where the tool is working. Water will then have to be introduced into the casing pipe, probably by hand because the casing will be sticking up above ground level, preventing drilling fluid from entering the hole. When this happens, the inefficiency of the "pumping" process through the drill pipe may not permit water to be pumped up high enough above the static water level in the casing pipe (which will go down as pumped) to come out of the discharge piece.
· The hole is drilled and cased to the maximum depth possible, to the bottom of the aquifer, or until the water-bearing layer has been penetrated far enough to produce the desired amount of water.
· Sink and develop the bottom section of the well. Most often the casing and well point are driven into aquifer.
NOTE: Where a single well is being sunk, the drill pipe could also be used as the casing if it is sealed tight to prevent water seepage down around the casing.
The driven and jetted sinking methods are different from the others in that they can sink the entire casing and well screen into the ground at the same time that the hole is being excavated They are used primarily to sink the casing and well screen into final position in holes that have already been sunk down to the water table where the aquifer is composed of loose, caving soil. They can also be used to sink wells from the ground surface, although the conditions which would allow them to be used economically are very specific and relatively uncommon.
1. The method
Techniques used in the sinking of driven wells are most commonly used not to drive well points and casing from the ground surface as the method describes but to drive casings down into place and well points and attached casing in from the top of the aquifer. It is a method that is used primarily along with and as a part of other methods.
FIG. 14-1. CONTINUOUS SLOT DRIVE POINT
This is a single piece of wire wrapped around and welded to a supporting frame attached to a steel point at one end and to the connecting pipe at the other. A cross-section of a bamboo variation of this type of screen is shown in Figure 15-4.
Make use of these various techniques when caving conditions prohibit the excavation of the complete hole for later casing.
This well sinking method is most effectively used in conjunction with hand augering where the aquifer is loose sand or gravel and the soil above it is non-caving. Hand-augering is more effective and faster in non-caving formations through which it would be difficult to drive a well point. Driving is then used in the lower water-bearing formation which will not support a handaugered hole.
2. Advantages and Disadvantages
These wells are easily driven, pulled out and put down elsewhere. In time and money expended to reach water, driven wells may be cheapest.
The limitations of a driven well include the following:
· Supplies are often unavailable. Drive points almost always have to be purchased because of the necessary high quality. Metal pipe and pumps are available in most countries, but not often in rural areas.
· Drive points will not penetrate hard rock and will penetrate clay only with great difficulty.
· A drive point can rarely be driven deeper than 15 meters.
The following equipment is required for constructing driven wells:
FIG. 14-3. BRASS-JACKED DRIVE POINT
A steel point attached to a pipe with holes in it, wrapped with a brass mesh preventing small soil particles from entering with the water.
· Drive well point. These are well screens with sharp steel points on their bottom ends so that they can be easily pounded into the ground. Because of the stresses put on them while they are being driven, drive points are invariably made of strong metal. It is possible to make a well point from a piece of iron or steel pipe by cutting and bending one end into a point, but commercially manufactured well points can withstand much greater stress. Above the pointed tip, all well points have a length of well screen through which water will enter the well. (See Figs. 14-1 and 14-2.)
· Metal pipe, Metal pipe is attached to the drive point and driven into the ground where it will act as the casing. Only metal pipe is strong enough to withstand the stresses put on it during the driving process.
· Drive couplings. These are special couplings in which pipe ends actually meet. (Questions have come up about the usefulness of this type of coupling because when driven, the connection loosens somewhat and cannot be tightened again because pipe ends are flush with each other already.) (See Fig. 14-3.)
FIG. 14-3. PIPE COUPLINGS
· Drive cap. This is installed On the pipe being driven to protect it. There are two kinds of drive caps: female drive caps (see Fig. 14-4a) which screw directly onto the threads or the top of the pipe and male drive caps (see Fig. 14-4b.) which screw into a drive coupling which screws onto the top of the pipe.
· Drive weight. This will be used to strike drive cap to drive it into the ground.
· Lifting device. This is an apparatus to lift drive weight to then let it fall on drive cap.
· A plumb bob or level is needed to check that the pipe being driven is going straight down.
· A water lifting device will be needed for final installation to allow people to get water from well.
FIG. 14-4. DRIVE CAPS ON TOOL STRINGS
Here are the major tools that are needed:
· Two pipe wrenches are needed to tighten metal pipe sections together.
· A pipe cutter or hack saw is needed to cut the metal pipe off at the desired level when it has been driven as far as desired.
· A metal file is needed to remove rough or sharp edges from the pipe after it is cut. This is not absolutely necessary, but it is always a handy tool to have around.
· Pipe threader. You will usually need to thread the metal pipe once it is cut so that you can attach the pump base plate to it. (Most commercial pumps have a screwed connection to the drop pipe.) Make sure that the pipe threader will work on the short section of pipe that is sticking out of the ground.
· Pipe dope or sealer should be put on metal pipe threads to make them watertight before the pipe is screwed into a coupling.
5. Driving Methods and Equipment
Following are five different methods and the equipment that can be used to drive wells.
· Use a sledge hammer to strike the drive cap directly. The equipment required includes a drive cap and a sledge hammer weighing 5-10 kilograms. Take care to hit the drive cap squarely because glancing blows may damage the pipe. Only a limited depth is possible because of the limited driving force. This method is physically very hard. (See Fig. 14-5.)
· Use a sledge to strike a driver which fits over the drive cap. The equipment required includes the drive cap, a sledge hammer, and a driver. A driver helps prevent damage from glancing blows.
· Use a weighted driver which fits over the drive cap. The driver is lifted and thrown or dropped to strike the drive cap. The equipment required includes a driver and drive cap. A driver can be fitted with handles to help in lifting and throwing, thus permitting two people to use it and enabling them to apply more force. (See Fig. 14-6.)
FIG. 14-5. SLEDGE HAMMER USED TO DRIVE WELL
· Use a steel driving bar attached to a rope which is lowered into the pipe to strike directly on the driving point. (See Fig. 147.)The equipment required includes a driving bar, rope, tripod, and pulley. This is one of the safest methods of driving because it does cause it does not weaken the pipe.
FIG. 14-6. WEIGHTED DRIVER
FIG. 14-7. STEEL DRIVING BAR ARRANGEMENT
(a) drive point can be driven down through the casing by a heavy weight which is lifted and dropped onto a reinforced head attached to the top of the drive point. The reinforced head shown here also has two sealing rings to seal it to the casing.
(b) This drive point is driven into place by a long heavy bar which, when dropped, strikes the back side of the steel point on the drive point. A special packing must then be wedged into place to seal the drive point to the casing.
· A 15-20 kg driving weight is used to strike a) a drive cap or b) a drive clamp attached to the pipe. The equipment required includes a drive cap or drive clamp, a driving weight, rope, pulley(s), and a tripod. These are all variations of the basic idea that a guided heavy weight strikes an instrument on the pipe to drive it.
a. The driving weight can have a-bar extending down from it which slides through a hole in the drive cap. (See Fig. 14-8a.)
b. The driving weight can slide up and down the metal pipe to strike a set of drive clamps attached to pipe. (See Fig. 14-8b.)
FIG. 14-8. HEAVY DRIVING WEIGHTS
6. The Sinking Process: From the Water Table
Here is a detailed description of the sinking process where a hole has already been sunk to the top of an aquifer from which it is desirable to draw water:
· Before removing the initial hole sinking tools from well, it would be useful if possible to try to sink the tools some distance into the aquifer to get an idea how deep it is. Only attempt this if your initial sinking tools are such that once sunk into the aquifer you are able remove them.
· Arrange and set up all tools, equipment and supplies. Make an effort to maintain an orderly well site as you proceed, to ensure the safety and convenience of further work.
· Attach the well point and extensions if needed, to the first pipe length. Pipe joints must be made up carefully both to insure a watertight joint and to prevent thread breakage. Attach a coupling to the top end of the first pipe length. Attach a pipe clamp just below the coupling on the top end of first pipe length. Lower the assembled pipe and screen into the hole,setting both ends of the pipe clamp on raised flat surfaces, such as wooden blocks on either side of the hole .
· Screw the coupling into one end of another pipe section and attach another pipe clamp below it as was done for first pipe section.
· Screw this second pipe section into the coupling on top of first pipe section now in the hole.
· Taking the weight of the pipe string on the pipe clamp on top of the second pipe section, lift the pipe string up just enough so that the first pipe clamp is not resting on the blocks.
· Remove the first pipe clamp and lower the pipe string until the second pipe clamp rests on the blocks.
· Continue adding pipe sections and lowering the pipe string until the well point rests on the bottom of the previously sunk hole.
· Plumb the pipe and drive it into the aquifer. plumbing the pipe will be much easier than if driving had been started from the ground surface, but you must still be careful that the pipe string is being sunk plumb. To tighten the joint, give the uppermost pipe a fraction of a turn with each blow until it is permanently set.
· Once you have driven the pipe as far as you can or want to into the water layer, you will need to develop the well. (See p. 202.)
· Before the pump is installed and any surface platform work is done, the gap between the pipe and the sides of the hole must be filled. This can be done with any material pulled out of the well up to a point three meters below the ground surface. The top three meters should be sealed with either mortar or puddled clay to prevent surface contamination from entering the water source.
7. The Sinking Process From the Ground Surface
Driving the entire well from the ground surface is most commonly used for extracting water from sands, especially those underlying beds of intermittent streams, and making use of the natural filtering properties of sandy beds of perennial rivers. Here is a detailed description of the major activities involved in sinking from the ground surface:
· Arrange and set up all tools, equipment and supplies. Clear the area immediately around the well site of all unnecessary tools. If a tripod is to be used, set it up and locate it so that the weight will be centered directly over hole. (It is easier to keep the pipe perfectly plumb if the weight is properly centered.)
· Place the well point in place and begin to drive it (p. 178). It is a good idea to dig or auger a shallow (5080 cm) hole in which to start the well point. Especially at the beginning, pipe plumb must be frequently checked. Later, when several lengths of pipe have been sunk, the whole length will be supported by the ground and will require infrequent checks of plumb. A support for the upper end of the pipe will initially help to hold it plumb and may later assist in aligning new sections of pipe so they can be screwed into couplings.
· Drive the pipe and add more pipe as needed. When the pipe has been driven so far that driving can no longer be accomplished, add another section of pipe.
- Remove the drive cap and screw a coupling in its place. Install a drive cap on top of the new pipe section. Screw the new pipe section into the coupling on top of the pipe set in ground. (See Fig. 14-9.)
- Where drive clamps are being used instead of a drive cap, a slightly different procedure is followed. To begin driving place clamps no more than 50 cm above ground on pipe (or at the bottom of a pipe section if the well screen sticks above the ground more than 50 cm). It is easier to keep the pipes plumb if the point of impact is closer to the ground surface. When the screen and pipe have been driven down to a point where the drive clamps almost touch the ground surface, move the clamps up 30 to SO cm. Reset them on the pipe and drive them down again. As more and more pipe is driven into the ground and the danger of driving the pipe string out of plumb decreases, the clamps can be moved. When the driving weight can no longer be raised far enough to provide sufficient striking force to drive the pipe string, a new length of pipe will need to be added. Screw a coupling onto the top of the pipe being driven and screw a new section of pipe into the top of the coupling. Make sure the weight can be raised above the coupling before the coupling is screwed on the pipe. You may want to slip new pipe through the driving weight before attaching it to the coupling.
· To determine whether water has been reached, the plumb line can be lowered into the pipe. If the line comes up wet, you have hit water. By comparing the depth the line reaches to the known depth of the pipe, you can also get an idea about whether earth or sand has to any significant degree entered the screen.
· You can also learn what kind of soil or formation you are driving through by the reactions of the pipe and driving weight when the pipe is struck by the weight.
FIG. 14-9. INSTALLING NEW PIPE SECTION
RELATION OF DRIVING TO SOIL CONDITIONS
· When you reach water, drive the pipe as far into the water layer as possible. The bottom of the well point should just touch the top of the impermeable layer on top of which the water sits. To be useful and not put too much stress on the well screen, the aquifer should be deeper than the height of the well screen. (See Well Screens, p. 193.) If you can drive the point six or seven meters into an aquifer without reaching the bottom, you have assured a very large supply of water and there may be no point in driving any further.
· If you don't reach water or have for some reason driven beyond the water bearing formation, you may want to lift the pipe string just a little or remove it completely from the hole. 1) You can sometimes lever the pipe up by using pipe clamps and crowbar or a stout stick. (See Fig. 14-10.) 2) You can use two jacks and pipe clamps to jack it out. (See Fig. 1411.) After it has been raised a few feet, the rest can be done by hand. Rotating the pipe Clockwise will assist its removal.
· Once you have driven the pipe as far as you can or want to into the water layer, you will need to develop the well. (See p. 202.) You may wish to prepare the top end of the pipe for connection or use with the particular water lifting device you are using before developing the well.
FIG. 14-10. LEVER PIPE UP
FIG. 14-11. JACK PIPE UP
1. The method
Jetted wells are sunk through the action of a fluid under pressure directed to the bottom of the hole to loosen soil particles and carry them to the surface.
Variations of this technique have been used in many different wells construction situations. A major use is to wash the casing into a loose or slightly caving formation by pumping water down through the casing and out the open bottom of the casing or well screen. Wells continue to be jetted from the ground surface only in areas where rock is not likely to be found and the larger drilling equipment is not available or too expensive.
In jetted techniques, drilling fluid is pumped down through the hollow drill rod and out through a hole in the jetting bit. (See Fig. 11-2g.) The faster and more powerful the spray, the better cutting action it will have. After the fluid has been directed at the bottom of the hole, it flows back up the hole carrying with it the soil that has loosened from the bottom of the hole. Once the fluid reaches the top of the hole, it is channeled through a small ditch into a large settling pit. The fluid stands in the settling pit long enough to allow drill cuttings to settle out of the fluid, before it is pumped back down the drill rod.
The volume of the settling pit should be at least three times the volume of the hole being drilled. It should be relatively shallow (0.7-1.0 meter is usually sufficient) and about twice as long in the direction of flow as it is wide and deep. For example: a settling pit two meters long, one meter wide and one meter deep could be used when drilling a 10 cm diameter well 85 meters deep.
The drilling fluid is usually a mixture of clay and water. The clay is needed to make a fluid of such consistency that it will tend to reinforce the hole walls by forming a kind of "mud cake" along them. Fluid with too much clay and accumulated drill cuttings will be thick and difficult to pump. The thickness of the fluid may need to be adjusted during drilling by adding more water and/or removing some of the accumulated cuttings from the settling pit. Water alone will often act effectively as a drilling fluid, especially as it thickens after accumulating some of the finer drill cuttings.
2. Advantages and Disadvantages
Here are the major advantages of using this technique:
· Loose, caving formations are easily penetrated.
· Few people are needed to operate the equipment.
The major disadvantages are:
· The method requires more special equipment than others.
· A large quantity of water is required for drilling.
· Hard clay and boulders may slow and stop drilling.
· Jetting bit: A shortened percussion type bit with water passages is helpful in loosening material at bottom of the hole. (See Fig. 14-12.)
FIG. 14-12. JETTING BITS
· Drill rod: Hollow drill rod is necessary to transmit the drilling fluid to the bottom of the hole. The smaller the diameter of the rod the lighter it will be.
· Swivel hose connection to top of drill rod: It is not absolutely necessary to have a swivel connection although some type of connection is needed through which fluid can be pumped into and down the drill rod. A swivel connection allows the tool string to be turned during jetting, thus helping to assure a straight, plumb hole and helping to loosen soil at the bottom of the hole.
· Flexible hose; This is needed to connect the pump with the connection on the top of the drill rod, allowing fluid to be pumped through the drill rod. A length is also needed between the pump and the setting pit from where the fluid is pumped.
· Pump: A pump is needed to move the drilling fluid through the drill rod and out the bit, where it can loosen and remove soil from the bottom of the hole. Large capacity hand pumps have been successfully used although motor pumps are easier and provide better jetting action. Diaphragm pumps are probably best suited to this kind of work because of their ability to move relatively large soil particles without damaging the pump.
· Tripod or other overhead support: Some type of overhead support is needed from which the tool string can be suspended and lifted or lowered when necessary.
· Hand tools: A number of hand tools can be used to connect and disconnect hoses and drill rod when necessary. Pipe wrenches, screwdrivers, and regular wrenches may be particularly handy, although shovels, rope and pulley will also be needed.
4. Sinking Process
· Arrange and set up all tools, equipment and supplies. Leave the area around the well clear of materials that do not have to be there. Set up the overhead tool string support and locate it directly over the proposed site. (See Fig. 14-13.)
· Dig out a settling pit as needed for the hole diameter and expected depth. A shallow channel should connect the settling pit and the hole so that drilling fluid coming up out of the hole will flow into the settling pit.
· Start the hole by digging as far as possible with a shovel or post-hole digger.
· Fill the settling pit and hole with drilling fluid or water.
FIG. 14-13. JETTING EQUIPMENT
· Assemble the tool string and suspend it from the overhead support. Attach the hoses between the drill rod and the pump and between the pump and the settling pit.
· Start the pump to begin the drilling operation. Begin by pumping slowly to allow time to carefully plumb the tool string as it sinks the first meter or two. As the drilling proceeds, more pressure may be necessary to get the needed cutting action with the heavier drilling fluid. A slight up and down percussion action may help to speed the drilling.
· Add more drill rod when necessary as the hole is deepened. The fluid circulation will have to be stopped to permit another length of drill rod to be added into the tool string. It may then be difficult to get the fluid circulation started again because the heavier soil particles tend to settle in the hole, clogging the bottom. Where the aquifer depth is known, it is possible to avoid this problem by assembling the entire casing and open bottom well screen above the hole, supported by a large scaffolding, so that it can all be sunk in one continuous operation.
· It may be necessary to add more fluid and clean some of the accumulated cuttings from the settling pit at some point during the drilling process so that drilling can proceed effectively.
· When a good water-bearing layer is reached, there is usually a noticable drop in the drilling fluid level and often a significant increase in the speed with which the hole is being sunk. The hole should be sunk as far as possible into the aquifer.
· Because the jetting operation tends to reinforce the hole walls with the drilling fluid, it may be possible to pull the drilling tools completely out of the well and then set the assembled casing and well screen into the bottom of the hole. Keep an accurate record of the depths at which water was first reached and where the drilling was stopped so that you can determine how much well screen should be installed and whether the casing and well screen have been lowered to the actual bottom of the hole. If the complete casing cannot be lowered into place it will have to be driven or washed down. You will want to jet the well to its final depth. It is possible that the well may begin to cave in as the tools are being pulled out, but if it has been jetted to the final depth, it will be much easier to drive or wash the casing and well screen into place than if the well has not been completely sunk.
The bottom section of a small diameter well usually consists of part of the casing pipe and a well screen which acts as the intake section. (See Fig. B.) In order for the well screen to function effectively, the aquifer material that surrounds the screen must be rearranged to allow as much water as possible to flow through it. This rearranging process is known as development.
For wells that are sunk into solid rock, the intake section could simply be the open bottom of the casing pipe but this is not a normal occurrence in relatively shallow wells.
The intake section or well screen is a continuation of the casing in that it reinforces the walls to keep the hole open. Simple screens can be made by making holes in the casing. However, commercially available well screens may incorporate useful features that cannot be obtained from homemade screens. (See Well Screen, p. 193.)
B. Considerations in the Construction of the Bottom Section
There are several considerations to bear in mind before beginning work on the bottom section:
· choosing the appropriate screen. One must consider the cost, material, local manufacture, opening size, diameter, length, and strength;
· properly placing the screen in the water bearing layer so that it can produce as much water as necessary;
· developing the aquifer material surrounding the screen to obtain the most efficient production.
C. Construction Procedure Outline
In general, there are only two steps involved in the construction of the bottom section:
· setting the screen in the desired place in the hole;
· developing the well.
Both of these steps must be done carefully and precisely in order to make full use of the rest of the work that has gone and will go into building the well. The amount of water that can be drawn from the well is almost completely controlled by how this section is built. Here are the activities involved in the two steps:
1. Set the screen in its designated place in the hole. Two general procedures can be followed, depending on the size of the well screen in relation to the casing pipe.
a. A pipe-size screen is attached directly to the bottom of the casing pipe and lowered into place in the hole. Usually the hole has been at least partially sunk and the screen and casing have penetrated to the desired depth by being driven or washed down. Where drilling fluid has been used in the sinking process, it may reinforce the hole enough to prevent its caving, thus permitting the screen and casing to be simply lowered into place.
b. A telescoping-size screen is lowered into Place through casing.
1) The most popular professional method is for the hole to be drilled to the desired depth and cased. The screen is then set in place on the bottom of the hole inside the casing and then the casing is pulled up to expose the screen. A special packing piece is next wedged into place between the screen and the casing to prevent the entry of unwanted aquifer material.
2) The screen can also be washed down into the aquifer from the bottom of the permanently placed casing.
NOTE: Telescoping screens into place may be difficult with locally made materials because of the difficulty of sealing the screen to the casing pipe once both have been set in their permanent places. Normally this is done with a special packing piece that is wedged into place.
2. Develop the Well
This is the process of using any one of a number of tools or methods to quickly move water in and out of the well screen, thereby removing the fine particles from the aquifer in the vicinity of the screen. It usually requires an average of 6 to 10 hours to complete.
D. Materials and Equipment
The following types of materials and equipment will be required for construction of the bottom section:
· Sinking equipment: The different sinking methods and ground conditions in which they are appropriate have been covered in detail in "Middle Section, Sinking Methods." Because people do not have to work in this kind of well in order to sink it, sinking methods need not change between the middle and bottom sections. The only difference between the two is that ground conditions beneath the water table are liable to be loose and caving.
· Well screen intake section: This refers to any portion of the casing that has holes in it which are intended to allow water to enter the well. (See section E on Well Screens, below.)
· Developing the well: For the equipment necessary to develop the well, see the section on Development, p. 202.
E. Well Screens
A well screen is a special section of casing with holes in it which is placed in the water bearing layer to allow water to enter the well. It should be strong enough to withstand stress while it is being installed as well as caving pressure in the hole. It should be made of material appropriate to the chemical and bacteriological characteristics of the local ground water, neither easily corroding nor rotting. The greater the open area of holes contained in the screen, the more efficiently it will work by letting in more water.
Well screens can either be commercially or locally made. The cost of commercially made screens and the time required for delivery to the well site usually make them unsuitable for isolated wells whose object is to supply only a few thousand liters of water per day to the local inhabitants. However, commercially manufactured screens offer advantages in quality. They can often be more easily installed and provide more water of better quality for a longer time than locally made screens. (See Fig. 15-1.)
Locally made screens are usually produced from the casing materials that will be used in the normal small diameter well sinking operation. There are some limitations concerning which materials can be used in various kinds of ground formations, but an appropriate solution can almost always be found. The cost of a locally made screen will simply be the cost of putting holes in a section of casing which has already been purchased for installation in the well. Perhaps their biggest disadvantage is the difficulty of producing the small opening sizes which may be necessary in fine sands. (See Fig. 15-2.)
FIG. 15-1. MANUFACTURED SCREENS
A new method has been developed which would enable the local manufacture of continuous slot well screens out of specially made plastic pipe. This pipe is extruded to provide lengthwise reinforcing ridges along its inside surface. A lathe equipped with a high speed grinder on which the grinding wheel is replaced by a small circular saw the desired slot width is used to cut a continuous slot. For more information see the article by Sternberg (Sternberg 1978) cited in the bibliography.
Manufacturers will provide well screen information on request. However, because shipping and cost considerations may prohibit the use of commercially produced well screen, the remainder of the discussion will relate to locally producable screens.
FIG. 15-2. LOCALLY MADE SLOTTED PLASTIC PIPE
Designing the bottom section of a small diameter well involves deciding 1) what the screen will be made of, 2) how long it will be, 3) what the diameter of the screen should be, 4) how big the openings are, 5) the opening shape, 6) the arrangement of the openings, and 7) exactly where this will be placed in the water-bearing formation.
In sinking with locally available hand powered equipment, certain limitations will influence how you design the bottom section.
1. Screen Material
Materials that are suitable for use as casing can all be modified for use as well screens by making holes in them.
· Commonly used materials such as steel or galvanized steel are suitable and often available, and can be driven into place where necessary. The quality of the water should be carefully checked for the possibility of corrosion or incrustation. Note that galvanized steel is only coated on the outside, so that once holes are made in it is no longer protected from rusting. This is a serious consideration because well life spans have been as little as three months in areas where steel casing was used in highly corrosive water.
· Plastic such as PVC and ABS is very useful for screen production where driving is not necessary. Or, alternately, the casing and the screen can be bailed down under constant downward pressure. Holes are easily cut into plastic, the material is so lightweight that it may be possible to lower it by hand. It will not corrode and is usually significantly cheaper than steel.
· Clay is another possibility. Clay pipe locally made and fired should be made with socket and joint fittings to enable certain location and prevent unwanted surface water entry. They are most successfully used where the hole is completed all the way to the bottom before the screen and casing installation. (See Fig. 15-3.)
FIG. 15-3. CLAY PIPE CASING AND SCREEN
· Wood has been used in some temporary wells. In these cases, long straight sections of tree trunks have been used for casing and for screens. If you must for some reason use wood, cut the screen openings lengthwise with the grain for greater strength.
· Bamboo, where it is available, has been used for casing and screens in wells whose life expectancy is only a few years. With the inner sections punched out, it can be made into a servicable pipe. Screens can be made in two ways. Long pieces of bamboo can be split lengthwise, arranged around circular spacers and wrapped with rope, (See Fig. 15.4) or narrow lengthwise slits can be cut with a circular saw in a solid piece of bamboo. As the bamboo gets wet, these slits will become smaller, an advantage in a well screen.
· All of the materials mentioned above have some limitation which makes them inappropriate in certain situations. Steel is easily corroded. Plastic will deform under too much stress. Clay is easily broken. Bamboo and wood will rot and taint the water. some materials, such as stainless steel, can be used in almost any situation but they are usually expensive.
FIG. 15-4. BAMBOO WELL SCREEN
2. Screen Length
Most small capacity well screens are between one and three meters long. The determining factor is the amount of open area the area of all the holes added together) that will permit water to enter. Two or three meters of locally made well screen will usually be sufficient while one meter of commercially produced continuous slot well screen is more than sufficient for use with a hand pump.
To check whether your screen is long enough:
· Determine the total amount of hole space in the screen. Assuming the holes are all approximately the same size and shape, you can figure out the area of one hole and multiply that figure by the number of holes.
· Next, estimate the maximum short-term demand for water. To do that, answer the question: How fast can someone possibly remove water from the well? That will be expressed in liters per minute or gallons per minute.
· Divide the first figure by the second figure. The screen should have at least 5.5 cm2 for each liter of water per minute, or 3.23 in2 for each gallon per minute, that can be taken from the well. These are absolute minimum figures. A greater area will be much better for well screens made of locally available materials.
3. Screen Diameter
When the screen is made of the same material as the casing, it will be the same diameter as the casing. It is simply a normal section of casing with holes.
It would be possible to make the screen a larger or smaller diameter than the casing and still firmly attach it to the casing by using reducing couplings or bushings, but there appears to be no advantage in doing so. If increasing the diameter would increase the amount of water that would flow into the well, it might be useful, but the increase in water flow with an increased diameter is so small as to make it insignificant.
4. Opening Size
This is probably the most difficult technical aspect to deal with because it is often just not possible to make your own screen openings as small as they should be.
Ideally, the openings should be big enough to allow the smaller particles of the surrounding soil through and yet hold out the larger soil particles.
5. Opening Shape
Generally the most effective opening shape is a long thin slit, easily made with a saw. This provides the greatest amount of open area in a small space and is not easily clogged. Small round or square holes, on the other hand, are more easily clogged and require more construction time. Depth and cross-sectional shape can also be important but can only be effectively dealt with in commercially manufactured screens. (See Fig. 15-5.)
6. Arrangement of Openings
Make and arrange the screen holes both to maintain material strength and also to permit the greatest flow of water.
FIG. 15-5. PUNCTURED METAL CASING USED AS WELL SCREEN
7. Location of the Screen in a Water Bearing Formation
Ideally, the screen should rest on the bottom of the water bearing layer and extend up from there. Often, however, it is not possible to sink the well that deep with hand powered equipment. In that case, the screen should be placed as far into the water bearing layer as possible. Some well construction specialists suggest that 6 to 7 meters is deep enough beneath the water table to assure adequate year-round yield. However, this will vary depending on several factors that are not easy to evaluate, such as aquifer permeability and surface recharge. (See Glossary.)
In some cases, the aquifer will be made up of layers of different kinds of soil. If sufficiently deep into the aquifer, the coarsest layer will produce the most water and, therefore, that should be where the screen is set.
G. Installation of Well Screen
There are two different general techniques for setting the screen in place in the hole. In the first, the screen is attached to the bottom of the casing and lowered into place with casing. In the second, the screen is telescoped through a casing that is already in place. Telescoping screens into place is a difficult procedure to complete successfully with only locally available materials. (See Fig. 15-6.)
Here are four possible approaches to setting a well screen and casing in place.
1. The casing and the screen can be lowered into a hole that is already sunk to the finish depth in the aquifer. This is only possible where the hole will stay open long enough to allow the screen and casing to be lowered into place, i.e., non-caving formations or where drilling mud has been used. In cases where minor caving will occur, it may be possible to drill the hole a little deeper than necessary so that caving material will accumulate below the level of the screen and not interfere with its installation. Where this will not work, another method of screen and casing installation must be used. Any appropriate casing material can be used because no particular strain is put on the casing while lowering it into place. With sections of clay pipe, for example, where one end fits inside the end of the next sections, the sections can even be lowered and placed one at a time. (See Fig. 15-7.)
2. The casing and the bottom screen can be sunk into their final position by operating a bailer inside the casing and screen to remove soil and allow the casing co sink. This method can be used to loosen caving soils which will not permit the hole to remain open. It is also employed in harder clay type soils although it will quickly reach a point where the casing will no longer sink because of too much friction with the surrounding ground material. It is useful where the casing is made of a material such as plastic that cannot be driven but can be helped along by adding weight to the top of the casing. When the desired depth is reached, the open screen bottom must be plugged. (See Fig. 15-8.)
FIG. 15-6. TELESCOPING SCREEN
FIG. 15-7. SCREEN AND CASING LOWERED INTO COMPLETED
FIG. 15-8. SCREEN AND CASING BAILED DOWN
3. The casinq and the screen can be driven into final position by any of the previously outlined driving techniques (See p. 178.) This method can be used in any unconsolidated formation whether it is caving or not, but is more useful in caving formations. In this method, an open bottom screen is alternately driven and bailed out as in the previous technique, but because the casing can be driven, it is not as likely to become jammed in the hole in harder layers. A hard casing and screening material must be used that will not easily deform when struck. When the desired depth is reached, the open screen bottom must be plugged. (See Fig. 15-9.)
4. Where pumps are available, the casing and screen may be washed in by pumping water down through the casing and out the bottom of the screen where it will pick up and carry soil particles back up to the surface between the casing and the hole walls. Some water will go out the screen openings but most will go out the bottom. This method can be used only where the casing is a continuous waterproof string. (See Fig. 15-10.)
FIG. 15-9. SCREEN AND CASING DRIVEN INTO PLACE
FIG. 15-10. SCREEN AND CASING WASHED DOWN
1. Basic Features
To develop a well is to remove the very small soil particles from the water-bearing formation which immediately surrounds the well screen. This increases the yield of the well and forms a graded filter around the screen. The filter prevents the entry of small soil particles which could eventually clog the well or damage the pumping mechanism.
A well can be developed once the screen and casing are in their permanent positions in the hole. By using the tools and methods described below, water is forced in and out of holes in the well screen. This in and out water motion tends to loosen the adjacent soil and carry with it through the screen openings all particles which are small enough to fit through them. Only the largest soil particles are left, deposited next to the outside of the screen. Farther from the screen, the predominant soil particles become smaller and smaller. (See Fig. 10-5.) When development is complete, a graded soil filter surrounds the screen, which both prevents the entry of small soil particles into the well and allows the water to flow as freely as possible toward the screen. This is possible because there are gradually more open spaces for the water to flow through as it gets closer to the screen.
The time required for development depends on the nature of the water bearing layer, the opening size of the well screen as related to aquifer particle size,and the type of equipment and degree of development desired.
This is probably the simplest method of removing the "fines" from the water-bearing formations. The well is pumped at a faster rate than normal until no more fine aquifer particles are removed with the water. While this does not really agitate the soil enough to create a real filter around the screen, overpumping is a useful technique. If a well will support overpumping, it should certainly operate at a capacity less than that with no problems.
It is strongly recommended that if a well is to be developed by overpumping, a separate pump should be used for the development process. Fine soil particles to be removed during development can cause an abnormally high rate of wear on the pump resulting in early pump failure.
This, too, is a relatively simple method of development which requires a water lifting device and a container in which water can be stored and then from which it will be allowed to flow easily back into the well. This involves pumping water to the surface and then letting it flow back into the well many times. The process provides a back and forth motion that can more effectively develop a water-bearing formation than overpumping. However, in many cases, the motion may not be strong enough to obtain maximum development.
Backwashing is more difficult to accomplish than might be expected. The water lifting device used to pull water to the surface may have to be completely removed from the hole to permit water to flow back down. Water will not flow back down a pump riser pipe because there is a foot valve at the bottom designed to prevent that. It must then flow down between the pump pipe and the casing. An exception is a turbine pump which will pump water without a foot valve, but these are expensive. Note, however, that even a little backwashing is better than none.
Surging is the most common method of well development. It involves forcefully moving water back and forth, in and out of the well screen, to remove the fine soil particles. The surging action is caused by a tool being lowered into the well casing to some depth beneath the water level where it is then moved up and down, causing the water to move back and forth. The closer the tool seals to the well casing, the more forceful the surging action can be.
There are a variety of tools that have been used to give this surging action.
· Bailer: If an open bottom screen is sunk into the water-bearing layer by bailing, the bailer's up and down motion also causes a surging action which will develop the area around the screen. The heavier the bailer is, the better it is, because it then has more force to push water back out of the screen. A bailer may operate more effectively for this purpose if it has accumulated soil, making it heavier and in some cases preventing water from coming up through the casing. A bailer can also be used to develop a formation once the well screen is already in place. Because bailers cannot form tight seals with the inside casing walls, they do not usually develop a formation as well as a surge block. (See below.)
· Swab: A swab is simply a series of rags carefully tied around a pipe and built up until they will fill the casing pipe. This is then lowered down into the water and operated in much the same way as a bailer.
· Surge block: A surge block is basically a flat seal that closely fits the casing interior and is operated like a plunger beneath the water level. Because it seals closely to the casing, it has a very direct positive action on the movement in the well. There are two basic types of surge plungers, solid and valve type. Both can be easily made from fairly readily available local materials. (See Fig. 15-11.)
FIG. 15-11. SOLID TYPE SURGE PLUNGER
A valve type surge plunger is made the same way as a solid type plunger but with two additions. Before assembly, several holes are drilled through the wood pieces and the sealing rings in such a way that they will all line up when the plunger is assembled. During the assembly a flexible sealing flap, with the same diameter as the wood, is added between the top wood piece and the metal washer. This acts as a flap valve over the holes that have been drilled through the rest of the plunger. (See Fig. 15-12.)
The action on the down stroke of a valve type plunger is milder than that of a solid plunger because some water will pass up through the holes in the plunger and may possibly be pumped to the surface by the up and down plunger action if the plunger seals well with the casing.
· Arrange and install necessary tools and equipment in an appropriate place. A plunger or swab will need to be beneath the water level.
· Apply an up and down motion, repeatedly raising and dropping the plunger 60 to 100 cm. The plunger should drop rapidly on the downstroke either as a result of being forced down or because of the weight of the connecting shaft.
· Surge for several minutes, then remove the plunger and use a bailer or sand pump to remove the accumulated fine particles. Be careful not to surge too long. If you do, the screen will fill up with so much fine material that only the upper portion of the screen can be developed.
FIG. 15-12. EXPLODED VIEW OF VALVE TYPE SURGE PLUNGER
A solid type surge plunger can be made from the same materials. Leave out the flexible sealing flap and do not drill holes through the wood supports and sealing rings.
· Continue surging and bailing until no more fine particles are removed with the water. The up and down motion of the plunger should at first be relatively slow and continue that way until the amount of fine material drawn into the well begins to decrease. The speed should then be gradually increased and each surging session should become longer and longer.
Exactly how long and how fast surging takes place will depend on how much material is being brought into the well, the ease with which it is brought up, and the kind of equipment you have.
Adding some kind of weight to the plunger or the connecting shaft will probably make it easier to work for a longer period of time, especially if you can use a rope and pulley to lift and then drop it in the well. If you can add some weight it is best to add it as close above the plunger as possible.
NOTE: It is a good idea to make the operating shaft long enough so that if it is dropped down into the well some of the shaft will still stick up above the casing to help in its removal.