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close this book Handbook for building homes of earth
View the document Table of contents
View the document Foreword
View the document Chapter 1: Introduction - Types of earth houses
View the document Chapter 2: Soils - And what can be done with them
View the document Chapter 3: Stabilization of soils
View the document Chapter 4: Where to build
View the document Chapter 5: Foundations
View the document Chapter 6: Lightweight roofs
View the document Chapter 7: Preparing the soil
View the document Chapter 8: Making adobe blocks
View the document Chapter 9: Making pressed earth blocks
View the document Chapter 10: Making walls with earth blocks
View the document Chapter 11: Making rammed earth walls
View the document Chapter 12: Roofs for earth houses
View the document Chapter 13: Floors
View the document Chapter 14: Surface coatings
View the document Suggested references
Open this folder and view contents Appendix

Chapter 5: Foundations

The foundation is that part of the house that is built below the ground surface and supports the house. A properly built foundation will keep the house from being damaged or twisted out of shape due to settlement of the earth, high winds, or frost action. A good house must have a good foundation; it is the most important part of the house.

TYPES OF FOUNDATIONS - House foundations may consist of separate piers or footings, which receive the load of the house through heavy beams that rest on them, or they may be continuous footings that run under all outside walls and load-bearing partitions. For earth walls, continuous footings are most commonly used. A foundation wall rests on the footing and extends upward and supports the walls. A typical continuous footing and foundation wall is shown in Figure 25. Not all such construction requires reinforcing steel.

FOOTINGS - Footings have three purposes: (1) to provide a solid, level base for the foundation walls; (2) to hold the weight of the house it supports on the earth so that the load will not be too great at any point for the soil to support; (3) to resist the "lifting" forces of hurricane or high winds which can tip a house over or move it off of its foundation.


Figure 25.

The size and depth of footings depend on: the type of material used to make them; the weight they must support; the load the soil will support, and, to some extent, the weather conditions of the area.

DEPTH OF FOOTINGS - In areas where freezing weather occurs, footings must be placed in the ground at least as deep as frost is ever found. Otherwise, the footing will shift and crack the house during or after freezes. The frost line is shown in Figures 26a and 26b as a heavy dash line. Figure 26b shows what can happen if the footing is too shallow. If no other means of knowing this depth is available, it can be roughly estimated on the basis of the resistance to digging during particularly long and severe winters. Ice particles can be seen in the soil. The footing should be placed below where ice is found.

In warmer climates, the footing depth needs to extend deep enough to reach good, solid earth free from vegetation (roots, etc.) and deep enough to prevent washing out or shifting due to rain and flooding. Usually, this depth averages from 12 to 18 inches. An exception to this is where the soil shows considerable shrinking and swelling as it aria and becomes wet during the year. If this is the case, the footing should extend to a depth where seasonal changes in moisture are minor. Sometimes this can be estimated by the appearance of the soil. The soil is usually very crumbly in the area where the moisture changes seasonally and it takes on a more solid appearance below this depth.

The best and easiest way to find out the proper depth of footing is to see what depth was used for similar houses in your area. If the houses are holding up well and have not settled or cracked, the footing depth used for them will do for your house. If you have any doubt, go deeper.

SIZE OF FOOTING - The size of the footing will depend upon the strength of the soil and the weight of the house. As a general rule, most shallow clays and shallow silty soils should not be loaded above 2,000 pounds per square foot of footing area. Firm sandy and gravelly soils may carry from 4,000 pounds to 6,000 pounds per square foot.

If there is any question about the value of the soil, a loading tat should be performed. A simple loading tat is described in Appendix A. Use it if you can. If you can't, then the strength of the soil can be very generally estimated using the 'simple strength test" given in Appendix A.

Once the bearing value or strength of the soil is known, and the weight of the house has been determined, the width and thickness of the footing can be easily determined from Table 4. The dimensions given in Table 4 can be varied somewhat depending on local experience but do not reduce any satisfactory values you find. The building in Figure 27 shows the effect on an earth wall of an under-designed foundation.


Figure 26.

MATERIALS FOR THE FOOTINGS - Since the footings are buried in the soil, they should consist of materials that cannot be damaged by decay or insect attaches. Use concrete, stone, brick, concrete block and similar materials. Recently, some footings have been made with stabilized soil. However, they have not been in place long enough to determine how suitable they might be. On the basis of what is now known, it is better to avoid the use of stabilized soil for footings except in arid or semi-arid regions.

CONCRETE FOOTINGS - Footings made from poured concrete are the most popular type because they are easy to make and work very well; however, they will probably be the most expensive type. Usually forms are not needed: as a trench can be dug to the correct depth and width of the footing and the concrete can be poured directly into the trench to form the required footing thickness. (See Figure 28.) If the soil i. sandy or tends to slide or cave in, then simple forms are required to insure that the footings are wide enough and that the soil does not fall in the freshly poured concrete. These forms may consist of boards with width equal to the thickness of the footing and held in place by enough wooden stakes to keep the boards from moving or bowing. The forms should always be removed, but not earlier than 48 hours after the concrete has been poured. It is best to cure the concrete for several days by covering it with wet sacks, leaves, etc.


Figure (See Table 4.)

 

TABLE 4

FOOTING WIDTHS AND THICKNESSES FOR CLAY FOUNDATION SOILS

IF SOIL STRENGTH IS

A

B

C

D

E

F

USE W AND T

W

T

W

T

W

T

W

T

W

T

W

T

Kind of House

                       

One Story - 6 inch thick walls - Light Roof

18"

7"

12"

1/8"

12"

8"

12"

8"

12"

8"

12"

8"

One Stow - 6 inch thick walls - Earth Roof

27"

8"

15"

9"

12"

10"

12"

10"

12"

10"

12"

10"

One Story - 9 inch thick walls- Light Roof

                       

One Story - 9 inch thick walls - Earth Roof

36"

18"

18"

9"

12"

11"

12"

11"

12"

11"

12"

11"

One Stair - 12 inch thick walls - Light Roof

                       

One Story - 12 inch thick walls - Earth Roof

36"

18"

18"

10"

12"

12"

12"

12"

12"

12"

12"

12"

One Story - 15 inch thick walls - Light Roof

36"

8½"

21"

10"

15"

12"

15"

12"

15"

12"

15"

12"

One Story - 15 inch thick wall - Earth Roof

86"

9"

24"

10"

18"

11"

15"

12"

15"

12"

15"

12.

Two Story - 9 inch & 6 inch walls - Light Roof

36"

9"

24"

10"

18"

11"

15"

12"

15"

12"

15"

12"

Two Story - 9 inch & 6 inch wall - Earth Roof

No*

 

27"

9"

21"

11"

18"

12"

15"

12"

12"

12"

Two Story - 12 inch & 9 inch walls - Light Roof

No

 

30"

9"

24"

10"

18"

12"

15"

18"

15"

13"

Two Story - 12 inch & 9 inch walls - Earth Roof

No

 

33"

9"

24"

11"

21"

11"

18"

12"

15"

13"

Two Story - 15 inch & 12 inch walls - Light Roof

No

 

36"

10"

27"

11"

24"

12"

18"

13"

18"

13"

Two Story - 16 inch & 12 inch walls - Earth Roof

No

 

No*

 

30"

10"

27"

11"

21"

13"

18"

14"

Two Story - 18 inch & 16 inch walls - Light Roof

No

 

No

 

36"

11"

30"

12"

24"

13"

21"

14"

Two Story - 18 inch & 16 inch walls - Earth Roof

No

 

No

 

36"

11"

33"

12"

27"

13"

24"

13"

* Do Not Build on Foundation Soil This Soft.

 

When digging for footings it is always better to dig on the "safe" side; that is, too deep, rather than too shallow. If the excavation is too deep it can be filled with extra concrete, sand or earth, but the sand and earth must be tamped in and not just loosely placed.

Concrete footings do not always require reinforcing steel but it is better to have them lightly reinforced to resist cracking that occurs with changes in temperature. Conditions where reinforcing is considered absolutely necessary include occasional "soft" spots in the bottom of the trench, and areas subject to earthquakes, hurricanes, or swelling of the soil. Reinforcing is also needed if the footing projects beyond the foundation wall more than 2/3 of its thickness. In these cases, it is desirable to seek the services of an engineer or architect for advice concerning the kind. size and location of reinforcement.


Figure 28.

The concrete for the footing should have enough water to be workable but at the same time it should not be so wet that the mortar tends to flow away and leave only the larger rocks. A good concrete mix for the footing uses:

1 part portland cement

3 parts clean washed sand

4 parts clean gravel

about 5 to 7 gallons of water for every 94 pound bag of cement

A good way to estimate whether or not you have the correct moisture is to place some of the mix in a bucket and rod it about 25 times with a smooth steel or wood rod. Then from a height of 6 inches above the surface, drop a smooth round rock about 4 inches in diameter into the mix. If it dents the mix about 1½ inches, the water content of the concrete is about right.

If reinforcing rods are used, they are placed in the excavation and securely supported at the proper height before the concrete is poured. The top finish of the footing is left very rough to provide a good bond for the foundation wall. It is usually a good idea to leave a small channel or groove in the center of the footing as shown in Figure 28 to provide additional bond between the footing and the foundation wall if the footing is made of concrete. If at all possible in earthquake areas, short lengths of steel reinforcing rod should be placed in the footing so they will project into the foundation wall and tie it securely to the footing. This usually requires careful spacing of these rods.

Sometimes the amount of concrete for the foundation can be greatly decreased by placing large stones in the trench and pouring the concrete around them to form the footing. If the stones are large it is a good idea to make the footings somewhat thicker than for regular concrete footings. Roughly, the thickness should be about l½ times the size of the largest stones. For example, if 6'' stones are used, the thickness should be 9".

MASONRY FOOTINGS - Bricks, concrete blacks, or rubble stone laid in mortar also make good footings. When using bricks, concrete blocks, or any regularly shaped materials, it is necessary that the bottom of the trench be leveled accurately; otherwise, the foundation wall may be unlevel. Either tamped sand can be used to level the bottom of the trench or a thin bed of concrete can be poured. For rubble stone footings, the trench does not have to be as level, but care must be taken to bring the height of the stones up so that the final surface - after being covered with a thin layer of mortar - will be level. Figure 31 shows a rubble stone footing.

Masonry footings may often be much cheaper to construct than concrete footings, since less cement is required. However, they are harder to lay and require more time to complete. Also, it is difficult to properly reinforce them. For this reason, they are not usually recommended for areas of hurricanes, high winds, and where the soil tends to swell.

Mortar used in the masonry or rubble footings should be of a high grade that will not fall apart with time. The following mix works well:

4 parts portland cement

1 part lime

12 parts clean sand

sufficient clean water to form workable mix

If masonry cement is available, use one part masonry cement, three parts sand and sufficient clean water to form a workable mix.

FOUNDATION WALLS - The foundation walls, which rest on the footings and support the walls of the house, must be strong, have a flat surface to start the earth wall on, be straight and they must be level.

Poured concrete, concrete blocks, burned brick, structural clay tile, or stones laid in mortar all make good foundation wall materials.

Again the most suitable, but probably the most expensive foundation wall is made from poured concrete. In earthquake or hurricane areas or where the soil swells considerably, it is advisable to reinforce the foundation walls in the same way as footings are reinforced. The amount, size and spacing of the steel reinforcing should be determined by a competent engineer or architect.

Well-stabilized earth, either in the form of blocks or rammed-in-place earth, may also be used. Since experience with these materials is limited, however, it is recommended that they be used only in areas of light rainfall and on sites that are well sloped to drain water away from the footings.

SIZE OF FOUNDATION WALLS - In constructing the foundation wall, two things must be considered - the thickness of the wall, and the height of the wall above the ground level.

Foundation walls should be made at least as thick as the earth walls that they support. Sometimes they are thicker, but this is true only when they will be used to support other loads, such as the interior floors.

The height of the foundation wall should be sufficient so that rain splash will not reach the earth blocks and cause them to erode or wash out. This height depends on the amount of rainfall and width of roof overhang. For dry areas (rainfall less than 15 inches per year) and wide roof overhangs, the foundation walls should extend 8 inches above the ground surface. For high rainfall areas and small roof overhang, 18 inches above the ground is required and under normal rainfall conditions, 12 inches is required. Even when a stucco or surface finish is used over the earth walls, the foundation wall should extend to the heights given above.


Figure 29.

CONCRETE FOUNDATION WALLS - When using poured concrete for the foundation walls, use forms (shuttering) to hold the concrete until it hardens These forms are set directly on top of the footing and must be solidly braced against the walls of the trench or at the ground surface to prevent movement when the concrete is poured. They should also be thick enough and braced well enough to prevent bulging.

Wood sheathing 1½ to 2 inches thick and braced about every 6 feet works well, or 1" tongue-and-groove boards braced every 2½ feet. If plywood is used, a thickness of 5/3" to ¾" is satisfactory. A typical wood form is shown in Figure 29. Constructing wood forms is a misleading job. They always seem too strong, and therefore wasteful, while they are being built. Once the concrete is poured, it is too late to correct the mistake of not making them strong enough. A bulging form is difficult or impossible to repair. Do not try to saw money by making the forms too weal.

The concrete forms should be carefully placed in the correct position and then well braced to prevent movement as shown in Figure 29. Once the forms have been braced, the top of the foundation wall should be located by driving nails and drawing lines on the inside of the forms. This can be done accurately using the water hose method given in Chapter 3. This height should be marked on the forms all the we, around the house as shown in Figure 30.

If reinforcing steel is to be placed in the foundation wall, it is easier to set only one side of the forms first (usually the inside form), mark the correct height of the wall on the form and then place and tie all of the reinforcing steel before erecting the remaining forms. This helps to line up the steel and get it to the correct height.


Figure 30.

After the forms have been placed and checked for correct al alignment and strength, the footing inside of the form should be swept clean, then lightly sprinkled. It is a good plan to leave a few small openings in the bottom of the forms so trash can be swept out. Be sure to close these openings before the concrete is poured. To keep the concrete from sticking to the forms, they should be wetted with water just before the concrete is poured, or better still, mopped with light oil before the forms are erected, not after.

Once the concrete pouring begins, it must be continued until the entire foundation wall is completed. Otherwise, undesirable joints in the concrete will be formed. The concrete should be "rodded" or tamped into the forms with a smooth steel or wood rod to pack it down solidly. Otherwise, you might get air pockets which weaken the wall seriously. Proper rodding is important over the entire foundation wall, but it is especially important on the outside face because not enough rodding can result in holes that look had. The concrete can be the same as that used for the footing with perhaps a little more water to make it easier to work. Placing ease may be estimated by the method outlined earlier under "concrete footings." If you use the method outlined earlier under "concrete footings," the rock should dent the mix about 2-2½ inches.

The gravel in the concrete should not be too large or it will get wedged between the form and the reinforcing steel. The largest size stone in the concrete should be about ½ of the distance from reinforcing bars to the edge of the form or between adjacent reinforcing bars, whichever is smaller. Remove the forms two days after the wall is poured. Keep the concrete moist by sprinkling or covering with wet sacks for a few more days. If there are any voids in the concrete, patch them as soon as the forms are removed. Use a grout or mortar mixture and smooth it out with a trowel or straight stick.

MASONRY FOUNDATION WALLS - Requirements for masonry foundation walls are the same as for poured concrete walls; that is, the walls should be straight, level and strong.

For laying the walls, a good, strong mortar should be used. The following mix has been found to be good:

4 parts cement

1 part lime

12 parts clean sand

sufficient water to make a workable mix.

The way to lay the blocks or bricks is the same as described in Chapter 10 (page 100) for laying earth blocks. Use the string lines on the batter boards to guide you in obtaining straight walls. The water hose method should be used to make sure that each course of blocks or bricks in the wall remains level during construction. If it is absolutely necessary to use masonry foundation walls in earthquake or hurricane areas, they can be reinforced by using mesh especially made for this purpose, heavy woven fence wire, or even small (3/8" or less) reinforcing bars. The reinforcing is placed between horizontal joints and sufficiently protected with mortar to prevent rusting. Again, a competent architect or engineer should design the correct amount, type, and placement of the reinforcing. Also, if hollow tile or blocks are used, it is a good idea to fill many (sometimes all) of the holes with mortar to obtain extra strength.

RUBBLE FOUNDATIONS - Often, a large supply of stones, broken brick, or other suitable materials is available for building a rubble foundation. This type of foundation works well and is economical, although it is harder to reinforce. Instead of building a footing and foundation wall, the rubble foundation can be constructed as one solid piece which tapers up so that it is the width of the earth wall at the top. (See Figure 31.) The width at the bottom is determined in the same way as for footings using Table 4.

Only hard, durable materials should be used in a rubble foundation. Mortar of the same type described for masonry foundation walls is suitable for holding the rocks together. Often, during the clearing of the site and digging for the foundation, suitable rocks and other materials can he saved for construction of the rubble foundation.


Figure 31.

Construction requires patience and it is more difficult to obtain the correct level and straightness using rubble. The largest stones available should be placed at the bottom of the foundation and the top should be covered with a thin layer of mortar to provide a smooth, level surface for the first layer of the wall.

BACKFILLING THE FOOTING EXCAVATION - Since the excavation for the foundation wall and footing is wider than the foundation wall, it must be backfilled. Preferably, the backfill material should he the native earth removed from the excavation. It should be placed back into the excavation as close as possible to the same moisture content and compactness of the surrounding, undisturbed soil. This always means tamping the soil back in place. Tamp it in thin, equal layers on both sides of the foundation wall so that there is no possibility of damaging or forcing the wall to lean. You can use the same kind of tamper described elsewhere for rammed earth.