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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 11: Making rammed earth walls

A rammed earth wall, you might say, is a large pressed earth block. Because it is much bigger than a pressed earth block and it is made right on top of the foundation wall, it is harder to do well. But if you do it right, you will save a lot of effort. You don't have to move a lot of small blocks around and lay them in the wall. A rammed earth wall will make as good a wall as pressed blocks will make.

Don't try to build a rammed earth house without knowing exactly what you should do. It will be worth your while to spend a lot of time studying this chapter before you start.

After finding the right kind of soil for rammed earth, the most important thing to do is to build a form or mold to ram the earth in.

The next most Important thing is to ram the earth correctly.

HOW TO BUILD FORMS - The forms for rammed earth are a lot like those used for poured concrete. They are simply rectangular boxes without tops or bottoms into which earth is pounded. The forms must be strong because they will have to stand a lot of abuse before your walls. are finished. Since they will have to be moved often, they must not be too heavy for a couple of men to lift.

The forms can be made from metal, but since most builders will want to use wooden forms, we will talk about them only. A simple form can be made like the one shown in Figure 72. It uses 2" thick lumber (2"x6", 2"x8" or 2"x10") nailed to 2"x"4 braces or studs spaced 2' to 3' apart. You'll need two panels and several long (about 28"-30") 5/8" round bolts to hold them together. The bolts should be long enough to extend through the forms and studs with the threaded portion sticking out a couple of inches. After the forms are removed, the bolts are driven out of the wall and the holes filled with tightly rammed soil mix.

The form in Figure 72 are used for making straight wall sections. To make corner sections, make some special forms from the same type of wood.

In building a rammed earth wall, the bottoms of the panels are clamped tightly over the foundation wall or a section of finished wall. If they are the right thickness or width, the bottoms of the forms will be right, also. To space the top of the forms, use 2"x2'' wood "spacers" cut to exactly the same width as the wall.

To keep the earth from coming out of the fortes at the ends, use a piece called an ''end gate." End gates should always be as thick as the rest of the forms - 2" in this case. End gates also serve as spacer blocks at the ends of the forms so they should be as wide as the walls. In most cases end gates will be used right at the end of the forms, but they should be made so they will fit any place inside the forms in case you have to ram short wall sections. Try to space end gates so they'll be at least 8" from the nearest bolt. If you don't leave enough room, it will be difficult to ram the soil correctly.


Figure 72. Form for ramming.

Always nail a beveled piece of wood on the end gate so that it faces the inside wall. When the earth is rammed in the forms, the beveled piece will form a groove in the end of the wall. When the next section of wall is rammed, the groove will be filled with earth to form a solid joint that bonds the sections together.

The form shown in Figure 72 will be, fairly heavy for two men to handle. If a smaller form is made, it will be lighter, of course, but you will have to move it too often.

Plywood, 1" thick, will do as well as 2" planks, and will make lighter forms. It must be braced better, though. Use 2"x4" studs spaced 18" apart in both directions. The bolts should be at least ½" in diameter.

Plywood is easiest to use in making forms, hut it is more costly than ordinary lumber and sometimes is hard to get. It will make nice, smooth walls, but it gouges easily if accidentally struck with the corner of the rammer


Figure 75. Thin metal strips such as these can be wed instead of bolts to tie rammed earth forms together.

Other thicknesses of lumber can be used, of course. Bolt and stud spacing can be based on those shown in Figure 72.

Instead of bolts, which might be hard to get, you can use thin metal strips to hold the forms together. Small slots are cut in the forms for the metal strips to go through. Metal peg. or large nails hold the strips to the forms and wedges can be driven to pull the strips tight as shown in Figure 75. Instead of pulling the metal strips out of the wall a, you do with bolts, just leave them in place. If you are going to use a plaster surface coating leave them sticking out a short distance to bond the surface coating to the wall. If not, cut them off flush with the surface of the wall.


Figure 76. This is a special form that was designed to use the metal de strips shown in Figure 75.

Of course, if you use strips, you will need many, but they are not costly. They can, be cut from thin pieces of sheet metal or even from heavy metal cans. A form that has been especially designed to use these metal strips is shown in Figure 76.

Another type of form that has been used is shown in Figure 77. The top bracing and rods are strong enough to hold the bottom of the form together. You'll need only a few bolts and you don't have the bother of removing the bolts from the wall. But the forms are harder to make and they also must be sturdier. Don't try them unless you are an experienced builder.


Figure 77. Form for ramming.

Here are some hints that might help you when you make forms:

1. Don't make your forms deeper than 2 or 2½ feet. If they are creeper, it will be difficult to ram the earth correctly at the bottom of forms.

2. Use seasoned lumber for your forms. Green lumber will warp.

3. Keep your forms oiled with a light-weight oil. This will stop warping, and keep the soil from sticking to the lumber.

4. When your forms are not being used, stack them flat in a protected and well-drained area so they won't warp.


Figure 78.

5. Make your bolts with crank-type handles as shown in Figure 78.

6. You can crank the bolts off and on faster than you can tighten the nuts with a wrench.

7. If several houses are being built, line the inside of your forms with thin sheet metal. They will last twice as long.

8. Drive a couple of nails partially in your spacer block and bend them over to fit on the bolts as shown in Figure 78. When you set the forms up, stick the bolts through the nails to hold the spacer blocks in place while you are tightening up the bolts.

9. If you can't find suitable bolts, use heavy wire to hold the forms together. Thread the wire around the studs and twist it tight with a rod or heavy nail. When the forms are removed, cut the wire off flush with the wall.

10. Every place where you have an unprotected corner, nail a small strip of wood, cut on a 45° angle, to the inside comer of the forms. This is called a chamfer strip, and it will help to "round off" the sharp corners.

 

Rammed Earth Tampers

Most rammed earth tampers are the hand type, but if you want to tamp soil faster, an air tamper - which requires expensive equipment will be better.

HAND TAMPERS - The weight of hand tampers is very important. The heavier the tamper, the faster the earth can be rammed. So use the heaviest tamper your laborers can handle all day long without overtiring. Small laborers cannot lift hand tampers weighing more than 16-18 pounds for long without tiring. Larger men may be able to work efficiently with tampers weighing 20 to 30 pounds.

The size of the striking face is regulated depending on the weight of the tamper. A good rule to follow is this: the tamper should weigh two pounds for every square inch of tamper face. A tamper with a 3"x3" square face (this is the most popular size) should weigh 18 pounds. For a 4"x49" square face the tamper should weigh 32 pounds.

A square tamper with a flat striking face is the best to use. Round-shaped and other curved faces may not gouge the forms as badly as square ones, but they don't compact as well, either. If you care to, you can make a few tampers with special shaped faces for compacting in small areas, such as around the end gates of the forms.

Figure 79 shows a sturdy metal tamper that is easy to make. Tampers can also be made of wood hut they are a little harder to use because they are bulky. The striking face of wood tampers should be covered with a piece of metal to prevent rapid wear and splitting

This hammer can be guided easier and therefore it does not damage the forms as much as a one-man tamper.

AIR-TAMPERS - With an air-tamper one mall can tamp soil in one-half to one-third the time that he could using a hand tamper. Many types of air-tampers are available. Get a light one that doesn't weigh more than 25- 30 pounds. It should be a long-stroke machine, of moderate feed, that delivers powerful blows. A six-inch square tamping face can be used with this type of machine.

A constant air pressure of approximately 70 pounds per equal-e inch is necessary. An air compressor with a free air delivery of 24 to 30 cubic feet per minute will operate one tamper

THICKNESS OF TAMPED LAYERS - A good rammed earth wall should be well tamped from top to bottom. Layers which are placed too thick will be loose at the bottom and will wash out <luring rains. The thickness of each loose layer of earth before tamping should not be more than 1" more than the width of the tamping face. For example. don't try to tamp more than 4" of loose soil with a 3"x3" square-faced tamper. After tamping, a 4" loose layer should be approximately 2½" thick

CONSTRUCTION PROCEDURE - The construction procedure for rammed earth walls is very similar to that used for earth blocks. The first step is to install the termite shields, if required. Bend them down 80 they are Oat against the side of the foundation wall. Then, place the forms over the foundation wall and draw them up tight against it with the bottom row of bolts, wires, or whatever you may be using. The end gates and spacer blocks are then inserted and the top row of bolts is tightened. You are now ready to start ramming. Figure 80 shows a rammed earth' wall under construction.

When a section is completed, the forms are moved, fastened tightly at their new location, and ramming continues. Ram a complete section around the house before starting the next layer. Joints between layers should be staggered like those in pressed blocks so they do not form a single, weak line in the wall. This method is shown in Figure 81.


Figure 80.

Figure 81 also shows how the beveled piece of wood on the end gate forms a good vertical joint between sections.

The first sections to be rammed should be the corners of the house. Particular care should be taken to make sure that corner forms are perfectly plumb or vertical. This is most important and should be checked often. A RAMMED EARTH WALL THAT IS BUILT LEANING CAN NEVER BE STRAIGHTENED.

Use string lines stretched between the corners to line up wall forms and assure straight walls. The method shown in Figure 61 works well, but place the string line slightly above the tops of the forms so it will not touch the forms.


Figure 81.

Protect newly-constructed wall sections until they gain strength. At night and when rain threatens, all walls exposed to rain should be protected with mats, heavy cloth, or waterproof paper. It is very important that this protection cover the top of the wall, since erosion starts there first. Walls stabilized with cement, lime, or other cementing-type stabilizers should be protected and kept moist during the entire curing period. Sacks or mats, moistened several times daily are good for this.

Some soils - particularly the sandier ones - may have a tendency to crumble when the next section is rammed on top of an earlier lift. If this happens, wait until the lower section gains enough strength to prevent this. For soils having cementing-type stabilizers in them, wait 3 or 4 days if possible. In general, you can ram on top of any wall section as long as it doesn't crumble or crack.

Before ramming a new section, scratch the top of the lower completed section about ½" deep with a pointed wood or steel stake. If the completed section is dry, moisten the top of it slightly. This will improve the bond between sections.

NUMBER OF WORKERS NEEDED - Once the soil mix is prepared, 2 to 4 workers can be used to construct the wall. Two men can do the ramming but, if the forms are short, the work will be more efficient if only one man stands in the forms. One worker is required to shovel the prepared soil mix into the forms. One worker, supplied with soil, can ram about 3 cu. ft. of soil in an hour.

Since the workmen operating tampers stand inside or on top of the forms, scaffolding or supports are not required for them. But as the wall increases in height, scaffolding may be necessary for the worker shoveling soil mix inside the forms. A worker on the ground shovels the mix onto the scaffolding. This requires a total of three or four workers. An alternative is for the man tamping the soil to pull up additional soil as needed in a bucket and pour it into the forms.

Scaffolding or other supports will also be necessary when the wall is high and the forms must be moved around.

TAMPING THE SOIL - REGARDLESS OF HOW GOOD THE SOIL MIX IS, A RAMMED EARTH HOUSE WILL NOT LAST LONG UNLESS IT IS TAMPED ENOUGH AND UNLESS THE MOISTURE CONTENT OF THE MIX IS RIGHT. The two go together, because unless the moisture content is right, the earth cannot be tamped right. For this reason, the moisture content should be checked often, not only when it is mixed, but also when it is being tamped. Mixes which are too wet or too, dry should be remixed. Cement-stabilized mixes which have dried out should not be used.

RAMMING SHOULD BE CONTINUED ON EACH LAYER UNTIL THE NOISE FROM THE RAMMING TOOL CHANGES FROM A DULL THUD TO A CLEAR, RINGING SOUND.

Workers operating the tampers have a hard but important job. Their work should be checked often. An easy way to check for soft spots is to shove a sturdy knife blade into various spots in the rammed layer. Soft spots found in this manner should be rammed some more. The important places to check are directly against the forms, in corners, and around beveled strips on end gates.

If the soil tamper will not ring regardless of the number of tamps, then something is wrong with the mix. Probably it is too wet, so check it closely.


Figure 82.

DOOR AND WINDOW OPENINGS - Openings for doors and windows can be done two ways in rammed earth construction. One way is to set the door and window frames first and ram the earth around them. If this is done the frames should be solidly braced as shown in Figure 82 otherwise they will be forced out of place by the high pressures produced by ramming. Brace door and window frames across the diagonals also so the openings will remain square. Frames should be as wide as the walls and should have ties sticking into the wall to hold them firmly in place.

The other way is to ram the earth first leaving an opening in the wall for the frames. When this method is used, the beveled strip is removed from the end gate. The end gate is then accurately positioned at the spot where the opening is to occur. Wooden nailer blocks must be placed in the wall so that door and window frames can be securely attached to them. The blocks are placed on top of a tamped layer and adjacent to the end gate. The next layer is tightly rammed around the block to hold it securely in position. The result is shown in Figure 83. Another method also shown in Figure 83 uses a single piece of timber the full height of the opening. Lightly nail this piece to the end gate so the end gate can be easily removed from it after each layer is completed. When using this method, the door frame does not have to be as wide as the wall, but the unprotected edges of the wall should be chamfered as explained on page 119.


Figure 83.

Sometimes you may run into a situation like this:

You will be ramming against a short section of w 11 (say 3' long) which is adjacent to a window opening. The pressure from ramming against this short section may cause it to slide toward the open space. To keep this from happening, strongly brace the short section against the wall on the opposite side of the opening or against stakes driven in the ground.

If a section does slide, tear it down and ram a new section in place.

WINDOW SILLS - For window sills in rammed earth follow the same rules as for pressed blocks.

LINTELS AND TIE BEAMS - Lintels and tie beams are constructed in the same way as for pressed blocks. There is one precaution to be noted, however. The pressures from ramming the earth on top of a lintel may be great enough to cause it to break or permanently sag. To keep this from happening. put sturdy braces beneath the lintel during ramming as shown in Figure 84, or design the building so that there is no earth wall over the door and window openings. This method is often preferred.

REINFORCEMENT - Barbed wire, woven wire or small metal reinforcing rods may be used to reinforce a rammed earth wall. This is particularly necessary if the building is being constructed in an area that has earthquakes or high winds and is made of stabilized earth. Reinforcing is not as effective in unstabilized earth walls as it is in stabilized material. Reinforcement around door and window openings always strengthens a wall. It eliminates damaging cracks at these points.


Figure 84.

Reinforcement should be placed no closer than two inches from the outside of the wall. Place it in lengths a little longer than the forms. Turn up the extra length against the end gate so that it can be bent down into the new section after the end gate is removed. Securely fasten the reinforcement in the new section to that extending from the previously compacted section.