Earth as building material

Contents - Previous - Next

Earth is one of the oldest materials used for building construction in rural areas. Advantages of earth as a building material are:

• 1 It is resistant to fire.
• 2 It is cheaper than most alternative wall materials and is readily available at most building sites.
• 3 It has very high thermal capacity that enables it to keep the inside of a building cool when the outside is hot and vice versa.
• 4 It is a good noise absorbent.
• 5 It is easy to work using simple tools and skills.

These qualities encourage and facilitate self-help and community participation in house building.

Despite its good qualities, the material has the following weaknesses as a building material:

• 1 It has low resistance to water penetration resulting in crumbling and structural failure.
• 2 It has a very high shrinkage/ swelling ratio resulting in major structural cracks when exposed to changing weather conditions.
• 3 It has low resistance to abrasion and requires frequent repairs and maintenance when used in building construction.

However, there are several ways to overcome most of these weaknesses and make earth a suitable building material for many purposes.

Soil Classification

Soil and earth are synonomous when used in relation to building construction. It refers to subsoil and should not be confused with the geological or agricultural definition of soil, which includes the weathered organic material in topsoil. Topsoil is generally removed before any engineering works are carried out, or before soil is excavated for use as a building material. Mud is the mixture of one or several types of soil with water.

There are several ways in which soil may be classified: by geological origin, by mineral content (chemical composition), by particle size or by consistency (mainly related to its moisture content).

Particle Size

Soils are grouped and named according to their particle size, as shown in Table 3.5.

Grading

The soil materials in Table 3.6 seldom occur separately and this necessitates a further classification according to the percentage of each that the soil contains. This is shown in the soil classification triangle from which it can be seen that, for example, a sandy clay loam is defined as soil which contains 50 to 80% sand, 0 to 30% silt and 20 to 30% clay.

Only a few mixes can be used directly as found for building construction with good results. However, many mixes can be improved to make good building material by correcting the mix and/or adding stabilizers.

Table 3.5 Classification of Soil Particles

Gravel, sand and silt are sometimes subdivided into coarse, medium and fine fractions.

Figure 3.10 Soil classification triangle.

The clay fraction is of major importance in earth construction since it binds the larger particles together. However, soils with more than 30% clay tend to have very high shrinkage/swelling ratios which, together with their tendency to absorb moisture, may result in major cracks in the end product. High clay soils require very high proportions of stabilizer or a combination of stabilizers.

Some soils produce unpredictable results due to undesirable chemical reactions with the stabilizer. Black cotton soil, a very dark coloured clay, is an example of such a soil. Generally soils which are good for building construction purposes, are characterized by good grading, i.e. they contain a mix of different sized particles similar to the ratios in

Table 3.6, so that all voids between larger particles are filled by smaller ones. Depending on use, the maximum size of course particles should be 4 to 20mm.

Laterite soils, which are widely distributed throughout the tropical and subtropical regions, generally give very good results, especially if stabilized with cement or lime. Laterite soils can be best described as highly weathered tropical soils containing varying proportions of iron and aluminium oxides, which are present in the form of clay minerals, and usually large amounts of quartz. Their colours range from ochre, through red, brown or violet to black. The darker, the harder, heavier and more resistant to moisture it is. Some laterites harden on exposure to air.

Table 3.6 Soil Gradings Suitable for Construction

If the soil at hand is not suitable it may be improved by adding clay or sand. The best soils for construction are sandy loam and sandy clay loam. Sandy clay gives fair results if stabilized.

Plasticity Index

Clays vary greatly in physical and chemical characteristics. Because of the extremely fine particles it is very difficult to investigate the properties, but some of them can be conveniently expressed in terms of plasticity by using standard tests.

Depending on the amount of moisture in a soil, it may be liquid, plastic, semi-solid or solid. As a soil dries, the moisture content decreases, as does the volume of the sample. With a very high moisture content the soil will flow under its own weight and is said to be liquid. At the liquid limit the moisture content has fallen so that the soil ceases to flow and becomes plastic, and is continuously deformed when a force is applied, but retains its new shape when the force is removed. A further reduction of the moisture content will eventually cause the soil to crumble under load and not deform plastically. The moisture content at this point is known as the plastic limit. The numerical difference between the moisture content at liquid limit and at the plastic limit is called the plasticity index. Both the liquid limit and the plasticity index are affected by the amount of clay, and the type of clay minerals present.

A high liquid limit and plasticity index indicates a soil that has great affinity for water and will therefore be more susceptible to moisture movements, which can lead to cracks.

Soil Testing Methods

As indicated above, some soils are more suitable as building material than others. It is therefore essential to have a means to identify different types of soil. There are a number of methods ranging from laboratory tests to simple field tests. Laboratory soil tests are recommended if production of buildings on a large scale (i.e., several houses) is intended.

Since soils can vary widely within small areas, samples of the soil to be tested must be taken from exactly the area where soil is going to be dug for the construction. Soil samples should be collected from several places distributed over the whole of the selected area. First remove the topsoil (any dark soil with roots and plants in it), usually less than 60cm. Then dig a pit to a depth of 1.5m and collect soil for the sample at various depths between 0.8 and 1.5m. The total volume required for a simple field test is about a bucketful whereas a complete laboratory test requires about 5O kg. Mix the sample thoroughly, dry it in the sun, break up any lumps and pass it through a 5 to 10mm screen.

In the laboratory the classification by particle size is done by sieving the coarse-grained material (sand and gravel) and by sedimentation for fine-grained material (silt and clay). The plasticity index is determined with the Atterberg limit test.

Soil tests will only give an indication of the suitability of the soil for construction purposes and the type and amount of stabilizer to be used. However, other properties such as workability and behaviour during compaction may discard an otherwise suitable soil. Therefore soil tests should be combined with tests on the finished products, at least where design and use call for high strength and durability.

Normally for small projects a simple sedimentation test combined with a bar shrinkage test gives enough information about the proportions of various particle sizes and the plastic properties of the soil.

Simple Sedimentation Test

This test gives an impression of the grading of the soil and allows the combined silt and clay contents to be calculated. Take a large clear glass bottle or jar with a flat bottom and fill it 1/3 full with soil from the sample. Add water until the bottle is 2/3 full. Two teaspoons of salt may be added to dissolve the soil more rapidly. Close the bottle and shake it vigorously and allow the contents to settle for one hour. Shake it again and let it settle for at least 8 hours.

The soil sample should now show a fairly distinct line below which the individual particles can be seen with the naked eye. Measure the thickness of the combined silt and clay layer above the line and calculate it as a percentage of the total height of the soil sample.

Figure 3.11 Simple sedimentation.

The test tends to give a lower figure than laboratory tests due to some silt and clay being trapped in the sand and because some material remains suspended in the water above the sample.

The main disadvantage with this test is that the silt and clay fractions cannot be determined separately. Because silt behaves differently from clay this could result in mistaken conclusions about the soil's suitability for stabilization and as a building material.

Bar Shrinkage Test

This test gives an indication of the plasticity index of the soil, since the shrinkage ratio of the soil when dried in its plastic state is related to its plasticity index.

A wooden or metal box without a top and with a square cross section of 30 to 40mm per side and a length of 500 to 600mm, is filled with soil from the sample. Before filling, the soil should be mixed with water to slightly more than the liquid limit. The consistency is right when a V-shaped groove cut in the soil will close after about 5 taps on the box. Grease or oil the box, fill with the soil and compact it well, paying special attention to the corners. Smooth the surface by scraping off the excess soil. Place the box in the shade for seven days. The drying can be hastened by placing the box at room temperature for one day and then in a 110°C oven until the soil is dry.

If the soil bar after drying has more than three large cracks in addition to the end gaps the soil is not suitable. Measure the shrinkage ratio by pushing the dried sample to one end of the box and calculate the length of the gap as a percentage of the length of the box. The soil is not suitable for stabilization if the shrinkage ratio is more than 10% i.e. a gap of 60mm in a 600mm long box. The higher the shrinkage ratio, the more stabilizer has to be used. Shrinkage ratio is counted as follows (see Figure 3.12).

Shrinkage ratio = [(Length of wet bar) - (Length of dried bar) x100 / Length of wet bar

Figure 3.12 Box for bar shrinkage test.

Soil Stabilization

The main weakness of earth as a building material lies in its low resistance to water. Overhanging eaves and verandas help considerably, but tropical rains of any intensity can damage unprotected walls. Because of the clay fraction, which is necessary for cohesion, walls built of unstabilized soil will swell on taking up water and shrink on drying. This may lead to severe cracking and difficulty in getting protective renderings to adhere to the wall.

However, the quality as a building material of nearly any inorganic soil can be improved remarkably with the addition of the correct stabilizer in a suitable amount. The aim of soil stabilization is to increase the soil's resistance to destructive weather conditions in one or more of the following ways:

• 1 By cementing the panicles of the soil together, leading to increased strength and cohesion.
• 2 By reducing the movements (shrinkage and swelling) of the soil when its moisture content varies due to weather conditions.
• 3 By making the soil waterproof or at least less permeable to moisture.

A great number of substances may be used for soil stabilization. Because of the many different kinds of soils and the many types of stabilizers, there is not one answer for all cases. It is up to the builder to make trial blocks with various amounts and kinds of stabilizers.

Stabilizers in common use are:

• Sand or clay
• Portland cement
• Lime
• Bitumen
• Pozzolanas (e.g., fly ash, rice husk ash, volcanic ash)
• Natural fibres (e.g., grass, straw, sisal, sawdust)
• Sodium silicate (water-glass)
• Commercial soil stabilizers (for roads)
• Resins
• Whey
• Molasses
• Gypsum
• Cow dung

Many other substances may also be used for soil stabilization although their use is not well documented and test results are scarce.

Sand or clay is added to improve the grading of a soil. Sand is added to soils which are too clayey and clay to soils which are too sandy. The strength and cohesion of the sandy soil is increased while moisture movement of a clay soil is reduced. Improved grading of the soil material does not stabilize the soil to a high degree, but will increase the effect of and reduce the required amount of other stabilizers. The clay or clayey soil must be pulverized before mixing with the sandy soil or sand. This may prove difficult m many cases.

Portland cement greatly improves the compressive strength and imperviousness and may also reduce moisture movement, especially when used with sandy soils. As a rough guide, sandy soils need 5 to 10% cement for stabilization, silty soils 10 to 12.5% and clayey soils 12.5 to 15%. Compaction when ramming or pressing blocks will greatly influence the result.

The cement must be thoroughly mixed with dry soil. This can be rather difficult especially if the soil is clayey. As soon as water is added the cement starts reacting and the mix must therefore be used immediately (I to 2 hours). If the soil - cement hardens before moulding, it must be discarded. Soil-cement blocks should be cured for at least seven days under moist or damp conditions.

Non-hydraulic lime or slaked lime gives best results when used with fine soils, i.e., silty and clayey soils. Lime decreases moisture movement and permeability by reaction with the clay to form strong bonds between the soil particles. The amount of lime used varies from 4 to 14%. Lime breaks down lumps and makes it easier to mix clayey soils. Curing at high temperatures makes the cementing molecules stronger and that should be an advantage in the tropics. The curing time is longer than for soil-cement.

Combination of lime and cement is used when a soil has too much clay for cement stabilization or too little clay for an extensive reaction with the lime. Lime will make the soil easier to work and the cement will increase the strength. Equal parts of lime and cement are used. Mixing the dry soil with lime first, makes the soil more workable. Blocks are cured for at least 7 days under moist conditions.

Bitumen (or asphalt) emulsion and cutback are mainly used to improve impermeability of the soil and keep it from losing its strength when wet, but may cause some decrease in dry strength. They are only used with very sandy soils, since it would be very difficult to mix them with clayey soils. Bitumen in its natural form is too thick to be added to soil without heating, so it has to be thinned with other liquids to make it workable. The easiest way is to mix it with water to make an emulsion. After the emulsion has been added to the soil the water will separate leaving a bitumen film on the soil grains.

If the bitumen emulsion is fast-settling, i.e., the water separates too quickly before it is mixed into the soil, the bitumen must instead be dissolved in kerosine or naphtha. This mix is called cutback and should be handled with care since it represents a fire hazard and explosion risk. After a soil has been treated with cutback it must be spread out to allow the kerosene to evaporate.

The bitumen content used is 2 to 4%; more may seriously reduce the compressive strength of the soil.

Combination of lime and pozzolana makes a binder which may be almost as good as Portland cement. It is used in the same way as a combination of lime and cement, but 2 to 4 parts of pozzolana are mixed to one part of lime and the curing time is longer than for ordinary cement.

Natural fibres, used in a mixing ratio of about 4%, greatly reduce moisture movement, but will make dry soil blocks weaker and more permeable to water.

Sodium silicate or water-glass is best used to coat the outside of soil blocks as a waterproofing agent.

Cob

Cob is used extensively in tropical Africa, where suitable soils are obtainable over wide areas. The best soil mix consists of gravel, sand, silt and clay in roughly equal proportions. Sometimes chopped grass or straw is added to reduce cracking. If the clay content is high, sand may be added. Laterite makes an excellent material for cob walling.

When a suitable soil has beed found the topsoil is removed and the subsoil dug up. Water is slowly added to the loose soil, which is kneaded by treading, until the soil has a wet plastic consistency. Natural fibres are added for stabilization if required.

The wet cob is rolled into balls or lumps about 20cm in diameter which are then bedded on the wall to form courses about 60cm high. The outside of the wall may be scraped smooth. In arid and semi-arid climates this type of wall may last for years if built on a proper foundation and protected from rain by a roof overhang or verandah.

Wattle and Daub (Mud and Wattle)

This method of building small houses is very common where bamboo or stalks (e.g., sisal) are available. It consists of a framework of split bamboo, stalks or wooden sticks supported by wood or bamboo poles. The soil, prepared as cob, is daubed on either side of the laths which act as reinforcement. Most soil is suitable for this construction, but if it is too clayey, the cracking may be excessive. To minimize cracking, stones are mixed with the soil or laid in the wooden skeleton. When mudding the inside of a building, the soil is often taken from the floor. Although this increases the ceiling height, it also makes flooding during the rainy season much more likely.

During drying, the weight of the soil is transferred to the wooden structure, with the total weight of the construction eventually resting on the poles.

Wattle-and-daub construction generally has a short lifespan due to erosion of the soil, and the uneven settling of poles and damage by fungi and termites. However, the durability can be improved considerably (20 to 40 years) by using a proper foundation, raising the building off the ground, applying a surface treatment and by using termiteresistant or treated poles.

Clay/ Straw

The technique of building walls of clay/straw has been highly developed in China where grain storage bins of up to 8m diameter, 8.5m height and 250 tonnes holding capacity have been constructed with these materials.

Any type of straw can be used, but it must be of good quality. The clay should be of strong plasticity, containing less than 5% sand. Some lime may be added for stabilization if the sand content is a bit too high.

First, the straw bundles are produced. The straw is pruned levelled at the root ends and then divided into two halves, which are turned in opposite direction and placed together so that they overlap by about two thirds of the length of the straw. The straw bundle is then spread out flat and soaked with clay mud. A thorough covering of each straw is essential for the final strength. The straw is then twisted together and any excess mud removed. The final clay/ straw bundle should be thick in the middle and tapered at both ends, have a length of 80 to 100cm and a diameter at the middle of about 5cm. The ideal proportion of straw and clay is 1:7 on a dry-weight basis.

The clay/straw bundles are placed on the wall either straight and flat or slightly twisted together. Walls for grain bins should have a thickness in centimetres equal to the internal diameter in metres +12, i.e., a 6m diameter bin should have a wall thickness of at least 18cm. It is important to compact the wall thoroughly during the construction to ensure high density, strength and durability. The wall must be built in separate layers, usually about 20cm, which are left to dry out to about 50% moisture content before the next layer is added.

Rammed Earth

This consists of ramming slightly damp soil between stout formwork with heavy rammers. It makes fairly strong and durable walls and floors when made thick enough with properly prepared, stabilized soil.

When used for walls the soil may contain some cobble, but the maximum size should be less than one-quarter of the thickness of the wall. When cement is used for stabilization it must be mixed with the dry soil by hand or in a concrete mixer, until the dry mixture has a uniform greyish colour. The amount of cement required is approximately 5 to 7% for interior walls, 7 to 10% for foundations and exterior walls and 10 to 15% for floors. The amount of stabilizer required will vary however with the composition of the soil, the type of stabilizer and the use. For this reason trial blocks should be made and tested to determine the correct amount of stabilizer.

Water is sprinkled on the soil while it is being mixed. If the soil is sticky from a high clay content, hand mixing will be necessary. When the correct amount of water has been added, the soil will form a firm lump when squeezed in the hand and just enough moisture should appear on the surface to give a shiny appearance.

After the mixing has been completed the soil should be placed in the formwork immediately. The formwork can be either fixed or sliding but must be stout. The soil is placed in layers of about 10cm and each layer thoroughly compressed with a ram weighing 8 to 10 kg before the next layer is placed. If water shows on the surface during ramming the soil mix is too wet.

If cement or pozzolana has been used for stabilization the product should be cured for 1 to 2 weeks in a moist condition before it is allowed to dry out. This can be done by either keeping the product enclosed in the formwork or by covering it with damp bags or grass which are watered daily.

Adobe or Sun-dried Soil (mud) Blocks

The best soil for adobe is one which can, when plastic, be easily moulded into an egg-size ball, and when allowed to dry in the sun becomes hard, shows little deformity and no more than very fine cracks. If wide cracks develop, the soil does not contain enough silt or sand and sand may be added as a stabilizer.

Preparing the Soil

When a suitable soil has been found all topsoil must be removed. Then the soil is loosened to a depth of l5cm. Water and sand, if needed, are added and worked into the loose soil by treading barefooted while turning the mass with a spade.

Water is added slowly and the soil mixed thoroughly until all lumps are broken up and it becomes homogeneous and plastic. When it is the right consistency for moulding it is cast in a wooden mould made with 1 to 3 compartments and with dimensions as shown in Figure 3.13.

Before the mould is initially used, it should be thoroughly soaked in oil. Because of shrinkage the finished blocks will be smaller than the moulds, and depending on bonding, will give a wall thickness of about 230mm, 270mm and 410mm.

Figure 3.13 Wooden moulds for making adobe blocks. Made of sawn timber 100x25mm.

Moulding the Blocks

To prevent sticking, the mould must be soaked in water, before being placed on level ground and filled with mud. The mud is kneaded until all corners of the mould are filled and the excess is scraped off. The mould is lifted and the blocks are left on the ground for drying. Each time the mould is dipped in water before repeating the process.

After drying for three or four days the blocks will have hardened sufficiently to be handled and are turned on edge to hasten drying. After another ten days the blocks can be stacked loosely in a pile. Adobe blocks should dry out as slowly as possible to avoid cracks, with a total curing time of at least one month.

The quality of the blocks depends largely on the workmanship, especially the thoroughness with which they are moulded. If the quality is good, only one in ten blocks should be lost due to cracking, breakage or deformities.

Stabilized-Soil Blocks

When a suitable soil has been found the topsoil should be removed and the subsoil dug out and spread out to dry in the sun for a few days.

Large particles and lumps must be removed before the soil is used by breaking the larger lumps and passing all the soil through a 10mm screen. If the proportion of gravel in the soil is high a finer screen, 4.5 to 6mm, should be used. The wire screen, usually about one metre square, is rocked in a horizontal position by one man holding handles at one end, the other end being suspended in ropes from above. The amount of loose dry soil needed will normally be 1.4 to 1.7 times its volume in the compacted blocks.

Mixing

The amount of stabilizer to be used will depend on the type of soil, the type of stabilizer and the building component being made. Table 3.7 and 3.8 gives a guide line to the necessary minimum mixing ratio of soil-cement for blocks compacted in a mechanical press. For blocks compacted in a hydraulic press the cement requirement can be reduced considerably, whereas a slight increase will be needed for handrammed blocks. The correct proportion of stabilizer is determined by making test blocks with varying proportions of stabilizer as described later.

Table 3.7 Cement: Soil [Ratio related to shrinkage ratio in the bar shrinkage rest

Table 3.8 Cement: Soil ratio related to the combined silt and clay content in the simple sedimentation test

Table 3.9 Batching for Stabilized-Soil Blocks

The importance of thoroughly mixing the dry soil first with the stabilizer and then with the moisture, in two distinct steps, cannot be emphasized too strongly.

The quantity of cement and dry soil is measured with a measuring box, bucket or tin, never with a shovel, and put either on a clean, even and hard surface for hand mixing or into a drum-type mixer (concrete mixer). They are mixed until the dry mixture has a uniform greyish colour. Water is added, preferably through a sprinkler, while the mixing is continued. When the correct amount of water has been added, the soil, when squeezed into a ball, should retain its shape without soiling the hand. The ball should be capable of being pulled apart without disintegrating, but it should disintegrate when dropped from shoulder height on to a hard surface.

Compaction by Hand-ramming

Moulds with one or more compartments can be made either from hard wood or steel. The mould should have hinges at one or two corners so that it can be opened easily without spoiling the block. The mould has no bottom and is preferably placed on a pallet rather than directly on the ground when moulding the block.

The mould is treated as often as required with oil to make the block surface smooth and to prevent the block from sticking to the mould. The soil mixture should be placed in layers in the mould and each layer thoroughly compacted with a flat-bottomed ram weighing 4 to 5 kg. Each block may need as many as 80 good blows with the ram. The top of the block is leveled off and the block and mould carried to the curing store where the mould is removed, and then the whole process is repeated.

Compaction with a Mechanical Press

There are a numerous number of mechanical blockmaking machines on the market. Both motor driven, which can make several blocks at a time, and hand-operated.

Figure 3.14 Mould for hand-rammed stabilized-soil blocks made of 20mm plane timber.

They all consist of a metal mould in which a moist soil mix is compressed.

Figure 3.15 Mechanical press for block making.

The moulding for a hand-operated press is done as follows:

• a The inside of the compaction-chamber is cleaned and oiled and a pallet is placed in the bottom, if required.
• b A measured amount of soil mix is poured into the compaction-chamber and the soil compacted into the corners by hand.
• c The lid is closed and the handle pulled down. The amount of soil mix is correct if the handle can be moved down to stop a little above a horizontal level.
• d The block is ejected and carried on the pallet, to the curing site and the pallet is returned to the press for reuse.

Curing of Blocks

Soil-cement blocks should be placed on the ground in the shade, as close together as possible and be kept damp (e.g., with wet grass). After one or two days the blocks can be carefully stacked and again kept damp for one to two weeks. After this period the blocks are allowed to air-dry for two to three weeks in an openly stacked pile before use.

Testing of Blocks

In the laboratory, dry strength and wet strength are determined by crushing two well-cured blocks in an hydraulic press, the first in a dry state, and the second after having been soaked in water for 24 hours. Durability is tested by spraying the blocks with water according to a standard procedure and making observations for any erosion or pitting.

In order to find out how much stabilizer is required, the following simple weather resistance test carried out in the field may give a satisfactory answer.

At least three different soil mixes having different stabilizer-soil ratios are prepared and at least three blocks are made from each of the different mixes.

Mixing, compaction and curing must be done in the same way as planned for the whole block production. At the end of the curing period three blocks are selected from each set, immersed in a tank, pond or stream all night and dried in the sun all day. This wetting and drying is repeated for seven days.

The correct amount of stabilizer to use is the smallest amount with which all the three blocks in a set pass the test. A few small holes can be allowed on the compaction surface, but if many holes appear on all surfaces the blocks are too weak. If the blocks have passed the test and the dry block produces a metallic ring when tapped with a hammer, they will have satisfactory durability and hardness.

If the blocks fail the test, the reason may be any of the following:

Unsuitable soil; insufficient amount of stabilizer; incorrect type of stabilizer; inadequately dried or lumpy soil; lumpy cement; insufficient mixing of the stabilizer; too much or too little water added; not enough compaction; incorrect curing.

Comparison of Masonry Units Made of Various Materials

There are many methods of making bricks and blocks, several of which are suitable for local production since they are labour intensive but do not require especially skilled labour.

The decision on which method of block or brick making to use depends on several factors, such as:

• the raw materials available;
• the characteristics of the soil;
• raw material and production costs;
• the requisite standards of stability, compression strength, water resistance, etc. 3 N/mm² is often regarded as the minimum compressive strength for use in one-storey buildings;
• the existing facilities for the maintenance of production tools and machines;
• the needed productivity.

Contents - Previous - Next