Earth, soil, laterite
When referring to earth or soil in building construction, both terms mean the same material. Mud is a wet, plastic soil mixture, with or without additives, which is used to make mud bricks (adobe) or monolithic mud walls.
Soil is the loose material that results from the transformation of the underlying parent rock by the more or less simultaneous interaction of climatic factors (sun, wind, rain, frost) and chemical changes, brought about by biological agents (flora and fauna) and migration of chemical substances through rain, evaporation, surface and underground water.
Of the various soil types that occur in the tropics and sub-tropics, laterites are of special interest in conjunction with building construction. These are highly weathered soils, which contain large, though extremely variable, proportions of iron and aluminium oxides, as well as quartz and other minerals. They are found abundantly in the tropics and sub-tropics, where they generally occur just below the surface of wide grasslands or forest clearings in regions with high rainfall. The colours can vary from ochre through red, brown, violet to black, depending largely on the concentration of iron oxides.
The special characteristics of laterites, by which they differ from other soils, are:
• Soft occurances tend to harden on exposure to air, which is why blocks have traditionally (eg in India) been cut in situ, allowed to harden and then used for masonry wall construction (hence the name was derived from "later", the latin word for "brick").
However, irrespective of the type of soil, it is always composed of particles of different size and nature, as summarized in the following chart.
In addition to the solid particles, soil also comprises:
• Air, which is a weakening factor and undesirable in building construction, as it also entraps micro-organisms and water vapour, both of which can cause deterioration of the building component.
• Water, without which the soil cannot be used for building, but which can carry dissolved substances (salts) that may create problems.
Most soils are suitable for use as building materials, though in various cases, the addition or removal of certain constituents is required to improve their quality. Several tests need to be carried out in order to identify the characteristics of the soil and its appropriateness for building construction. The procedures are described under Soil Testing.
It must be stressed that, contrary to common belief, building with earth is not a simple technology. The mere fact that natives of many countries have been building their houses with earth since thousands of years does not mean that the technology is sufficiently developed or known to everyone. It is indeed the lack of expertise that brings about poor constructions, which in turn gives the material its ill reputation. However, with some guidance, virtually anyone can learn to build satisfactorily with earth, and thus renew confidence in one of the oldest and most versatile building materials.
Soil constructions are found in all parts of the world, though to a lesser extent in areas of extreme rainfall.
Buildings can consist entirely or partially of soil, depending on the location, climate, available skills, cost and use of the buildings. The construction can be monolithic or made of various components (bricks, renders, infills).
In areas where there is a large diurnal temperature variation (arid zones or highlands) the walls and roofs are preferably thicker than in more uniform climates (humid zones), where the need for materials of high thermal capacity is less.
Soil can be used for all major parts of the building:
• Hard varieties of laterite, with good particle size distribution (sand to gravel), lightly compacted, for small buildings in dry regions.
• Similar laterite as aggregate in concrete.
• Stabilized air-dried soil blocks, with 10 % cement content, laid in laterite-cement mortar, only in dry regions.
• Base course same as for foundations.
• Direct moulding, without shuttering, just by pressing moist earth by hand.
• Rammed earth construction by tamping lightly moistened soil in shuttering (similar to concrete) for monolithic walls. Stabilization with straw, cement, lime, bitumen, cow dung, etc. as required.
• Straw clay construction, similar to rammed earth, but with straw (any kind) as the major ingredient and clay as the binder. (Good thermal insulation, eg for highland regions).
• Daubed earth applied on a supporting substructure, eg wooden or bamboo frame with wickerwork or plaited straw (wattle and daub).
• Masonry constructions, using air-dried mud blocks (adobe) laid in a mud mortar (with addition of some sand). Rain protective rendering required.
• Masonry constructions, using compressed, air-dried stabilized soil blocks laid in soil-cement or soil-lime mortar. In areas of moderate rainfall, no rendering required.
• Renders, using soil with or without additives, such as binders (cement, lime, gypsum), waterproofing agents (bitumen, plant extracts, chemicals), fibrous material (plant or animal fibres, cow dung), or using plain cow dung.
• Paints based on soil mixes.
• In reasonably dry areas, with good drainage and low water tables: subbase of well compacted, clay-rich soil, covered by large sized gravel (to break capillary action), topped by small sized gravel and a layer of sand, the surface layer made of a silty soil, mixed with 5 % linseed oil and compacted with tamper or vibrator.
• Same as before, but surface layer of stabilized soil bricks or tiles, laid on the sand bed and jointed with soil cement mortar.
• Traditional rural house floors (Asia, Africa) made of compacted stone or earth and smoothened with a mixture of soil and cow dung, or only cow dung (for resistance to abraison, cracks and insects).
• Other surface hardeners: animal (horse) urine mixed with lime, ox blood mixed with cinders and crushed clinker, animal glues, vegetable oils, powdered termite hills, crushed shells, certain silicates and other synthetic products.
• Traditional flat roof with timber sub-structure covered with soil (same as for rammed earth walls) and compacted well, only suitable for dry regions.
• Fibre-soil reels laid moist between timber purlins, on flat or sloped roofs, evened out with a fibre-soil layer and covered with roofing felt or bitumen coat; not recommended in termite prone areas.
• Grass roofs, requiring a water and rootproof membrane, gravel to drain water and ventilate roots and a soil layer on which grass grows, providing favourable indoor climate and sound-proofing, as well as air-purification; suitable for all climates.
• Soil brick vaults and domes, constructed with or without formwork, such that each brick rests on the layer below, passing on the compressive forces in a curved line within the thickness of the structure; a traditional construction found in most arid and semi-arid regions.
• Availability in large quantities in most regions,
• hence low cost (mainly for excavation and transportation) or no cost, if found on the building site.
• Easy workability, usually without special equipment.
• Suitability as construction material for most parts of the building.
• Fire resistance.
• Favourable climatic performance in most regions, due to high thermal capacity, low thermal conductivity and porosity, thus subdueing extreme outdoor temperatures and maintaining a satisfactory moisture balance.
• Low energy input in processing and handling unstabilized soil, requiring only 1 % of the energy needed to manufacture and process the same quantity of cement concrete.
• Unlimited reuseability of unstabilized soil (ie recycling of demolished buildings).
• Environmental appropriateness (use of an unlimited resource in its natural state, no pollution, negligible energy consumption, no wastage).
• Excessive water absorption of unstabilized soil, causing cracks and deterioration by frequent wetting and drying (swelling and shrinkage) as well as weakening and disintegration by rain and floods.
• Low resistance to abraison and impact, if not sufficiently stabilized or reinforced, thus rapid deterioration through constant use and possibility of penetration by rodents and insects.
• Low tensile strength, making earth structures especially susceptible to destruction during earthquakes.
• Low acceptability amongst most social groups, due to numerous examples of poorly constructed and maintained earth structures, usually houses of the underprivileged population, thus qualifying earth as being the "poor man's material".
• On account of these disadvantages, lack of institutional acceptability in most countries, which is why building and performance standards often do not exist.
• Avoidance of excessive water absorption can be achieved by selection of the most appropriate type of soil and/or correcting the particle size distribution; also by adding a suitable stabilizer and/or waterproofing agent; good compaction; and more important, by good design and protective measures.
• Resistance to abraison and impact is generally improved by the same measures as above; waterproofing agents, however, do not necessarily impart higher strength and hardness; hence special additives may be needed and special surface treatment.
• Soil constructions in earthquake zones require careful designing to minimize the effect of destructive forces, but also the use of additional materials, which possess high tensile strength (especially for reinforcements).
• Building important public buildings and high standard housing with earth can be convincing demonstrations of the advantages of the technology and thus improve its acceptability.
• By eliminating the major disadvantages, the lack of institutional acceptability can be overcome. Because of the importance of the material, methods of testing and improving soils for building construction are dealt with in more detail.
Whether the aim is to build a single house or to start a production unit for stabilized soil blocks, it is essential to test the soil used, not only in the beginning, but at regular intervals or each time the place of excavation is changed, as the soil type can vary considerably even over a small area.
Basically there are two types of tests:
• indicator or field tests, which are relatively simple and quickly done,
• laboratory tests, which are more sophisticated and time consuming.
In certain cases, soil identification on the basis of experience can be sufficient for small operations, but normally some indicator tests are indispensable. They provide valuable information about the need for laboratory tests, especially if the field tests give contradicting results. Not all the tests need to be carried out, as this can be tiresome, but just those that give a clear enough picture of the samples, to exclude those that show deficiencies. This is not only necessary to achieve optimum material quality, but also to economize on costs, material, stabilizers, manpower and energy input.
It should further be remembered that soil identification alone does not provide assurance of its correct use in construction. Tests are also necessary to evaluate the mechanical performance of the construction material.
• The soil is best excavated directly at the building site and several holes are dug in an area that is big enough to supply all the required soil.
• First, the topsoil containing vegetable matter and living organisms is removed (unsuitable for construction).
• The soil samples are then taken from a depth of up to about 1.5 m for manual excavation, or up to 3 m if a machine will be doing the work.
• A special device, an auger, is used to extract samples from various depths. Each different type of soil is put on a different pile.
• The thickness of each layer of soil, its colour and the type of soil, as well as an accurate description of the location of the hole should be recorded on labels attached to each bag of soil taken for testing.
Indicator or Field Tests
The implementation of these simple tests should preferably follow the order presented here.
Duration: few minutes
Immediately after removal, the soil should be smelt, in order to detect organic matter(musty smell, which becomes stronger on moistening or heating). Soils containing organic matter should not be used or tested further.
Duration: few minutes
After removing the largest particles (gravel), a sample of soil is rubbed between the fingers and palm of the hand. A sandy soil feels rough and has no cohesion when moist. A silty soil still feels slightly rough, but has moderate cohesion when moist. Hard lumps that resist crushing when dry, but become plastic and sticky when moistened indicate a high percentage of clay.
Similar tests can be done by crushing a pinch of soil lightly between the teeth (soils are usually quite clean!).
Duration: few minutes
A slightly moist ball of soil, freshly cut with the knife will reveal either a dull surface (indicating the predominance of silt) or a shiny surface (showing a higher proportion of clay).
Duration: few minutes
When the knife easily penetrates a similar ball of soil, the proportion of clay is usually low. Clayey soils tend to resist penetration and to stick to the knife when pulled out.
Equipment bowl of water or water tap
Duration: few minutes
When washing hands after these tests, the way the soil washes off gives further indication of its composition: sand and silt are easy to remove, while clay needs to be rubbed off.
Equipment: two screens with wire mesh of 1 mm and 2 mm
Duration: half an hour
With the help of the screen the dry gravel and sand particles should be separated on a clean surface to form two heaps. Crushing of clay lumps may be necessary beforehand. By comparing the sizes of the heaps a rough classification of the soil is possible.
A. The soil is either silty or clayey if the "silt + clay" pile is larger; a more precise classification requires further tests.
B. Similarly the soil is sandy or gravelly, if the "sand + gravel" pile is larger.
C. and D. Further sieving with a 2 mm mesh screen will reveal whether the soil is gravelly or sandy.
In the case of sandy or gravelly soil, a handful of the original material (before sieving) should be moistened, made into a ball and left to dry in the sun. If it falls apart as it dries, it is called "clean", and thus unsuitable for earth constructions, unless it is mixed with other materials.
If the soil is not "clean", the silt and clay pile should be used for the next tests.
Water retention test
Duration: 2 minutes
A sample of the fine material is formed into an egg-sized ball, by adding just enough water to hold it together but not stick to the hands. The ball is gently pressed into the curved palm, which is vigorously tapped by the other hand, shaking the ball horizontally.
• When it takes 5 - 10 taps to bring the water to the surface (smooth, "livery" appearance), it is called rapid reaction. When pressed, the water disappears and the ball crumbles, indicating a very fine sand or course silt.
• When the same result is achieved with 20 - 30 taps (slow reaction), and the ball does not crumble, but flattens on pressing, the sample is a slightly plastic silt or silty clay.
• Very slow or no reaction, and no change of appearance on pressing indicate a high clay content.
Dry strength test
Equipment: oven, if no sun available
Duration: four hours for drying
2 to 3 moist samples from the previous test are slightly flattened to 1 cm thickness and 5 cm Φ and allowed to dry completely in the sun or in an oven. By attempting to pulverize a dry piece between thumb and index finger, the relative hardness helps to classify the soil:
• If it is broken with great difficulty and does not pulverize, it is almost pure clay.
• If it can be crushed to a powder with a little effort, it is a silty or sandy clay.
• If it pulverizes without any effort, it is a silt or fine sand with low clay content.
Equipment flat board, approx. 30 x 30 cm
Duration: 10 minutes
Another moist ball of olive size is rolled on the flat clean surface, forming a thread. If it breaks before the diameter of the thread is 3 mm, it is too dry and the process is repeated after re-moulding it into a ball with more water. This should be repeated until the thread breaks just when it is 3 mm thick, indicating the correct moisture content. The thread is re-moulded into a ball and squeezed between thumb and forefinger.
• If the ball is hard to crush, does not crack nor crumble, it has a high clay content.
• Cracking and crumbling shows low clay content.
• If it breaks before forming a ball, it has a high silt or sand content.
• A soft spongy feel means organic soil.
Duration: 10 minutes
With the same moisture content as the thread test, a soil sample is formed into a cigar shape of 12 to 15 mm thickness. This is then progressively flattened between the thumb and forefinger to form a ribbon of 3 to 6 mm thickness, taking care to allow it to grow as long as possible.
• A long ribbon of 25 to 30 cm has a high clay content.
• A short ribbon of 5 to 10 cm shows low clay content.
• No ribbon means a negligible clay content.
Equipment: cylindrical glass jar of at least 1 litre capacity, with a flat bottom and an opening that can be just covered with the palm; centimetre scale
Duration: 3 hours
The glass jar is filled quarter full with soil and almost to the top with clean water. The soil is allowed to soak well for an hour, then with the opening firmly covered, the jar is shaken vigorously and then placed on a horizontal surface. This is repeated again an hour later and the jar then left standing undisturbed for at least 45 minutes.
After this time, the solid particles will have settled at the bottom and the relative proportions of sand (lowest layer), silt and clay can be measured fairly accurately. However, the values will be slightly distorted, since the silt and clay will have expanded in the presence of water.
Linear shrinkage test
Equipment long metal or wooden box with internal dimensions 60 x 4 x 4 cm (l x b x h), open on top; oil or grease; spatula
Duration: 3 to 7 days
The inside surfaces of the box are greased to prevent the soil from sticking to them. A sample of soil with optimum moisture content is prepared (ie when squeezing a lump in the hand, it retains the shape without soiling the palm, and when dropped from about 1 metre height, breaks into several smaller lumps). This soil mix is pressed into all corners of the box and neatly smoothened off with the spatula, so that the soil exactly fills the mould. The filled box is exposed to the sun for 3 days or left in the shade for 7 days.
After this period, the soil will have dried and shrunk, either as a single piece or forming several pieces, in which case they are pushed to one end to close the gaps. The length of the dried soil bar is measured and the linear shrinkage is calculated as follows:
((Length of wet bar) - (Length of dried bar))/(Length of wet bar) x 100
To obtain good results in construction, the soil should shrink or swell as little as possible. The more the soil shrinks, the larger is the clay content, which can be remedied by adding sand and/or a stabilizer, preferably lime.
Wet sieving test
Equipment: a set of standardized sieves with different meshes (eg 6.3 mm, 2.0 mm, 0.425 mm and 0.063 mm); flat water container below the sieves; 2 small buckets, one filled with water; stove or oven for drying samples; 2 to 5 kg balance with an accuracy of at least 0.1 g
Duration: 1 to 2 hours
A 2 kg soil sample is weighed dry, placed in the empty bucket and mixed with clean water. The water-soil mix, well stirred, is poured into the sieves, which are placed in descending order one on top of the other, with the finest mesh at the bottom, below which is the flat container. The bucket is rinsed clean with the remaining water, which is also poured into the sieves.
Each sieve will have collected a certain amount of material, which is dried by heating on the stove or in the oven, then weighed accurately and recorded. The fine particles in the bottommost container is a mixture of silt and clay, which cannot be separated by sieving. This is carried out by the next test.
Equipment: a 1-litre graduated glass measuring cylinder, with an inside diameter of about 65 mm; a circular metal disk on a stem, which can be lowered down inside the cylinder; a rubber tube and heat resistant drying dishes for siphoning; a watch; a pinch of salt; stove or oven and balance, as in previous test
Duration: 1 to 2 hours
A dry sample of 100 g of the fine material from the previous test is carefully weighed and put into the cylinder. A pinch of salt is added, to improve dispersion of the clay particles, and water is filled up to the 200 mm mark. With the cylinder kept firmly closed with the palm of the hand, the contents are shaken vigorously until a uniform suspension of the grains is achieved. The cylinder is placed on a firm level surface and the time taken.
After 20 minutes, the metal disk is carefully lowered down to cover the material that has settled at the bottom of the cylinder, without disturbing it. The clay, which is still in suspension, is removed by siphoning off the liquid, which is subsequently dried out and the residue weighed. The weight in grams is also the percentage of clay in the sample.
Grain size distribution analysis
With the results of the wet sieving and siphoning tests of one sample showing the relative proportions of the various constituents, as defined by their particle sizes, several points can be plotted on a chart. A curve is then drawn so that it passes through each point successively, giving the grain size distribution of that particular soil sample. This can tee repealed for other samples on the same chart, showing the range of soil types analyzed.
The chart below shows an example of a gravelly soil (G) and a clay soil type (C). The horizontally shaded area indicates the types of soils that are suitable for rammed earth construction, while the vertically shaded area shows appropriate soils for compressed block production. The overlapping area is thus good for most soil constructions, so that a curve (I) running through the middle symbolizes a soil of ideal granulation.
The purpose of this exercise is to determine whether the available soil is suitable for building. If the soil is too gravelly, the gaps between the particles are not properly filled, the soil lacks cohesion and is consequently very sensitive to erosion. If the soil is too clayey, it lacks the large grains that give it stability, and is thus sensitive to swelling and shrinkage. An optimum grain size distribution is one in which the proportion of large and small grains is well balanced, leaving practically no gaps, and sufficient clay particles are present to facilitate proper cohesion.
If the tests reveal a poor grain size distribution, it can be corrected to some extent by:
• sieving the gravelly fraction, if the soil contains too much coarse material;
• partly washing out the clayey fraction, if finer particles are in excess;
• mixing soil types of different granular structure.
Atterberg limit tests
These tests, developed by the Swedish scientist Atterberg, are needed to find the respective moisture contents at which the soil changes from a liquid (viscous) to a plastic (mouldable) state, from a plastic consistency to a soft solid (which breaks apart before changing shape, but unites if pressed), and from this state to a hard solid. While the previous tests determined the quantity of each soil constituent, the Atterberg tests show which type of clay mineral is present. This has an influence on the kind of stabilizer required.
For all practical purposes, the determination of the "liquid limit" and "plastic limit" is sufficient, the other Atterberg limits are not so important. However, the determination of the Atterberg limits is usually carried out with the "fine mortar" fraction of the soil, which passes through a 0.4 mm sieve. This is because water has little effect on the consistency of larger particles.
Liquid limit test
Equipment a curved dish, about 10 cm in diameter and 3 cm deep, with a smooth or glazed inner surface; a grooving tool (as illustrated); a metal container with tightly fitting cover (eg large pill box), a drying oven which maintains a temperature of 110° C; a balance, accurate to at least 0.1 g, preferably to 0.01 g.
Duration: about 10 hours
A sample of fine soil (about 80 g) is mixed with drinkable water to a consistency of a thick paste and evenly filled into the dish such that the centre is about 8 mm deep, gradually diminishing towards the edge of the dish.
This is divided into two equal parts by drawing the grooving tool straight through the middle, making a V-shaped groove (of 60° angle) and a 2 mm wide gap at the bottom. Alternatively, a knife can be used.
The dish is held firmly in one hand and tapped against the heel of the other hand, which is held 30 to 40 mm away. The motion must be a right angles to the groove. If it takes exactly 10 taps to make the soil flow together, closing the gap over a distance of 13 mm, the soil is at its liquid limit.
If it takes less than 10 taps, the soil is too moist; more than 10 taps means that it is too dry. The moisture content must then be corrected, whereby moist soils can be dried by prolonged mixing or adding dry soil. The process is repeated until the liquid is found.
With an accurate balance, it is sufficient to take just a small sample of soil, scraped off from a point close to where the groove closed. The sample is put into the container, which is tightly covered and weighed before the moisture can evaporate. The soil container is then put into the 110°Coven until the veil is completely dry. This may take 8 -10 hours and can be checked by weighing several times, until the weight remains constant.
Knowing the wet (W1) and dry weight (W2) of the soil and container, and the weight of the clean dry container (WC), the liquid limit, expressed as the percentage of water in the soil, is calculated as follows:
Liquid Limit=Weight of Water/Weight of oven dried soil x 100
L=(W1-W2)/(W2-WC) x 100 %
Some examples of liquid limits are:
Sand: L = 0 to 30
Plastic limit test
Equipment: a smooth flat surface, eg glass plate 20 x 20 cm; a metal container, drying oven and balance, as for the liquid limit test.
Duration: about 10 hours
About 5 g of fine soil is mixed with water to make a malleable but not sticky ball. This is rolled between the palms of the hands until it begins to dry and crack. Half of this sample is rolled further to a length of 5 cm and thickness of 6 mm.
Placed on the smooth surface, the sample is rolled into a thread of 3 mm diameter (see illustration for Thread test). If the sample breaks before the diameter reaches 3 mm, it is too dry. If the thread does not break at 3 mm or less, it is too moist. The plastic limit is reached, if the thread breaks into two pieces of 10 - 15 mm length. When this happens, the broken pieces are quickly placed in the metal container and weighed (W1).
The next steps of drying and weighing the soil and container are the same as for the liquid limit test, determining the values W2 and WC. The whole procedure is repeated for the second half of the original sample. If the results differ by more than 5 %, the tests must be repeated one again.
The plastic limit is calculated in the same way as the liquid limit:
Plastic Limit = Weight of Water/Weight of oven dried soil x 100
P=(W1-W2)/(W2-WC) x 100 %
The plasticity index (PI) is the difference between the liquid limit
(L) and plastic limit (P):
The simple mathematical relationship makes it possible to plot the values on a chart. The advantage is that the areas can be defined in which certain stabilizers are most effective.
It should, however, be noted that laterite soils do not necessarily conform to this chart. There is in fact no substitute for practical experimentation, using the recommended stabilizers to begin with, and starting with small dosages. The choice of soil stabilizers is dealt with in detail in the next chapter.
[Ukrainian] [English] [Russian]