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GENERAL OBSERVATIONS
At the surface of the solid part of the planet's crust (land), soil forms a layer of loose material, varying in thickness, which supports living creatures and their structures and plant life.
Soil is formed from bed-rock as a result of very long processes of weathering and the very complex manner in which particles migrate. These result in an infinite number of types of soil, with infinite variations in characteristics.
Topsoil or agricultural soil, which contains a high proportion of organic matter, forms a layer above the bed-rock, which may be more or less weathered. When the upper layers of earth are made up of loose material and contain little organic matter, they can be used for building.
COMPOSITION
Soils are made up of varying proportions of four types of material: gravels, sands, silts and clays. Each of these behaves in a characteristic way: thus, for example, when exposed to variations in humidity, some will change in volume, others will not.
The first two of these types of material are stable, the other two unstable. This notion of stability, i.e. the ability to withstand alternate humidity and dryness without its properties changing, is of fundamental importance in a building material.
a) Gravels are made up of pieces of rock of varying hardness, the size of which ranges between approximately 2 and 20 mm. They form a stable constituent of the soil. Their mechanical properties undergo no detectable change in the presence of water.
b) Sands are made up of mineral particles, the size of which ranges between approximately 0,06 and 2 mm. Also stable constituents of the soil, they lack cohesion when dry, but have a very high degree of internal friction, i.e. very great mechanical resistance to movement between the particles which make them up. When moistened, however, they display apparent cohesion as a result of the surface tension of the water occupying the voids between the particles.
c) Silts are made up of particles the size of which range between approximately 0.002 (2 y) and 0,06 mm; they have little cohesion when dry.
As their resistance to movement is generally lower than that of sands, they display cohesion when wet; when exposed to different levels of humidity they swell and shrink, changing perceptibly in volume.
Gravels, sands, and to a lesser extent silts, are therefore characterized by their stability in the presence of water. When dry, they have little to no cohesion and therefore they cannot be used on their own as the principle materials of a building.
d) Clays, which form the finest fraction of soils (less than 2 y), have completely different characteristics than those of the other particle types. They consist mainly of microscopic clay mineral particles, including - amongst others - kaolinites, illites and montmorillonites. Clay particles are coated in a film of absorbed water and because they are so minute they are very light in weight compared with the surface tension forces occurring in the film of absorbed water. Thus volume forces are low relative to surface forces.
The film of absorbed water which adheres strongly to the clay-layers, links the micro-particles of the soil together, and it is this which gives clay its cohesion and most of its mechanical strength. This can be eliminated only by very advanced dessication. Clay lends soil its cohesion, acting as a kind of natural binding agent between the coarser particles which form its skeleton.
Unlike sands and gravels, however, clays are unstable and are very sensitive to variations in humidity. They are greatly attracted to water and as their moisture content rises, the films of absorbed water become thicker and the total apparent volume of the clay increases. Conversely, during the shrinkage which occurs as they dry out, cracks can appear in the clay mass, reducing its strength. When next exposed to moisture, these cracks form channels through which water can penetrate to the heart of the material. It is this "swelling-shrinkage" characteristic, i.e. variations in volume of clayey soils according to their moisture content, which has to be contended with!
What has been said so far applies to moisture contents below the "liquid limit" and at which clay has cohesion. With high moisture contents, clays "liquifly" and lose all cohesion.
EXPLOITATION
Usable layers or deposits of soil are rarely found et the
surface of the ground (except in arid areas), because here soils contain too
much organic matter. The depth of this organic topsoil rarely exceeds 1 to 2
metres. On the other hand usable soil is rarely found at great depths, where
there are too many stones, or even solid rock. The depth or height of usable
layers of soil varies greatly, from a few centimetres to several metres.
GENERAL OBSERVATIONS
Soil properties change from one soil to another depending on the nature of the particle fractions making them up and the complex way in which these mix together. It is often the dominant particle fraction of a soil which characterizes its fundamental properties and dictates its behaviour.
One can distinguish on the one hand between chemical properties, which are linked to the presence of salts, oxides and sulphates, and on the other between physical properties, which are numerous, and which include colour, structural stability, adhesion, apparent dry density, moisture content, porosity or the proportion of voids, absorption capacity, capillary potential and range, permeability, linear shrinkage, dry strength and many more. Understanding its chemical and physical properties enables one to define the quality and performance of a soil for building purposes.
At the same time, it is not always necessary to have an exhaustive knowledge of the chemical and physical properties of a soil. What is important, however, is to have a thorough grasp of three fundamental properties, which are:
- the texture or particle size distribution of the soil, i.e. the quantity of stones, gravels, sands, silts and clays present, expressed in percentage terms;- the plasticity of the soil or the ease with which it can be shaped;
- the compressibility of the soil, or the extent to which voids, and therefore its porosity, can be reduced to a minimum.
TEXTURE OR PARTICLE SIZE DISTRIBUTION
This is measured by particle size analysis for the coarse fraction (gravels, sand, silts) and by sedimentation analysis for the fine fraction (clays).
Gravels and sands give the material its strength, whilst clays bind it together; silts fulfil a less clear intermediate function.
When we define an optimum curve, we are attempting to make best use of the qualities of the various types of materials making up the soil.
FIGURE
PLASTICITY
Plasticity defines the extent to which a soil can be distorted without any significant elastic reaction, typically cracking or crumbling, occurring.
The plasticity of a soil, as well as the limits between different states of consistency, are defined by measuring the "Atterberg limits".
These are carried out on the fine fraction of the soil (particle size diameter superior to 0.4 mm). The amount of water, expressed in percentage terms, corresponding to the point at which the material passes from a plastic to a liquid state is known as the Liquid limit (LL). The point at which it passes from a plastic to a solid state, is known as the Plastic limit (PL). At LL, the soil begins to display some resistance to shearing. At PL, the soil ceases to be plastic and becomes crumbly. The plasticity Index (Pl), which is equal to LL - PL, determines the extent of the plastic behaviour of the soil. Combining LL and PL defines the sensitivity of the soil to variations in humidity. The plastic properties of a soil can be shown on a plasticity diagramme.
The following are examples for certain soils:
- sandy: PI from 1 to 10; LL from 0 to 30
- silty: PI from 5 to 25; LL from 20 to 50
- clayey: PI > 20; LL > 40
FIGURE
COMPRESSIBILITY
The compressibility of a soil defines its maximum capacity to be compressed for a given amount of compaction energy and at a given moisture content (the optimum moisture content or OMC). When a force is applied to a quantity of soil, the material is compressed and the proportion of voids decreases. The more the density of a soil can be increased, the lower its porosity will be and the more difficult it will be for water to penetrate. This property results from the tighter overlapping of the particles which lowers the risk of the structure being modified in the presence of water.
The moisture content must be high enough to lubricate the particles and enable them to move around in such a way as to occupy as little space as possible. At the same time the moisture content must not be too high, or the voids would be full of water, and therefore impossible to compress.
The compressibility of a soil is measured by the "Proctor test". It can be shown on a compressibility diagramme, showing the relationship between the optimum moisture content and the optimum dry density, for a given amount of compaction energy.
FIGURE
SOIL MECHANICS
As we have already seer), soil is made up of inert materials (gravels, sands, silts) and active materials (clays). The former act like a skeleton and the latter like a binding agent, much the same as a cement does. The structure of a soil is thus comparable to that of concrete with a different binding agent. The proportion in which each type of material is present will determine the behaviour and properties of different soils.
The following curve gives an approximate indication of the types of soil which are recommended for the manufacture of compressed earth blocks. As can be seen, the proportions of each type of material can vary considerably depending on the qualities of each, which differ quite widely, particularly for clays. Knowing the proportions of each, es shown on a particle size distribution curve, is an important indicator but is rarely enough for soil selection purposes.
FIGURE
PROPORTIONS OF VARIOUS KINDS OF MATERIAL
Gravels: 0-40%
Sands: 25-80%
Silts: 10-25%
Clays: 8-30%
It is generally accepted that many soils which fall outside the recommended areas can still give acceptable results in practice. On the other hand, soils which do conform will in most cases give good results. The shaded areas are guidelines for the user and not specifications to be rigidly applied.
TYPICAL SOILS
Texture influences the properties of a soil since each fraction of particles has its own particular characteristics and these can define that of the soil if it is present in sufficient quantities. A proportion of 8% clay is enough to lend cohesion and plasticity to the soil. 40 to 50% clay fines gives a soil with the properties of a clay.
Proportions can vary greatly, resulting in a virtually infinite number of types of soil. One can distinguish, however, between four main types of soil texture.
Each of these types is described by reference to the interpretations of field tests.
Gravelly soil
Description: very rough texture, not sticky, little cohesion (the "cigar" formed by hand with such soil breaks at a short length and "biscuits" of soil crumble easy), little or no shrinkage.
FIGURE
Sandy soil
Description: gritty texture, not sticky, little cohesion (the "cigar" breaks at a short length and "biscuits" of soil crumble easily), little or no shrinkage.
FIGURE
Silty soil
Description: smooth texture, sticky, cohesive (the "cigar" breaks at a long length and "biscuits" of soil are difficult to break up), fairly significant shrinkage.
FIGURE
Clayey soil
Description: very smooth texture, very sticky, highly cohesive (the "cigar" breaks at a very long length and "biscuits" of soil are very difficult to break up), significant shrinkage.
FIGURE
Moistening a soil accentuates its reactions and properties. Water has the effect of introducing mechanical forces due to phenomena of capillarity. The finer the particle size, the greater these tensile forces will be and the more absorption will occur.
In the case of clays (active materials), electrostatic forces also intervene and these lend the material great cohesion and plasticity, plasticity being the capacity to change shape without breaking.
Wetting a soil enables one to determine its absorption capacity, as well as its cohesion and its plasticity. Thus typical soils will each behave in a particular way.
Observing samples which have been molded and allowed to dry can
also be useful. Samples made from gravelly, sandy and silty soils will have lost
all cohesion, whereas those made from a clayey soil will have retained great
cohesion, but they may also have cracked during shrinkage.
A soil will react very differently depending on the amount of water it has absorbed. One can differentiate between four broad hydrous states: dry, moist, plastic and liquid. Each hydrous state has a corresponding application. These will be dependent on a number of factors linked not only to the nature of the soil and the building system used, but also to the broader context (whether the region is arid or not, traditions, skills, etc.)
PRELIMINARY INVESTIGATION
Existing data should preferably be gathered before starting work on site. These might include maps or descriptions emanating from geological, pedological, topographical, or agronomical surveys or from roadworks.
It is often very useful to question the people living in the area: they may be able to supply conclusive information, particularly if earth is being used for building in the locality, suggesting that there are usable deposits.
Earth for building very often lies below a layer of organic soil; it ist therefore important not to embark upon uncontrolled sampling and to remember that organic soil, which is unusable for building, is often the sole means of subsistence of the farmers in the area.
TAKING SAMPLES
Samples can be taken from bore holes or open trenches, but most commonly from a combination of the two, complementing each other. One might also be able to locate existing road cuttings, escarpments, etc. Samples should be taken from homogenous layers; depending on the scale of the project, these can be chosen "by eye" or by using a statistical sampling system.
The sample size and weight will depend on the number of tests to be carried out. in principle 1 to 2 kg is enough for field testing. To test for compressibility (using the Proctor test), 6 to 10 kg will be needed, and to make a standard block (measuring 29.5 × 14 × 9 cm) approximately 10 kg will be needed.
The sample taken must be representative. Different soils should not be mixed together and rather than trying to create an "average" soil, a greater number of samples should be taken. Samples should be labelled and given an identification card listing any data which might help to pinpoint the soil (where the sample was taken and by whom, the use to which it is put, etc.).
PROSPECTING EQUIPMENT
Digging equipment can be manual (spades, pick-axes, trowels, drills, etc.) or mechanized (boring machines).
Some drilling tools enable one only to dig out the soil, but others simultaneously extract cylindrical core samples. Sometimes a small pick (as used for geological purposes) or a penknife can suffice for surface sampling.
Warning: containers must be sturdy enough not to split. Canvas
or plastic can be used to preserve the original moisture level of the sample. In
all cases, an easily accessible and robust label must be used to enable samples
to be identified.
GRAIN SIZE DISTRIBUTION
This test consists in filtering the soil through a series of standard mesh sieves placed one above the other in decreasing order (i.e. the finest mesh at the bottom) and in determining the proportion of matter left in each sieve.
FIGURE
SEDIMENTATION ANALYSIS
The grain size distribution analysis obtained by passing the sample through sieves is incomplete. It may suffice for most roadworks applications, but is insufficient for the purposes of building with earth, which requires analysis of the texture of fines of a diameter inferior to 0.08 mm. This requires sedimentation analysis which exploits the different speeds at which particles of soil suspended in water will settle. The coarsest will settle first and the finest last. Variations in density are measured at regular intervals and at a given height (density diminishes as the liquid clears). The speed at which the particles settle depending on their size enables one to calculate the proportions of the various sizes of particles.
FIGURE
ATTERBERG LIMITS
A soil can have various states of consistency: liquid, plastic or solid. A Swedish researcher named Atterberg defined these various hydrous states and the boundaries separating them as limits and indices, expressed as percentages by weight of the moisture content. Five limits can be measured:
- the liquid limit,
- the plastic limit,
- the shrinkage limit,
- the absorption limit,
- the adhesion limit.
The first two of these are the most important; the other three, although of interest, are rarely used. Atterberg limits are determined using the "fine mortar" fraction of the soil, i.e. that which passes through a 0.4 mm mesh sieve, as this portion, which constitutes the "mortar" for the coarser particle, is the portion that might be affected by water, modifying its consistency.
FIGURE
PROCTOR TEST
For efficient soil compaction, the compaction process must be carried out on a material the moisture content of which lubricates the particles, thus enabling them to move around in such a way as to take up as little space as possible.
If the moisture content is too high the soil may swell and the pressure of the compacting machine will be dissipated by the water trapped between particles. If, on the other hand, the moisture content is too low, the particles will be insufficiently lubricated and it will not be possible to compact the soil to its minimum volume.
The Optimum Moisture Content (OMC) at which maximum dry density is obtained is determined by the Proctor test (after the American entrepreneur who perfected it). The results are recorded on a diagramme showing the dry density (pd), expressed in kg/m³, on the axis, and the moisture content (MC), expressed in percentage by weight, on the abscissa. The three principal variables which affect the maximum dry density obtainable are the texture of the material, its hydrous state and the compaction energy used.
FIGURE
CHEMICAL ANALYSIS
The presence of organic matter and soluble salts is harmful. Chemical analysis can be:
- quantitative, i.e. detecting the presence of substances and quantifying them (by filtration or using a spectrometer). This type of analysis requires sophisticated equipment, which makes it very expensive;- qualitative, i.e. detecting only the presence of substances without attempting to quantify them. The presence of organic matter, sulphates and chlorides is often checked using this far simpler and cheaper form of analysis.
FIGURE
TOUCH/SMELL/WASHING
Method
1. Take a small quantity of dry soil and rub it in the palm of
the hand feeling its texture.
2. Moisten the soil; if it begins to give off a
musty smell, it contains organic material.
3. Gently rub the moistened soil,
again feeling its texture.
4. Gently wash the soil off the palm of the hand,
noting how sticky it is.
Interpretation Texture:
A soil which feels coarse when dry will feel smooth when
moistened if it contains lumps of clay. Sand on the other hand will feel gritty,
as will to a lesser extent silt.
Washing:
- if the soil is not sticky and washes off easily, it has a high gravel and/or sand content;
- if the soil is sticky and difficult to wash off, it has a high silt content;
- if the soil is very sticky and very difficult to wash off (leaving traces of colour), it has a high clay content.
CIGAR TEST
Method
1. Remove all gravel from the sample.
2. Moisten and knead it well until a smooth paste is obtained.
3. Leave to stand for 30 minutes, or more if possible, to allow it to become very smooth.
4. Roll between the hands into a cigar shape 3 cm in diameter.
5. Place the «cigar» across the palm of the hand and push it gently forward with the other hand.
6. Measure the length of the piece which breaks off.
7. Repeat several times.
This test enables one to observe the cohesion of the soil and thus above all the quantity and quality of the clays present.
Interpretation
Average the lengths measured:
- less than 5 cm: the soil contains too much sand,
- more than 15 cm: the soil contains too much clay,
- between 5 and 15 cm: the soil is good.
BISCUIT TEST
Method
1. Proceed as with the cigar test, by removing all gravel and kneading the sample well until a smooth paste is obtained.
2. Mould it into flat biscuit-shaped discs approximately 3 cm in diameter and 1 cm thick.
3. Leave to dry and observe any signs of shrinkage (away from the sides of the mould and/or any cracking).
4. Break the "biscuit" noting how hard it is.
Interpretation
Shrinkage:
If the biscuit is cracked or if there is a clear gap between the dried sample and the sides of the mould, the soil contains too much clay.
Breaking:
- very hard to break; breaks with an audible crack: the soil has a high clay content;- brittle, but breaks fairly easily and can be crumbled between the thumb and forefinger: a good, sandy-clayey soil;
- breaks readily and is easily reduced to powder: the soil has a high sand or silt content.
SEDIMENTATION ON JAR TEST
Method
1. Take a transparent cylindrical jar or bottle of at least 1/2 litre capacity and fill it with approximately 1/4 soil and 3/4 water.
2. Seal the top using your hand and shake well.
3. Leave to stand for at least 30 minutes and observe the sedimentation layers.
Interpretation
Coarse material (gravels) will be deposited on the bottom, followed by sands, then silts, with clays at the top.
The depth of each layer gives an indication of the proportions of each type of material. These proportions are only approximate: the layer of gravel, which contains many voids, will seem relatively "deep" compared to that of clay, which will have very few voids. Nevertheless, the test shows if the soil has a reasonable distribution of all types of material or if on the contrary it contains too much of one type.
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