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CLOSE THIS BOOKSmall-Scale Manufacture of Stabilised Soil Blocks (ILO - WEP, 1987, 204 p.)
CHAPTER II. RAW MATERIALS, TESTING AND STABILISERS
VIEW THE DOCUMENTI. RAW MATERIALS
VIEW THE DOCUMENTII. QUARRYING THE RAW MATERIAL
III. SOIL TESTING PROCEDURES
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENTIII.1. Preliminary on-site tests
VIEW THE DOCUMENTIII.2. Further soil testing procedures
VIEW THE DOCUMENTIII.3. Laboratory testing methods
IV. SOIL STABILISERS
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENTIV.1. Principles of soil stabilisation
VIEW THE DOCUMENTIV.2. Soil stabilisation methods

Small-Scale Manufacture of Stabilised Soil Blocks (ILO - WEP, 1987, 204 p.)

CHAPTER II. RAW MATERIALS, TESTING AND STABILISERS

I. RAW MATERIALS

The basic raw material needed to produce stabilised soil building blocks is soil containing a minimum proportion of silt and clay to provide cohesion. Not all soils are suitable for building purposes. The soil characteristics and climatic conditions of the area must be assessed. For example, a dry, semi-desert climate requires different soil blocks from those used in temperate, rainy or monsoon areas.

Soils are variable and complex materials, whose properties can be modified to improve performance in building construction by the addition of various stabilisers.

All soils consist of disintegrated rock, decomposed organic matter and soluble mineral salts. A soil can be graded into fractions according to a system of soil classification widely used in civil engineering. Such classification, based on particle size, is provided in Table II.1:

Table II.1 Soil classification according to particle size1

Diameter of particle (mm)

Name of fraction

60 - 20

Coarse gravel

20 - 6.0

Medium gravel

6.0 - 2.0

Fine gravel

2.0 - 0.6

Coarse sand

0.6 - 0.2

Medium sand

0.2 - 0.06

Fine sand

0.06 - 0.02

Coarse silt

0.02 - 0.006

Medium silt

0.006 - 0.002

Fine silt

Less than 0.002

Clay

1 See British Standards Institution, BS1377, 1975.

Soils can also be classified in terms of being heavy or light to work and handle, depending on the texture of the soil. There are seven main types of soil; clay soils, heavy loams, medium loams, sandy loams, sandy soils, chalk and limestone soils, and peat soils. Figure II.1 illustrates the composition of the more common soils with respect to sand and the combined silt and clay content.

It is possible to measure the proportions of silt, sand and clay within a soil, with the help of the triangular diagram represented in figure 11.2. This triangular, soil classification chart was originally developed by the Public Roads Administration of the United States. For example, the soil indicated at point × of the chart would be classified as a clay soil with the following constituents: 10 per cent silt; 50 per cent clay; and 40 per cent sand.

Soil fractions fall into four separate and distinct parts:

- the gravel fraction which can occur in six different shapes: rounded, irregular, flaky, angular, elongated, or elongated and flaky;

- the sand fraction (fine aggregate fraction of a soil) can be subdivided into four main zones - one to four - in ascending order of fineness. The zone number is determined by the amount of fine particles passing a 0.6 mm sieve;

- the silt fraction generally consists of fine ground rock which will hold together when damp and compressed. Too much water may make the soil spongy, but not sticky. Therefore careful analysis must be performed before it can be decided whether such soil can be used in block making; and

- the clay fraction which is further described below.1

1 The composition of clays is described in detail in Grimshaw, 1971.

The clay fraction is of major importance in the study of soil stabilisation because of its ability to provide cohesion within a soil. Mineralogically, clay may contain a variety of components such as kaolinite, vermiculite, illite, chlorite and montmorillonite. Clay minerals usually impart plasticity to the clays. Montmorillonite is extremely plastic and sticky, while kaolin is less so, and chlorites and vermiculites not at all.


Figure II.1. Soil composition Sand with silt and clay proportions


Figure II.2. Soil classification triangle

Kaolinite and montmorillonite represent opposite ends of the spectrum of the clay fractions. They differ in their ability to expand and contract when subjected to changing moisture conditions. For example, a typical black cotton soil from the Sudan having a combined silt and clay fraction of about 55 per cent cent, with the clay fraction containing montmorillonite, has a linear drying shrinkage of about 18 per cent. This type of soil also expands a great deal when moistened. On the other hand, a laterite soil with a predominance of kaolinite, has a low level of linear shrinkage.1

1 These characteristics are described in detail in Prescott and Pendleton, 1966.

The production of good quality, durable stabilised soil blocks requires the use of soil containing fine gravel and sand for the body of the block, together with silt and clay to bind the sand particles together. A suitable type of stabilising agent must also be added to minimise the linear expansion that occurs when water is added to the clay fraction. The stabilising agent has other beneficial aspects which are described in a later section.

II. QUARRYING THE RAW MATERIAL

For small-scale, on-site manufacture of stabilised soil building blocks, a minimum of 700 tonnes per year of suitable soil is required for each block making machine.

The quarry should be as close as possible to the manufacturing site in order to minimise the trouble and expense of transporting the raw material. Sufficient soil must be available from the quarry site to meet the required scale of production.

Test holes

Trial holes must always be dug before major excavation commences to test the suitability of the soil and estimate available quantities. A cross section of the soil layers and zones, known as the soil profile, is illustrated in figure II.3.

The top soil (zone 1), usually dark in colour, contains fibrous materials and rotting vegetation; the lower layers of this zone may smell when wet and be very friable when dry.

Zone 2 soil should have a beige colour and will be very sticky if it contains a high clay fraction. Under wet conditions, clay soils will induce the formation of puddles of water and will be slippery and greasy to the touch.

The sandy soil (found in zone 3) is much easier to excavate, will not retain any free water and will feel gritty to the touch.

Several test holes should be dug close to one another. It is advisable to excavate a minimum amount of soil: a 15 cm diameter hole, 2 to 3 metres deep should usually be sufficient to obtain a full soil profile and detailed analysis of the clay and sand fractions.

Soils can vary widely even within a small area. For this reason, one should not be satisfied with what is found in a single test hole and should instead dig several holes in an area big enough to supply all of the soil that is needed. The number of holes to be dug must be determined in each case. Test holes are made according to the following steps.

One square metre of top soil should first be removed with a spade in order to expose the zone 2 soil layer. The depth of the top soil, which may vary between 15 cm to several metres should be recorded for future reference.


Figure II.3. Soil profile

The further excavation of a small diameter test hole is best achieved with a screw auger or bucket auger which are normally operated by two men. Figures 11.4 and 11.5 illustrate these two types of hand-operated, soil drilling equipment. Each of these tools can be fitted with varying lengths of screwed tubes to allow excavation of different depths. The operators must apply vertical pressure to the auger head via the screwed tube at the same time as rotating the cross handle.


Figure II.4. Hand screw auger

When in use, the screw auger is rotated into the ground to a depth of about 20 cm, then lifted out, and the soil removed from the cutting blade flights. The bucket auger collects the excavated soil within its bucket-shaped flights and is emptied after removal from the ground. A hole of about 15 cm diameter is cut with the screw auger, whereas the smallest bucket auger produces a hole of about 25 cm diameter.

Whatever the type of auger used, an accurate depth record of soil conditions must be kept, along with a site-plan view of the location of the test holes. An example of such a site-plan is shown in figure II.6.

The screw auger can be manufactured locally in a blacksmith’s shop by first cutting annular rings from 6 mm thick mild steel plate. These rings are then opened up to form the auger flights and welded to a centre shaft. The bucket auger (figure 11.5) is more difficult to manufacture locally.

Quarrying equipment and tools

Different types of excavating tools can be used in a quarry, depending on the size of the proposed project. For a large project, a bulldozer can be brought on site to remove the top soil quickly (zone 1 in figure 11.3). It is recommended that this top soil should be stockpiled so that it can be replaced and re-used for agricultural purposes after excavation. Excavation of zone 2 or 3 (see figure 11.3) may require a mechanical drag line shovel (figure 11.7).


Figure II.5. Bucket auger

In view of the scales of production covered by this memorandum (up to a daily output of 400 blocks per block making machine), it is more economical to use wheelbarrows and the various hand tools available on the market. Hand digging has been found to be reasonably efficient even for medium-size brick works producing up to 10,000 fired bricks per day.


Figure II.6. Soil survey site-plan


Figure II.7. Mechanical drag line shovel

A major advantage of hand digging over mechanised excavation is that unwanted materials (e.g. large rocks and stones, uncrushable objects, tree roots) can be easily discarded when excavating. This is not easily achieved with mechanised excavation.

The spade or shovel is the most common type of handtool used for digging. The most common types are illustrated in figure 11.8.

Type (a): a clay digging spade with a slightly rounded blade which can be used to dig both clay and heavy loams;

Type (b): a spade with a square-ended blade suitable for cutting through fibrous materials and skimming weed growth (e.g. top soil growth and grass);

Type (c): a builder’s shovel, with upturned edges to prevent spillage; this is a very efficient handtool, ideal for general lifting and mixing duties;

Type (d): this type of shovel is slightly curved and has a pointed cutting edge; it was originally developed to handle asphalt; it is used in the building industry, although it is not very efficient for digging or mixing materials together; and

Type (e): a pick-hoe which has many uses for digging, breaking up hard ground and lumps; it is very efficient for both excavating and mixing duties.

Spades or shovels with shafts of different lengths and blades of different sizes are widely available. The standard shape of a spade is 29 cm long and 19 cm wide, whereas the shovel blade is 29 cm long and 24 cm wide.

The shafts of spades and shovels should have a gentle crank just above the point where they are joined to the blade to allow easier use and maximum leverage. The strapped or tubular socket should be securely attached Co the shaft. Metal treads welded to the upper edge of the blade makes digging, especially that of heavy soils, less painful to the foot.


Figure II.8. Hand digging tools


Figure II.9. Handle shapes

The spades and shovels illustrated in figure 11.8 show three different types of handle shapes. These are illustrated in more detail in figure 11.9. They are:

- type (a); A T-shape handle is less expensive but does not offer the same control as either type (b) or (c); this type of handle can easily be broken;

- type (b): A D-shape handle allows full control of the spade or shovel but has a limited life because the handle joints are exposed to water which can cause premature rotting and splitting of the hold piece; in addition, the steel assembly pin might corrode, become weaker and split the wood.

- type (c): A ‘YD’-shape handle is the most comfortable shape to hold, being slightly larger than the ‘D’-shape. It affords greater control and is therefore most efficient to use; it is, however, the most expensive of the three types; shaped metal shields are employed to protect the assembly joints.

III. SOIL TESTING PROCEDURES

A detailed investigation of the raw material is always desirable and a thorough laboratory analysis should always be carried out for large-scale production. It is not essential, however, to use sophisticated tests to determine the suitability of a soil for small-scale production. Simple preliminary tests can be conducted on site to obtain an indication of the components of a soil sample, its silt/clay and sand fractions, and to investigate soil mouldability, an essential characteristic in the manufacturing of stabilised soil blocks.

For soils which appear to be suitable at first sight, further tests should be carried out to determine the nature of the soil and to select a suitable stabilisation procedure.

III.1. Preliminary on-site tests

Soil samples from zone 2 and zone 3 soils (obtained from test holes) should be tested in the way described below:

Smell test: Damp soil emitting a musty odour indicates the presence of organic material and is therefore not suitable for block making. Such soil should be discarded.

Colour appearance: The dark brown crumbly humus in the soil is organic matter. Soil of this colour should in general be discarded. Light brown to black colouring indicates that the soil contains at least a small proportion of organic matter but that it may be suitable for stabilising. The colour test does not, however, work in all cases. For example, black cotton soils are dark brown to black in colour but do not contain much organic material.

A reddish to dark brown colour indicates the presence of iron oxides which are acceptable for soil stabilisation purposes. White to yellow colouring is an indication of the predominance of lime-based compounds or sand. This type of soil can be stabilised.

Pale brown colouring is characteristic of the presence of clay; lime might be needed as a stabilising agent for this type of soil.

Shine test: A small piece of dry soil is rubbed with the back of a finger nail in order to identify the main component in the sample. The soil surface is abrasive to the touch and the soil remains dull if sand or silt is predominantly present. On the other hand, a sample containing clay shines and is smooth to the touch.

Thread rolling test: This test requires adding sufficient water to a small quantity of soil so that the sample can be easily moulded by hand. The soil sample is then rolled out on a flat clean surface into a thread with the palm of the hand or the fingers (see figure II.10). The reduction of the thread to about 3 mm in diameter indicates the presence of a high clay fraction. On the other hand, the breaking of the thread at a larger diameter indicates the presence of a moderate sand fraction. This test is also used to determine the plastic limit of a soil (see section III.3).

Hand moulding test; After having removed stones and any foreign bodies larger than about 6 mm diameter, the soil sample is moistened and formed into a cube with an edge of about 2.5 cm. If a cube is formed easily, a high clay fraction is present. Although good adhesion and mouldability of such soil are advantageous in the block making process, too much clay will make the soil sticky to work with, and its high shrinkage may lead to cracks within the manufactured soil blocks.


Figure II.10. Thread rolling test

Next, the moulded test ‘cube’ is allowed to dry out in the sun for one day. The occurrence of any surface cracks indicates a high clay fraction, which may give similar cracking problems in the blocks. On the other hand, the splitting of the cube into several pieces indicates the presence of too much sand or silt. Blocks produced from such soil may also fall apart.

III.2. Further soil testing procedures

The preliminary on-site testing methods described above will indicate whether a soil is likely to be suitable for stabilised soil block production. These tests may not, however, be sufficient. Other tests may be necessary, especially if the preliminary tests are not conclusive.

Sophisticated laboratory methods of soil testing, including chemical and sieve analysis and determination of the plastic limit, liquid limit and the optimum moisture content for maximum soil density have all been evolved by soil engineers. However, these laboratory tests are expensive and time-consuming and are only deemed necessary for large-scale projects. For a small project, fairly effective but simple on-site tests requiring simple equipment which may be locally manufactured can be conducted.

After preliminary on-site tests on soil samples obtained from test holes, the holes producing a priori good quality soil should be opened up in order to collect a larger sample for more detailed examination. The following on-site tests may then be performed:

Particle size distribution: This test gives a quantitative measure of the individual soil fractions. It requires four sieves and a tray similar to those illustrated in figure II.11; these sieves nest onto one another for proper site sieve analysis.

The four sieves must have different wire mesh sizes (e.g. 6 mm, 2 mm, 0.2 mm and 0.06 mm). The 0.06 mm mesh may be difficult to obtain and could be replaced by an open weave cloth. The fifth container is a catchment tray. The test should be performed according to the steps noted below.

A sun-dried soil sample of 2 kg is first weighed out and placed inside the 6 mm sieve located on top of the nest of sieves. By shaking the nest of sieves simultaneously, all the fine particles pass through this sieve and, depending on their fineness, some will rest on intermediate sieves, while those passing the 0.06 mm sieve will fall into the catchment tray.


Figure II.11. Site sieves

Once the transfer of material from one sieve to another has ceased, the separated fractions of soil lying on top of each sieve and in the catchment tray are removed, weighed and recorded. A simple particle size distribution is thus obtained for soil sampling.

The fraction of soil retained on the sieves may be classified as follows:

Sieve mesh size

Designation of the fraction retained on the sieve

6 mm

Coarse and medium gravel

2 mm

Fine gravel

0.2 mm

Coarse and medium sand

0.06 mm

Fine sand

Catchment tray

Combined silt and clay

The results of the sieve analysis give an indication of the type of stabilising agent best suited for the soil. Ideally, there should be an even distribution of each soil fraction in order to manufacture good-quality stabilised soil building blocks. If this were to be the case, about five per cent cement would be needed as a stabilising agent. In practice, it is generally found that one fraction is larger than the others. For example, if there is a high fraction of coarse and medium sand and a low silt/clay fraction (e.g. less than about 20 per cent), about four to six per cent cement should be used to stabilise the soil. Conversely, if the silt/clay fraction is high, (e.g. above about 30 per cent), about six to eight per cent lime can be used as a stabilising agent. However, there may be a high proportion of silt present which would affect the linear shrinkage properties of soil; in this case, cement may be required.

Sedimentation bottle test: This test gives more information on the finest particles contained within a soil sample. It is performed in the manner noted below.

A wide-necked, straight-sided and flat-bottomed bottle or jar is needed for this test. The bottle is first filled to one-third with clean, uncontaminated water (see figure 11.12(1)). Approximately the same volume of dry soil (which has passed through the 6 mm sieve) and a teaspoonful of common salt are added. Salt facilitates the dispersion of soil particles (see figure 11.12(2)).

The lid is then firmly fixed on the bottle and the contents well shaken. When the soil and water have been mixed, the bottle is placed on a flat surface for about half an hour. Then, the bottle should be shaken again for two minutes and replaced on the level surface. Two or three minutes later, the water will start clearing. The finer particles fall more slowly and are thus deposited on top of the larger size particles. Two or three distinct layers will be observed, with the lowest layer containing fine gravel, the central layer containing the sand fraction and the top layer containing the combined silt and clay fraction. Figure 11.12(3) illustrates this layer formation in a bottle. The individual percentages can be determined by direct measurement of the depth of each layer.

Linear shrinkage mould test: This test indicates the linear shrinkage of a soil sample as it dries. This information will help determine the best type and amount of stabiliser required. This test requires first the construction of a linear shrinkage mould with the following internal dimensions: 40 mm × 40 mm × 600 mm. Figure 11.13 illustrates the mould required together with leading dimensions.

The first step in this test is to lubricate the internal faces of the mould with some type of oil or grease. Ideally, silicone grease is preferred but any type of mould release oil or grease could be used. The lubricant reduces soil drag on the internal faces of the mould occurring as the soil sample dries out and shrinks.

The soil sample which passed through the 6 mm sieve is mixed with water until a wet puddingy mix is obtained (this occurs near the liquid limit - see section III.3). This mix is then packed into the mould cavity, ensuring that the mould is completely full (absence of air pockets) and the top open surface is smooth. The mould is then placed to dry either in the sun for about five days or under shading for about ten days. In either case, it must be protected from rain.

If the soil has a high clay content, the sample will shrink and hog up out of the mould. This is illustrated in figure 11.14 which shows the shrinkage properties of black cotton soil. A soil sample which shrinks and cracks across the width of the mould (see figure 11.15) indicates a high sand fraction and low silt and clay fractions.


Figure II.12. Sedimentation bottle test


Figure II.13. Drawing of linear shrinkage mould

The linear shrinkage can be determined by subtracting the length of the dry soil sample from the length of the mould cavity. This shrinkage is usually expressed as a percentage of the original mould cavity length.

III.3. Laboratory testing methods

Until 1939, the science of soil mechanics was almost entirely in the research stage, and with the exception of the liquid limits, there were no standard tests to determine the engineering properties of a soil. Since then, increased knowledge of soil properties and its frequent use in practical engineering has led to a convergence of soil testing methods used in different countries, and to the formulation of national standards.

A large number of simple or sophisticated laboratory tests are currently used in various countries.1 However, the following laboratory tests should be sufficient for assessing materials for the production of stabilised soil building blocks. These tests are briefly discussed below.

1 Some of these tests are described in Akroyd, 1962.

Optimum moisture content (OMC): This characteristic of soils is defined2 as the moisture or water content at which a specified amount of compaction will produce the maximum dry density. With relation to soil, a low moisture content will affect the extent to which the soil can be compacted under pressure. In this case, individual soil particles cannot come into close contact with one another, thus allowing the presence of some air spaces between them. If, on the other hand, the moisture content of a soil is high, there will be a greater flow of particles when pressure is applied but these particles will be separated by a film of moisture. Ultimately, as the soil dries, the water evaporates, leaving air spaces between the particles. Consequently, high and low moisture contents will result in poor compaction, which is synonymous with low density. The relationship between dry density and percentage moisture content is illustrated in figure 11.16.

2 The definition may be found in British Standards Institution, BS924, 1975.


Figure II.14. Linear shrinkage of a high silt/clay soil


Figure II.15. Linear shrinkage of a sandy soil

A compromise can be found between extremes of moisture content to minimise air voids and therefore to obtain maximum compaction and density. The moisture content corresponding to the highest dry density is defined as the optimum moisture content.

It may be shown that the OMC and the density of a soil depend upon the type and quantity of stabilising agent employed and the method of compaction used.1 Therefore, the optimum moisture content should be determined on the basis of a prior knowledge of the type and quantity of stabilising agent which is intended to be used for a given amount of soil and of the selected compaction method.

1 See Lunt, 1980.

Liquid limit (LL): The liquid limit is defined as the moisture content at which a soil passes from the plastic to the liquid state. The method employed to determine the liquid limit consists first of placing a soil-water paste in a standard cup. The paste is then divided into two halves with a grooving tool. The moisture content at which the two halves will flow together when the cup is given a standard number of blows is finally determined. This moisture content corresponds to the liquid limit of the mixture.


Figure II.16. Typical density/moisture curve

Plastic limit (PL): The plastic limit is defined as the moisture content at which the soil becomes too dry to be in a plastic condition. The plastic limit is determined by rolling a thread of soil to 3 mm in diameter between the fingers and a glass plate. The soil will be at its plastic limit if the thread just crumbles under this rolling action.

Plasticity index (PI): The plasticity index is defined as the numerical difference between the liquid limit and the plastic limit:

PI = LL - PL.

Particle size distribution; This test relates to the quantitative determination of the particle size distribution in a soil down to the fine sand fraction. The combined silt and clay fraction can be obtained by a wet sieving method1 or by a pipette method to determine the individual silt and clay fractions. The procedure involves the preparation of a soil sample by wet sieving to remove the silt and clay fractions, followed by dry sieving of the remaining coarser material.

1 This method is described in West and Dumb let on, 1972.

Chemical tests: There are two distinct chemical tests employed to check the suitability of a soil:

- determination of the organic matter content; and
- soil chemical analysis.

These two tests are briefly described below.

Organic matter testing: Organic matter takes the form of humus which usually occurs in the top soil layer or zone 1. This organic matter will seriously impair the setting or hardening of cement or will affect the pozzolanic reaction between hydrated lime and the stabilisation of the soil.

The best method to check the presence of organic impurities consists in determining the pH value of a soil (i.e. the level of acidity or alkalinity of a compound2). The pH of a soil sample is determined in the following manner. The sample is shaken vigorously with excess distilled water in a glass container and allowed to settle. A chemical indicator is then added to the supernatant water. The resulting change of colour of the indicator indicates the pH of the soil. The following colour changes indicate the degree of acidity or alkalinity of a sample:

- red: high degree of acidity (pH lower than 5,5);
- orange to yellow: low degree of acidity (pH between 5.5 and 6.5);
- brownish: neutral sample (pH between 6.5 and 7.0);
- green to green-blue: low alkalinity (pH between 7 and 8);
- blue; high degree of alkalinity (pH greater than 8).

2 The testing method for determining the presence of organic materials is described in the British Standards Institution BS1924, 1975.

Soils with pH readings above 10 and below 4.5 are rare. They should not be used for soil stabilisation projects because they have high impurity levels. Their use requires high proportions of stabiliser and will therefore considerably increase production costs.

Chemical analysis: The chief purpose of a full chemical analysis is to identify all the elements present and their proportions. In some instances, it may reveal the presence of an unsuspected mineral which might affect the stabilisation process. The results can also be used to determine whether the soil can be classified as a true laterite, a lateritic or a non-lateritic soil.

Table 11.2 provides the percentage of various chemical compounds present in soil samples from four countries. The following remarks can be made regarding the suitability of these soils for block making:

- the sum of the fractions of alumina, silica and iron oxide must be greater than 75 per cent; this is the case for the four soil samples;

- the percentage loss on ignition (LOI) must be less than 12 per cent. Higher figures will indicate the presence of organic matter which would affect the hardening of a stabilised soil block; thus, the Kenya soil sample would be suspect and might not be found suitable for a soil stabilisation project;

- soluble salts in a clay may influence the plasticity of the soil and will affect the long term strength of a stabilised soil block; these salts are often compounds of potassium and sodium; a combination of potassium and sodium oxides greater than 2 per cent constitutes an undesirable amount of soluble salts; thus, the Egyptian soil sample would be suspect.

- the four soil samples may be classified as lateritic or non-lateritic soils according to the value of the following ratio:

Table II.2. Chemical soil analysis (percentage)

Chemical component

Chemical symbol

Soil type



Jamaica red

Kenya red coffee

Sudan black cotton

Egypt

Alumina

Al2O3

17.20

32.90

9.18

18.30

Silica

SiO2

62.50

36.20

76.80

51.30

Phosphorus pentoxide

P2O5

0.01

0.22

0.01

0.15

Sulphur trioxide

SO3

0.01

0.01

0.01

0.83

Potassium oxide

K2O

0.25

0.36

0.45

1.17

Calcium oxide

CaO

0.35

0.41

1.85

2.59

Titania

TiO2

0.93

1.52

0.68

0.98

Manganese oxide

Mn2O3

0.04

0.33

0.05

0.05

Iron oxide

Fe2O3

8.39

10.72

3.54

8.19

Sodium oxide

Na2O

1.13

0.27

0.33

3.32

Magnesia

MgO

0.55

0.24

0.46

1.79

Loss on ignition

LOI

9.40

18.10

6.24

11.66

The following table indicates the classification of soils according to the value of the above ratio:1

Soil types

Value of ratio

Laterite

1.33 or less

Lateritic soil

1.33 to 2.0

Non-lateritic soil

2.0 and above

1 Most soil engineers, chemists and geologists working in the field of soil stabilisation use this method of soil classification.

The four soil samples from table 11.2 may thus be classified as follows:

Kenya sample: true laterite
Egypt sample: lateritic soil
Jamaica sample: non-lateritic soil
Sudan sample: non-lateritic soil

IV. SOIL STABILISERS

Methods to improve the natural durability and strength of a soil - commonly referred to as soil stabilisation - are practised in many countries. These methods are not new, since stabilisers (e.g. natural oils, plant juices, animal dung and crushed ant hill materials) have been used for many centuries. In recent years, scientific rather than ad hoc techniques of soil stabilisation have also been introduced, developed largely from early methods devised for the stabilisation of earth roads.

IV.1. Principles of soil stabilisation

The silt and clay fraction of a soil reacts to the application of water, swelling when taking in water and shrinking on drying out. This movement can produce cracking of walls and accelerate erosion, which, if serious, may lead to structural failures. Furthermore, the movement often causes the crumbling of protective renderings which may have been applied to the surface of the wall.

The aim of soil stabilisation is to increase the soil resistance to the erosive effects of local weather conditions, including changes in the temperature, humidity and rain.

A better soil resistance to erosion can be achieved in one or more of the following ways:

- by increasing the density of a soil;

- by adding a stabilising agent that either reacts with or cements the soil particles together; and

- by adding a stabilising agent which acts as a waterproofing agent.

The use of the correct stabilisation method might improve the compressive strength of a soil by as much as 400 to 500 per cent and increase its resistance to erosion.

IV.2. Soil stabilisation methods

There are seven main methods of soil stabilisation. These are described and assessed in this section.

(i) Manual or mechanical stabilisation method: This method increases, through mechanical means, the density of a soil and therefore improves its durability. The easiest way of increasing soil density is to ram or tamp a slightly moistened soil mix in a mould in order to eliminate the air pockets;

It was shown in section III.3. that the highest block density may be achieved by compaction once the soil has reached an optimum moisture content. A standard test1 may be used to determine the OMC value for a given type of soil.2 The latter may then need to be moistened or dried in order to achieve this value before the soil can be used for block making. For example, with a compaction pressure of 3 MN/m2 on a soil containing about 50 per cent silt and clay, a maximum dry density of 1980 kg/m3 may be achieved with an OMC value of 12 per cent (see curve in figure 11.16).

1 The British Standard Institution, BS1377, 1975.
2 It may be noted that this value will generally change with the addition of a stabilising agent.

Manual compaction methods vary from foot treading to hand tamping equipment, with compacting pressures varying between 0.05 to about 4 MN/m2. Mechanical equipment may achieve compacting pressures of several thousands MN/m2. However, such equipment is outside the scope of this memorandum as it is not economically feasible for small-scale production.

(ii) Cement stabilisation; Ordinary Portland cement (OPC)3 is the type of cement most widely used in the world today. It is made from a mixture of limestone and clay, heated to around 1,500°C. Gypsum is then added and the resulting mix ground to a fine powder. Portland cement hydrates when water is added and produces a cementitious compound independently of any aggregate.

3 For example, OPC manufactured to British Standard 12: see British Standards Institution BS12, 1971.

When cement is added to a high-sand-fraction soil, the sand particles act as a filler. Thus, after the water is added to the mix, hydration occurs and the soil particles are embedded in a matrix of hard cementitious gel. The small proportion of lime released during the hydration process may react further with the small clay fraction of the soil mix, forming additional cementitious bonds within the soil-cement mix.

For effective stabilisation, it is important that the clay fraction is not so high as to swamp the small percentage of cement added to the soil mix. Therefore, it is necessary to increase the cement content of a soil mix as the clay fraction of a soil increases. The relationship between the linear shrinkage observed and the cement to soil ratio required has been established by the non-governmental organistion VITA.1 Table 11.3 shows that the cement to soil ratio varies between 5.56 per cent and 8.33 per cent as the measured shrinkage varies between 15 mm and 60 mm (by means of the shrinkage test).

1 See Volunteers in Technical Assistance, 1977.

Table 11.3. Cement to soil ratio

Measured shrinkage (mm)

Cement to soil ratio

Under 15

1:18 parts (5.56 per cent)

15-30

1:16 parts (6.25 per cent)

30-45

1:14 parts (7.14 per cent)

45-60

1:12 parts (8.33 per cent)

It may be noted that, for a given shrinkage, cement to soil ratio is a function of the compacting pressure exerted. For example, a CINVA-Ram machine exerts a compacting pressure of about 2 MN/m2 (see Chapter IV). If this pressure is increased to about 10 MN/m2 (e.g. using a different machine), the cement dosage could be reduced to between 4 and 6 per cent for soils with a shrinkage of up to 25 mm. Above this shrinkage value, 6 to 8 per cent lime (see below) could be used for effective stabilisation.

(iii) Lime stabilisation: The production of hydrated lime is carried out in two stages.

The first stage requires the calcination of limestone (or shells or coral) in a kiln at 900°C. This stage expels carbon dioxide and produces quick lime or calcium oxide. The second stage involves slaking or hydrating quick lime with a certain volume of water which causes the production of hydrated lime or calcium hydroxide.

Both quick and hydrated limes can be used to stabilise soils containing a high clay fraction.1

1 Lime is a caustic material that can cause damage to the eyes and skin. Careful handling is therefore advised, especially with quick lime which can react explosively if mixed incorrectly with water.

When lime is used as a stabiliser for soils with a high clay content, four reactions are supposed to occur:

- a cation exchange (a chemical exchange of ions takes place, giving the clay a lower affinity for water); the resulting mix is thus characterised by a lower moisture movement;

- flocculation or agglomeration follows as a result of the cation exchange; this results in the formation of clusters of the microscopically small soil particles, making the mix more viscous or stiff;

- carbonation of the lime itself, as it reacts with the carbon dioxide from the air, gives rise to a hardening effect; and

- a pozzolanic reaction (i.e. a chemical reaction between the clay and the lime, yielding hydrated calcium silicate aluminate compounds similar to some of those found in Portland cement). The rate at which this pozzolanic reaction proceeds is a function of the temperature. Thus, it is very low in temperate climates, but usually fast in the tropics.

The first two reactions take place as soon as the lime is added to the soil. The last two reactions are slower, causing the strength of lime stabilised soil blocks to develop over weeks, months or even years.

It has been suggested that when lime is used as a stabiliser instead of cement, the dosage should be double.2 However, research at the United Kingdom Building Research Establishment shows that such doubling is not necessary if a sufficiently high compacting pressure (e.g. a higher pressure than that provided by the CINVA-Ram press) is applied on a high clay content soil. Thus, the volume of air voids brings the lime and soil particles into closer contact, and the stabilising reactions can take place as fully as possible. For example, tests show that wet compressive strengths between 3.0 MN2 and 3.5 MN/m2 may be obtained with compacting pressures in the range of 8 to 14 MN/m2. This is illustrated in figure 11.17 with blocks made from Sudanese black cotton soil, tested over a wide range of compaction pressures. Eight per cent of lime is used as the stabilising agent with a soil which has a high silt and clay content of 58 per cent and a linear shrinkage of 11 per cent.

2 See Volunteers in Technical Assistance, 1977.

The main advantage of lime over Portland cement as a stabilising agent is that relatively simple equipment is required for its production, thus facilitating local manufacture. However, it has often been found that hydrated lime is more costly than Portland cement in countries where both materials are available. In rural areas, the difficulty of obtaining cement will often dictate the use of lime.


Figure II.17. Strength/compaction pressure curve of lime stabilised soil building blocks

(iv) Bitumen and bitumen emulsions: In its natural form, bitumen or asphalt is too thick to be added to the soil. It is usually warmed to change it into a fluid and mixed with organic solvents, such as benzine, to make it thinner. It is emulsified with water for the production of a bitumen emulsion. This emulsion is mixed with a soil so that, when the moisture dries out, the bitumen reverts back to its natural state. This results in binding soil particles together. Little extra strength is gained by the soil. The main advantage of the operation is the waterproofing of the blocks which can then better withstand rain or humid weather conditions.

The most suitable soils for bituminous stabilisation are sands and sandy soils. Soils with a high clay fraction would require uneconomically large amounts of bituminous emulsion in order to obtain satisfactory results.

Stabilisation with a bituminous emulsion is not usually recommended because material costs are high. Furthermore, the heat of tropical sun tends to soften the block surface so that anyone touching the wall might get dirty from a bitumen deposit.

(v) Gypsum plaster: Gypsum plaster (or plaster of Paris) is produced by heating gypsum rock to about 170°C. At this temperature, 75 per cent of crystallisation water is driven off, leaving a white powder. The latter gets hard after mixing with water and settling over a short period of time. This material is usually employed for finishing internal wall surfaces and occasionally as a mortar. It is slightly soluble in water. Occasionally, gypsum plaster is used as a soil stabiliser for medium range clay-content soils. However, blocks made from such a mix are not very durable due to their low water resistance. They should therefore be used only for internal walls.

Gypsum plaster soil blocks were used in Australia for external walling. They required a protective covering or cladding of metal sheeting on the external faces of the walls. These protected gypsum plaster soil blocks developed sufficient strength to act as load bearing blockwork.

(vi) Chemical stabilisers: Different chemical compounds have been tested as stabilising agents. However, they require the application of sophisticated production techniques which are outside the scope of this memorandum.

(vii) Other stabilisers: Many so-called ‘stabilisers’, such as animal dung ant heap material, bird droppings and animal blood, have been used for the manufacture of stabilised soil blocks. These waste materials generally contain nitrogenous organic compounds which, when wetted, form a gluey substance which helps bind together soil particles.

Chopped straw, grasses and natural organic fibres, although not active stabilisers, are used as reinforcement material to minimise linear shrinkage problems which occur with high clay content soil.

Agricultural waste, such as rice husk ash, cotton stalks, ash from burnt crushed sugar cane (bagasse ash), skimmed lime sludge from a sugar refining process (which burns spontaneously, leaving a black filter cake mud), resins and oils, are also used to a limited degree for soil stabilisation.

The above materials are often used in the production of sun-dried adobe blocks in rural areas. Although they provide only a small increase in strength to the blocks, they are a useful addition to a village-scale block production unit. Most of these ‘stabilisers’ are readily available within a rural community.

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