Stabilizing a soil is to lend it properties which are irreversible in the face of physical constraints. A great many parameters intervene, depending as much on the design of the building, on the quality of the materials used, on economic aspects of the project, or on issues of durability. For stabilization to be successful, the process used must be compatible with these various imperatives.
NATURE OF THE PROBLEM
When building with earth, one is confronted with two basic options.
- The type of soil available on site dictates the building system.
- The building system, having been predetermined, dictates the use of a particular type of soil.
In the first instance, architecture, in other words the design, takes account of the site context and determines the building systems which will ensure the durability of the buildings; architectural choices act as a "stabilizer". This is the first approach to be preferred and used.
In the second instance, it is the manufacturing technique, often alien to the site, which ensures the durability of the materials used, more or less independently of the building systems; the process and the addition of material(s) act as a «stabilizer».
In this chapter, we deal with the second instance, i.e. the improvement of the soil by adding stabilizers (materials). Every kind of soil, however, has a corresponding suitable stabilizer.
There are more than a hundred products in use today for stabilization. These stabilizers can be used both in the body of the walls and in their outer "skin": in renders, for example. Stabilization has been practiced for a very long time, but despite this, it is still not an exact science and to date no "miracle" stabilizer is known among the multitude of products available, some of which should not even be considered, either because of their inefficiency, or because they are prohibitively expensive.
Only two characteristics of the soil itself can be treated: its structure and its texture.
There are three ways of treating the structure and the texture of a soil:
- reducing the volume of voids between the particles, i.e. affecting its porosity;
- blocking up the voids which can't be eliminated, i.e. affecting its permeability;
- improving the links binding the particles together, i.e. affecting its mechanical strength.
The main objectives being pursued are:
- obtaining better mechanical performances: increasing dry and wet compressive strength;
- reducing porosity and variations in volume: swelling and shrinking with moisture content variations;
- improving the ability to withstand weathering by wind and rain: reducing surface abrasion and increasing waterproofing.
There are three stabilization processes:
- Mechanical stabilization: the properties of the soil are modified by treating its structure: compaction of the soil modifies its density, its mechanical strength and its compressibility, its permeability and its porosity.
- Physical stabilization: the properties of a soil can be modified by treating its texture: a controlled mix of the various particle fractions.
- Chemical stabilization: Other materials or chemical products are added to the soil and modify its properties.
WHEN TO STABILIZE?
There is a tendency at present to stabilize systematically, but stabilization is not obligatory. One can manage very well without and build with earth well without stabilizing. Builders' achievements are there to prove it. Stabilization can entail a significant additional cost: between 30 and 50% of the cost price of the material.
- Do not stabilize material which is not going to be exposed to water: protected walls, rendered walls, internal walls, good design following the logic of earth as a building material.
- Do stabilize when the material is going to be exposed: bad design, failing to take account of the fundamental principles of building with earth, or location constraints: a damp site, or walls exposed to driving rain, for example.
There are two ways of increasing density:
- Either subjecting the soil to mechanical manipulation in order to force out as much air as possible, by kneading or compressing the soil. The texture of the soil does not change, but its structure does as the particles are redistributed. The soil is not just compressed in its original state: it is first broken up to make it more uniform, and then compressed.
- Or filling as many voids as possible with other particles. For this second approach, the texture must be perfect: the void left between each group of particles is filled by another group of particles. Here the texture is being directly treated.
In this instance a soil is reinforced by the addition, generally speaking, of fibres of organic origin (straw), animal origin (hair, wool), mineral or synthetic origin (synthetic fibres). This approach creates a network of omni-directional fibres which improves notably tensile and shearing strengths and also helps to reduce shrinkage.
A strong, inert, three-dimensional matrix is introduced into the soil. This causes consolidation by cementation (i.e. the formation of a "skeleton") which coats the particles and resists movement within the material. Portland cement is the principal example of this kind of stabilizer, or certain glues or resins. The main consolidation reactions occur within the stabilizer itself and between the stabilizer and the sandy fraction of the soil. Secondary reactions, however, can be observed between the stabilizer and the clay fraction. Clay affects the efficiency of the stabilizer and can modify the mechanical behaviour of the material.
In this instance, the inert matrix introduced into the soil includes clays. Two mechanisms giving the same results are known:
- An inert matrix is formed by the clays: the negative and positive charges of the plate-like clay particles, or their chemical composition, are used to bind them together through the intermediary of a stabilizer, which acts as a binding agent or a catalyst in this binding effect. Certain chemical stabilizers work in this way, including certain acids, polymers, flocculants, etc.
- An inert matrix is formed with the clays. A stabilizer reacts with the clay and precipitates a new, insoluble, inert material: a kind of cement. This is a pozzolanic reaction, notably as obtained with lime.
This slow reaction depends essentially on the quantity and quality of the clay present.
This way of stabilizing helps to reduce water erosion and the swelling and shrinkage occurring as a result of repeated alternate wet-dry cycles. There are two known ways of water-proofing:
- All voids, pores, cracks and crazing are filled with a material which is not water-sensitive. Bitumen is an example of one of the products which works best in this way. This method of stabilization is particularly well-suited to sandy soils which display good volume stability and which are little affected by water. It can also be used for silty and clayey soils which demand more stabilizer because their specific surface area is greater.
- A material which expands and seals off access to pores as soon as it comes into the slightest contact with water is dispersed throughout the soil. One example of such a material is bentonite.
In this instance, the state of the interstitial water is modified and the sensitivity of the plate-like clay particles to water is reduced. This process uses chemical products (calcium chloride, acids' quaternary amines or resins) or ion exchange helps to eliminate as much absorption and adsorption of water as possible.
Sands and gravels: these are added when the soil is not usable in its natural state, often because it contains too much clay. By correcting its texture, they increase its density.
Fibres: adding these to reinforce the soil is very common in traditional adobes but incompatible with the CEB compression process as they render the mix too elastic. Bitumen: this has a water-proofing effect but needs to be mixed in evenly, which demands a process using a great deal of water, similar to adobes.
Resins and chemical products: these often combine several methods of stabilizing. Their efficiency in most cases depends on very specific soils and procedures and their use should be carefully considered beforehand. Their availability is often erractic and their cost generally high and variable and these factors should be taken into account.
Cement and lime: cement has a cementation effect whereas lime has a bonding effect. These two stabilizers will be considered in more detail below.
Cement is probably one of the best stabilizers for CEBs. Adding cement before compaction improves the characteristics of the material, and particularly its resistance to water, thanks to the irreversible nature of the links it creates between the largest particles. Cement mainly affects sands and gravels, as in concrete or in a sand-cement mortar. This means that it is not necessary, and indeed it may be harmful, to use soils which have too high a clay content (> 20%). Its use does not require too much water which corresponds to the humid compression state of CEBs.
EFFICIENCY AND HOW MUCH TO USE
In general, at least 5 to 6% cement will be needed to obtain
satisfactory results. Compressive strength is highly dependent on the amount
With low proportions (2-3%) certain soils perform less well than when left unstabilized.
Given similar local conditions, there may be no guarantee that a CEB will use less cement than a cement block.
Best results are obtained with sandy soils.
The presence of iron oxides allows stabilization to occur efficiently with little cement, as a result of pozzolanic reactions or hardening effects.
The presence of organic matter is risky.
Water containing salts must in all cases be avoided.
These are very harmful, especially calcium sulphate (anhydride and gypsum).
On the stabilized material
Using a high proportion of cement, quite apart from economic considerations, cannot improve a poor soil. The plasticity index should be fairly weak (max pl: 15 to 20%), illustrating the efficiency of cement with relatively sandy soils.
Variations in size: cement-stabilization reduces the extent of shrinkage and swelling.
HOW TO PROCEED FOR CEMENT STABILIZATION
Note: Ways and means of processing are dealt with in detail in the chapter on PRODUCTION.
An even mix is crucial. Lumps or sods of clay are therefore to be avoided. Care must be taken during screening with no prior preparation as apart from stones and gravel, there is a risk of also removing lumps of clay and thereby modifying the properties of the soil. To prevent lumps reforming after disintegration, the soil should be kept dry.
Even with a well-prepared soil, the cement must be mixed in as thoroughly as possible, otherwise, as it is generally used in low proportions (4 to 8%), it will not be evenly distributed. Mixing should be done in two stages: dry and wet mixing. The cement will begin to act on contact with water, which is why water should be added to the dry mix at the last moment before compaction in order to keep the time before it is used (retention time) to a minimum, as this greatly affects the quality of the blocks (see below). The moisture content of the mix will be slightly drier than the OMC for sandy soils and slightly wetter for soils containing too much clay.
Cement stabilized blocks must be kept in a humid environment for at least 7 days. The surface of the blocks must not be allowed to dry out too quickly, as this causes shrinkage cracks. The blocks must be sheltered from direct sun and wind and kept in conditions of relative humidity (RH) approaching 100% by covering them with waterproof plastic sheets. After 28 days there will be no further significant increase in the strength of the cement. High temperatures will increase the strength obtained and temperature can be raised using black plastic sheeting.
Examples: compared with 14 days'curing at 100% relative humidity, blocks cured for 7 days at 100% RH and 7 days at 95% RH blocks will achieve 25% less strength. Blocks cured for 7 days at 40°C will be 1.5 to 2 times stronger that blocks cured for 7 days at 20°C
After curing, water must be allowed to evaporate and the clay fraction to shrink. To prevent shrinkage occurring too quickly, exposure to wind and direct sun must be reduced. Drying out will take approximately 14 days.
Stabilizing soil with non-hydraulic lime (quicklime or slaked lime) is commonly used for roadworks, although mainly for temporary roads. Lime stabilization has the advantage of reacting in a very positive way with clayey soils with a relatively high moisture content, which is often the case for site access roads, for example. Lime will above all form links with the clays present, and hardly at all with the sands. The use of this stabilizer is therefore on the whole not recommended for the manufacture of CEBs, which requires fairly low moisture contents and soils with a relatively high sand content. It should be considered only if cement stabilization is impossible. Results with lime are better than with bitumen or than with resins etc. Hydraulic limes, which more closely resemble cement, are not considered here.
EFFICIENCY AND HOW MUCH TO USE
Adding 2 to 3% lime immediately provokes a lowering of the plasticity of the soil and fragments lumps. For ordinary stabilization purposes, the amounts generally used range from 6 to 12%, i.e. equivalent to the amounts of cement used, but it should be noted that in the case of lime, there is an optimum quantity to be used for each type of soil.
Best results are obtained with clayey soils (20 to 40% and even 70%).
This slightly reduces the stabilizing effects, but lime can be capable of neutralizing some of the organic matter present.
These are harmful and to be avoided.
The soil becomes less plastic, but given its use in association with clayey soils with plasticity indices of 18 to 30%, the soil-lime mix remains sufficiently plastic.
Dry density falls and the OMC rises, which means that the Proctor curve flattens out and moves to the right, indicating reduced sensitivity to water.
This will be highly dependent on the amounts used and will tend to increase over time.
Variations in size
Because lime creates links with clays, it reduces swelling and shrinkage.
HOW TO PROCEED FOR LIME STABILIZATION
This is an important stage and must be carefully carried out. The more finely the clay has been broken down, the more actively the lime will be able to attack it. This can prove to be a difficult operation, as clay displays great cohesion.
Too wet a soil can be dried and fragmented using quicklime. Stabilization will work efficiently if at least 50% of the clay lumps are ground down to a diameter below 5 mm.
This must be very carefully done to ensure that the soil and the lime are thoroughly mixed. With very plastic soils, a two-stage process can be used, at one or two days' interval, enabling the lime to break down lumps of soil; at the same time, this two-stage approach can reduce the effect of the lime on strength.
If one proceeds with the material wet, the mix can be left to react after mixing. At least two hours must be allowed, and preferably 8 to 16 hours. Higher strengths will be obtained. If one proceeds with the material in a plastic state, the lime-soil mix (whether quicklime or slaked lime) should be left to react for several weeks. This is particularly the case for renders which become progressively smoother and more sticky.
Dry density is very sensitive to the way compaction is carried out, particularly when high amounts of lime are used. The exothermic reaction provoked by the quicklime consumes nearly 1% of the moisture content per % of added quicklime. The moisture content will therefore need to be corrected to make it close to the OMC.
Curing and drying
An increase in compressive strength can be observed as the curing time is prolonged. This phenomenon extends for several weeks and persists for many months. Complete curing takes six months, but the blocks are usable after 56 days (theoretically).
Curing conditions for lime are identical to those for cement, i.e. a hot, humid environment. Wet curing prevents the evaporation of untrapped water from within the blocks which is vital for the lime-clay reactions to occur.
Curing in the sun under plastic sheeting raises temperatures and relative humidity.
PRINCIPLES (see examples of calculations)
Stabilization calculations always refer to the weight of dry materials. The proportions of stabilizer used corresponds to the percentage by weight of the stabilizer compared with the weight of the earth (including any sand or gravel which may have been added).
As it is difficult to weigh accurately on site, weight is converted into volume.
For this, the density of the material when loose and dry (p) being used in the mix must be known (see formula 1). Once (p) is known, it is very easy to convert quantities into volumes of loose, dry material (see formula 2).
CALCULATION OF THE DENSITY OF MATERIAL WHEN LOOSE AND DRY
Dry the sample (of earth, sand, etc.) and weigh 1 litre of this sample. If you do not have accurate scales (±10 g), then weigh out a larger amount (5 or 10 litres...), in order to reduce the degree of innacuracy. The results in grams/litre are equivalent to those of kg/m³ (see formula 1).
AMOUNT OF EARTH, SAND AND GRAVEL
Containers of known capacity must be available (e.g. wheelbarrow » 60l capacity, buckets between 10 and 15 l capacity) to allow precise calculations. The amounts required will be multiples of these container capacities.
dry density of sack of cement (kg/m³)
dry density of dry earth (kg/m³)
dry density of sand (kg/m³)
weight of cement (kg)
weight of earth (kg)
weight of sand (kg)
volume of dry sand (m³)
volume of dry earth (m³)
percentage of cement = degree of stabilization (%)
percentage of earth (%)
percentage of sand (%)
METHOD OF CALCULATION FOR AN EARTH/CEMENT MIX
The weight of the cement is obtained by determining the volume of earth and the degree of stabilization (the percentage of cement used), (see formula 3).
The volume of earth is obtained by determining the weight and the percentage of cement used (see formula 4).
Results must be rounded off to make volumes equivalent to multiples of container capacities, and then the percentage of cement recalculated using these values (see formula 5).
METHOD OF CALCULATION FOR AN EARTH, SAND (OR GRAVEL) AND CEMENT MIX
The weight of the cement is obtained by determining the volumes of earth and sand (or gravel) and the degree of stabilization (see formula 6).
The volume of earth and the volume of sand (or gravel) are obtained by determining the weight and percentage of cement and the percentages of earth and sand (see formulae 7 and 8).
The volumes of earth and sand (or gravel) must be rounded up or down to the nearest whole site measuring out volume and the rate of stabilization then recalculated (see formula 9).
The percentages of earth and of sand (or gravel) can then be precisely recalculated.