Soil Stabilisation
Soil stabilisation is a process of treating a soil in such a manner as to maintain, alter or improve the performance of the soil as a construction material. The changes in the soil properties are brought about either by mechanical blending of soil type or by the incorporation of additives . So, in general there are two types of stabilisation namely mechanical stabilisation and chemical stabilisation.
Mechanical Stabilisation
Mechanical stabilisation is achieved by intelligently blending locally occurring materials to obtain a desired grading. Compaction of soil is also form of mechanical stabilisation. It is well known that a dense, well-graded mass offers the maximum resistance to lateral displacement under a load. If the well-graded material is compacted, densification of the mass takes place. The mechanical strength of the mass is due to the internal friction and the cohesion. Internal friction is supplied by the coarser particles (gravel, sands and silts) where cohesion is due to the clay friction.
Chemical Stabilisation
1. Soil-Lime Stabilisation
During the last few decades, the use of soil-lime stabilisation has gained in popularity in the USA, Africa and Australia. Soil mixtures are used as sub-base or base courses. Addition of lime to soil generally results in decreased soil density, changes the plasticity properties of soil and increase soil strength. These changes are the result of several reactions.
The first of these reactions is alteration of the water film surrounding the clay minerals. The strength of the linkage between two clay minerals is dependent on the charge, size and hydration of attracted ions. The calcium ion (lime) serves to bind the soil particles close together. This in turn decreases plasticity and results in a more open and granular structure.
A second process by which lime changes a soil is that of flocculation of the soil particles.
The third process by which lime affects soil is reaction of lime with soil components to form new chemicals. The two principal components of soil which react with lime are alumina and silica. This reaction is a long term one and one that results in greater strength if lime-soil mixtures are cured for a longer period. This action is known as ‘pozzolanic action’.
Clayey soils are the most amenable to lime treatment. Recent experience in both USA and the UK has shown that soils in which sulphates are present should be avoided. In bother countries, examples have been reported of lime stabilised clays swelling to a marked degree in the months following construction. The cause of this swelling has been traced to a reaction between sulphates in the soil and the calcium silica-alumina hydrates formed as lime reacts with the soil. The reaction produces the minerals which can absorb large quantities of moisture, swelling as they do so. This reaction can occur in the presence of as little as 0.3 percent of sulphate in the soil and is reported to be activated in situations where the soil is in or near a saturated condition.
2. Soil-Cement Stabilisation
The addition of cement to soil to improve its strength has been in vogue since quite some time. The material is very popular in the USA, UK and in African countries. The principal advantages with soil-cement are that almost all soils are amenable to this technique. It is a scientifically designed engineering material and cement itself is a standard material whose quality is tested and assured. Because of its very high flexural strength, it has a very high load spreading property. Thus, soil cement can spread the load over a wide area and bridge over the locally weak spots of the underlying sub-grade or sub-base. In view of its high flexural rigidity, it is often classed a semi-rigid pavement. The durability of soil-cement is of high order and its strength is known to increase with age. The main disadvantages are the higher cost than lime-soil and the need for a high degree of quality control. Because of volumetric changes that take place when cement hydrates, early shrinkage cracks are formed in soil-cement layers, affecting their overall performance.
3. Soil-Bitumen Stabilisation
The addition of a bituminous binder to a soil improves its properties considerably. Firstly, if the soil lacks cohesion, the bitumen coats the soil particles, bounds them together and supplies cohesion. Secondly, bitumen being a water proofing material, the mixture become less prone to the adverse effects caused by ingress of water. Soil-aggregate mixtures or cohesive soils can be made to benefit from the action. A third procedure is to a spray a bituminous binder on a dry surface of a low-cost road (earth-gravel), with a view to prevent dust and to stop the entry of moisture into the road.
The success of this type of stabilisation lies in selecting the right quantity and type of binder. An excess of the binder will result in too thick a binder film around the soil particles and destroy part of the internal friction. When aiming at the waterproofing, it is seldom necessary to fil up the entire void space. The gradation of the soil particles also has an important bearing on the satisfactory performance of the stabilisation technique.
Bitumen stabilisation is generally used for stabilising cohesive fine-grained soil and most useful in areas where climate is hot and dry. It is due to the reasons that water is not needed at any stage during the mixing and laying process and in arid regions the bearing capacity of soil and sub-bases is often high, so that quite thin road bases are required, thus offsetting the relatively high cost of the binder.
4. Chloride Stabilisation
Calcium chloride has been used extensively as a dust palliative and moisture stabiliser for decades. Its wide use is reported in the USA and Canada.
Calcium chloride has deliquescent and hygroscopic properties. By the former is meant the ability of a material to absorb moisture from the air and thus to dissolve and become liquid. By the latter is meant the ability of a material to absorb and retain moisture without necessarily becoming liquid. These properties render the material ideally suitable as a dust-palliative on untreated low-cost roads. In dry climate regions, the moisture evaporates from the road during the day, but if calcium chloride is present, the moisture can be regained in the night. Another property of calcium chloride is that it lowers the vapour pressure of water in which it is dissolved. An increase in the surface tension of water is noticed when calcium chloride is present in a soil-water mixture. As the surface tension of the pore water rises, the rate of evaporation falls. When some evaporation takes place, the pore water content is itself reduced and this in turn causes surface tension to rise further. The water films then close on the soil particles and grip them together. If calcium chloride is added to a soil, it is observed that its unit weight increases. This is because of the lubricating effect of the chemical. It is well known that chlorides dissolved in water lower the freezing temperature of water. This makes the chemicals extremely useful in frost-susceptible locations.
5. Sodium Chloride Stabilisation
Sodium chloride, commonly known as salt is available in natural state as rock salt and sea water. Its use as a stabiliser derives from the many properties listed under calcium chloride. It is, however, less hygroscopic and inferior to calcium chloride as a lubricant for aiding compaction. An important beneficial effect is the crystallization of the salt forming a compact and hard surface, which improves the stability of the layer and prevents evaporation. The quantity of sodium chloride is roughly the same as calcium chloride, e.g. 0.5 percent by weight.
6. Other Chemicals/Materials Used for Soil Stabilisation
Several other chemicals/materials have been used for chemical stabilisation of soil. Some of them are:
· Lime fly-ash mixture – fly ash is a waste product of thermal plants
· Sodium silicate
· Sodium silicate
· Lignin – a by product in paper manufacturing industry
· Resins – wood products
· Molasses – a waste product from sugar manufacturing plants