The term soil stabilization means the improvement of the stability or bearing power of the soil by use of controlled compaction, proportioning and the addition of suitable admixture or stabilizers. Stabilization deals with physical, physico-chemical and chemical methods to make the stabilized soil serve its purpose as engineering material.
The basic principles in soil stabilization are stated below:
1. Evaluate the properties of given soil.
2. Deciding the method of supplementing the lacking property by effective and economical method of stabilization.
3. Designing the stabilized soil mix with stability and durability values.
4. Considering the construction procedure by adequately compacting the stabilized layers.
Effects of Stabilization:
Soil stabilization may result in any one or more of following changes:
(i) Increase the stability, change in properties like density or swelling, change in physical characteristics.
(ii) Change in chemical properties.
(iii) Retaining the desired minimum strength by water.
(i) Proportioning Technique – Various locally available materials and aggregates are mixed in suitable proportion and compacted to serve desired objective. For example – the stability of fine-grained soil could be improved by adding gravel or sand and vice versa.
(ii) Cementing Agent – The strength of stabilized soil can be increased by the addition of cementing agent like Portland cement, lime or lime fly ash. Bituminous materials also impart binding effect to non-cohesive soils.
(iii) Modifying Agent – If a small quantity of stabilizer improved the properties like swelling and high plasticity to make the soil a useful construction material, then such stabilizer may be called a modifier. The most common modifier in case of highly plastic soils is lime.
(iv) Waterproofing Agent – A compacted and stable soil may become weaker by ingress of water or when subjected to soaking condition. If the absorption of water can be stopped by use of some waterproofing agent, it will be possible to make use of such materials with advantage. The most useful agent is bitumen.
(v) Water Repelling Agent – Same function as of waterproofing agent may be performed by some water repelling or retarding agents like vinsol resin or other resinous materials.
(vi) Water Retaining Agent – Some non-cohesive soils have sufficient stability with presence of some moisture. But the soil may become loose when completely dried. In such cases some agent is used to retain the required quantity of moisture such as calcium chloride.
(vii) Heat Treatment – Thermal stabilization has different useful aspects as regards to clayey soil. They may reduce the swelling and heat treated soil may be used as a soft aggregate in mechanical stabilization.
(viii) Chemical Stabilization – Use of certain chemicals with quantity 0.5% of soil by weight impart useful changes in soil. But considerable investigation and care is required.
(ix) Mechanical Stabilization – Correctly proportioned material (soil and aggregates) when compacted to get a mechanically stable layer, the method is called mechanical stabilization.
Two basic principles of mechanical stabilization are:
If the granular soil containing negligible fines is mixed with a certain proportion of binder soil, it is possible to increase the stability. Similarly, the stability of a fine-grained soil could be considerably improved by mixed suitable proportion of granular material to get a suitable gradation.
Mechanical stabilization has been successfully applied for sub-base and base course construction. It is also useful in surface course for low cost roads such as village roads when traffic and rainfall are low.
In all the above methods the adequate compaction of the stabilized layers is the most important requirement.
Properties of Soil Aggregate Mixtures:
(iii) Changes in volumes
(iv) Stability with variation in moisture content
(v) Good drainage
(vi) Less frost susceptibility
(vii) Case of compaction
It is generally believed that stability of a soil aggregate mixture can be increased by increasing the dry density. Hence, proportioning of mixes is done to attain maximum dry density.
Therefore, it may be seen that proportioning of the mix affects the properties considerably. With proper proportioning it is possible to attain a mix with best combination of the desirable properties.
Three Typical Stages of Mixes:
Factors Affecting Mechanical Stabilization:
1. Mechanical strength of aggregates
3. Properties of soil
4. Presence of salts, mica, etc.
Methods of Granular Mixes (Designing):
(i) Trial method
(ii) Combining from sieve analysis
(iii) Arithmetic method
(iv) Proportioning by triangular Chart method
(v) Rothfutch’s method
Design of Mix:
The factors to be considered in design of mix are:
3. Index properties and
The particle distribution that gives the maximum density. Theoretical gradation for maximum density is given by –
Fuller assumed the particles as spherical. While deriving the equation for maximum density, the value of gradation index for spherical shape is ƞ = 0.5
The value of ƞ for angular or flaky particle will be less than 0.5 upto 0.3 or even less.
When different materials like aggregates, sand and fines are available, the ratio of each size to produce a mix of maximum density is called proportioning.
Design of Mixtures:
In case of the natural local material does not fulfill the requirements for a stable mixture, it may be necessary to combine two or more materials from different sources in order to secure a properly proportional mixture for the purpose at hand. There are, broadly speaking, three methods to determine the proportions in which materials should be mixed.
The method is to make up a series of mixtures, the proportions of which are based on experience with the materials.
These mixtures are then tested and one which best meets the grading and plasticity requirements is selected for use in the field.
Combining for Plasticity Index:
In this method it is a proportion of the materials by means of formula or with the help of tables so that Plasticity Index (PI) of the mixture may be expected to be within the allowable range. Then a trial mixture is made. Liquid limit is within the allowable limits and if the gradation is reasonably close to the specification.
Recommendations vary for application of the formula should the PI of one of the materials be zero.
Different recommendations are that values of 0, 1 or 3 be used when PI is actually zero. This formula is intended as an approximation, and should be used only as a guide.
Another method of combining for plasticity index is described below:
Percentage of soil binder to be added = K x R x percentage of aggregate passing IS : 425 micron sieve
K = factor based on plasticity index of binder, plasticity index of aggregate and plasticity index of desired mixture.
Knowing the plasticity index of the binder soil, in the left hand column cross the table given in Bulletin No. 5 of Highway Research Board, horizontally the column containing value of plasticity index of the mixture desired and find value of K under the heading of the plasticity index for the aggregate.
For example – for plasticity index of binder soil 14 and plasticity index of desired mixture 2 and for non-plastic aggregate (Plasticity index = 0), the value of K is 0.17.
Soil Stabilization with Lime:
Soil-lime has been widely used either as a modifier for clayey soils or as a binder. In several cases both actions of lime may be observed. When clayey soils with high plasticity are treated with lime, the Plasticity index is decreased and the soil becomes friable and easy to be pulverised, having less affinity with water. All these modifications are considered desirable for stabilization work. Lime also imparts some binding action even in granular soils.
In fine-grained soils there can also be Pozzolanic action resulting in added strength.
When a clay is treated with lime, the various possible reactions are base exchange, coagulation or flocculation, reduction in thickness of water film around clay particles, cementing action and carbonation.
Fine clay particles react with lime and get flocculated or aggregated into larger particle groups which are fairly stable even under subsequent soaking. Plastic clay soils tend to agglomerate more than silty and sandy soils.
Due to this flocculation; the lime-treated clays indicate a different grain size distribution. The changes in plasticity, characteristics of soil-lime mixture also take place simultaneously; total lime required for the changes depends on several factors including soil type.
The maximum dry density of soil-lime mix is decreased by 2 to 3 per cent in terms of untreated soils; however this decrease in dry density with the addition of small proportion of lime does not cause reduction in strength.
Soil-lime is quite suitable as sub-base course for high types of pavements and base course for pavements with low traffic. As in the case of soil-cement, soil-lime also cannot be used as a surface course even for light traffic in view of its very poor resistance to abrasion and impact. Soft lime is quite suitable in warm regions; but it is not suitable under freezing temperatures.
Factors Affecting Properties of Soil-Lime:
The various factors on which the properties of soil-lime depend are soil type, lime content, compaction, curing and additives, if any.
Various soil properties affect the base exchange characteristics and Pozzolanic action. The proportion of increase in strength in a soil-lime mix depends on the Pozzolane in the soil.
Generally, an increase in lime content causes a slight change in liquid limit and a considerable increase in plastic limit resulting in reduction in plastic limit.
The rate of increase in plastic limit is first rapid and then the rate decreases beyond a certain lime content. This point is often termed lime fixation point.
This is the approximate lime content that is considered to be used up for modification of clay. During this range, the increase in stability of the clay-lime may not be noteworthy. When the lime content in the mix is further increased, there is a high rate of increase in stability.
However, when the lime content is increased beyond a certain proportion, the stability values generally start decreasing.
With proper lime treatment it is possible to make the clay almost non-plastic with plasticity index reducing to practically zero.
Increase in lime content also causes considerable reduction in swelling and increase in shrinkage limit, all these changes are desirable for stabilization of clay.
Types of Limes:
After long curing periods, all types of limes produce almost the same effects. However, quick lime (CaO) has been found to be more effective than hydrated lime [Ca(OH)2]. But quick lime has a tendency to cause skin burns of unprotected workmen, and hence need careful handling and protection. Hydrated lime is therefore commonly used in stabilization work, either as a dry powder or by mixing with water.
The compacted density is important as regards the strength of soil-lime is concerned. Hence, compaction is done at OMC and maximum density is aimed at.
The strength of soil-lime increases with curing period upto several years. The rate of increase in strength is rapid during the initial period of curing, which also depends on the curing temperature.
At low temperature the rate of strength gain decreases, considerably. Below the freezing point practically there is no gain in strength. The humidity of the surroundings during curing also affects the strength.
Addition of lime alone with soil often does not increase the strength of the mix as desired. Portland cement and Pozzolanic materials like fly ash and surkhi are most promising materials in this respect.
Lime-fly ash stabilization is cheap and is a method with considerable scope for the construction of low cost roads in warm regions and where fly ash is available as a waste product.
Chemical additives like sodium metasilicate, sodium hydroxide and sodium sulphate are also found to be useful additives to soil-lime.
Design of Soil-Lime Mix:
If lime is used mainly as a modifier for highly plastic clay, then the lime content may be decided based on lime fixation limit or at a higher value to reduce the plasticity index and swelling values upto the desired limits. Some strength tests also may be considered a criterion for the design of mix.
However, the compressive strength of soil-lime specimens without additives, at seven days may be quite low.
Construction of Soil-Lime Base Course:
There is no recommended limit of test values for the soil to be stabilized with lime. Even highly plastic soils can be suitably modified by lime. Fresh stock of hydrated lime or quick lime are stored near the site.
Lime — fine powder
Plants and equipment needed are scarifying, pulverising, mixing (in situ) and compaction.
(i) Preparation of sub-grade.
(ii) Pulverization of the soil to be stabilized.
(iii) Addition of part of lime as dry powder or as slurry with water and mixing.
(iv) Allowing the mixture to age for about a day or preconditioning the soil, and remixing when pulverization becomes easy.
(v) Adding rest of the lime, water if necessary and remixing.
(vi) Spreading to desired grade and compacting.
(vii) The soil-lime is protected from drying out and is allowed moist-curing.
(viii) Field control tests include checking moisture content at the time of compaction and checking dry density soon after compaction.