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Sunday, 22 January 2012

STABILIZATION of SOIL


        Stabilization in a broad sense incorporates the various methods employed for modifying the properties of a soil to prove its engineering performance.  Stabilization is being used for a variety of engineering works, the most common application being in the construction of road and airfield pavements, where the main objective is to increase the strength or stability of soil and to reduce the construction cost by making best use of locally available materials.

Principles Of Stabilization:

            Natural soil is both a complex and variable material.  Yet because of its universal availability and its low cost winning it offers great opportunities for skilful use as an engineering material.

            Not uncommonly, however the soil at any particular locality is unsuited, wholly or partially, to the requirements of the construction engineer.  A basic decision must therefore be made whether to:
·         Accept the site material as it is and design to standards sufficient to meet the restrictions imposed by its existing quality.
·         Remove the site material and replace with a superior material.
·         Alter the properties of existing soil so as to create a new site material capable of better meeting the requirements of the task in hand.

The latter choice, the alteration of soil properties to meet specific engineering requirements is known as “Soil stabilization.”

It must also be recognized that stabilization not necessarily a magic wand by which every soil property is changed for the better.  Correct usage demands a clear recognition of which soil properties must be upgraded, and this specific engineering requirement is an important element in the decision whether or not to stabilize.  Properties of soil may be altered in many ways, among which are included chemical, thermal, mechanical and other means.

The chief properties of a soil with which the construction engineer is concerned are: volume stability, strength, permeability, and durability.
Methods of stabilization may be grouped under two main types:
1.    modification or improvement of a soil property of the existing soil without any admixture.
2.    Modification of the properties with the help of admixtures.

Compaction and drainage are the examples of the first type, which improve the inherent shear strength of soil.
Examples of the second type are: mechanical stabilization, stabilization with cement, lime, bitumen and chemicals etc,.
LIME STABILIZATION:

          Stabilization of soils with hydrated lime is applicable to far heavier clayey soils and is less suitable for granular materials and second it is used more widely as a construction expedient that is to prepare a soil for further treatment or to render a sufficient improvement to support construction traffic.  As a temporary measure such modification or stabilization need not necessarily affected to the standards required for permanent construction.  Quick lime or lime slurries may also be used for excessively wet or dry conditions respectively.  It is therefore a very versatile stabilizer.

         In roads lime stabilization is widely used for sub-base construction or sub grade improvement; nevertheless there is no sound reason why these roles should not be interchangeable.

MATERIALS:

          The materials to be considered are lime, soil and water and it is important that the type of lime to be used is clearly defined.  It is unfortunate that the term “lime” is used to describe calcium hydroxide (agricultural lime) calcium hydroxide (slaked lime or hydrated lime) and calcium oxide (quick lime).  The term is used here and in general engineering practice to mean hydrated lime.

LIME:

          Calcium hydroxide is most widely used for stabilization.  The stabilizing effects ultimately depend on chemical attack by the lime on clay minerals in the soil to form cementitius compounds (calcium silicate) and carbonate doesn’t do this. Lime is prepared by heating calcium carbonate (natural limestone) in kilns until carbon dioxide is driven off.  The calcium oxide discharged from the kiln is known as “Quick lime” and because of lumpy condition and high heat of hydration, which makes it difficult to handle and store, particularly in humid climates it is usual slake the quick lime immediately forming hydrated lime (calcium hydroxide) as very fine powder.  It is important to note that the hydration process involves a large reduction in density and this expansion is the basis of deep stabilization techniques using lime piles.  Hydrated lime poses much less of a storage problem as it is no longer so susceptible to humidity: but both forms will revert to carbonate on prolonged exposure to air. The mean particle size is about 1/10th that of cement.  On addition of lime to soil two main types of chemical reaction occur:

·         Alteration in the nature of the absorbed layer through base exchange phenomenon and
·         Cementing or puzzolanic action.

Lime reduces the plasticity index of highly plastic soils making them more friable and easy to be handled and pulvarised.  The plasticity index of soils of low plasticity generally increase in the optimum water content and a decrease in the maximum compacted density, but the strength and durability increases.

          The amount of lime required may be used on the unconfined compressive strength or the CBR test criteria.  Normally 2 to 8% of lime may be required for coarse grained soils and 5 to 10% for plastic soils.



SPECIFICATION REQUIREMENTS FOR LIME



         Property
                                     Lime

          Quick lime
               (Cao)
       Hydrated lime
           (Ca(OH)2)

Calcium and magnesium oxides
Carbon dioxides-at kiln
                    -elsewhere

Fineness


Not less than 92 percent

Not more than 3 percent
Not more than 10 percent

Not less than 95 percent

Not more than 5 percent
Not more than 7 percent

Not more than 12 percent on 180*180
Cement standard sieve.




Construction Sequence for Lime Stabilized Bases:

1.    Shaping the Sub-grade and scarifying the soil.
2.    pulvarising the soil.
3.    Adding and mixing lime.
4.    Compacting.
5.    Finishing
6.    Curing.
7.    Adding wearing surfacing

There are three methods for carrying out these operations:

·         Mix in place method
·         Traveling plant
·         Stationary plant method.

Mix in place method:
In this method, the subgrade is first shaped to the required grade and is cleared of undesirable materials.  It is then scarified to the required depth of treatment and the soil is pulvarised until atleast 80% of the material (excluding stones) passes a 4.75mm sieve.  If another soil is to be blended, it is mixed with the loose, pulvarised soil.  The pulvarised soil is spread and shaped to proper grade.  Calculated amount of lime is then evenly distributed over the surface and intimately mixed.  Water is added as required for compaction and the soil lime water is turned into an intimate mixture.  No strict time limitation for completion of job is however necessary since soil lime cementation reactions and are slow.  It is fairly easy to process coarse grained soil.  Adding lime in proportions of1 to 4% can facilitate Pulvarisation and mixing of plastic clays.

            Mix in place method is considered cheaper and more adaptable to different field conditions, but the processing of soil is not so thorough and accurate as with other methods.


Traveling Plant method:

In this method, the pulvarised soil is heaped into a window and the lime is spread on the top.  An elevator to a mixer carried on a traveling platform where water is added and mixing is done lifts the soil and lime.  The mixture is then discharged on to the subgrade.  It is spread with a grader and compacted.  A uniform subgrade surface with controlled depth of treatment is possible.  The plant is however costly.



Stationary plant method:
In this method, the excavated soil is brought to a stationary mixing plant.  At the plant lime and water are added and mixed with the soil.  The mixture is then transported back to the desired location, dumped, spread and compacted.  Similar to traveling method, the method affords an accurate proportioning of materials and thorough mixing.  The method is slower and may prove expensive due to additional haulage of soil.


MIX DESIGN:

            The mix design procedures start from an estimate of the likely lime requirement followed by detailed tests as necessary for the particular circumstances.  These should be based on a knowledge of the appropriate properties, mechanisms criteria etc. as described below.

Properties:

            The properties of lime-stabilized soils vary in a similar manner to that found with cement-stabilized soils.  The differences lie mainly in the effect of additive content, the effect of time and the effect of temperature.

The unconfined compressive strength of soil lime mixtures increase with increasing lime content to a certain level usually about 8% for clay soils.  The rate of increase then diminishes until no further strength gain occurs with increasing lime content: in contrast to cement stabilization where the increase in strength continues to quite high cement contents (20%).  Because with lime soil mixtures there is no rapid cementation akin to the setting of concrete the effect of delay in compaction is far less important with lime stabilization and indeed, an enhanced stabilizing effect may be obtained by leaving the material loose or by breaking up lightly compacted material and recompacting after 24-hours delay.  Because there is, in general no urgency for compaction, the process of lime stabilization is more flexible in the field.  However it was pointed out that where a rapid increase in optimum moisture content occurs as a result of lime stabilization “it may be more economical to compact quickly than to add extra water”.


The gain in strength with time of a compacted soil-lime mixture broadly follows the pattern for soil-cement mixtures but the effect of temperature is more marked.  The more rapid gain in strength with increasing temperature may be one reason for the widespread use of lime in warmer climates.


Lime has an almost instantaneous effect in most cases on the plasticity of a clay and therefore upon the strength. Picture  shows a four-fold increase in strength after six minutes for clay mixed with lime: by contrast the change in strength with cement is delayed until the initial hydration set takes place.  Lime improves texture, rendering a clay more workable, so much so that lime stabilization is often used for this purpose alone in clayey soils as a preliminary to shaping and compaction or to cement stabilization without regard to any possible strength increase in the compacted state.




Reaction Mechanism

            Lime reacts with the clay minerals of the soil, or with any other fine, pozzolanic component such as hydrous silica, to form a tough water-insoluble gel of calcium silicate, which cements the soil particles.  The cementing agent is thus exactly the same as for ordinary Portland cement, the difference being that with the latter the calcium silicate gel is formed from hydration of anhydrous calcium silicate (cement) whereas with the lime the gel is formed only after attack on and removal of silica from the clay minerals of the soil.  The with contrast cement stabilization is that the latter is essentially independent of soil type: as illustrated by picture which shows the rate of gain of strength for cement stabilized soils is different for each soil type.

            The silicate gel proceeds immediately to coat and to bind clay lumps in the soil and to block off the soil pores. In time, this gel gradually crystallizes into well defined calcium silicate hydrates such as tobermorite and hillebrandite, the microcrystals of which can also interlock mechanically.  Note that reaction proceeds only whilst water is present and able to carry calcium and hydroxyl ions to the clay surface (i.e. whilst pH is still high).  The reaction thus ceases on drying, and very dry soils will not react with lime. (Or cement).
            The mechanism of the reaction can be represented thus:

NAS4H  +  CH  ---à NH  +  CAS4 H  --à NS  +  degradation product

Where S = Sio2 H =  H2O  A  =   Al2O3  C = CaO  N  =  Na2O


Criteria:

            The criteria developed for soils treate with lime fall into two broad groups as does the usage of the material.  Where the lime treatment aimed at “modifying” the soil properties by reducing plasticity improving workability increasing grain size etc.. The lime treatment is aimed at permanent and substantial “stabilization” of a soil then the criteria are based on strength bearing capacity etc..
            Lime modification of soil has been used for three main purposes: to reduce the plasticity of an otherwise acceptable mechanically stable material to improve the workability of a soil and its resistance to deflocculation and erosion and to produce a rapid increase in strength in wet clay soil as a construction expedient.  Criteria are not always available to measure the adequacy of the treatment.  For the first named purpose the liquid and plastic limit and plasticity index are determined with varying amounts of lime added to the soil until the normal plasticity requirements for an untreated material are met.  In most cases there would be in addition an increase in UCS and bearing capacity but this is not usually taken into account.

            The procedure for evaluating the effectiveness by plasticity changes may be misleading, however, in kaolinitic or illitic soils where only small and slow changes in plasticity index occur.  For these soils a better procedure is to adopt a strength test.

  




SUGGESTED CRITERIA FOR SOIL-LIME

Process
Purpose
Requirement
Lime “modification”
Improvement of access on wet site.
Improvement of workability and pulvarization.
Large increase in plastic limit. Rapid increase in bearing strength.
Large and rapid decrease in plasticity increase in proportion passing 3/16 in. sieve.
Lime “stabilization”
Improvement of subgrade material.
Improvement of base material.
Increase in bearing capacity
Decrease in swell
Decrease in plasticity
Increase in strength or bearing capacity (min CBR 880).

SUGGESTED LIME CONTENTS

Soil Type
Content for modification
Content for Stabilization
Fine crushed rock
Well graded clay gravels
Sands
Sandy clay
Silty clay
Heavy clay
Very heavy clay
Organic soils
2-4 percent
1-3 percent

Not recommended
Not recommended
1-3 percent
1-3 percent
1-3 percent
not recommended
Not recommended
~3 percent

Not recommended
~ 5 percent
2-4 percent
3-8 percent
3-8 percent
not recommended


Design Procedure:

            Mix design therefore, consists of adding varying amounts of lime to the soil and observing the effect, after a suitable curing period. on the plasticity, aggregations, strength or bearing capacity, when a suitable additive level may be determined.  A useful guide is to allow 1 percent of lime (by weight of dry soil) for each 10 percent of clay in the soil.  For closer determination, two samples prepared at _+- 2 percent of this lime content will usually reveal the optimum economic percentage.  While the changes in plasticity are accepted fairly readily, there is, unfortunately, a conservative attitude to the improvements in strength, bearing capacity and stress-strain behavior.



Summary:

            Addition of lime to a soil with inadequate mechanical stability will improve strength, bearing capacity and resistance to water softening.  In clay soils, lime will often cause rapid changes in plasticity and this in effect will “dry out” the soil.  This is the basis of the use of lime stabilization as a construction expedient or for pre-treatment prior to cement stabilization.  Lime stabilization is in general more tolerant of construction delay than cement stabilization and more suitable for clay soils.