A SEMINOR ON SOIL STABILISATION

Prepared by D.ARUNA

INTRODUCTION

Stabilisation in a board sense, incorporates the varios methods employed for modifying the properties of a soil to improve its engineering peformance .

Stabilisation is being used for a variety of engineering works, the most common application being in the construction of road and air-field pavements.

where the main objective is to increase the strength or stability of soil and to reduce objective is to increase the strength or stability of soil and to reduce the construction cost by making best use of the locally available materials.

CLASSIFICATION OF THE METHODS OF STABILISATION

1.Stabilisation additives without.

2.Stabilisation with additives

Classifications may be based on the treatment given to soil, on additives used, or on the process involved.

Broadly speaking, soil stabilization procedures may be brought under the following two heads

Stabilisation with out additives may be “mechanical” re-arrangement of particles through compaction or addition or removal of soil particles.

. It may be “drainage”- drainage may be achieved by the addition of external load

Stabilisation with additives may be cement stabilization (that is soil cement), Bitumen stabilization, or chemical stabilization

STABILISATION OF SOIL WITH OUT ADDITIVES

Some kind of treatment is given to the soil in this approach; no additives are used.

The treatment may involve a mechanical process like compaction and a change of gradation by addition or removal of soil particles

MECHANICAL STABILISATION

Mechanical stabilisation involves two operations

1. changing the composition of soil by addition or removal of certain constituents, and

2. densification or compaction. The particle size distribution and composition are the important factors governing the engineering behavior of a soil.

Significant changes in the properties can be made by addition or removal of suitable soil fraction for mechanical stabilisation .

Where the primary purpose is to have a soil resistant to deformation and displacements under loads soil materials can be divided into two fractions

The granular fraction retained on a 75 micron IS Sieve and the fine soil fraction passing a 75 micron sieve.

Mechanical stabilization has been largely used in the construction of cheap roads.

Typical examples are given in Table 31.1.

Table 31.1 Typical Gradation Specifications of Mechanically.

Is Sieve

Percentage passing

Base Surfacing Base or surfacing

Max. Size Max. Size Max. Size

40 mm 20mm 20mm 10 mm

40 mm 100 - - -

20 mm 80-100 100 100 -

10 mm 55-80 80-100 80-100 100

4.75 mm 40-60 50-75 60-85 80-100

2.0 mm 30-50 35-S60 45-70 50-80

1.18 mm - - 35-60 40-65

600 micron 15-35 15-30 - -

300 micron - - 20-40 20-40

75 micron 5-15 5-15 10-25 10-25

Note:

(i) For bases: Liquid limit not exceeding 25% and plasticity indes not exceeding 6.

(ii) For surfacing: Liquid limit not exceeding 35% and plasticity index between 4 and 9.

STABILISATION BY DRAINAGE

Generally speaking, the strength of a soil generally decreases with an increase in pore water and in the pore water pressure.

Addition of water to a clay causes a reduction of cohesion by increasing the electric repulsion between particles

For a given total stress, an increase in pore water pressure results in a decrease of effective stress and consequent decrease in strength.

The methods used for drainage for this purpose are

1. Application of external load to the soil mass,

2. Drainage of pore water by gravity and/or pumping

4. Application of a thermal gradient.

using well points, sand – drains, etc

3. Application of an electrical gradient or electro

-osmosis; and,

STABILISATION OF SOIL WITH ADDITIVES

The mode and degree of alternation necessary depend on the nature of the soil and its deficiencies.

If additional strength is required in the case of cohesionless soil, a cementing or a binding agent may be added and if the soil is cohesive, the strength can be in creased by making it moisture – resistant, altering the absorbed water films, increasing cohesion with a cement agent and adding internal friction.

Swelling and shrinkage may also be reduced by cementing, altering the water adsorbing capacity of the clay mineral and by making it moisture – resistatnt.

BITUMEN STABILISATION

Asphalts and tars are the bituminous materials which are used for stabilization of soil, generally for pavement construction. These materials are normally too viscous to be incorporated directly with soil.

bitumen stablisation is classified under the following four types: (i) Sand bitumen

(ii) soil –bitumen

(iii) water – proofed mechanical stablisation and

(iv) oiled earth.

CHEMICAL STABLISATION

Calcium chloride

Sodium chloride

Sodium silicate.

Calcium chloride is used as a water retentive additive in mechanical stabilized bases and surfacing

The stabilising action of sodium chloride is somewhat similar to that of calcium chloride, but it has not been so widely used

The sodium silicate solution in water, known as water glass, in combination with other chemicals, such as calcium chloride, is used as an injection for stabilizing deep deposits of soil.

CEMENT STABILISATION

Portland cement is one of the most widely used additives for soil stabilization.

Factors affecting the soil-cement

Nature of the soil

Cement contentCompactionMixingAdmixturesDesigning and testing soil – cement Field construction of soil-cement

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