Geotechnical Engineering

DHARM

SOIL STABILISATION 699

Change of gradation—addition or removal of soil particles

The engineering behaviour of a soil depends upon (among other things) the grain-size distri-

bution and the composition of the particles. The properties may be significantly altered by

adding soil of some selected grain-sizes, and, or by removing some selected fraction of the soil.

In other words, this approach consists in manipulating the soil fractions to obtain a suitable

grading, which involves mixing coarse material or gravel (called ‘aggregate’), sand, silt and

clay in proper proportions so that the mixture when compacted attains maximum density and

strength. It may involve blending of two or more naturally available soils in suitable propor-

tions to achieve the desired engineering properties for the mixture after necessary compaction.

Soil materials can be divided into two fractions, the granular fraction or the ‘aggregate’,

retained on a 75-micron I.S. Sieve, and the fine soil fraction or the ‘binder’, passing this sieve.

The aggregate provides strength by internal friction and hardness or incompressibility, white

the binder provides cohesion or binding property, water-retention capacity or imperviousness

and also acts as a filler for the voids of the aggregate.

The relative amounts of aggregate and binder determine the physical properties of the

compacted stabilised soil. The optimum amount of binder is reached when the compacted binder

fills the voids without destroying all the grain-to-grain contacts of coarse particles. Increase in

the binder beyond this limit results in a reduction of internal friction, a slight increase in

cohesion and greater compressibility. Determination of the optimum amount of binder is an

important component of the design of the mechanically stabilised mixture.

Mechanical stabilisation of this type has been largely used in the construction of low-

cost roads. Guide specifications have been developed based on past experience, separately for

base courses and surface courses.

The grading obtained by a simple rule given by Fuller has been found to be satisfactory:

Percent passing a particular sieve = 100

d

D

...(Eq. 17.1)

where d = aperture size of the sieve, and

D = size of the largest particle.
Suggested gradings for mechanically stabilised base and surface courses for roads are
given in Table 17.1.

If the primary aim is to reduce the permeability of a soil, sodium montmorillonite—a
clay mineral, called ‘‘bentonite’’—may be added. For example, the permeability of a silty sand
could be reduced from 10–4 cm/s to 10–9 cm/s by the addition of 10% of bentonite. However, it
must be remembered that bentonite is costly and its effectiveness may be reduced by flowing
water, and wetting and drying. Naturally available local clay can be blended with pervious
soils to result in a more nearly permanent blanket; this may be a much cheaper and superior
approach, if such a material is available in the proximity of the site.