Geotechnical Engineering

DHARM

SOIL STABILISATION 701

surface course also should be 76 mm thick with a minimum of one-third portion of sand in the
soil mixture to which brick aggregate is added at 50% of the soil mixture. The plasticity index
for the mix is recommended to be 9.5 to 12.5 for roads without surface treatments, and 8 to 10
for roads with surface treatments.
Although the grading is not conducive to producing the densest mix, it is supposed to
yield a satisfactory road surface under mixed traffic conditions prevailing in India.
Mehra’s method has been very popular, especially in the northern parts of the country.

17.3.2Stabilisation 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. The strength of a saturated soil depends
directly on the effective or intergranular stress. For a given total stress, an increase in pore
water pressure results in a decrease of effective stress and consequent decrease in strength.
Thus, drainage of a soil is likely to result in an increase in strength which is one of the
primary objectives of soil stabilisation.
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, using well-points, sand-drains,
   etc.,


3. application of an electrical gradient or electro-osmosis; and,


4. application of a thermal gradient.

Application of external load to the soil mass
The aim is to squeeze out pore water. The common load is by way of adding an earth sur-
charge. Other miscellaneous techniques are also used.

Drainage of pore water by gravity and/or pumping

Well-points are used to drain pore water either by gravity and/or pumping (Barron, 1948).
Vertical sand drains or sand piles (Rutledge and Johnson, 1958) are used to expedite
drainage of a soil stratum. The diameter of the sand-drains may be 40 to 50 cm and the spacing
may be 2 to 3 m. A drainage blanket is placed on top and a surcharge fill is placed on top of this
blanket.
A proper design of sand-drain installation involves the determination of diameter and
spacing of sand drains, the thickness of the drainage blanket, and, amount and duration of
surcharge fill loading (Terzaghi, 1943).

Application of electrical gradient or electro-osmosis
When a direct electric current is passed through a saturated soil, water moves towards the
cathode. If this is removed the soil undergoes consolidation. This phenomenon is called ‘‘electro–
osmosis’’.

In addition to electro-osmotic consolidation, passage of electric current can cause ion
exchange, alteration of arrangement of the particles, and electro-chemical decomposition of
the electrodes. The combination of these changes brought about in the soil is called ‘electrical
stabilisation’. This procedure has been successfully employed to increase skin friction of piles
(Casagrande, 1952 and 1953).