DHARM
706 GEOTECHNICAL ENGINEERING
Passing 75–μ sieve : 10 to 30%
Liquid limit : less than 40%
Plasticity Index : less than 18
Water-proof mechanical stabilisation
Small quantities of bitumen—1 to 3%—may be added to mechanically stabilised soils to make
them water-proof.
Oiled earth
Slow-curing and medium-curing road oils are sprayed to make the earth water-resistant and
resistant to abrasion. The oils penetrate a short depth into the soil without any mechanical
mixing.
Admixtures
The addition of small quantities of phosphorus pentoxide or certain amines was found to im-
prove the effectiveness of asphalt as a soil stabiliser.
The construction of soil-asphalt is very much similar to that of soil-cement; usual thick-
ness ranges from 15 to 20 cm as with soil-cement.
17.4.4Chemical Stabilisation
Chemical stabilisation refers to that in which the primary additive is a chemical. The use of
chemicals as secondary additives to increase the effectiveness of cement and of asphalt has
been mentioned earlier.
Lime and salt have found wide use in the field. Some chemicals are used for stabilising
the moisture in the soil and some for cementation of particles. Certain aggregates and disper-
sants have also been used.
Lime stabilisation
Lime is produced from natural limestone. The hydrated limes, called ‘slaked lines’, are the
commonly used form for stabilisation.
In addition to being used alone, lime is also used in the following admixtures, for soil
stabilisation:
(i) Lime-fly ash (4 to 8% of hydrated lime and 8 to 20% of fly-ash)
(ii) Lime-portland cement
(iii) Lime-bitumen
The use of lime as a soil stabiliser dates back to Romans, who used it in the construction
of the ‘Appian way’ in Rome. This road has given excellent service and is maintained as a
traffic artery even today.
There are two types of chemical reactions that occur when lime is added to wet soil. The
first is the alteration of the nature of the adsorbed layer through ion exchange of calcium for
the ion naturally carried by the soil, or a change in the double layer on the soil colloids. The
second is the cementing action or pozzolanic action which requires a much longer time. This is
considered to be a reaction between the calcium with the available reactive alumina or silica
from the soil.