DHARMSOIL STABILISATION 707
Lime has the following effects on soil properties:
Lime generally increases the plasticity index of low-plasticity soil and decreases that of
highly plastic soils; in the latter case, lime tends to make the soil friable and more easily
handled in the field.
It increases the optimum moisture content and decreases the maximum compacted den-
sity; however, there will be an increase in strength. About 2 to 8% of lime may be required for
coarse-grained soils, and 5 to 10% for cohesive soils.
Certain sodium compounds (e.g., sodium hydroxide and sodium sulphate), as secondary
additives, improve the strength of soil stabilised with lime.
Lime may be applied in the dry or as a slurry. Better penetration is obtained when it is
used as a slurry. The construction of lime-stabilised soil is very much similar to that of soil-
cement. The important difference is that, in this case, no time limitation may be placed on the
operations, since the lime-soil reactions are slow. Care should be taken, however, to prevent
the carbonation of lime. Lime stabilisation has been used for bases of pavements.
Salt stabilisation
Calcium chloride and sodium chloride have been used for soil stabilisation. Calcium chloride is
hygroscopic and deliquiscent. It absorbs moisture from the atmosphere and retains it. It also
acts as a soil flocculant. The action of sodium chloride is similar.
The effect of salt on soil arises from colloidal reactions and the alteration of the charac-
teristics of soil water. Salt lowers the vapour pressure of pore water and also the freezing
point; the frost heave will be reduced because of the latter phenomenon.
The main disadvantage is that the beneficial effects of salt are lost, if the soil gets leached.Lignin and chrome-lignin stabilisation
Lignin is one of the major constituents of wood and is obtained as a by-product during the
manufacture of paper from wood. Lignin, both in powder form and in the form of sulphite
liquor, has been used as an additive to soil for many years. A concentrated solution, partly
neutralised with calcium base, known as Lignosol, has also been used.
The stabilising effects of lignin are not permanent since it is soluble in water; hence
periodic applications may be required. In an attempt to improve the action of lignin, the ‘Chrome-
lignin process’’ was developed (Smith, 1952). The addition of sodium bichromate or potassium
bichromate to the sulphite waste results in the formation of an insoluble gel.
If the lignin is not neutralised, it is acid and acts as a soil aggregant; when neutralised
as with Lignisol, it acts as a dispersant. Chrome lignin imparts considerable strength to soils
as a cementing agent (Lambe, 1962).
Stabilisers with water-proofers
It is well known that cohesive soils possess considerable strength when they are dry. When
they have access to water, they imbibe it and lose strength. Water-proofers, i.e., chemicals
which prevent the deleterious effects of water on soils, are useful in such cases. Siliconates,
amines and quaternary ammonium salts fall in this category.
Water-proofers do not increase the strength, but help the soil retain its strength even in
the presence of water.