Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 73

*3.12 THIXOTROPY OF CLAYS

When clays with a flocculent structure are used in construction, these may lose some strength
as a result of remoulding. With passage of time, however, the strength increases, though not
back to the original value. This phenomenon of strength loss-strength gain, with no change in
volume or water content, is called ‘Thixotropy’. This may also be said to be “a process of soften-
ing caused by remoulding, followed by a time-dependent return to the original harder state”.
The loss of strength on remoulding is partly due to the permanent destruction of the
structure in the in-situ condition, and partly due to the reorientation of the molecules in the
adsorbed layers. The gain in strength is due to the rehabilition of the molecular structure of
the soil. The strength loss due to destruction of structure cannot be recouped with time.

‘Thixis’ means the tough, the shaking, and ‘tropo’ means to turn, to change. Thus, thix-
otropy means “to change by touch”; it may also be defined, basically, as a reversible gel-sol-gel
transformation in certain colloidal systems brought about by a mechanical disturbance fol-
lowed by a period of rest.

The loss in strength on remoulding and the extent of strength gain over a period of time
are dependent on the type of clay minerals involved; generally, the clay minerals that absorb
large quantities of water into their lattice structures, such as montmorillonites, experience
greater thixotropic effects than other more stable clay minerals.

For certain construction situations, thixotropy is considered a beneficial phenomenon,
since with passage of time, the earth structure gets harder and presumably safer. However, it
has its problems—handing of materials and equipments may pose difficulties. Thixotropic
influences have affected piles, a type of foundation construction, driven in soils. The distur-
bance may cause temporary loss in strength of the surrounding soil. Driving must be fully
done before thixotropic recovery becomes pronounced. Thixotropic fluids used in drilling op-
erations are called ‘drilling muds’.

3.13 ILLUSTRATIVE EXAMPLES

Example 3.1: In a specific gravity test with pyknometer, the following observed readings are

available:

Weight of the empty pyknometer = 7.50 N

Weight of pyknometer + dry soil = 17.30 N

Weight of pyknometer + dry soil + water filling the remaining volume = 22.45 N

Weight of pyknometer + water = 16.30 N

Determine the specific gravity of the soil solids, ignoring the effect of temperature.

The given weights are designated W 1 to W 4 respectively.

Then,

the weight of dry soil solids, Ws = W 2 – W 1

= (17.30 – 7.50) N = 9.80 N

The specific gravity of soil solids is given by Eq. 3.1:

G =

W

WWW

s

s−−() 34

(ignoring the effect of temperature)