A laboratory investigation of shear wave velocity in stabilised soft soils.

The stabilisation of soft clay soils is intended to increase their shear strength and to
reduce their compressibility. The possibility exists of using geophysical methods to
monitor changes in these properties.
Laboratory experiments were carried out on stabilised clays to study the relationships
between shear wave velocity, and hence small strain shear stiffness, and shear strength or
one-dimensional compressibility. One artificial clay, Speswhite kaolin, and two natural
clays, from Malaysia and Sweden, were used as the base clays. Either ordinary Portland
cement or a 1: 1 mix of the cement with unslaked lime was added to the base clays in order
to stabilize them.
In the first part of the investigation, samples of stabilised clay were initially
subjected to a non-destructive bender element test to obtain the shear wave velocity and
then to an unconfined compressive strength test or vane shear strength test. It was evident
that small stabiliser amounts (less than 10 % of the dry weight of the base clay) could
significantly improve both the strength and stiffness of the originally soft material. In
addition, good correlations between the shear strength and the shear wave velocity (or
small strain shear stiffness) of the stabilised clays were established.
In the second part of the investigation, an instrumented oedometer was used to
simultaneously monitor shear wave velocity and one-dimensional compression during tests
on samples cured for a set period. Lateral stresses were also measured. Complementary
tests were conducted in standard oedometers, to study the effect of the curing period. In
these tests yield stresses were identified and corresponded to the onset of changes in shear
wave velocity. After yield, the constrained moduli could be correlated with shear wave
velocity. Tests were also carried out on samples of clay in which a central stabilised
column had been created. Equal strain predictions of the compression of these samples,
based on the results of separate tests on the two components, were relatively successful.
The results of the research suggest that shear wave velocity measurements could be
useful in practice to enable the shear strength and post-yield compressibility of stabilised
clay soil to be estimated.