Page 178 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
Soil Fabric and Structure 173

‘‘clay skins,’’ technically called ‘‘argillans,’’ that cover surfaces of the individual
blocks or peds.

8.4 SENSITIVITY

8.4.1 Sensitive Clays

Sensitivity is more than skin-deep, as all clays are somewhat sensitive, which
means that they lose a substantial portion of their strength when their floccu-
lated structure is disturbed. This is illustrated in Fig. 8.4(c). However, floccu-
lation still may triumph in the long run if the soil then is left undisturbed.
A loss of strength upon remolding or stirring and regain with aging is called
thixotropy.

Thixotropic behavior explains the ‘‘setup factor’’ that occurs from pile driving in
cohesive soils, such that if driving stops for a few hours penetration cannot easily
be restarted. After a day or so the pile-bearing capacity commonly is increased by
a factor of two, so load tests normally are conducted after sufficient time has
elapsed to allow thixotropic setting.

As previously discussed, thixotropy also is a vital attribute of drilling muds
that are circulated down through a drill pipe and come back up in the
annular space between the pipe and the edges of a borehole. Similar muds are
used to hole slurry trench excavations open for constructing cast-in-place
retaining walls.

Muddy streams and rivers usually supply sufficient agitation to keep clays
dispersed. They then can slowly settle out in quiet water such as in a lake. This is
in contrast to settling in salt water, where the clay is flocculated and quickly
settles. Deltas in freshwater lakes therefore are sandy while those in sea water
contain more clay.

8.4.2 Measuring Sensitivity
Sensitivity is defined as the ratio of strength measured before remolding to that
measured after remolding at the same soil moisture content and density.

Sensitivity normally is determined from laboratory unconfined compression tests.
An undisturbed cylindrical sample is obtained by pushing a thin-walled steel tube;
it is then extruded, and loaded vertically in a compression machine until it fails.
The soil sample then is remolded without allowing any loss of water, pressed into
a cylindrical mold so that it is the same size as the original specimen, and again
tested in unconfined compression. and the ratio of the initial strength to the
remolded strength is the sensitivity. The strength after remolding is called a
residual strength.

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