Page 179 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
174 Geotechnical Engineering

Table 8.1 Sensitivity Descriptive term
The Rosenqvist 52 Insensitive
terms for sensitivity 2–4 Moderately sensitive
4–8 Sensitive
8–16 Very sensitive
16–32 Slightly quick
32–64 Medium quick
464 Quick

Sensitivity also can be quickly measured in the field with the vane shear test.
In this test, two vertical blades arranged to make an X are pushed into soil and
twisted. The torque to cause failure is measured, and twisting continued to obtain
a residual shear strength and strength ratio.

The sensitivity of clays typically ranges from about 2 to 4, and emphasizes
the importance of avoiding unnecessary disturbance. Terms used to describe
sensitivity are shown in Table 8.1.

8.4.3 Chemically Dispersed Clays

Clays are routinely dispersed in the laboratory as part of the procedure for
particle size analysis, discussed in the preceding chapter. In this case the clay
particles are separated as in Fig. 8.4(d) and the electrical charge is modified so
that the particles repel instead of attracting one another. The details of this action
are described in Chapter 10.

Dispersed clays can remain dispersed in a laboratory flask for months or years
without settling out, but opposing forces of gravity and Brownian motion
eventually cause the suspension to develop a density gradient. There normally
is no discrete X-ray diffraction spacing because the clay particles are randomly
separated and too thin.

Naturally dispersed clay can occur in alkaline playa lakes where the clay mineral is
sodium-saturated smectite. This is the most expansive of naturally occurring
expansive clays. Drying draws particles together and results in shrinkage cracks,
and wetting re-expands the clay.

8.4.4 Quick Clays

A particularly troubling but fortunately unusual clay is called quick clay. Unlike
quicksand, which is a condition brought on by rising seepage water, quick clay is a
clay that is exceptionally sensitive and turns to liquid if disturbed.

Quick clay is clay that has been deposited and flocculated in salt water, usually in
estuaries, and then leached of excess salt so that the flocculated structure becomes

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