Page 264 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 259

therefore is between 6 and 19 cm, depending on filling of the shrinkage cracks and
amount of lateral elastic compression of the soil.

12.8.9 Controlling Volume Change with a
Nonexpansive Clay (n.e.c.) Layer

One of the most significant discoveries for controlling expansive clay was by
Dr. R. K. Katti and his co-workers at the Indian Institute of Technology, Mumbai.
Katti’s group conducted extensive full-scale laboratory tests to confirm field
measurements, such as shown in Fig. 12.9, and found that expansion can be
controlled by a surficial layer of compacted non-expansive clay. A particularly
severe test for the design was the canal shown in Fig. 12.10. The most common
application of Katti’s method is to stabilize the upper meter (3 ft) of expansive
clay by mixing in hydrated lime, Ca(OH) 2 . If only the upper one-third, 30 cm
(1 ft), is stabilized, volume change will be (0.15 þ 0.08) 30 ¼ 7 cm (3 in.), a
reduction of about 60 percent. If the upper 60 cm (2 ft) is stabilized, the volume
change will be 0.08 30 ¼ 2.4 cm (1 in.), a reduction of over 85 percent.
Stabilization to the full depth has been shown to eliminate volume change
altogether. The next question is, why?

An answer may be in the curves in Fig. 12.8, as a loss of clay structure greatly
increases clay expandability. As a result of shrink-swell cycling and an increase in
horizontal stress, expansive clays are visibly sheared, mixed, and remolded, so by
destroying soil structure expansion probably begets more expansion. According to
this hypothesis, substituting a layer of nonexpansive clay for the upper highly
expansive layer may help to preserve the structure and integrity of the underlying
layer. It was found that using a sand layer or a foundation load instead of densely

Figure 12.10
The Malaprabha
Canal in India was
successfully built
on highly
expansive clay
using Katti’s
method, by
replacing the upper
1 m of soil with
compacted
nonexpansive clay
(n.e.c.) to
replicate the
conditions shown
in Fig. 12.9 for the
underlying soil.

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