Page 262 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 257

more than if the clay were not disturbed. The reason for this has not been
investigated, but an argument may be made for a time-related cementation effect
of edge-to-face clay particle bonding, which would prevent water from entering
and separating the clay layers.

12.8.5 Relation to PI

Figure 12.8 shows the conclusions of several researchers who related clay expand-
ability to the plasticity index or PI. Curves A and B show results for remolded
samples, and curves C and D are from undisturbed samples where surcharges were
applied to more or less simulate floor and foundation loads, respectively. It will be
seen that the lowest expandability is shown by curve D, which is for undisturbed
soil under the foundation load.

12.8.6 Relation to Moisture Content

Generally expansion pressure decreases as the soil moisture content increases,
and expansion stops when the smectite clay is fully expanded. This depends on the
relative humidity of the soil air and occurs well below the point of saturation of
the soil itself. This is illustrated in Fig. 12.9, where seasonal volume change
occurs between 30 and 70 percent saturation. Expansion will not occur in a clay
that already is wet, which is not particularly reassuring because damaging
shrinkage still can occur if and when the clay dries out.

Figure 12.8
Data indicating that
remolding and a
loss of structure
greatly increase
swelling pressures.
All but curve D,
which has a
surcharge pressure
of 1000 lb/ft 2
(44 kPa) have a
surcharge
pressure of 1 lb/in. 2
(6.9 kPa).
(Modified from
Chen, 1988.)

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.