Page 256 - Geotechnical Engineering Soil and Foundation Principles and Practice
P. 256
Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 251
Figure 12.4
The plastic limit
test. As the soil
thread crumbs,
pieces are
collected in a metal
container for oven-
drying to determine
the moisture
content.
averaged. See AASHTO Specification T-90 or ASTM Specification D-424 for
details of the plastic limit test.
12.4 DIRECT APPLICATIONS OF LL AND PL TO FIELD SITUATIONS
12.4.1 When a Soil Moisture Content Exceeds
the Plastic Limit
The liquid limit and plastic limit tests are more diagnostic than descriptive of
soil behavior in the field because the tests involve continual remolding. However,
there are some important situations where remolding occurs more or less con-
tinuously in the field. One example is soil in the basal zone of a landslide. As a
landslide moves, it shears and mixes the soil. This mixing action can occur if the
soil moisture exceeds the plastic limit. If through chemical treatment such as with
drilled lime (quicklime) the plastic limit is increased, the landslide stops.
12.4.2 When a Soil Moisture Content Exceeds the
Liquid Limit
Exceeding the soil liquid limit in the field can generate harmful and potentially
devastating results, as the soil may appear to be stable and then when disturbed
can suddenly break away, losing its thixotropic strength and becoming
transformed into a rapid churning, flowing mudslide that takes everything in its
way. The rate of sliding depends on the slope angle and viscosity of the mud; the
lower the viscosity and steeper the slope, the faster the slide. The most devastating
mudslides in terms of loss of life therefore occur in mountainous terrain where the
mud moves faster than people can get out of the way and escape almost certain
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.
