Page 272 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 267

other rock that readily breaks into layers may render a coarse-grained soil
unsuitable for certain purposes, since alternate wetting and drying may cause it to
disintegrate partially or completely. This characteristic can be determined by
submerging thoroughly dried particles in water for at least 24 hours and observing
slaking or testing to determine a loss of strength.

12.11.3 Fine-Grained vs. Coarse-Grained Soils
As shown in Table 12.2, fine-grained soils are defined as having over 50 percent
passing the No. 200 sieve. The percentage finer can be estimated without the use
of a sieve and weighing device, by repeatedly mixing a soil sample with water and
decanting until the water is clear, and then estimating the proportion of material
that has been removed. Another method is to place a sample of soil in a large
test tube, fill the tube with water and shake the contents thoroughly, and then
allow the material to settle. Particles retained on a No. 200 sieve will settle out of
suspension in about 20 to 30 seconds, whereas finer particles will take a longer
time. An estimate of the relative amounts of coarse and fine material can be made
on the basis of the relative volumes of the coarse and fine portions of the
sediment.

12.11.4 Fine-Grained Soils

Field identification procedures for fine-grained soils involve testing for dilatancy,
or expansion on shaking, plasticity, and dry strength. These tests are performed
on the fraction of soil finer than the No. 40 sieve. In addition, observations of
color and odor can be important. If a No. 40 sieve is not available, removal of the
fraction retained on this sieve may be partially accomplished by hand picking.
Some particles larger than this sieve opening (0.425 mm, or nominally 0.5 mm)
may remain in the soil after hand separation, but they probably will have only a
minor effect on the field tests.

DiIatancy
1
3
3
For the dilatancy test, enough water is added to about 2 cm ( in. ) of the
2
minus-40 fraction of soil to make it soft but not sticky. The pat of soil is shaken
horizontally in the open palm of one hand, which is struck vigorously against the
other hand several times. A fine-grained soil that is nonplastic or has very low
plasticity will show free water on the surface while being shaken, and then
squeezing the pat with the fingers will cause the soil structure to dilate or expand
so that the soil appears to dry up. The soil then will stiffen and finally crumble
under increasing pressure. Shaking the pat again will cause it to flow together and
water to again appear on the surface.

A distinction should be made between a rapid reaction, a slow reaction, or no
reaction to the shaking test, the rating depending on the speed with which the pat
changes its consistency and the water on the surface appears or disappears. Rapid

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.