Page 161 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
156 Geotechnical Engineering

of seepage is important to prevent water from emerging on the earth slope where it
might lead to piping and failure. Geotextile filters generally are more expensive
but are easier to install than are layers of sand, and are less likely to be damaged
or compromised during construction.

Design
Protective filters act as a drain while resisting clogging by fine particles. They also
cannot permit a breakthrough, and may be required to provide insulation against
frost action. The finer sizes of particles in a soil filter tend to control its
performance. Generally the filter F 15 size is compared with the D 85 size for the
base soil. (To avoid confusion the filter size is designated with F instead of D.)
A conservative and acceptable guide for design is F 15 /D 85 55.

An additional requirement for the retention of clay, for example in the core of an
earth dam, is that F 15 50.5 mm.

Example 7.5
Is the sand in Fig. 7.2 an appropriate filter for an earth dam constructed from the glacial till
in the same figure?
Answer: The sand has F 15 ¼ 0.12 mm, and the till has D 85 ¼ 0.4 mm. Then 0.12/
0.4 ¼ 0.355, so the filter should perform adequately. In addition F 15 50.5 mm so there
should be little or no clay penetration.
Question: What if the dam is constructed from the loess in the figure?

7.5.7 Geotextile Filters
The apparent opening size (AOS) of geotextile fabrics is defined as O 95 , which is
the size for 95 percent of glass beads of a particular size grade to pass through
during sieving (ASTM Designation D-4751). One criterion in regard to filtration
of soil is that O 95 /D 85 52 or 3, where D 85 is for the soil.

7.5.8 Grouting
Grouting is pumping of a fluid under pressure into a soil so that it either (a)
permeates the soil, referred to as injection grouting, or (b) displaces the soil, called
compaction grouting. The determination of whether a grout will inject into the soil
pores or displace the soil is mainly dependent on the relations between the
respective particle sizes.

Injection grouting is a common remedial treatment used to solidify loose
foundation soil and rock underneath buildings, dams, and other structures.
Injection grouting also is used to seal leaks under dams or lagoons, to seal off and
contain buried hazardous wastes, and to seal off the groundwater aquifers in
preparation for tunneling.

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.