Page 163 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
158 Geotechnical Engineering

grout. The sand still may be a candidate for compaction grouting or injection grouting with
chemical solutions, depending on the properties that are required.

As a general guide:

Gravel or very coarse sand can be injection grouted with cement and/or fly
ashs.
Medium to fine sand can be compaction grouted with cement/fly ash or
injection grouted with sodium silicate or specially ground fine cement.
Silt can be compaction grouted.
Clay cannot be grouted, but expansive clay can be stabilized by a diffusion
process of hydrated lime, which is much slower than the other processes.

Partly because of the difficulty in controlling injection grouting and knowing
where the grout goes, compaction grouting has become increasingly popular in
recent years.

7.6 DESCRIBING PARTICLE SHAPE

7.6.1 Particle Shape and Engineering Behavior
The shapes of soil grains can influence engineering behavior, as round grains
obviously are more likely to slip and roll than angular fragments that mesh or
interlock together. For this reason crushed rock normally creates a stronger
surface of a ‘‘gravel’’ road than do the more rounded particles of gravel. On the
other hand gravel, having been through many cycles of pounding against a beach
or river bottom, is more likely to be harder and less likely to degrade into dust.

The main effect of angularity is harshness, or the tendency for the soil to dilate or
increase in volume during shearing, a matter that can be quantified with strength
tests.

Grain shapes closely relate to their mineralogy and origin; quartz sand grains
derived from disintegration of granite tend to be round, whereas grains of feldpar
derived from the same rock are more angular, and grains of mica are flat. Alluvial
gravel generally is well rounded, sand less so, and silt not at all. Dune sand not
only shows rounding, but the grain surfaces are etched from repeated impacts.

The measurement of shapes of individual grains can be time-consuming, but
measurement of grain profiles can be digitized and automated. A chart that can be
used to estimate shape, or ‘‘sphericity,’’ is shown in Fig. 7.6. Sphericity
theoretically is the ratio of a grain surface area to that of a sphere, but can be
approximated by dividing the intermediate grain width by its length. As this does

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