Page 174 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
Soil Fabric and Structure 169

Figure 8.2
Simulated random
sedimentation of
spherical particles
tends to create
a stacking
arrangement and
vertical orientation
of voids, resulting
in low lateral stress
and high horizontal
compressibility.

Although not yet confirmed by measurements, an open arrangement from particle
stacking may help explain low lateral stresses and collapsibility of loess that
suddenly densifies under its own weight when it becomes saturated with water
(Handy, 1994). It also may increase the responsiveness to horizontal compaction
that occurs during compaction grouting, pile driving, and the construction of
Rammed Aggregate Piers.

8.2.3 Granular Soils Containing Clay
Although the single-grained structure applies only to particles that are silt-size
and larger, microscopic examinations show that coarse particles commonly are
coated with clay oriented flat against the grain surfaces. The presence of clay
coatings is revealed under a polarizing microscope that can block out light coming
through the host grains so that the clay coatings appear as bright fringes. In
tropical soils the coatings are iron oxides and hydroxides, which gives these soils
their red color.

Although clay coatings probably decrease the coefficient of sliding friction
between coarser grains, the clay also increases contact pressures because the
affinity of clay for water can create suction. The amount of suction is sensitive to
the water content, which explains why the strength of clayey granular soils is
reduced by wetting.

8.2.4 Clay Bridges

Even a moderate clay content causes clay to become concentrated at the particle
contacts, forming clay connectors or bridges. These are illustrated in Fig. 8.3(a).
This phenomenon was originally described by Grim for molding sands, which are
mixtures of sand and clay used in metal casting. Clay is added to increase cohesive
strength because sand without clay readily crumbles.

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