Page 172 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
Soil Fabric and Structure 167

8.1.2 Overview of Granular Soil Fabric
The fabric of granular soils can be investigated by impregnating the soil with
plastic and viewing cut faces. Another preparation procedure is to cut and grind
thin sections that may be examined under a polarizing microscope, which enables
identification of the minerals.

Such studies often reveal a preferred horizontal orientations of pores and flat or
elongated grains. More recent studies indicate vertical stacking of grains on top of
one another, which is in contrast to the widely hypothesized close-packing
arrangements that are analogous to crystal structure.

8.1.3 Overview of Cohesive Soil Fabric

Because their small particle size is difficult to observe under a microscope, clay
fabric mainly has been defined by inference from crystal structure and physical
properties. For example, clay crystals are known to be flat, and stirring a
suspension of clay in water makes it more liquid, like stirring a milk shake. If the
liquid is allowed to stand it sets or stiffens. Then, like a milk shake, it will respond
and turn back into a liquid with stirring. The inference is that there must be some
interparticle bonding or optimization of particle contacts that is disrupted by
stirring. In a milk shake, tiny ice crystals can temporarily become disorganized; in
a clay suspension, it is the clay particles that are temporarily disorganized. This
property is called thixotropy, which is from Greek for touch-sensitive.

Clay fabric is directly observed under a scanning electron microscope, but the
requisite vacuum dehydrates the clay and can change its structure. Freeze-drying
in which a sample is first frozen and then evacuated for a long period of time can
help to preserve the structure.

Soil mixtures containing both granular and cohesive components combine both
elements of fabric, but normally one will dominate. The transition occurs at about
25 to 30 percent clay, which is sufficient to separate and prevent contact between
larger soil particles.

8.2 GRANULAR SOIL FABRIC

8.2.1 Single-Grained Structure
A soil that shows no discrete relationship between one grain and another is said to
have a single-grained structure. A random single-grained structure is shown in
Fig. 8.1(a). However, in many instances elongated particles tend to be aligned
parallel and horizontally as illustrated in Fig. 8.1(b). In this case the pores also
are oriented horizontally, reducing shearing strength in a horizontal direction.
Preferred orientation of flat particles occurs during sedimentation because it is
unlikely that particles will stand on end.

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