Page 175 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
170 Geotechnical Engineering

Figure 8.3
(a) Clay bridges
add cohesive
strength to
granular soils,
but (b) grain
separation occurs
at about 25 to
30 percent clay.

The molding sand studies showed that with an increasing clay content there
becomes a point where the coarser grains no longer are in contact, and the soil
behaves more like clay than a sand or silt. That percentage varies depending on
the gradation of the coarse material, but usually is about 25–30 percent clay as the
sand changes from granular to cohesive. The separation of sand particles is
illustrated in Fig. 8.3(b).

Loess Studies
Loess is almost entirely silt and clay, with the clay content gradually increasing
with distance from a source. Loess containing less than 16 percent 0.002 mm clay
has over a 90 percent probability of being collapsible, which is a phenomenon that
is limited to low-density granular soils, but if the clay content exceeds 32 percent
the soil is very unlikely to be collapsible but behaves as a moderately expansive
clay (Handy, 1973).

8.3 COHESIVE SOIL FABRIC

8.3.1 Cardhouse Structure

Clay particles in suspension in water tend to combine into flocs as a result
of edge-to-face and edge-to-edge attractions between individual crystals,
creating an open, ‘‘cardhouse’’ structure (Fig. 8.4(a)). Slow sedimentation may
result in stacking similar to that which occurs with granular soils and a loose
‘‘honeycomb’’ structure (Fig. 8.5), and scuba divers describe a transition from
water to soil such that they have difficulty feeling where the water leaves off and
the soil begins.

8.3.2 Clay Particle Orientation under Overburden Pressure
As the cardhouse structure of Fig. 8.4(a) becomes compressed under load, the
clay particles tend to become oriented flat, as in Fig. 8.4(b). The most extreme
examples of pressure-induced orientation are shales.

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