Characteristics of Geologic Materials and Formations - Geotechnical Engineering Investigation - page 392

Capillarity : Negligible.
Frost heaving : Essentially nonsusceptible.
Liquefaction and piping : Potential increases with increasing fineness. Loose fine sands are
most susceptible; gravel is nonsusceptible.

Rupture Strength
Strength is derived from intergranular friction.
Failure criteria : General shear failure of shallow foundations does not occur because
compression occurs simultaneously with load application, and a deep failure surface can-
not develop. Failure occurs by local shear, the displacement around the edge of a flexible
foundation, or punching shear, i.e., failure by the rupture of a deep foundation. In slopes,
failure is relatively shallow in accordance with the infinite-slope criteria. Collapse of soil
structure occurs in lightly cemented loose sands.

Deformability
Response to load is immediate as the voids close and the grains compact by rearrange-
ment. Deformation is essentially plastic, with some elastic compression occurring within
the grains. The amount of compression is related to gradation, relative density, and the
magnitude of the applied stress. Susceptibility to densification by vibrations is high and
the materials are readily compactable.
Crushing can occur in grains of shell fragments, gypsum, or other soft materials, even
under relatively low applied stresses.

Silts (Inorganic)
Hydraulic Properties

Permeability: slow draining

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Capillarity: high

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Frost heaving susceptibility: high

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Liquefaction and piping susceptibility: high

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Rupture Strength
Strength is derived from intergranular friction and apparent cohesion when silt is partially
saturated. Strength is destroyed by saturation or drying. Upon saturation, collapse may
occur in lightly cemented formations, such as loess.

Deformability
Slow draining characteristics result in some time delay in compression under applied
load. Compaction in fills, either wet or dry, is relatively difficult.

Clays
Hydraulic Properties
Permeability : Clays are relatively impervious, but permeability varies with mineral com-
position. Sodium montmorillonite with void ratios from 2 to as high as 15 can have k

10 −8 cm/s 2 . It is used as an impermeabilizing agent in drilling fluid for test boring or in a
slurry trench cutoff wall around an excavation. Kaolinite, with void ratios of about 1.5, can
have k values 100 times higher than montmorillonite (Cornell University, 1951).
Capillarity : It is high, but in excavations evaporation normally exceeds flow.