Environmental Engineering Reference
In-Depth Information
Grain Minerals
Types
The predominant granular soil mineral is quartz, which is essentially stable, inert, and
nondeformable. On occasion, sands and silts will include garnet, magnetite, and horn-
blende. In climates where mechanical disintegration is rapid and chemical decomposition
is minor, mica, feldspar, or gypsum may be present, depending on the source rock.
Shell fragments are common in many beach deposits, especially in areas lacking quartz-
rich rocks, and offshore, in the middle latitudes, calcareous or carbonate sands are com-
mon (see
Section 7.4.6).
The weaker minerals such as shells, mica, and gypsum have low
crushing strengths; calcareous sands can have deleterious effects on concrete.
Identification
Simple tests to identify grain minerals include the application of hydrochloric acid to test
for calcareous materials, and the determination of specific gravity
(
Table 5.5).
Silt
General
Although it consists of bulky particles, silt is often grouped with clays as a fine-grained
soil since its particle size is defined as smaller than 0.074 mm. Nonplastic silt consists of
more or less equidimensional quartz grains and is at times referred to as “rock flour.”
Plastic silt contains appreciable quantities of flake-shaped particles.
Silts are classified as inorganic, ranging from nonplastic to plastic (see
Figure 3.12),
or
organic, containing appreciable quantities of organic matter. The smooth texture of wet silt
gives it the appearance of clay.
Properties
Dilantancy:
Silts undergo changes in volume with changes in shape, whereas clays retain
their volume with changes in shape (plasticity). Grains are fine, but compared with clays,
pore spaces are relatively large, resulting in a high sensitivity to pore-pressure changes,
particularly from increases due to vibrations. Because of their physical appearance and ten-
dency to quake under construction equipment, silts are often referred to as ”bull's liver.”
Stability
: When saturated and unconfined, silts have a tendency to become ”quick” and
flow as a viscous fluid (liquefy).
”
Apparent Cohesion
” results from capillary forces providing a temporary bond between
particles, which is destroyed by saturation or drying. For example, a moist, near-vertical
cut slope will temporarily stand stable to heights of 10 ft. or more but will collapse when
wetted or dried.
5.3.3
Clays
Characteristics
General
Clays are composed of elongated mineral particles of colloidal dimensions, commonly
taken as less than 2
m in size (Gillott, 1968). Behavior is controlled by surface- rather than
mass-derived forces. A spoon sample of lacustrine clay is illustrated in
Figure 5.9.
µ
Mass Structures
Clay particles form two general types of structures: flocculated or dispersed, as shown in
Figure 5.10.
