Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 312

DID YOU KNOW?

Poisson's effect (see Chapter 11) is also applicable in the realm of structural geology. Rocks,
like most materials, are subject to Poisson's effect while under stress. On a geological time
scale, excessive erosion or sedimentation of Earth's crust can either create or remove large
vertical stresses upon the underlying rock. This rock will expand or contract in the vertical
direction as a direct result of the applied stress, and it will also deform in the horizontal direc-
tion as a result of Poisson's effect. This change in strain in the horizontal direction can affect
or form joints and dormant stresses in the rock (Engelder, 2013).

Many sands and gravels are composed primarily of quartz. A value of 2.66 is commonly assumed
for the specific gravity for these soils. Specific gravities of sands and gravels derived from granites
or limestones might be higher. Soils with a high percentage of silt-size particles generally have a
specific gravity of about 2.68, because quartz is usually a major constituent, and small additional
amounts of clay minerals slightly increase the value. Clay soils may have specific gravity values
ranging from about 2.60 to 2.80. An average value of 2.7 is commonly assumed. Soils that contain
a large amount of micaceous flakes and soils with significant amounts of hematite or magnetite may
have quite high specific gravities, ranging from 2.75 to 3.3. Test data are usually required to accu-
rately determine specific gravities for these unusual soils.

12.5 SOIL PARTICLE CHARACTERISTICS
The size and shape of particles in the soil, as well as density and other characteristics, relate to sheer
strength, compressibility, and other aspects of soil behavior. Engineers use these index properties to
form engineering classifications of soil. Simple classification tests are used to measure index prop-
erties (see Table 12.2) in the lab or the field. From Table 12.2, we see that an important division of
soils (from the engineering point of view) is the separation of the cohesive (fine-grained) from the
incohesive (coarse-grained) soils. Let's take a closer look at these two important terms.

TABLE 12.2
Index Property of Soils
Soil Type Index Property

Cohesive (fine-grained) Water content

Sensitivity
Type and amount of clay
Consistency
Atterberg limits

Incohesive (coarse-grained) Relative density

In-place density
Particle-size distribution
Clay content
Shape of particles

Source: Adaptation from Kehew, A.E., Geology for
Engineers and Environmental Scientists , 2nd ed.,
Prentice-Hall, Englewood Cliffs, NJ, 1995, p. 284.