Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 313

Cohesion indicates the tendency of soil particles to stick together. Cohesive soils contain silt and
clay. The clay and water content makes these soils cohesive through the attractive forces between
individual clay and water particles. The influence of the clay particles makes the index properties
of cohesive soils somewhat more complicated than the index properties of cohesionless soils. The
resistance of a soil at various moisture contents to mechanical stresses or manipulations depends
on the soil's consistency , the arrangement of clay particles, and is the most important characteristic
of cohesive soils.
Another important index property of cohesive soils is sensitivity . Simply defined, sensitivity is
the ratio of unconfined compressive strength in the undisturbed state to strength in the remolded
state (see Equation 12.10). Soils with high sensitivity are highly unstable.

Strengthin undisturbedconditi on

Strengthinremoldedcondition

Sensitivity =

(12.10)

Soil water content is an important factor that influences the behavior of the soil. The water con-
tent values of soil are known as the Atterburg limits , a collective designation of so-called limits of
consistency of fine-grained soils which are determined with simple laboratory tests. They are usually
presented as the liquid limit (LL), plastic limit (PL), and shrinkage limit (SL). The plastic limit is the
water level at which soil begins to be malleable in a semisolid state, but molded pieces crumble easily
when a little pressure is applied. When the volume of the soil becomes nearly constant (solid) with fur-
ther decreases in water content, the soil has reached the shrinkage limit. The liquid limit is the water
content at which the soil-water mixture changes from a liquid to a semifluid (or plastic) state and tends
to flow when jolted. Obviously, an engineer charged with building a highway or building would not
want to choose a soil for the foundation that tends to flow when wet. The difference between the liquid
limit and the plasticity limit is the range of water content over which the soil is plastic and is called the
plasticity index . Soils with the highest plasticity indices are unstable in bearing loads.
Several systems for classifying the stability of soil materials have been devised, but the best
known (and probably the most useful) system is called the Unified System of Classification. This
classification gives each soil type (14 classes) a two-letter designation, primarily based on particle-
size distribution, liquid limit, and plasticity index.
Cohesionless coarse-grained soils behave much differently than cohesive soils and are based
on (from index properties) the size and distribution of particles in the soil. Other index properties
(particle shape, in-place density, and relative density, for example) are important in describing cohe-
sionless soils, because they relate to how closely particles can be packed together.

12.6 SOIL STRESS AND STRAIN
If you are familiar with water pressure and its effect as you go deeper into the water (as when diving
deep into a lake), it should not surprise you that the same concept applies to soil and pressure. Like
water, pressure within the soil increases as the depth increases. A soil, for example, that has a unit
weight of 75 lb/ft 3 exerts a pressure of 75 psi at a 1-foot depth and 225 psi at 3 feet, etc. As you might
expect, as the pressure on a soil unit increases, the soil particles reorient themselves structurally
to support the cumulative load. This consideration is important, because the elasticity of the soil
sample retrieved from beneath the load may not be truly representative once it is delivered to the
surface. The importance of taking representative samples cannot be overstated. The response of a
soil to pressure (stress) is similar to what occurs when a load is applied to a solid object; the stress
is transmitted throughout the material. The load subjects the material to pressure, which equals the
amount of load, divided by the surface area of the external face of the object over which it is applied.
The response to this pressure or stress is called displacement or strain . Stress (like pressure), at any
point within the object, can be defined as force per unit area.