Environmental Engineering Reference
In-Depth Information
Shear stress (
τ
) is the stress causing shear.
Shear strength (S or s) is a characteristic value at which a material fails in rupture
or shear under an applied force.
Dilatancy is the tendency of the volume to increase under increasing shear or
stress difference.
Strength of Geologic Materials
Components: Friction and Cohesion
Friction is a resisting force between two surfaces as illustrated in Figure 3.18 . It is often the
only source of strength in geologic materials and is a direct function of the normal force.
Cohesion results from a bonding between the surfaces of particles. It is caused by electro-
chemical forces and is independent of normal forces.
Influencing Factors
Strength is not a constant value for a given material, but rather is dependent upon many
factors, including material properties, magnitude and direction of the applied force and
the rate of application, drainage conditions in the mass, and the magnitude of the confin-
ing pressure.
Stress Conditions In Situ
Importance
A major factor in strength problems is the existence of stress conditions in the ground, pri-
marily because normal stresses on potential failure surfaces result from overburden pres-
sures.
Geostatic Stresses
Overburden pressures, consisting of both vertical and lateral stresses, exist on an element
in the ground as a result of the weight of the overlying materials. Stress conditions for
N
P
w
T
f
N
FIGURE 3.18
Frictional force f resisting shearing force T [ P
force
applied in increments until slip occurs; N
normal
force component including block weight W; T
shearing stress component; f
frictional resistance;
α
T = N tan
T max = Nf = N tan
angle of obliquity [resultant of N and T ];
φ
friction angle, or
α max at slip; S max
maximum
shearing resistance
T max ].
 
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