Shear Strength Applications in Plastic and Limit Equilibrium - Unsaturated Soil Mechanics in Engineering Practice - page 628

Figure 12.64 Relationship between angle of internal friction, φ , and Terzaghi's bearing capacity
factors (from Terzaghi, 1943).

unsaturated soil mechanics. The above discussion illustrates
that the same plasticity theory applies for both saturated and
unsaturated soils. The most important information required
when using the bearing capacity theory for unsaturated soils
pertains to the selection of appropriate shear strength param-
eters and a reasonable design matric suction value.

unsaturated soils. The linear shear strength parameters are
based on the assumption that the failure surface of the soil
is planar.
Two of the shear strength parameters can be combined
with the result that the shear strength equation has a form
similar to that used for saturated soils. The required shear
strength parameters for a conventional bearing capacity
design are the total cohesion c [i.e., c

12.4.2 Selection of Soil Parameters and Design Suction
The shear strength parameters used in engineering practice
for bearing capacity design for saturated soils have ranged
from effective shear strength parameters to the undrained
shear strength, c u . In other words, the design can be based
on either an effective stress analysis or a total stress analysis.
Likewise, it is possible to use either a stress state variable
approach or a total stress approach when dealing with unsat-
urated soils.

c +

u w ) tan
φ b ] and the effective angle of internal friction φ .Itis
necessary to estimate the φ b angle for the soil and have an
approximate design matric suction value. It is difficult to
assess an appropriate design value for matric suction. It is
reasonable to use a measured value of matric suction only
if there were some assurance that the matric suction in the
soil can be maintained. Generally, a factor of safety would
be applied to measured values of matric suction. Another
option is to consider the likely long-term hydrostatic
condition of the groundwater table (Fig. 12.65).
Footings are commonly placed well above the groundwater
table. It may be reasonable to assume that negative pore-water
pressures will be maintained immediately below a footing if
adequate surface and subsurface drainage is provided around
the structure. It must be realized that there may be fluctu-
ations in the groundwater table as a result of building the
structure. The groundwater table may also be lowered, but

= (u a −

12.4.3 Stress State Variable Approach
The shear strength parameters for a soil with matric suction
are (1) effective angle of internal friction φ , (2) effective
cohesion c , and (3) angle of shear strength change with
respect to matric suction, φ b . It is also possible to use
a nonlinear shear strength function with respect to matric
suction; however, a linear shear strength parameter will be
used to present the primary concepts of bearing capacity in

Rail Rail

Tie

Ballast
Subballast
Subgrade

Excessive
evaporation
Eqilibrium with
water table

Flooding of
dessicated soil

Pore-water pressure

Figure 12.65 Pore-water pressure profiles that might exist below a spread footing.