Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 567

high-air-entry disk. The soil specimen has a net vertical nor-
mal stress of σ n −

u a and a matric suction of u a − u w at the

end of the consolidation process (Fig. 11.60).
Shearing of the soil specimen is achieved by horizontally
displacing the top half of the direct shear box relative to the
bottom half. The soil specimen is sheared along a horizon-
tal plane between the top and bottom halves of the direct
shear box. The horizontal load required to shear the spec-
imen divided by the nominal area of the specimen gives
the shear stress on the shear plane. The pore-air and pore-
water pressures are controlled at constant values during the
shear process. The shear stress is increased until the soil
specimen has failed. The failure plane has a shear stress
designated as τ ff , corresponding to a net vertical normal

Figure 11.61 Typical shear stress versus shear displacement plot
from direct shear test (from Gan, 1986).

stress of σ f − u a f

(i.e., equal to σ − u a ) and a matric

suction of u a − u w f (i.e., equal to u a −

path. The low coefficient of permeability for unsaturated
soils results in “times to failure” that are reduced by direct
shear testing. Other problems associated with testing unsat-
urated soils in a direct shear apparatus are similar to those
common to testing saturated soils (e.g., stress concentra-
tions, definition of the failure plane, and rotation of principal
stresses).

u w ) (Fig. 11.60).
Figure 11.61 shows a typical plot of shear stress versus
horizontal displacement for a direct shear test.
The failure envelope can be obtained from the results of
direct shear test results without constructing Mohr circles.
The shear stress at failure, τ ff , is plotted as the ordinate,

and σ f −

u w f are plotted as the abscis-
sas to give a point on the failure envelope (Fig. 11.62). A

u a f

and u a −

line joining points of equal magnitude of σ f − u a f deter-

mines the φ b angle (e.g., a line joining points A , B , and C
in Fig. 11.62). Similarly, a line can be drawn through the

11.8 TYPICAL LABORATORY TEST RESULTS

The presentation of shear strength results on unsaturated
soils focuses mainly on the shear stress versus matric suc-
tion relationship (i.e., τ versus u a −

points of equal u a − u w f to give the angle of internal fric-

tion φ (e.g., a line drawn through points A 1 , A 2 , and A in
Fig. 11.62).
The direct shear test is particularly useful for testing unsat-
urated soils because of the reduced length of the drainage

u w plane). The failure
envelope on the shear strength versus matric suction plane is
used to obtain the φ b shear strength parameter or the nonlin-
ear relationship between shear strength and matric suction

Figure 11.60 Stress conditions at various stages of consolidated drained direct shear test.