Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 564

Figure 11.56 Stress conditions at various stages of unconfined compression test.

test can be theoretically related to the major principal stress
through use of the D pore pressure parameter. Figure 11.56
illustrates typical changes in the stress state variables that
would occur during unconfined compression if the pore pres-
sures were measured.
Figure 11.57 illustrates two possible stress paths that could
be followed by an unsaturated soil specimen during the
unconfined compression. The initial stress state is repre-
sented by stress point A where the soil has a zero net confin-
ing pressure (i.e., σ 3 −

For the case of increasing matric suction during compres-
sion (e.g., the soil dilates), the stress state in the soil will
move backward from point A to a point (or plane) some-
where behind point A . This stress path is not shown in
Fig. 11.57.
The stress paths illustrated in Fig. 11.57 are associated
with unsaturated soil specimens. The soil may be unsatu-
rated in the field or become unsaturated during sampling
due to the release of the overburden pressure. Unconfined
compression test results on the unsaturated soil apply to
the condition where the total confining pressure σ 3 is zero

0) and a matric suction u a −
u w . The matric suction can increase, decrease, or remain
constant during undrained compression depending on the A
parameter of the soil. Matric suction will most commonly
decrease during undrained compression and the stress state
in the soil will mo ve forward from point A to point B along
the stress path AB (Fig. 11.57). The stress state of the soil
at failure is represented by point B . The pore-air pressure
is assumed to increase slightly during compression. This
caus es t he net confining pressure to decrease along stress

u a =

(Fig. 11.58a). The deviator stress at failure, σ 1 − σ 3 f ,

is referred to as the unconfined compressive strength q u .
The unconfined compressive strength is commonly taken as
being equal to twice the undrained shear strength c u .How-
ever, the undrained shear strength for the unsaturated soil
increases as the confining pressure increases. As a result, the
compressive strength of the unsaturated soil may not satis-
factorily approximate the undrained shear strength c u at a
confining pressure greater than zero.
Soil samples may remain saturated in the field in some
cases even though the pore-water pressure is negative. The
pore-water pressure may be negative because the soil is
from some distance above the groundwater table, or it may
be negative due to the release of overburden pressure, or
both. The shear strength equation for saturated soils can
be used even though the pore-water pressures are negative.
The undrained shear strength c u for the soil remains con-
stant even when a variety of confining pressures are applied
(Fig. 11.58b). For a saturated soil, the unconfined compres-
sive strength q u is twice the undrained shear strength c u
regardless of the magnitude of the confining pressure.

path AB to a negative value [i.e., σ 3 − u a f

u af ]. The
matric suction at failure will be less than the initial matric

=−

suction [i.e., u a − u w at A ].

In the case of constant matric suction being maintained
during compression, the stress state of the soil co uld move
from point A to point B 1 along the stress path AB 1 .The
stress path therefore lies in a plane of constant matric suc-
tion. Stress point B 1 represents the stress state of the soil at
failure. The pore-air and pore-water pressures are assumed to
remain constant during compression in order for the matric
suction to be constant. As a result, the net confining pres-
sure remains equal to zero until failure is reached [i.e.,
σ 3 −

u w f at B < u a −

u a = σ 3 − u a f

= 0].