Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 537

(2) consolidated undrained, (3) constant water content,
(4) undrained, and (5) unconfined compression tests.
The tests were performed using a modified triaxial cell.
The pore-air and pore-water pressures were either measured
or controlled during the strength tests. Bishop (1961b) pro-
vided details on how the pore pressures could be controlled
and measured using a modified triaxial apparatus. Tests con-
firmed that pore-water pressures could be measured directly
through a saturated porous ceramic disk sealed onto the
base pedestal below a soil specimen. Pore-water pressure
measurements were made by balancing the pressure in the
measuring system through use of a null-type, no-flow indi-
cator. This direct measurement, however, was limited to a
negative gauge pressure above 90 kPa. Bishop and Eldin
(1950) successfully measured pore-water pressures down to
negative 90 kPa in a saturated soil specimen while perform-
ing a consolidated undrained test with a carefully deaired
measuring system. It was suggested that the axis transla-
tion technique was required when the pore-water pressures
became lower than 1 atm negative.
The axis translation technique translates highly negative
pore-water pressure to a pressure that can be measured with-
out cavitation occurring in the water of the measuring sys-
tem. A ceramic disk with an air-entry value greater than the
matric suction being measured was required in order to pre-
vent the passage of pore-air into the measuring system. A
single layer of coarse glass fiber cloth with a low attraction
for water was placed on top of the soil specimen for pore-air
pressure measurement or control.
Constant-water-content (or CW) triaxial tests were per-
formed on compacted shale and compacted boulder clay by
Bishop et al. (1960). The shale had a clay fraction of 22%
and was compacted at a water content of 18.6%. A series of
triaxial tests on the saturation specimens of the compacted
shale gave an angle of internal friction φ of 24.8 ◦ and an
effective cohesion c of 15.8 kPa. The boulder clay had a
clay fraction of 18% and was compacted at a water content
of 11.6%. The saturated boulder clay showed an effective
angle of internal friction φ of 27.3 ◦ and an effective cohe-
sion c of 7.6 kPa. The tests on the compacted boulder clay
were performed at a strain rate of 3.5

Colloidon membranes

Sintered bronze

Plastic tube

System for applying a constant negative pore-water pressure

To

Vacuum

Constant head overflow tube

Figure 11.20 Modified direct shear equipment for testing soils
under low-matric-suction conditions: (a) modified direct shear box
with colloidon membrane; (b) system for applying constant nega-
tive pore-water pressure (after Donald, 1956).

specimens were then sheared at a rate of 0.071 mm/s. The
results from four types of sand are presented in Fig. 11.21.
The shear strength at zero matric suction is the strength due
to the applied total stress. The shear strength increases to a
peak value as matric suction is increased. Following the peak
value for shear strength there was a decrease to a fairly con-
stant shear strength value. The shear strength of the sands
appears to increase at the same rate as for an increase in
total stress as long as the specimens remain saturated. The
increase in strength decreases once the sands start to desat-
urate. The strength of the sand is decreased once matric
suction increases beyond some limiting value.
The water content versus matric suction desorption curves
(i.e., SWCCs) were also measured for each of the sand mate-
rials. A comparison of the SWCCs and the shear strength
envelopes showed that there was a correlation between the
air-entry value for each soil and the point at which the shear
strength envelope became nonlinear. The residual conditions
on the SWCC were also shown to correspond quite closely
with the point where the shear strength became essentially hor-
izontal after passing the peak strength. These relatively simple
direct shear strength tests provided considerable insight into
the behavior of unsaturated soils.
An extensive research program on unsaturated soils was
conducted at Imperial College, London, in the 1950s and
early 1960s. Bishop et al. (1960) proposed testing techniques
and presented the results of five types of shear strength tests
on unsaturated soils classified as (1) consolidated drained,

10 -5 %/ s, and 15
% strain was considered to represent failure. The laboratory
results were presented in terms of stress points designated

×

as σ 1 + u a f at fail-

ure (where σ 1 is the major principal stress and σ 3 is the
minor principal stress). Figure 11.22 shows a typical plot of
constant-water-content test results on compacted shale. The

σ 3 / 2 u w f and σ 1 + σ 3 / 2 − −

condition when the σ 1 − u w f ratio reached a

maximum value was considered to be the failure condition.
Pore-air diffusion through the rubber membrane around
the triaxial specimen was prevented by surrounding the
membrane on the specimen with mercury rather than
water. Triaxial test results of a consolidated drained test on
unsaturated loose silt were used to verify the significance
and application of σ

σ 3 / σ 3 −

− u a and u a − u w as stress variables.