Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 578

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Figure 11.77 Multistage direct shear test results on compacted glacial till specimen No. GT16-
N4; shear stress versus horizontal displacement (from Gan et al., 1988).

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Figure 11.78 Multistage direct shear test results on compacted glacial till specimen No.GT-16-
N5; shear stress versus horizontal displacement (from Gan et al., 1988).

The test results on the Indian Head glacial till exhibit signifi-
cant nonlinearity in the failure envelope with respect to matric
suction. The slope of the shear strength envelope (i.e., non-
linear φ b ) is plotted in Fig. 11.79b for various matric suction
values.
Figure 11.80 presents a summary of the results obtained
from five unsaturated specimens tested using the multistage
direct shear test procedure. The results fall within a narrow
band that forms a curved failure envelope. The φ b angle
starts at a slope equal to φ (i.e., 25.5 ◦ ) at matric suctions
close to zero and decreases significantly at matric suctions
in the range of 50-100 kPa. The φ b angles reach a fairly
constant value ranging from 5 ◦ to 10 ◦ when the matric suc-
tion exceeds 250 kPa. The scatter in the failure envelopes
appears to be primarily due to slight variations in the initial
void ratios of the soil specimens.
Nonlinearity of the failure envelope was also observed by
Escario and Saez (1986) on direct shear tests performed on
several compacted soils. The properties, initial conditions,

consolidation times, and displacement rates associated with
two of the soils tested are tabulated in Table 11.7. The tests
were performed using a modified direct shear apparatus. The
consolidated drained testing procedure was used along with
the axis translation technique.
Figures 11.81 and 11.82 present the results on compacted
red clay of Guadalix de la Sierra and Madrid clayey sand
specimens, respectively. The test results exhibited nonlin-
earity in the failure envelope with respect to matric suction.
The failure envelopes with respect to the net normal stress
appear to be essentially linear.

11.9 SELECTION OF STRAIN RATE

The strength testing of unsaturated soils is commonly per-
formed at a constant rate of strain. An appropriate strain
rate should be selected prior to commencing the test. The
selected strain rate must ensure the equalization of induced