Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 571

11.8.3 Nonlinear Shear Strength versus Matric
Suction
The nonlinearity of the shear strength versus matric suc-
tion relationship became increasingly apparent as unsatu-
rated soils were tested over a wider suction range (Fredlund
et al., 1987). The reanalyses of the triaxial test results on the
compacted Dhanauri clay using a planar and a curved failure
envelope were compared. The CD and CW triaxial tests on
Dhanauri clay compacted at two densities were presented
by Gulhati and Satija (1981). The Dhanauri clay consisted
of 5% sand, 70% silt, and 25% clay. The liquid limit of the
soil was 48.5% and the plastic limit was 25%. The saturated
effective shear strength parameters (i.e., c and φ ) for spec-
imens compacted at two different densities were measured
using consolidated undrained triaxial tests (Table 11.4). The
consolidated drained and constant-water-content tests on the
unsaturated compacted specimens were performed at strain
ratesof1 . 3

suction was nonlinear (Fredlund et al., 1987). Figures 11.68
and 11.69 present the results for compacted Dhanauri clay
at low and high densities, respectively. The results are
plotted on the shear strength versus matric suction plane
corresponding to zero net normal stress at failure [i.e.,

σ f −

u a f 0]. The shear strength parameters c and φ

obtained from the consolidated undrained tests on the satu-
rated specimens (see Table 11.4) were used in the analysis.
The curved failure envelopes have a cohesion intercept of
c and a slope angle φ b equal to φ starting at zero matric
suction. The φ b angle begins to decrease significantly at
matric suction values greater than 50 kPa for the low-density
specimens. The decrease in φ b begins at matric suction
values of 75-100 kPa for the high-density specimens. The
φ b angle reaches a relatively constant value of 11 ◦ when
the matric suction exceeds 150 kPa for the low-density
specimens. The φ b angles for the high-density specimens
appear to reach a relatively constant value of 9 ◦ when
the matric suction exceeds 300 kPa.
There is good agreement between the failure envelopes
for the consolidated drained and constant-water-content
test results when assuming a curved failure envelope with
respect to matric suction. In other words, the assumption of
a curved failure envelope leads to the conclusion that there
is a unique failure envelope when the soil is tested using
different stress paths. The uniqueness of the curved failure
envelope is demonstrated at both densities. It is also appar-
ent that specimens prepared at different densities should be
considered as different soils.
Thu et al., (2006) studied the characteristics of the
shear strength, volume change and pore-water pressure of
compacted silt (i.e., coarse kaolin) during shearing under
constant-water-content conditions. A series of CW triaxial
tests were carried out on statically compacted silt specimens.

=

10 − 4 % /s , respectively.
The test results on the unsaturated soil specimens were
reanalyzed by Ho and Fredlund (1982a) using the assump-
tion of a planar failure envelope and the results are summa-
rized in Table 11.4. It appears that the linear interpretation
of the failure envelope results in different c and φ b param-
eters for the same soil tested using different test procedures
(i.e., CD and CW tests). In other words, the results give the
impression that different test procedures may yield different
shear strength parameters. It appears that the assumption of
a planar failure envelope when analyzing the data suggests
there may be nonuniqueness in the shear strength parame-
ters. In addition, the c values obtained from the analysis of
the unsaturated soil tests do not agree with values obtained
from triaxial tests on saturated soil specimens (Table 11.4).
The reanalysis of the shear strength data on Dhanauri clay
showed that the failure envelope with respect to the matric

10 − 4 and 6 . 7 × ×

Table 11.4 Triaxial Tests on Compacted Dhanauri Clay

CU Tests on Analysis of Test Results on

Saturated Specimens Unsaturated Specimens (Ho and Fredlund, 1982a)

Initial Volume-

c (kPa) φ (Deg) c (kPa) φ b Mass Properties Type of Test (Deg)

Low density CD 20.3 12.6

1478 kg/m 3

ρ d = 7.8 29.0

w = 22.2 % CW 11.3 16.5

CD 37.3 16.2

High density
ρ d 1580 kg/m 3 = 7.8 28.5

w = 22.2 % CW 15.5 22.6

Note : ρ d = dry density, w = water content, CU = consolidated undrained, CD = consolidated drained, CW = constant

water content.
Source : Data from Satija (1978).