Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 575

deviate from the effective angle of internal friction (i.e., 32 ◦ )
as the air-entry value of the soil was exceeded (Fig. 11.72).

considerable dilation and postpeak strain softening. The vol-
umetric strain remained essentially constant when the devi-
ator stress was constant.
Figure 11.72 shows the projection of shear stresses at fail-
ure, τ f , onto the τ versus u a −

11.8.4 Undrained and Unconfined Compression Tests
Six series of undrained and unconfined compression tests
(i.e., UC) on unsaturated compacted specimens were per-
formed by Chantawarangul (1983). The soil was clayey sand
consisting of 52% sand, 18% silt, and 30% clay. The soil had
a liquid limit of 30%, a plastic limit of 19%, and a shrink-
age limit of 16%. The soil was compacted using a miniature
Harvard apparatus to give high- and low-density specimens
at various water contents. The water contents were generally
on the dry side of optimum. The high- and low-density spec-
imens correspond to dry density values of approximately
1800 and 1700 kg/m 3 , respectively.

u w plane for the CW tests.
The effective angle of internal friction, φ , of the compacted
kaolin was found to be 32 ◦ .The φ b angle was found to be
the same as the effective angle of internal friction, φ (i.e.,
32 ◦ ), at low matric suction values. The φ b angle decreased
to as low as 12 ◦ at high matric suctions. The matric suction
at the inflection point of the relationship between φ b and
matric suction increased with an increase in the net confin-
ing stress, as shown in Fig. 11.73. In other words, the net
confining stress had an effect on the nonlinearity of the φ b
angle for the CW tests on compacted kaolin specimens. The
shear strength versus matric suction relationship started to

700

σ 3
σ
σ
σ
σ
σ 3

- u a
- u a
- u a
- u a
- u a
- u a

= 50 kPa

b

= 12 °

= 100 kPa

3

600

= 150 kPa

3

= 200 kPa

3

500 = 250 kPa

3

b

= 32 °

= 300 kPa

Inflection

400

300

200

100

Inflection

0

0 50 100 150 200 250 300

Matric suction ( u a - u w ), kPa

Figure 11.72 Intersection line between failure envelope and the τ f versus matric suction plane.

40

σ 3 - u a = 300 kPa

30

σ 3 - u a = 250 kPa
σ 3 - u a = 200 kPa

σ 3 - u a = 50 kPa

20

σ 3 - u a = 100 kPa
σ 3 - u a = 150 kPa

10

0

0 100 200 300

Matric suction ( u a - u w ), kPa

Figure 11.73 Nonlinearity in relationship between φ b and matric suction for constant-water-