Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 570

(a)

Figure 11.66 Intersection line between failure envelope and the

τ versus u a − u w plane for compacted shale (data from Bishop

et al., 1960).

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 saturation specimens of compacted shale
gave an effective angle of internal friction φ of 24.8 ◦ and
an effective cohesion c of 15.8 kPa. The laboratory results
were reanalyzed by Fredlund et al. (1978), and the results
are presented in Fig. 11.66. The φ b angle was 18.5 ◦ for the
compacted shale when assuming a constant effective angle
of internal friction during the interpretation of the data.
The boulder clay had a clay fraction of 18% and was
compacted at a water content of 11.6%. The saturated boul-
der clay showed an effective angle of internal friction φ
of 27.3 ◦ and an effective cohesion c of 7.6 kPa. The tests
on the compacted boulder clay were performed at a strain
rate of 3.5

(b)

(c)

10 -5 %/ s, and 15% strain was considered
to represent failure conditions. The laboratory results were
reanalyzed by Fredlund et al. (1978) and the results are pre-
sented in Figs. 11.67. The average φ b angle was 22.0 ◦ for
the boulder clay when assuming a constant effective angle
of internal friction during the interpretation of the data.

×

Figure 11.65 Stress versus strain curves and presentations of
failure envelope for Tappen-Notch Hill Silt specimen No. E-3:
(a) deviator stress versus strain curve; (b) failure envelope pro-
jected onto τ versus σ -u a plane; (c) line defining slope of shear
strength with respect to matric suction (from Krahn et al., 1989).

Figure 11.67 Intersection line between failure envelope and τ versus u a − u w plane for com-

pacted boulder clay (data from Bishop et al., 1960).