Shear Strength Applications in Plastic and Limit Equilibrium - Unsaturated Soil Mechanics in Engineering Practice - page 654

c . The equivalent increase in cohesion for each substratum
was computed from the matric suction profile and a φ b angle
of 15 ◦ [i.e., (u a − u w ) tan φ b ].

A parametric study was also undertaken using various
percentages of the hydrostatic negative pore-water pressure.
The matric suction used in each analysis was limited to 1 atm
or a computed cohesion increase of 27 kPa. The results
of a slope stability study on cross section A-A are shown
Fig. 12.93. A factor of safety of 1.0 corresponds to a matric
suction profile of approximately 10-20% of hydrostatic con-
ditions. A significant increase in the factor of safety (i.e.,
approximately 25%) is obtained when the matric suction
profile is increased up to 40%. The smaller increase in the
factor of safety beyond the 40% profile is related to the limit
of 101.3 kPa placed on the potential matric suction value.
The critical slip surface tends to penetrate deeper into the
slope as the cohesion of the soil increases. The increase in
cohesion is due to the increase in matric suction.
Slope stability analyses were also performed on example
problem 1 using the measured matric suctions shown in
Fig. 12.92. The equivalent increase in cohesion for each sub-
stratum was computed from the matric suction profiles with
a maximum measured value of 85 kPa. The calculated fac-
tors of safety were 1.07 and 0.98 based on the matric suction
profiles measured on November 29, 1980, and October 27,
1981, respectively.

Figure 12.95 Cross section A - A for example 2.

Fig. 12.94. A steep and high cut slope exists behind a residen-
tial building. A proposed high-rise residential building above
the slope prompted a detailed investigation for the stability of
the slope under the new conditions. The cut slope under con-
sideration is below a major access road and the cross section
of concern, A - A , is shown in Fig. 12.95. The slope is inclined
at 60 ◦ to the horizontal and has an average height of 30 m.
The stratigraphy consists entirely of weathered rhyolite. The
water table lies well below the ground surface.
The shear strength parameters of the soil comprising exam-
ple 2 were measured in the laboratory on undisturbed samples.
The properties of the soils are summarized in Table 12.10.
In situ matric suction measurements were obtained using

12.7.1.2 Example 2 (Steep Slope behind Residential
Buildings)
A high, steep cut slope existed behind a residential build-
ing in Hong Kong. The site plan for example 2 is shown in

Figure 12.94 Site plan for slope stability example 2.