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benefits if there is sufficient space to construct long, shallow embankments, and the cohesion
is strong enough to support the loads expected to be present. Soil with undrained cohesion of
3.5 kPa is very soft material and it would not be possible to construct an embankment with it.
Steeper slope angles require less land area for construction but require soils with greater
undrained cohesion. Undrained cohesion of 60 kPa is generally the accepted minimum
allowable strength to ensure plant equipment can drive over the embankment. If additional
strength is required in an existing embankment but there is little land area available to extend
the toe of the embankment, then the use of soil stabilisation can become expensive.
Sensitivity Data
140
120
Factor of safety
100
80
1V:20H
60
10V:1H
40
20
0
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Sensitivity Range
Figure 6.5: Sensitivity plot
For presentation purposes, the undrained cohesion values have been normalised to values
from -1 to 1, with -1 being the lowest value (1 kPa) and 1 being the highest value (300 kPa).
This is a default function when performing sensitivity analyses in SLOPE/W to allow
different variables to be analysed with the same x-axis.
Zero cohesion could not be analysed in SLOPE/W using undrained conditions since the angle
of shearing resistance equals zero and therefore there are no shear strength parameters used in
the analysis. Without any shear strength (𝜏𝑓 ) there would be no soil - only water). This is
represented in Coulomb’s equation for shear strength:
𝜏𝑓 = 𝑐 + 𝜎 tan 𝜙
6.1
𝜏𝑓 = 0 + 𝜎 tan 0 = 0
71