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back-analysis of case studies of embankment failures. Azzouz et al. (1983) reviewed the
same failures as Bjerrum and some additional failures. They also considered 3
Dimensional (3D) effects. Azzouz et al. (1983) concluded that the correction factors
obtained when 3-D effects were included gave better estimates of the actual strength as
measured by the SHANSEP technique, and recommended use of the “average correction
curve including end effects” for conventional 2-Dimensional analysis shown in Figure 6.27.
On balance it seems wise to apply the Azzouz et al. (1983) correction using the equation
S u
S uv where S u
design undrained shear strength; S uv
undrained strength measured
by the vane;
correction factor from Figure 6.27, but to also calibrate the vane with good
quality laboratory testing on samples from the site. For design of embankments on soft clays
it is recommended the Ferkh and Fell (1994) method be adopted, but with more conservative
factors of safety than recommended in the paper as detailed in Section 11.3.2.
6.1.7.3 Self boring pressuremeter
The undrained strength of cohesive soils can be measured using the Self Boring
Pressuremeter (SBPM).
However the measured strength is affected by disturbance, being larger the greater the
disturbance, and the extent of disturbance is unpredictable. In view of this use of the
SBPM to estimate Su is not recommended.
6.1.8
Shear strength of fissured soils
6.1.8.1 The nature of fissuring, and how to assess the shear strength
Fissures are discontinuities in soils which have similar implications to the strength and per-
meability that joints have in rock masses - they reduce the soil mass strength, and (usually)
Figure 6.27.
Vane shear correction factors (Azzouz et al., 1983).
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