Environmental Engineering Reference
In-Depth Information
1984), there was evidence from back analysis of failures, or from laboratory testing and
mapping of the fissures, that the mass strength could be considerably lower than the fully
softened. Table 6.2 summarises the data. Raby Bay and Plantes Hill both had failures which
could be back-analysed (albeit with rather poor information on pore pressures at the time of
failure).
Stark and Eid (1997) present data from 14 cases which show back-analysed mobilised shear
strengths, for fissured clays as being about the average of the fully softened and residual
strengths for soils with a liquid limit between 50-130%. They explain this in terms of pro-
gressive failure, rather than the nature of the fissures controlling the shear strength, which is
considered more likely by the authors.
It is recommended that the following procedure be followed to assess the shear strength
of fissured clays.
(a) Map the fissures for orientation, continuity and surface characteristics as detailed in
(b) Analyse the orientation using stereo projection methods as one does for rock joints.
MacGregor et al. (1990) give an example;
(c) Carry out laboratory tests to determine the peak, fully softened and residual strengths
and, if possible, the strength along fissures (this procedure is described below);
(d) Assess the strength of the soil mass parallel to the fissure surfaces accounting for the
continuity and nature of the fissure surfaces eg. polished slickensided fissures would
be assigned residual strength, and if they were 50% continuous, the strength determined
as approximating to the average of residual and fully softened strength. Normal to the
fissures, and for rough fissure surfaces, fully softened strength would apply.
This procedure is highly judgemental, and should where practicable, be backed up with
back analyses of failures in the same geological materials.
The undrained strength is also affected by the presence of fissures. This is best assessed
by using the procedures described in Section 6.1.6 to estimate the strength from the effec-
tive stress strengths (which have been assessed allowing for the fissures).
CPT, CPTU and Vane Shear Tests do not allow the soil to fail along the fissures, so they
will generally measure the soil substance strength, not the fissured mass strength, and
hence will over-estimate the strength.
6.1.8.2
Triaxial testing of fissured soils
The shear strength of a fissured soil mass can be determined by triaxial testing, provided
the sample is sufficiently large to contain a representative selection of the fissures, and the
Table 6.2.
Shear strengths of fissured soils from three projects in Australia.
Shear strengths
Peak
Residual
Adopted
Project
References
c
c
R
R
c
Raby Bay
(1)
Moon (1992)
5
24
2
8
0
13
Plantes Hill
(2)
MacGregor et al. (1990)
10
25
0
11
5
18
Botany Bay
(3)
Thorne (1984)
25
24
0
13
5
15
Notes:
(1)
Residual soils derived from tuff. Strengths are those used while the first author was
involved in project. One failure was back analysed to give c
0 kPa,
13°.
(2)
Residual soil derived from basalt.
(3)
Marine clay fissured by desiccation and differential consolidation.
