Environmental Engineering Reference
In-Depth Information
Figure 11.3.
Examples where non circular failure surfaces must be analysed.
11.2.2
Some common problems
Some common problems in LEA (which are often simply omissions and/or occur through
lack of proper care) include:
-Use of circular rather than non circular analysis, where failure surfaces will clearly fol-
low weak surfaces - e.g. bedding surface shears. Figure 11.3 shows examples where
non circular analysis must be used;
-
Omitting to model cracks in the ground surface and the water pressure which will often
develop in the cracks due to rainfall or surface water flowing into the crack. This can
have a significant effect on calculated factors of safety, particularly for smaller failure
surfaces near a dam crest;
-
Not modelling anisotropy of strengths, e.g. in fissured clays or stratified soil and rock;
-
Not clearly showing on the drawing of the cross section and failure surface the prop-
erties used for that particular analysis. If the two are separated confusion is likely to
occur.
These features can all be modelled with modern computer programs. If the program being
used cannot model these features, consideration should be given to purchasing a program
which can.
A more subtle problem, which is an extension of the discussion on the different meth-
ods of calculation described above, is where essentially translational sliding in rock is
being modelled. Figure 11.4 shows such a slope, where the rock is relatively strong and
with a few, widely spaced, near vertical joints AB, CD, EF (e.g. sandstone or conglomer-
ate), with potential sliding on a weak layer (e.g. claystone) OBDF. The driving force for
instability is the groundwater in the joints and the gravity forces along OBDF.
Use of a conventional computer program to analyse the stability of OAB, OCD, or OEF
will overestimate the factor of safety substantially even if the failure surfaces are pre-defined,
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