Environmental Engineering Reference
In-Depth Information
11
Analysis of stability and deformations
11.1
METHODS OF ANALYSIS
The analysis of the stability of dams and other slopes is usually carried out using Limit
Equilibrium Analysis (LEA). In most situations more detailed analysis using numerical
methods, e.g. finite element analysis, is not necessary. However if it is necessary to model
the deformations of the dam or the effects of strain weakening, as in progressive failure,
numerical methods must be used. Comprehensive reviews of slope stability analysis have
been written by Fredlund and Krahn (1977), Whitman and Bailey (1967), Fredlund (1984),
Graham (1984), Mostyn and Small (1987), Duncan (1992, 1996a and b), Morgenstern
(1992), and Hungr (1997).
Whether carrying out limit equilibrium or numerical analysis, it is important to formu-
late the problem correctly and, in particular, to establish whether undrained strengths
(total stress analysis) or drained strengths (effective stress analysis) is used. Table 11.1
summarises the shear strengths, pore pressures and unit weights which should be used.
The reason for using total stress analysis (Su,
u ) in low permeability zones for end of con-
struction, rapid drawdown and staged construction condition is that it is difficult to predict
the pore pressures accurately. Undrained strengths should also be used for contractive soils as
described in Section 6.1.3.
If in doubt as to whether undrained or drained conditions will be most critical, it is wise
to check both and adopt the results from the lowest strength case. Undrained conditions can
occur even in relatively slow rates of change of reservoir levels. Cooper et al. (1997) describe
a situation where annual cycles of rise of reservoir level were sufficient to induce undrained
loading conditions in the downstream poorly compacted (contractive) clay zone of the con-
crete corewall Embankment No.1 at Hume Dam. This is described in Section 11.6.1.
Effective stress methods can be used for the end of construction and have the advantage
that monitoring of pore pressures can assist in assessing the stability. Section 11.4.2 describes
the methods for estimating the pore pressures in the partially saturated soil as it is compacted
in the dam and loaded by the weight of the embankment as construction continues.
It is important to model the foundation of the dam in the analysis of stability, particu-
larly where the dam is founded on soil or on rock with low strength surfaces within it, e.g.
bedding surface shears. Most failures of dams have occurred where such weak zones in
the foundation have not been recognised.
11.2
LIMIT EQUILIBRIUM ANALYSIS METHODS
11.2.1
General characteristics
Table 11.2 Summaries the characteristics of the methods of slope stability analysis.
These methods all employ the same definition of the factor of safety, F:
Shear Strength of the Soil
Shear Stress re
F
(11.1)
quiredfor equilibrium
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