SMALL DAMS PETITS BARRAGES

� Simple Bishop (1955)
� Spencer (1967)
� Janbu (1957)
� Morgestern and Price (1965)
Determination of most critical slope surface
The following approach must be followed to select the most critical surface:
� Determine a series of safety factors with various radii but same centre point and plot
on scale.
� Redo above with different centre points encroaching to the lowest safety factor. Plot of
safety factors contours to be made.
� More than one critical surface must be identified for e.g. rockfill dams where surface
slopes are occurring.
Selection of shear parameter
The selection of shear parameter to be tested meeting site conditions is normally based
on specific test methods. A summary and explanation are given in Table 5.13.
Test method
Table 5.13 – Summary of Triaxial Tests
Application in stability analysis Remark
At low test pressure
UU Test represents end of construction condition cavitation occur during
(Undrained, for core zones. Also applicable to impervious shear. Stress curves
Unconsolidate foundations where consolidation is slow in may have curves if
d) comparison with rate of earthfill placement. materials are
saturated.
CU
(Consolidated,
Undrained)
Test represents behaviour of impervious or
semi-pervious materials of embankments or
foundations which consolidated during
construction and which were exposed to stress
change during sudden draw down conditions.
Test also used to analyse stationary conditions
in downstream slope.
Test applicable to impervious and semi-
Undrained stress
increases with
decreasing moisture
content and increases
in consolidation
stresses.
CD
(Consolidated,
Drained)
pervious soils with hydrostatic pore pressures,
before or after shear, under slow increase in
load is revealed. Also used to evaluate shear
values under setting out conditions where
excess hydrostatic pressure during the life of the
dam is determined.
CU strength is higher
in value than CD
strength for expansion
soils.
5.8.4 Minimum Safety Factors Against Slip Circle Failures
73