Environmental Engineering Reference
In-Depth Information
Table 11.8.
Hume No.1 embankment results of limit equilibrium
stability analysis (Cooper et al., 1997).
Factors of safety
Case
Effective stress
Undrained strength
Peak strengths
1.52 (1.52)*
1.23 (1.14)
Residual strength
1.06 (0.98)
0.97 (0.89)
Note: The figures in brackets assume no strength in the concrete
core. For values marked “*”, the critical failure surface does not
pass through the wall.
strengths taking account of pore pressures generated during undrained shearing using the
formula:
ccos P sin
12A in
′
o
S
(11.22)
u
f
1
K
2
o
′
where P
(11.23)
o
vo
where c
,
are the effective stress peak strengths, A
f
Skemptons pore pressure coeffi-
cient at failure, K
o
effective vertical stress.
A
f
was in the range 0.36 to 0.7 for the triaxial tests. A value of 0.7 was adopted.
The results of the analysis are given in Table 11.8.
The numerical analysis underestimated the actual displacements using the undrained
strengths derived from peak strengths and overestimated them using the undrained
strengths derived from the residual strengths. The analysis showed a yield surface devel-
oped in the 2 m of saturated fill just above the foundation downstream of the core wall
and was able to replicate movements at other parts of the dam not affected by the satura-
tion of the lower part of the fill.
From this it was concluded that:
the rest earth pressure coefficient,
vo
-
The behaviour of the dam was due to marginal slope stability;
-
The use of effective stress analysis significantly over-estimates the factor of safety;
-
Undrained strength analysis more correctly models the behaviour but allowance must
also be made in this case for strain weakening.
It should be noted that the investigations had shown that the movements could not be
explained by consolidation, collapse mechanisms or internal erosion and piping.
Subsequent investigations of an area where No.1 Embankment joins the concrete spill-
way section confirmed the need to use undrained strengths in stability analysis. In that area
borehole inclinometers confirmed the development of shear surfaces in the saturated, soft-
ened fill. Both areas have had berms constructed to improve the stability.
11.6.2
Eppalock Dam
Eppalock Dam is a 47 m high central core earth and rockfill dam which was built between
1960 and 1962.
Figure 11.10
shows a cross section through the dam.
The rockfill in the dam was placed in lifts 2 m to 4 m thick, spread with a light tractor.
There was no rolling. The core was compacted by sheepsfoot roller, probably to
98%
