Environmental Engineering Reference
In-Depth Information
Either drained or IC-U
envelope is used, whichever
gives lower strength
Normal stress
effective stress
on base of slice after drawdown
NORMAL STRESS -
σ
Figure 11.22.
Composite shear strength envelope.
US Corps of Engineers (1970), and Duncan et al. (1990), and is to allow for possible
drainage (the drainage paths may be short in these low stressed areas) and softening
under the low confining stresses. It will also allow for the cracking, dilation and soft-
ening of the earthfill which occurs because of settlements during construction and
under repeated drawdowns.
(iii) It uses the effective stress after drawdown to estimate the strengths as recommended by
Duncan et al. (1990). They demonstrated that the use of the effective stresses prior to
drawdown as recommended by US Corps of Engineers (1970) and (USBR, 1987) is con-
servative. However the authors caution that these effective stresses should be estimated
conservatively and the default position would be to use the effective stress prior to
drawdown which is more readily estimated.
(iv) It uses undrained strengths above the normal maximum operating level (after allow-
ing for cracking and softening) because effective stress strengths in this area, where con-
fining stresses are low, greatly under-estimate the real strength of the partially saturated
soil and, if used, could give misleadingly low strengths in the upper part of the dam.
Dams which have experienced larger than normal settlements should be assumed to
have cracking and softening present.
11.5
DESIGN ACCEPTANCE CRITERIA
11.5.1
Acceptable factors of safety
Most dams are designed and the safety of existing dams is assessed using factors of safety
as acceptance criteria. The factors of safety adopted are reasonably universal, with for
example similar values used by US Bureau of Reclamation (USBR, 1987), US Corps of
Engineers (1970), Building Research Establishment Guide to the Safety of Embankment
Dams in the United Kingdom (BRE, 1990) and Norwegian Geotechnical Institute (1992).
Table 11.6
lists what can be regarded as baseline minimum factors of safety applicable
to design of new dams with high consequence of failure, with the qualifications listed in
the notes.
