Environmental Engineering Reference
In-Depth Information
12.6.4.3
Recommended approach
As a general guide, for most projects it will not be necessary to use other than simplified
methods. Only for high consequence of failure projects, where stability and/or deforma-
tions show marginal safety, should it be necessary to use numerical methods and then it
will usually be sufficient to model the post earthquake deformations.
More advanced dynamic methods are expensive and should only be done by very experi-
enced persons who understand the limitations of the analysis and the need to use well con-
sidered properties.
There should not be an over-reliance on stability and deformation analysis at the expense of
good engineering judgment and the consideration of the general design principles given in
Section 12.7. At best, the analysis methods are approximate and controlled by the quality of
data put into them, the limitations of the methods themselves and of those doing the analysis.
12.7
DEFENSIVE DESIGN PRINCIPLES FOR EMBANKMENT DAMS
The concept of “defensive design” of embankment dams for earthquake was developed by
Sherard (1967) and Seed (1979a) and endorsed by Finn (1993), ICOLD (1986c, 1999) and
ANCOLD (1998).
The general philosophy is to apply logical, commonsense measures to the design of the
dam, to take account of the cracking, settlement and displacements which may occur as
the result of an earthquake. These measures are at least as important (probably more so)
as attempting to calculate accurately the stability during earthquake or the likely defor-
mations. The most important measures which can be taken are:
(a) Provide ample freeboard, above normal operating levels, to allow for settlement
or slumping or fault movements which displace the crest. For example, one might
adopt a narrow spillway with large flood rise and large freeboard instead of a wide spill-
way with small flood rise and thus usually a lower freeboard, provided the costs were
similar.
(b) Use well designed and constructed filters downstream of the earthfill core (and cor-
rectly graded rockfill zones downstream of a concrete face for concrete face rockfill
dams) to control erosion if the core (or face) being cracked in the earthquake. Filters
should be taken up to the dam crest level, so they will be effective in the event of large
crest settlements, which are likely to be associated with transverse cracking. For larger
dams, full width filters (2.5 m to 3 m) might be adopted instead of narrower (1.5 m
say) filters placed by spreader boxes. This would give greater security in the event of
large crest deformations.
(c) Provide ample drainage zones to allow for discharge of flow through possible cracks
in the core. For example ensure that at least part of the downstream zone is free drain-
ing or that extra discharge capacity is provided in the vertical and horizontal drains
for an earthfill dam with such drains. In this regard some embankment dam types are
inherently more earthquake resistant than others. In general the following would be
in order of decreasing resistance:
- concrete face rockfill;
- sloping upstream core earth and rockfill;
- central core earth and rockfill;
- earthfill with chimney and horizontal drains;
- zoned earth-earth rockfill;
- homogeneous earthfill.
(d) Avoid, densify, drain (to be non-saturated) or remove potentially liquefiable materials
in the foundation or in the embankment.
Figure 12.39
shows the principles for new
