Environmental Engineering Reference
In-Depth Information
There are a number of other less important measures which are listed by Sherard
(1967), Seed (1979a) and Finn (1993). These include:
(g) Use a well-graded filter zone upstream of the core to act as a crack stopper, possibly
only to be applied in the upper part of the dam. The concept is that, in the event that
major cracking of the core occurs in an earthquake, this filter material will wash into
the cracks, limiting flow and preventing enlargement of the crack. If well-designed fil-
ters are provided downstream, this upstream filter is of secondary importance.
(h) Provide crest details which will minimize erosion in the event of overtopping, e.g. by
having a wide crest, and if the core is a highly dispersive soil, it could be modified with
lime to make it non dispersive (in reality this is not likely to make a lot of difference).
The common practice of using steeper slopes near the crest (usually to provide for
camber of the crest) and the use of wave walls in concrete face rockfill dams will make
the crest more susceptible to damage.
(i) Flare the embankment core at abutment contacts, where cracking can be expected, in
order to provide longer seepage paths. Just as (or more) important is to consider the
detailing of the contact with concrete walls and the provision of filters downstream of
the contacts. This detailing is discussed in Section 13.6.
(j) Locate the core to minimize the degree of saturation of materials (e.g. use sloping
upstream core). Finn (1993) also suggests positioning chimney drains near the central
section of the embankment). These measures are intended to reduce to a minimum the
extent of saturated zones which are more likely to reduce strength on cyclic loading.
They are particularly relevant where sand, silty sand and sand-gravel soils are used in
the dams as these are most susceptible to liquefaction. If all materials in the dam are
well compacted these requirement is not important.
(k) Stabilize slopes around the reservoir rim (and appurtenant structures such as spill-
ways) to prevent slides into the reservoir or onto the structures.
(l) Provide special details if there is likelihood of movement along faults or seams in the
foundation.
(m) Site the dam on a rock foundation rather than soil foundation (particularly if it is
potentially liquefiable) where the option is available.
(n) Use well graded (densely compacted) sand/gravel/fines or highly plastic clay for the
core, rather than clay of low plasticity (if the option is available) (Sherard, 1967).
Seed et al. (1985) suggest that the slopes of concrete face rockfill dams should be flat-
tened to limit displacements in earthquake. We are not convinced this is necessary for
well-compacted rockfill.
When assessing an existing dam, the use of these “defensive design” measures is seldom
practical (except in remedial works). However, it is useful to gauge the degree of security
the existing dam presents by comparing it with this list. Where the dam fails to meet many
or most of these features, particularly (a) to (e), this may be a better guide to the fact that
the dam may not be very secure against earthquake than a lot of analysis.
Dams which have well designed and constructed filters, have adequate stability against
normal loads, and do not have liquefiable or strain weakening zones or foundations, will
be able to withstand the loading from very large earthquakes.
