3.2.9 Stabilizing and heightening
A number of older gravity dams are underdesigned by present-day stand-
ards, having undesirably low stability. The underdesign is a reflection of
past limitations on the understanding of loads, materials and dam behavi-
our. In some cases stability margins have been eroded by revised loading
criteria, e.g. uprated flood maxima. In others, distress has resulted from
progressive deterioration. Remedial action to improve stability can be
taken by
- vertical prestressing of the dam or
- construction of a downstream supporting shoulder or berm.
(a) Prestressing
Prestressing provides an additional vertical load with a resultant line of
action close to the upstream face. It requires the introduction of highly
stressed steel-strand cables or tendons into the dam, anchored at crest level
and at depth within the foundation. A necessary condition for prestressing is
therefore the presence of rock competent to accept the high anchorage
loads imposed. The principle of prestressing is illustrated in Fig. 3.10.
152 CONCRETE DAM ENGINEERING
grouted anchorage
formed in competent
rock horizons
Fig. 3.10 Stabilizing and heightening: vertical prestressing
