3.8 Upgrading of masonry and concrete dams
The upgrading of older masonry or concrete dams is in most instances the
response to a recognized inadequacy in design and/or construction stand-
ards when reviewed in context with present-day good practice. The most
common deficiencies relate to overturning stability under more extreme
loading conditions, with the possibility of unacceptable levels of tensile
stress being generated at the heel. Concerns over sliding are rather
less common, but almost all concerns over stability can be identified with
slenderness of the dam profile, i.e. excessive stresses attributable to under-
design. A common feature in underdesigned profiles is inadequate provi-
sion for the effects of internal seepage and uplift pressure, the
incorporation of internal relief drains not becoming near-universal prac-
tice until c. 1950–55.
Upgrading the stability of a gravity dam by direct methods such as
prestressing or constructing a downstream supporting shoulder was dis-
cussed in Section 3.2.9. In the case of dams which do not have adequate
provision for uplift relief, but have an internal inspection gallery stability
can be enhanced by drilling a screen of relief drains from crest to gallery
and, if required, from gallery into the underlying rock foundation.
Gallacher and Mann (2002) describe and discuss major upgrading
work undertaken to stabilize the 49 m high masonry-faced Tai Tam Tuk
dam, Hong Kong. Stability of the dam was deemed acceptable under
normal loading conditions but inadequate for seismic loading, and stabi-
lization was accomplished by drilling internal relief drains. The paper dis-
cusses the influence of the installed drainage screen on seepage and uplift
and the consequent effects on stability under extreme loading.
An alternative approach to the alleviation of seepage-related prob-
lems lies in the application of a protective upstream membrane. Reserva-
tions have been expressed with regard to the long-term durability
and integrity of such membranes, but the availability of durable high-
performance synthetic membranes has led to a reappraisal of the tech-
nique and to increasing use of upstream membranes, particularly in Italy.
Scuero and Vaschetti (1998) describe a proprietary system and it’s success-
ful application to dams of different type. A later development has been
the underwater application of a protective membrane of this type to an
arch dam in California, described in Scuero, Vaschetti and Wilkes (2000).
The latter also give a useful summary of the alternative approaches to
controlling seepage and deterioration.
Cases of in-service deterioration of the exposed faces of concrete
dams are not uncommon. This is particularly the case with dams built
before 1955–60, in a time of limited understanding as to the nature and
characteristics of mass concrete and of what constituted good concreting
practice. In most instances the deterioration is associated with internal
180 CONCRETE DAM ENGINEERING
