seepage, particularly at badly executed construction joints, resulting in the
leaching out of hydrated cementitious compounds and some regression in
strength and impermeability/durability. The damage is generally localized
and quite superficial in relation to the overall integrity of a gravity or
massive buttress dam. The longer-term effects are considerably more
serious in the case of Ambursen-type buttress dams and thin multiple-arch
structures (see Figs 1.5c) and d)), where the nominal hydraulic gradient
through the slender concrete sections is very much greater, giving rise to
high rates of leaching and attrition. It is for this reason, inter alia, that
dams of these types are no longer favoured.
Superficial physical deterioration of mass concrete can also be
caused by severe freeze-thaw action or the action of aggressive reservoir
water, particularly on concrete mixes which had a high initial
water/cement ratio or were poorly compacted.
The in-service deterioration of masonry in gravity dams is generally
confined to degradation of the bedding mortar in the joints. It is therefore
relatively superficial and of little immediate significance to the overall
integrity of a massive dam.
Masonry sourced from a dense, competent parent rock is almost
invariably highly durable. Dressed masonry was generally limited to
upstream and downstream faces, providing a highly durable and aestheti-
cally attractive external finish. Cost considerations normally dictated that
the interior or hearting be of cyclopean masonry, i.e. very large semi-
dressed or undressed rocks bedded in concrete and/or with the intervening
void spaces packed with smaller stones and mortar.
Sims (1994) discusses the ageing of masonry dams with particular ref-
erence to examples from Indian experience. Bettzieche and Heitefuss
(2002) describe in-service rehabilitation of older masonry dams in
Germany with regard to the retrospective drilling of relief drainage and
driving of inspection or drainage galleries. They describe the successful
employment of drill-and-blast techniques and, in the case of the 51 m high
Ennepe dam a tunnel-boring machine, for driving internal galleries. The
successful application of underwater working techniques to the in-service
rehabilitation of bottom outlet works at the Ennepe and Moehne dams is
discussed in Heitefuss and Kny (2002).
Recent years have seen considerable advances made in regard to
rehabilitation options. Modern rehabilitation techniques are assuming
ever-greater importance in relation to cost-effective upgrading of ageing
masonry and concrete dams to conform to the standards of today.
amelia
(Amelia)
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