DESIGN FEATURES AND CONSTRUCTION 169
phase, e.g. rock excavation and foundation grouting etc. Detailed pre-
planning of all construction activities must therefore commence well in
advance of site preparation, with the objective of ensuring optimum avail-
ability and utilization of all resources, i.e. finance, materials, plant and
labour, throughout the construction period. It will be noted that several
major activities are subject to non-concurrent peaks and troughs as con-
struction progresses. A comprehensive account of the construction of a 40 m
high curved gravity dam is presented in Tripp, Davie and Sheffield (1994).
The initial phase, site preparation, embraces the provision of a site
infrastructure. This includes access roads, offices and workshops, and fixed
plant. Accommodation, commissariat and other facilities for the labour
force are also necessary on large or remote projects.
Preparations for river diversion, the second phase, may involve
driving a bypass tunnel through the flank of the dam. As an alternative,
the river may be diverted through a temporary gap formed by leaving a
low-level tunnel through a block or by the omission of a complete mono-
lith until the final closure stage is approached. Temporary cofferdams are
normally required to allow the construction of diversion and other works
on the river bed.
Foundation excavation and preparation is the third phase. Rock
excavation should be the minimum consistent with attaining a safe founda-
tion, the use of explosives being carefully controlled to avoid shattering
and fissuring otherwise sound rock. The finished rock surface should be
irregular, sound and clean. It is normal to provide a rise towards the toe,
the nominal slope of about 4–15% being determined by the rock
characteristics. Fissures are infilled with ‘dental’ concrete, and the grout
curtain to control underseepage installed. Additional shallow grouting to
consolidate the contact zone is carried out as necessary.
The construction operations of the fourth phase require particularly
careful planning and control. Individual monoliths are raised on either the
‘alternate block’ or the ‘shrinkage slot’ principle as illustrated in Fig. 3.21.
In either method the objective is to maximize shrinkage before pouring
abutting lifts of concrete in adjacent blocks.
The sequence of events within each concrete pour is formwork erec-
tion, surface preparation and placing concrete, with compaction by vibra-
tor. This is followed by an interval before formwork can be struck, and a
further appreciable interval for initial shrinkage to occur before the cycle
is repeated. Features which complicate or delay the concreting cycle
unnecessarily are expensive, and must be kept to the minimum. Curing of
the completed pours is effected by moist curing and/or by membrane
curing compounds.
The final phase of project execution involves the completion of any
ancillary structures, and installation and testing of valves, gates etc. It is
concluded with a controlled and carefully monitored initial impounding
(Chapter 7 should also be referred to).