GRAVITY DAM ANALYSIS 153
At suitable intervals along the crest holes are cored through the dam
and into the foundation. Prestressing cable or tendon assemblies are
inserted and a grouted lower anchorage formed as shown. The strands are
carried through an upper anchorage assembly at crest level, where they
are stressed to the desired load before being finally secured. The prestress
load is thus distributed into the dam from the upper anchorage, and
careful detailing with local reinforcement will be necessary to avoid over-
stress of the adjacent concrete.
The resultant prestress load operates as an adjunct to Pm, and so
improves sliding stability. Its positive moment with respect to the toe acts
to increase overturning stability. The compressive stresses generated inter-
nally must be superimposed on the stresses generated by other loads. Note
that for the reservoir full condition the effect of superposition is to elimi-
nate any upstream tensile stresses, but that under the reservoir empty con-
dition tensile stresses may develop at the downstream face due to the
prestress.
The design prestress, Pps(kN m^1 ), should satisfy the more demand-
ing of the stability modes, i.e. overturning or sliding, having regard to the
safety factor desired for each. Sliding stability generally proves the more
demanding criterion. Ppsis determined by the inclusion of a separate pre-
stress term alongside FOandFSFin the stability expressions of sections
3.2.2 and 3.2.3 and solving for Ppsin terms of the specified stability factors
of safety. For overturning (FO, from equation (3.19)),
Pps (kN m^1 ) (3.40)wherey 2 is the moment arm of Ppswith respect to the toe. For sliding (FSF,
from equation (3.25)),
Pps ∑V. (3.41)Equivalent expressions may be derived giving Ppsfor sliding in terms of FSS
orFLE.
The prestressing tendons are typically located at 3–7 m centres along
the crest. The prestress load required for each, PT(kN), is the appropriate
multiple of Pps. A recent UK instance of prestressing, at Mullardoch dam,
is described by Hinks et al.(1990).
Prestressing also provides a structurally efficient and economical
approach to heightening existing dams. In its absence additional concrete
must be placed to thicken as well as raise the profile in order to ensure
adequate stability. Prestressing is technically preferable, avoiding ques-
tions about compatibility and structural unity of old and new concrete in a
thickened profile.
(FSF∑H cAh)
tanFO(∑M ve) (∑Mve)
y 2