it is important that options are kept open, assessing the implications of
each with respect to resources, programme and cost, until a preferred solu-
tion is apparent. It may also be necessary to take account of less tangible
socio-political and environmental considerations in the determination of
that solution.
Four considerations of cardinal importance are detailed below.
- Hydraulic gradient: the nominal value of hydraulic gradient, i, for
seepage under, around or through a dam varies by at least one order
of magnitude according to type.
- Foundation stress: nominal stresses transmitted to the foundation
vary greatly with dam type.
- Foundation deformability: certain types of dams are better able to
accommodate appreciable foundation deformation and/or settlement
without serious damage.
- Foundation excavation: economic considerations dictate that the
excavation volume and foundation preparation should be minimized.
The first consideration is illustrated by reference to Figs 1.2–1.5 inclu-
sive. Notional values of gradient range from about 0.5 for a homogeneous
embankment (Fig. 1.2(a)), to 10 or more for a buttress or cupola dam
(Figs 1.4(b) and 1.4(e)). The ability of softer and weaker or more erodible
foundations to resist high hydraulic gradients safely is very limited.
In illustration of the second point, notional stress values for 100 m
high dams of different types are shown in Table 1.6.
The significance of excessive or non-uniform foundation deformabil-
ity, point 3 above, arises in relation to cracking and stress redistribution
within the dam. The relative structural flexibility of a well-designed
embankment dam may be advantageous.
With regard to the final consideration, the economic disincentive of
excessive excavation, particularly in relation to a concrete dam, is self-
evident.
It is inappropriate to generalize over considerations controlling
the choice of dam type beyond the four major points referred to. Their
32 ELEMENTS OF DAM ENGINEERING
Table 1.6 Notional foundation stresses:
dams 100 m in height
Dam type Notional maximum stress
(MN m^2 )
Embankment 1.8–2.1
Gravity 3.2–4.0
Buttress 5.5–7.5
Arch 7.5–10.0
