The situation shown in Fig. 1.7(d), with deep overburden under one-
half of the site, could well suggest the composite solution shown. An earth-
fill embankment is constructed where overburden depth is considerable,
the spillway being conveniently accommodated on a concrete gravity
section where the required excavation depth is reasonable, e.g. at Cow
Green dam (Kennard and Reader, 1975).
It will be appreciated that in all four illustrative instances the avail-
ability of alternative construction materials, and their relative cost, will
impact heavily upon the final decision as to type of dam.
Figure 1.7 should be studied in conjunction with Table 1.7, which
summarizes dam type characteristics in the context of selection of type.
The choice of type of dam may be influenced in part by factors which
assume a particular importance in relation to a specific site. An example
would lie in a situation where site conditions in a steep-sided valley might
favour an earthfill embankment, but a spillweir and channel of the
required size might prove disproportionately expensive to construct in the
hillside round the flank of the dam. In such a case the economic balance
might then tilt in favour of the gravity dam with an overflow crest, if the
overburden depth were not excessive.
It may also be noted that local factors may assume greater relative
importance for a specific site. Examples might include access, seismic risk,
34 ELEMENTS OF DAM ENGINEERING
Table 1.7 Dam selection: type characteristics
Type Notes and characteristics
Embankment
Earthfill Suited to either rock or compressible soil foundation and wide
valleys; can accept limited differential settlement given
relatively broad and plastic core. Cut-off to sound, i.e. less
permeable, horizons required. Low contact stresses.
Requires range of materials, e.g. for core, shoulder zones,
internal filters etc.
Rockfill Rock foundation preferable; can accept variable quality and
limited weathering. Cut-off to sound horizons required.
Rockfill suitable for all-weather placing.
Requires material for core, filters etc.
Concrete
Gravity Suited to wide valleys, provided that excavation to rock is less
thanc.5 m. Limited weathering of rock acceptable.
Check discontinuities in rock with regard to sliding.
Moderate contact stress. Requires imported cement.
Buttress As gravity dam, but higher contact stresses require sound rock.
Concrete saved relative to gravity dam 30–60%.
Arch and Cupola Suited to narrow gorges, subject to uniform sound rock of high
strength and limited deformability in foundation and most
particularly in abutments. High abutment loading. Concrete
saving relative to gravity dam is 50–85%.
