BUTTRESS DAM ANALYSIS 155
The selection of compacted rockfill for the supporting shoulder
offers the advantage, as opposed to earthfill, of good drainage and steeper
side slopes. A further material for shoulder construction is rolled dry lean
concrete (RDLC), introduced in Section 3.7.2. The construction of a
downstream rockfill shoulder to improve stability of the 45 m high Upper
Glendevon dam in the UK is described in Macdonald, Kerr and Coats
(1994).
3.3 Buttress dam analysis
3.3.1 GeneralButtress dams fall into two distinct groups, as identified in Section 1.4.
Massive diamond or round-headed buttress dams representative of
modern practice were illustrated schematically in Figs 1.4(b) and 1.4(c).
The earlier but now largely obsolete flat slab or Ambursen buttress dam
was also shown, in Fig. 1.5(c). The latter type is not considered further.
Relative to the gravity dam the principal advantages of the massive but-
tress dam lie in its obvious economy of material and in a major reduction
in uplift load. The buttress dam also offers greater ability to accommodate
foundation deformation without damage. The advantages listed are offset
by considerably higher finished unit costs (overall cost of the completed
dam per m^3 of concrete) for the reduced quantities of concrete employed.
This is attributable largely to the cost of the more extensive and frequently
‘non-repetitive’ formwork required. Significantly increased stresses are
also transmitted into the foundation of each buttress. The criteria for foun-
dation competence are therefore appreciably more rigorous than those
applicable to the gravity dam.
3.3.2 Buttress analysis and profile designButtress dam analysis parallels gravity dam practice in being conducted in
two phases, stability investigations preceding the determination of stresses
within the profile. The structural form of the buttress dam, detailed in the
example of Fig. 3.12, has two important consequences with respect to
primary loads. First, uplift pressures are effectively confined to the but-
tress head, resulting in the modified uplift distribution shown in Fig. 3.12.
Pressure relief drains are therefore only necessary in exceptional cases. As
a further consequence of the form the vertical component of the water
loadPwv, on the sloping upstream face is very much enhanced relative to
any gravity profile. Stability against overturning is therefore a less
meaningful design criterion.