ARCH DAM ANALYSIS 157
In structural terms the massive buttress dam is constructed from a
series of independent ‘units’, each composed of one buttress head and a
supporting buttress, or web. Each unit has a length along the axis of the
dam of about 12–15 m. Structural analysis is therefore conducted with
respect to the buttress unit as a whole. The sliding stability of one complete
unit is investigated in terms of FSS, sliding factor or, more usually, FSF, shear
friction factor, in accordance with the principles of these approaches as out-
lined in Section 3.2.3. The design minimum values for FSSandFSFare
normally comparable with those required of a gravity profile.
Stress analysis of a buttress ‘unit’ is complex and difficult. Modern
practice is to employ finite element analyses to assist in determining the
optimum shape for the buttress head to avoid undesirable stress concen-
trations at its junction with the web. An approximate analysis of the down-
stream portion of the buttress web, using modified gravity method
analysis, is possible if the web is parallel sided. The sides of the buttress
web are, however, generally flared towards base level, to increase contact
area and hence sliding resistance and to moderate the contact stress on the
foundation.
Profile design for a buttress dam is not subject to the simplifications
outlined in Section 3.2.7. A trial profile is established on the basis of previ-
ous experience, the selection of a round head or a diamond head being
largely at the discretion of the designer. The profile details are then modi-
fied and refined as suggested by initial stress analyses.
3.4 Arch dam analysis
3.4.1 GeneralThe single-curvature arch dam and its natural derivative, the double-
curvature arch or cupola with vertical and horizontal curvature, were
introduced in Section 1.4 and in Figs 1.4(d) and 1.4(e) respectively. The
valley shape and rock conditions which may favour consideration of an
arched dam in preference to gravity or rockfill alternatives were outlined
in Section 1.6.6 and Table 1.7.
Arch and cupola dams transfer the greater proportion of the water
load to the valley sides rather than to the floor, functioning structurally by
a combination of arch action and vertical cantilever action. Abutment
integrity and stability are therefore critical, and the importance of this
point cannot be overstated. Progressive abutment deformation or yielding
in response to arch thrust results in load transfer and stress redistribution
within the dam shell and in the abutment itself. In more extreme situations
of significant abutment yielding or instability local overstress of the dam
wall will ensue, destroying arch action and resulting in catastrophic