Civil Engineering June 2022 | Vol 30 No 5

50 June 2022 Civil Engineering GARIEP DAM The footprint of the dam site had been established on the basis of prior inves- tigation drilling undertaken by DWA which confirmed the competence and foundation engineering parameters for the dolerite sill on which the wall would be built. While DWA pressed for a double- curvature arch structure for the dam, based their own successful design and construction experience during the 1950s, the foreign consultants objected on the basis of the geometry of the site which did not lend itself to such a design. Objections, obviously largely based on the global experience of the day, designated arch dams to much narrower and deeper gorge situations. The final outcome was a compromise, with the flanks of the structure housing the gated spillways and power penstocks consisting of traditional gravity structures transitioning into the double-curvature arch section for the main ungated spillway. In parallel with the structural design of the wall, the foreign partners of the engineering teams were dealing with the final hydraulics and modelling of the spillway arrangements and testing of the overspill apron and riverbed stability in response to passing of the design floods. In the meantime, preliminary work including a construction village known as Oranjekrag, which remains to this day with all facilities and services infrastruc- ture, had been developed together with an access bridge across the river and an airport, while the existing Bethulie rail bridge which would be drowned by the dam was totally rebuilt together with railway deviation and station. Construction of Gariep Dam was awarded to a South African/French consortium in April 1966. The French partner, being experienced large dam builders, included some engineers who had been active at Kariba Dam (completed in 1959), while the South African partner, a mining group, handled the surface plant provision, crushing of dolerite aggregate, overhead cableways and craneage, and underground works for the power plant. Key elements of the dam commenced by September 1966, including provision of temporary cofferdams behind which to construct the foundations and initial wall sections comprising the (right bank) river diversion works required to pass floods during the first years and early stages of construction. In February 1967, with the first cof- ferdams in place, the site experienced a flood of 8 500 cumecs, the largest on the Orange River since 1934! While the main river flow was able to pass through the central river channel which was not yet blocked by construction, the flank cofferdams were over-topped and the river level rose to 5 m above their crests. Two years later, in February 1969, by which time the outer flanks of the wall were well advanced, the right flank river diversion fully functional and the main cofferdam protected the central parts of the rising wall, a further flood occurred causing over-topping once more. The Orange River was making its final at- tempts to breach the construction of this great venture! Roughly 18 months later, by September 1970, with the wall sufficiently advanced, the diversion outlets were sealed and the reservoir began to fill. In view of the complexity of the over- spill structure, discharge splitters, bridge piers and final roadway crossing the dam, much of the related concrete work comprised use of precast units, rather than attempting in-situ construction of complex shapes at these great heights. The left bank powerhouse (an Eskom undertaking in its entirety), comprising four 90 MW Francis turbines and related electrical works, was supplied by Japanese and German firms. Much of the related components, spiral cas- ings, and steel-lined penstocks were locally fabricated by heavy engineering works and transported to site, as were spillway gates, control valves and other mechanical equipment. Figure 2 Gariep Dam in November 2021 at about 75% capacity