Civil Engineering June 2022 | Vol 30 No 5

52 June 2022 Civil Engineering Key characteristics of Gariep Dam include: Q Q Height of 88 m Q Q Crest length of 914 m Q Q Contains 1.73 million m 3 of concrete Q Q Storage capacity of 5 343 million m 3 Q Q Reservoir surface area of 352 km 2 when full. ORANGE-FISH TUNNEL From the early days of conception of Orange River water transfer into the head- waters of the Great Fish catchment of the Eastern Cape, the tunnel alignment had been fairly well defined by the shortest possible route beneath the Zuurberg Plateau, which gave an acceptable gradient for a gravity fed conduit. Initial drilling investigations post World War II by DWA (1948 to 1953) were supplemented by further work (1964 to 1966) by the consulting engineers ap- pointed for detailed design and construc- tion monitoring of the tunnel. The result was a tunnel with a capacity for up to 25% of the Orange River flow, giving a lined diameter of 5.33 m and length of approximately 82.45 km at a gradient of 1:2000. It has been said that the only cer- tainty with regard to tunnelling is its uncertainty, and the objective of site investigation is to minimise that risk. The siting and planning of the tunnel therefore involved site investigations on a scale unequalled in South African civil engineering history. Tender information for this prodigious project finally included some 27 500 m of rock core, drilled from 280 vertical boreholes along the route – some up to 450 m deep. To this was added the results of experience and observations of trial exploratory adits advanced into the sedimentary and intrusive rocks which characterised the Beaufort Group forma- tions of the Karoo Sequence that would be encountered by the tunnel. In addition, an extensive programme of rock mechanics testing was implemented together with geotechnical data gathering, rock support tests and response to excavation by the more conventional drill-and-blast tech- nology of the day. All of this was provided as tender information to enable the provi- sion of realistic contract prices. The tunnelling construction was di- vided into three contracts, each of which featured appointments including con- sortia of both South African and foreign (mainly French and Italian) contractors: Q Q Inlet: comprising the intake tower in the basin of Gariep Dam, two shafts and 27.2 km of tunnel. Q Q Plateau: comprising three deep shafts and 31.1 km of tunnel. Q Q Outlet: comprising two shafts, 24.1 km of tunnel, underground control works, surface distribution works, and outfall into the headwaters of the Great Fish River. While early tunnel boring technology was available and considered in early planning, it was mainly geared towards soft rock tunnelling. Because of a significant hard rock dolerite component to the predicted formations to be traversed, all three ap- pointed contractors for the work, which included South African mining expertise, ultimately elected to undertake the work by the known conventional approach. In spite of the significant amount of pre-construction investigative data pro- vided, each section of the project was to ex- perience unexpectedly serious challenges that would impact significantly on both the construction schedule and final costs. Figure 3 Orange-Fish Tunnel intake tower in the Gariep Reservoir