Civil Engineering June 2022 | Vol 30 No 5

Civil Engineering June 2022 53 In addition, the tragic loss of life on the intended four-year project, which finally ran to six years, bore witness to this most hazardous of civil engineering aspects and simply confirmed the onerous global statistics related to such high-risk work. Significant challenges included: Q Q Inlet: a 900 ℓ/s water inrush on the heading 2S which took 18 months to manage and finally overcome. Q Q Plateau: a methane gas fire on heading 4S which cost the project more than 12 months. Q Q Outlet: low cover conditions over much of the final outlet heading, requiring heavy arch support, which reduced tunnelling advance to a pedestrian pace of around 100 m per month. It was a tribute to the innovation and staying power of the engineers and contractors that the project endured to a successful completion, as well as that of the client representative (Secretary for Water Affairs) on whose recommendation suitable modifying agreements to the con- tracts were drawn up and agreed, which covered the contractors for the extreme cost escalations that occurred during the years after the contracts were signed. All that remains visible today of those momentous years are the intake tower at Oviston, the outlet distribution structure, and the short open canal and outfall works in the shadow of Teebus Mountain which continues to transfer up to 25% of the Orange River flow to the Great Fish and Sundays River valleys. VANDERKLOOF DAM Vanderkloof Dam is built in a steeply sided gorge in a massive dolerite rock intrusion, far more suited to the double curvature arch dam approach than Gariep. It was established from early days of assessing the irrigation potential of the Orange River that this site had great potential for both power generation as well as irrigation of arable land in neigh- bouring river catchments. However, during the runup to construction commencement, several changes were necessary and, as a consequence of the qualified tender from the best-placed contractor as well as other unacceptably-high bids, DWA – having undertaken much arch dam work in recent years – was instructed to build the dam departmentally, amid outcries from the South African construction fraternity. Significant design changes were made in conjunction with comprehensive hy- draulic model testing of the intakes to the powerhouse, spillways and plunge pool. This work was done abroad under the auspices of the design team retained from the Gariep Dam assignment to achieve greater efficiency. Construction finally commenced in 1973 with rather less concern for river diversion, given that Gariep Dam was now functioning and available to afford flood control and attenuation if required. Nevertheless, it would take 14 months to construct the foundations and bring the wall to a safe level, implying that it was vulnerable to at least one flood season. Cofferdams were limited to earth embankments that would divert flows through a convenient shelf in the right flank rock face of the gorge. And again, the inevitable occurred! Flooding in March 1974 resulted in overtopping of both the earth cofferdams, which was repeated the following month, bringing an inevitable delay to resumption of work. Figure 4 Orange-Fish Tunnel outlet canal in the shadow of Teebus Mountain (November 2021, running at 26 cumecs)