Civil Engineering December 2021 | Vol 29 No 11

Civil Engineering December 2021 23 In order to allow a higher first stage grouting pressure, to produce greater penetration into the fine fissures that existed in the bedrock, the cut-off trench was first excavated and backfilled, then the grouting standpipes were installed through the imported clay cutoff. This allowed a significant increase in the overburden pressure, and therefore the pressure that could be specified for the grouting, particularly in the upper 5 m of the grout holes. RIVER DIVERSION CULVERT The river diversion culvert and upstream cofferdam were designed to reduce the risk of damage to the partly completed dam so that flood events during construc- tion would be manageable. They were designed to provide a 20% risk of exceed- ance during the expected construction period of 2 years and were therefore designed for the 1:10 year flood. This required the diversion culvert to consist of two 2 m × 2 m barrels with an 8 m high coffer dam. On completion of the dam the culvert was closed and provides passage for the two outlet pipes, a 250 mm for domestic supply to the water treatment works, and a 350 mm for emergency emptying of the dam as well as making provision for environmental releases. The dam closure methodology consisted of the installation of two sets of precast reinforced concrete planks with the void between the planks filled with mass concrete. After cooling for approxi- mately 3 months after dam closure, to allow concrete shrinkage to take place, the mass concrete was post grouted through a tube manchette system. SPILLWAY The spillway is a 25 m long ogee side channel spillway with a 5 m wide channel. Due to the proximity of the water treat- ment works downstream of the spillway, the design of the stilling basin needed to be accurate. It was designed and model tested by Stellenbosch University, which indicated that the spillway channel should widen to 9.5 m before its discharge into the basin to reduce the unit flow. A combination of the USBR II and III basins was developed by Stellenbosch University to control the hydraulic jump in the stilling basin and dissipate the energy of the discharge. This resulted in a novel design that suited the requirements. The model testing showed that riprap was required above the hardrock level to pre- vent erosion towards the water treatment works during extreme events. The spillway needed to be designed for large and unusual loads. There is signifi- cant turbulence in a spillway, as well as high water velocities that can cause large negative pressures or jets that can pres- surise and lift structures. The energy in a spillway is substantial, for a flood event of 100 m 3 /s the stilling basin needs to dissi- pate approximately 30 MW of energy, and in the spillway channel water will reach velocities of 90 km/h. This requires heavy concrete sections, rock anchors, careful attention to drainage design and concrete detailing to prevent potential failure. ENVIRONMENTAL, SOCIAL AND HEALTH AND SAFETY To minimise the environmental impact of the dam and the impact of siltation, it was conceived as an off-channel storage dam in a small section of the catchment. The instream flow requirements require envi- ronmental water releases be provided to maintain the aquatic environment within the river channel downstream of the dam. The section of the river channel on the downstream left bank was an area of significant environmental value and demarcated as a no-go area during construction. The design ensured that the water table was maintained to support the local ecosystem and several trees were planted to replace the protected trees removed during construction. Strong community engagement was key to success. Three households required relocation utilising a process of coopera- tive relocation. Community consultations highlighted the need to construct a pedestrian bridge across the spillway and provision for safe access across the dam wall. The agricultural lands within the basin also required compensation, a com- plex task where there are no title deeds. The overall contract, which included pipelines and pump stations, employed 109 local people for 17 900 person days, with R 5.2 million being spent on local wages and contractors. Possibly the major factor that led to project success was having an experienced Engineer’s Representative on site and experienced Dam Safety Office Approved Professional Person responsible for design, together with the teamwork between the professional team and the contractor. Dams are unusual structures with unusual requirements, hence there are relatively few people with experience in the design and construction of these structures. The experienced professional team was able to provide the requisite guidance to solve issues on site, accom- modating the contractor’s preferences while maintaining the design intent.  Water could reach velocities of 90 km/h in the spillway channel The dam has a 1:3 upstream slope with a 1:2 downstream slope