Civil Engineering December 2021 | Vol 29 No 11

SANRAL SA I CE 2021 Nat i onal Awards Category: Techn i cal Excellence T he Xhora Off-Channel Storage Dam provides assurance of supply to the Xhora Water Supply Scheme which is intended to provide potable water to 70 000 people in the Elliotdale District of Mbhashe Local Municipality within the Amathole District Municipality. The source of the water is the Xhora River with water being pumped to the dam from a diversion weir. The Xhora Off-Channel Dam is a 29.4 m high, 230 m long, zoned earthfill embank- ment dam. A double-barrelled diversion culvert was constructed to allow for flood water during construction. The diversion culvert was used for the outlet pipework on completion of the closure of the culvert. The 25 m long ogee side channel spillway is located on the right flank with a 5 m wide trapezoidal spillway chute, widening to 9.5 m before entering the stilling basin. There is a pedestrian bridge across the spillway to serve the local community. Due to the high cost of the pumped water from the diversion weir on the Xhora River, a comprehensive grout curtain was constructed to minimise leakage. The Category 2 dam is located on an unnamed tributary of the Xhora River and has a 6 km 2 catchment while the reservoir has a gross storage volume of 2.7 million m 3 . The spillway has been designed for a routed 1:100 return period flood of 87 m 3 /s and Safety Evaluation Flood of 165 m 3 /s. EMBANKMENT The dam has a 1:3 upstream slope with a 1:2 downstream slope, and an access road which slightly reduces the effective downstream slope. The crest is 7 m wide, allowing for road traffic and pedestrian access across the dam. The outer zones of the dam, or shells, which are identified as the zone 2B materials, are constructed from the less weathered sedimentary rocks within the dam basin. These were also used to construct the cofferdam. The central core, which acts as the low permeability zone of the dam, is constructed from the zone 1B/2A materials, and consists of the alluvial and colluvial materials within the reservoir basin, as well as the more weathered upper horizon of the siltstones and mudstones in the basin. For backfilling of the cutoff trench a more flexible zone 1A material was imported from a decomposed dolerite quarry 8 km away. The cutoff trench extends through the surface materials on which the dam is constructed down to competent rock. The low hydraulic gradient (flatter than 1:1.5) through the core, combined with a gradual transition between the zones, disposed of the need for a sepa- rating filter zone, but good drainage of the downstream shell was provided by a filter/ drainage blanket between its base and the floor of the dam. GROUTING It was necessary to ensure a relatively low permeability of the dam’s foundation. The grouting design required the primary holes to extend to a depth of up to 30 m. This is deeper than what would normally be implemented, but zones of leakage were detected to this depth. COMMENDATION Technical Excellence category (Sponsored by Maccaferri SA) KEY PLAYERS Client: Amathole District Municipality Consultant: Hatch Africa Contractors: Stefanutti Stocks Mfuraa Consortium Xhora Off-Channel Dam 22 December 2021  Civil Engineering The completed Xhora Off-Channel Storage Dam