Xhora Dam showcases design innovation

Located in the Elliotdale District of Mbhashe Local Municipality in the Eastern Cape, the Xhora Off-channel Storage Dam was built to give greater security of supply to the Xhora Water Supply Scheme (Xhora WSS), which provides potable water to 70 000 people.

Earth filled embankment dam

Materials are very important to the design and construction of an earth embankment dam. Hard rock and crushed rock products are required for the rip rap, as well as filters and drains within the dam. Therefore, a hard rock quarry was developed within the basin primarily utilising the igneous granophyre rock with the overburden indurated siltstone.

The embankment was constructed with different zones, where the central core acted as the low-permeability zone of the dam – available in the reservoir basin. However, materials available in the basin had a fairly low plasticity so, for backfilling of the cut-off trench, a more flexible clay material was imported from a decomposed dolerite borrow pit about 8 km away.

Construction

Dams can cause loss of life and extensive damage should they fail, and a spillway is a key component of a dam for the safe discharge of flood waters. A 25 m long ogee side-channel spillway on the right flank with a 5 m wide channel was constructed.

It has been designed for a 1:100 return period flood of 87 m 3 /s and safety evaluation flood of 165 m 3 /s. The spillway discharge curve was obtained using computational fluid dynamics software. Due to the proximity of the water treatment works downstream of the spillway, the design of the stilling basin needed to be accurate.

The novel design was developed and model tested by Stellenbosch University. The dam was fitted with outlet pipework, the stilling basin, twin box culvert as part of the river diversion works, water abstraction facilities and associated pipework, as well as a pedestrian bridge across the spillway for the local community.

A double-barreled diversion culvert was constructed to allow for floodwater during construction. The diversion culvert provides passage for the two outlet pipes, a domestic supply to the water treatment works, and for emergency emptying of the dam, as well as making provision for environmental releases. On completion of the dam, the culvert was closed through the installation of two sets of precast reinforced concrete planks, with the void between the planks filled with mass concrete post-grouted through a tube manchette system.

In the initial design process, the team debated how to abstract water for domestic consumption. The preferred water for domestic consumption should be drawn from the surface waters of the dam, which are oxygenated.

Typically, an intake tower is constructed with inlets at various levels to be operated so that water can be abstracted close to the surface as the reservoir level moves up and down. As the water demand on the scheme is relatively small, the cheapest solution was considered to be a floating intake.

The floating intake structure consists of a stainless-steel float filled with foam and anchored in place with chains that are connected to concrete blocks on the floor of the reservoir. The intake screen is attached to the float with a flexible steel-reinforced mining hose conveying water to the outlet pipework through the river diversion culvert.

This resulted in a substantially cheaper intake structure when compared with an intake tower. Due to the relatively shallow depths in the dam, it is a fairly simple process to replace the intake system, after approximately 25 years, utilising divers.

Grouting

A substantial portion of water in the dam is pumped from the Xhora River via a diversion weir. This makes the water rather expensive; it was therefore necessary to ensure a relatively low permeability of the foundation. Geotechnical investigations revealed that while the foundation rock was generally of low permeability, there were areas with moderately weathered rock and zones of high permeability.

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. The grouting consists of drilling holes into the rock foundation, pressure testing with water to determine if there are leakage paths within that section or stage (measured in lugeons), and then pumping in a cement grout to fill leakage paths and ultimately to backfill the hole.

The grouting was carried out in 4 m to 5 m stages, depending on the depth, into the rock. To avoid the need to construct and grout through a wide and anchored reinforced concrete pad, a less-used approach to the grouting was adopted. To allow for a higher first-stage grouting pressure and produce greater penetration into the fine fissures.

PROFESSIONAL TEAM

*Design, construction, monitoring and commissioning: Hatch

*Contractor: Stefanutti Stocks Mfuraa Consortium

*Geological and material investigations: Terreco Geotechnical

*Environmental: Pollution Control Technologies

*Institutional and social development: Thetha

*Health and safety: Sange Institute of Health and Safety

*Stilling basin design and model testing: University of Stellenbosch

*Blasting and quarry: Baydrive Mining and Civils

*Grouting and rock anchors: Wepex Concrete batch plant: Lafarge

*Joint laboratory: Road Lab