Dam Wall Structure- Maguga Dam
Three rockfill embankment dam options were provided for at tender stage, i.e., a central clay core, central asphalt core and upstream concrete face. The central clay core rockfill embankment was selected as the preferred option by most Tenderers and was constructed. The main embankment and saddle embankment both had similar sections. Filters were provided for both upstream and downstream of the clay core and seepage through the core is monitored at four flow monitoring stations downstream of the embankments.
The flow monitoring stations downstream of the main embankment are provided with flow monitoring instrumentation. The flow monitoring station downstream of the saddle dam is provided with a pipe at which manual flow measurements may be made. The spillway consists of a reinforced concrete labyrinth and concrete lined spillway chute. Water plunges from the flip bucket at the bottom of the concrete chute into a plunge pool before flowing into the Komati River.
Access and drainage galleries have been provided longitudinally below the labyrinth floor and beneath the chute floor on both sides. Transverse drains that drain towards the galleries are provided below the concrete chute lining. Seepage is monitored at flow monitoring stations near the downstream end of the galleries below the lower apron slab.
The outlet works consist of a free-standing intake tower over a shaft with two bottom outlets and eight multi-level intakes at 7.5m separation. The outlet tunnel and river Outlet Structure house two 1.8 m diameter penstocks. The intakes are provided with 1.8m dia butterfly valves to enable selection of the appropriate water draw off level. The volume of releases into the river from each penstock is controlled by a 1.8m dia and 400mm dia fixed cone sleeve valve. The penstocks each bifurcate upstream of the river Outlet Structure to convey water to the power station.
The Instrumentation Installed at Maguga Dam includes:
- Total Pressure Cells. These monitor the pressure in various zones of the embankment, particularly the contact between the core and rock or concrete foundation and total pressure within the clay core of the embankment.
- Foundation Piezometers. These monitor the water pressure within the foundation rock.
- Open Tube Piezometers. These monitor the water pressure within the rock foundation downstream of the toe of the dam.
- Embankment Piezometers. These monitor the pore water pressure within the clay core.
- Settlement gauges and inclinometers. These monitor the settlement in the clay core and movement of the clay core in an upstream/downstream direction and transversely along the centreline.
- Seepage Flow gauges. These monitor seepage through the clay core, through the foundations downstream of the clay core and downstream of the spillway grout curtain.
- Settlement beacons. These monitor vertical settlement and lateral movement at various points on the embankment. Accurate surveys are required to establish the movement.
- Extensometers and settlement gauges. These monitor relative settlement and relative movement of the filter layers and downstream rockfill shell.
- Water level recorders. These monitor the water level in the dam.
