Design and Construction of Sand Dam to Float Dredge over Buried Pipelines

Mineral sands mining by dredging is carried out within the Tomago Sand-beds aquifer, which is also used to supply potable water to the Newcastle region. In order to transfer a floating dredge and separator plant over twin 1 m diameter water supply pipelines, a large temporary lock system was constructed of sand, comprising 4 m high dam walls. The water level was raised to 3 m above original ground level to enable the mining plant to float across the pipelines.

Engineering design and construction of the lock system was required to prevent damage to the strategically important pipelines. The design included slope stability and seepage analyses as well as analysis of stresses imposed on the pipeline by the embankment. The necessity to control stress on the pipeline required spanning over the pipeline with a steel plate structure underlain by a void. The presence of the void within the embankment placed particular emphasis on the control of piping, which had potential to cause both filling of the void and embankment failure. After consideration of both cut-off walls and lining systems, piping was controlled using a combination of sand and gravel toe berms underlain by filter fabric , as well as baffles along the steel plate. A void was maintained below the steel plate utilising reinforced filter fabric.