2009 – The internal investigation of conduits through an earth dam

Tim Logan, Angus Swindon, Chris Topham

Edgar Dam is a 17m high saddle dam forming part of the Gordon River Power Development (GRPD) in south west Tasmania; the smallest of three dams, which created the current Lake Pedder. It is essentially a homogeneous embankment, designed and built between 1970 and 1972. It is assigned a “High A” Hazard Category. An unusual feature of the dam design is a reinforced concrete facing on the upstream face, crest and the upper portion of the downstream face provided primarily as protection against wave overtopping.The upstream facing is bedded on drainage material encapsulating a longitudinal drain 1.6m above the level of the bottom of the concrete. This drain is connected to four transverse drains (100 mm diameter PVC) which run through the body of the dam and discharge through the concrete slab on the downstream face. The screening level risk assessment for Edgar Dam identified piping through the embankment as the predominant failure mode, particularly related to the transverse drains and the uncertainty surrounding the competency of the backfill around the pipes. To address this, the condition of drain backfill has been assessed using geophysical logging, supplemented by an internal video inspection. The information has allowed a more detailed risk assessment to be performed and potential mitigation measures to be assessed.

Keywords: Risk Management, Dam safety, Conduits, Geophysical Logging.

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