2013 – Uplifting foundation experiences: borehole blowout, uplift concerns and foundation modelling at Clark Dam

Lyndon Johnson, Alan White and Chris Topham

The integrity of foundation drainage systems is a key factor in minimising uplift pressures under concrete gravity dams. Contemporary industry practice for foundation drainage systems (and modern criteria presented in the imminent release of the concrete gravity dam guidelines) will lead owners with older concrete dams to consider enlarging foundation drainage systems via borehole drilling in the foundation. This paper presents the cautionary tale of a dam owner that undertook foundation drilling works in the gallery of a 67-m high concrete gravity arch dam and experienced borehole “blowout” in one of the drilled holes. Water under 90% of full reservoir head issued from the borehole and needed to be controlled. The context of the works is presented, followed by a description of the blowout, the risk mitigation measures that were planned prior to the work, and which ultimately had to be initiated. Management of the incident is discussed, including the use of blowout protection collars and valves, subsequent investigatory drilling, and pressure grouting programme. Dam safety concerns associated with the incident and their management are presented. The paper concludes with some recommendations to manage these risks for other owners considering a drilling programme in a concrete dam foundation.

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