2016 – Root Cause Analysis (RCA) in Dam Design

Malcolm Barker

This paper will present the use of Root Cause Analysis (RCA) as a means of evaluating the causes for failure modes and is based on work completed for an upstream tailings storage facility (TSF) raise where significant transverse and longitudinal cracking was observed.
The design of the TSF was based on the use of a starter wall with perimeter discharge from spigots spaced at about 25 m centres along the upstream crest. The TSF was raised using an upstream design and during routine inspections two years after completion of the raise, transverse cracks of up to 30 mm were noted on the crest and longitudinal cracks up to 40 mm width were noted on the downstream slope of the raised embankments. Concerns were raised over the extent and depth of the transverse cracking and the risks they pose to piping, seepage and containment.

Field investigations including test pitting and material testing were completed to evaluate the depth and extent of the cracking. The findings from field investigations, together with a review of the historical aerial photographs and superposition of the cracks and the locations of the spigots were then used in a Root Cause Analysis workshop.

Discussions on all causes for the cracking, asking the question “why did the problem occur?”, and then continuing to ask “why that happened?” until the fundamental process element that failed was reached”.

During the workshop, the most significant contributors for the transverse and longitudinal cracking and the likely location, extent and size of the cracks were evaluated. This identified the potential for traditional structural hog and sag bending moments causing the transverse crest cracking with the potential for transverse cracking at the interface of the raise and the original tailings. This was not previously identified as a potential piping location. The longitudinal cracking was considered to be mainly owing to settlement of the upstream tailings.

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