CMC was awarded the Design & Construct contract by Clarence Valley Council to reconstruct the failed Shannon Creek Dam Tailwater structure, which had failed after two previous overtopping incidents following large storm events.
Shannon Creek Dam provides the main water supply for both Clarence Valley and Coffs Harbour City Councils, and the Tailwater structure is crucial to the overall main dam’s safety.
The D&C contract involved taking the principal’s gravity wall concept design through to final design and then construction, however upon undertaking preliminary geotechnical investigations together with flood event modelling it was quickly realised that council’s concept gravity wall design would not achieve either the design life criteria or future predicted overtopping events, plus there was likely failure of the existing abutments.
The CMC design team then worked proactively with council and their specialist dam consultants, to propose several alternative designs, each to meet the design requirements whilst considering constructability in the tight site constraints. The preferred alternative design was priced and negotiated with council in an ‘open book’ approach prior to award of a contract variation to complete that design and construct the replacement Tailwater Dam Structure. The resultant structure incorporated unique perforated concrete walls with anchored buttress supports and strengthened abutments.
Innovation: The proposed alternative design solutions incorporated close collaboration with CMC’s design team and construction team to produce the best outcome, including resource planning, formwork design, crane access, site access, on-site pre-casting, and management of sub-surface water flows.
Collaboration: With the requirement of a complete redesign, the best outcome for the project was achieved, through the close and open communication with council and their consultants, and the ‘Open Book’ approach to the contract variation.
