Cultana seawater pumped hydro-electric energy storage project – geotechnical optioneering design

This paper presents the concept geotechnical design carried out for the optioneering stage (feasibility stage business case) on the Cultana Seawater Pumped Hydro-Electric project. The site is located 10km south-west of Port Augusta in South Australia. The site spans east to west from the banks of the Spencer Gulf (RL +2m AHD) to a hill located west (RL+250m AHD). The site is underlain by varying geology ranging from Alluvial sands more than 30m thick near shore to interbedded Shale, Sandstone and Quartzite formations located within 2m of ground level at the hill.

The objective of this project is to provide between 100MW and 250MW of dispatchable peak electricity to the National Electricity Market in South Australia, as well as providing a range of grid support and ancillary services.

This paper discusses the geotechnical concept design for a 40m deep bunker style powerhouse structure located nearshore, a 3.4km onshore transfer pipeline connecting the powerhouse to the hill penstock pipeline, the penstock pipeline that transverses up and down the steep hill to a height of approximately 220m above ground level and a 2.9GL reservoir located on a flat plateau at the top of the hill.

Each of the four structures presented above had its own unique geotechnical risks relating to dewatering, foundation design and reservoir wall design. The powerhouse structure required a permanent retention system that was fully waterproof. A diaphragm wall was considered the most appropriate solution. The transfer pipeline intersected highly variable geology with sections comprising deeper soils, shallow caprock and bedrock. Trench excavations with rock breaking or hydraulic excavation was recommended. The penstock pipeline loads, and its serviceability criteria required deep foundations. The steep hill slope made access using typical piling machinery a challenge. A group micropile option was considered the safest solution. The reservoir was designed for daily rapid drawdown which would result in potential wall instability and erosion. A crushed sandstone wall with a clay core was designed to mitigate this risk. Each option was selected based on hydraulic considerations, cost effectiveness, constructability and safety in design.