The pumping station site sat at 9 metres below ground level, in poor soils. When excavations started Sydney Water hit the water table at around 3 metres in depth, meaning increased risk of subsidence in the pit walls and inundation at the base of the excavation.
Sydney Water contracted Coates Hire Engineering and Technical Services, to design, build, test and install a shoring system that would hold the sheet pile walls of the pit, while also dewatering the excavation site so the works could be carried out.
Temporary Works Engineer at Coates Hire Engineering and Technical Services Team, Darren Browne, says the pumping station at Wolli Creek was a complicated site which needed to be secured before workers could safely operate.
“There was nothing simple about the site due to the confined site boundaries, a number of existing services both overhead and below ground, the deep excavation depth and extremely challenging soil and groundwater conditions,” says Browne.
He says Sydney Water had to re-scope the project as they gained more information, but Coates Hire’s Mega Brace shoring system was adaptable to changing conditions. The pit walls were held-up by four Mega Brace hydraulic braces, installed incrementally as the excavation proceeded, creating a wall that was supported during the work.
The groundwater was controlled with a large spear-point system, installed to achieve sufficient suction lift at the required flow rates.
Sydney Water Construction Co-ordinator Colin Burrell says due to the soil quality and high water table at the site, the excavation required a fit-for-purpose shoring and dewatering design to ensure that the work could be carried out safely and economically.
“We had groundwater sitting at 3 metres below the surface, which the wall had to retain. The dewatering system needed to keep the pit dry at all times,” he says.
He says the project was completed without customer complaint or disruption to wastewater services. “The Coates Hire team was very professional and they were able to adapt their designs and their approach according to the new conditions. They allowed us to push forward on the project knowing that the shoring and dewatering issues were being dealt with.”