Retrofitting a pressure relief system to a tanked basement

A multi-storey development situated in the Botany Sands aquifer in the Eastern Suburbs of Sydney was constructed in the early and mid 2000’s in two stages. The lowermost 3rd level basement was designed as a partly tanked system based on an initial 5 months for the first stage then a subsequent 12 months of groundwater monitoring for the second stage.

A few years after completion the local groundwater table began to steadily rise, ultimately peaking at 1.5 m above the level of tanking. This resulted in a significant ingress of water through the upper basement walls and a significant increase in the uplift forces on the basement slab.

This paper summarises the challenges posed in developing and implementing a solution, the design alternatives and their strengths and weaknesses and the approvals process required. The adopted solution was a perimeter network of pressure relief outlets that were designed to intercept increased groundwater levels and limit the ingress of water and subfloor pressures to the original design level. The outlets were custom designed to be readily installed with minimal impacts to the existing structure, provide minimum outflow flow and be readily inspected and cleaned. The intercepted groundwater is collected in a separate system of collection pipes and discharged to other areas of the site for recharge. Any excess water that cannot be recharged is collected for offsite disposal.

The system was constructed in 2019 and has been engaged since. To date the system has worked as intended allowing the continued use of the basement levels for residents and the public.

This is the first publication on this system and the format of this conference does not allow the investigation, design, construction and operation to be explained in significant detail. It is planned that a journal publication will be written to explain this information in greater detail.