Water build-up behind retaining wall at strata carpark- Hunters Hill


Last December Stormwater Sydney was contracted for remediation work at a townhouse development in Hunters Hill with water build-up behind the retaining walls. The current ag line was chocked, and no gravel or geo fabric was evident.

To resolve this, we installed a new stormwater line with drainage/access pits that extended into the retaining wall allowing the pipes to be flushed out for future maintenance and drain to the building’s stormwater line.


The underground car park to the townhouse development exhibited water build-up behind the retaining walls. An opening had been created to monitor the problem. The existing ag line was choked, with no gravel or geotextile (filter) material being evident.

Over time sediment from the natural rock had washed down and couldn’t be removed efficiently or economically. The building’s structural engineer had requested that no more large openings be made, preventing access and cleaning of the void.

Stormwater works:

To resolve this issue, we proposed that at every 6 linear meters, drainage/access pits be cut out into the car park floor and be extended into the retained area behind the wall. This allowed the existing ag line to drain into these pits as well as being a flushing point for future maintenance.

The concrete retaining wall remained intact, with openings created at each pit location. These were located below the car park floor level and were filled with aggregate. The ag lines were extended to reach every pit for future cleaning. The pits were be installed to match the car park level. All four new pits were linked via a 100mm ag line draining to the existing stormwater system.

A louvre was also installed on the existing opening with a safety barrier installed inside the cavity to address falling debris. This barrier was designed on-site.

Stormwater Sydney provides design, management, and consulting services across Sydney. Inspection, design, cleaning, maintenance, repairs, construction, and advisory on stormwater & WSUD systems.

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