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Project Profile N21LA007

Specialised Resin and High-Strength Concrete Used to Seal Damaged Groundwater Pit

Strata Building

The problem

A three-storey residential apartment complex in Manly NSW was experiencing ongoing flooding in its basement due to a damaged groundwater pit that required immediate emergency works. The leaking water in the basement was flooding up to 1m deep and impacting the building’s lift shafts, electricals, storage and vehicles.

The building is situated in a high water table location, with the groundwater level approximately 1m above the building’s 400mm thick basement slab. The building was supported on piled footings that extend well below the damaged groundwater pit. Fortunately, the building has several below basement slab pits which meant the failed pit was able to be abandoned, the slab sealed and the groundwater drainage system diverted to a different pit which had a new pump system installed to capture the diverted groundwater and pump it to the surface. The 1.8m deep x 2.2m wide damaged pit was a 1-piece precast concrete unit that had formed a crack in its side and base and was subsequently leaking the sandy groundwater. The drainage system’s original pump could not cope with the increasing volume of sandy water and eventually stopped working. A portable pump had been installed to drain the excess water, however the pump had failed and as the water was flowing into the pit at a rate of 3L per second, filling the pit in approximately 12 minutes, the subsequent overflow had created an urgent flooding issue in the basement of the property. Furthermore, it was estimated that the original pump and mobile pump had extracted approximately 15 tonne of sand when they were operational, indicating there was a large, approximately 7m3, void underneath the basement slab which could seriously impact the slab’s stability.

Prior to Mainmark’s involvement in the project, several third-party contractors had attempted to remediate the damaged pit, including an attempt to hydro-vac the water and sand in the damaged pit by inserting a 560mm diameter sleeve into a newly formed cored hole in the slab, then filling the pit with approximately 2m3 of concrete. However, the remediation attempt failed as the basement slab was weakened by the coring and the concrete filling did not seal the leak in the pit, forcing all pipework to be diverted to the adjoining pits. Mainmark was approached by the building owners to provide an urgent solution to stop the water ingress by sealing the pit and reduce the possibility of structural failure of the basement slab.

The solution

The client required Mainmark to provide a costeffective solution to stop water ingress and seal the openings in the groundwater pit slab that was to be abandoned after the slab access hatch and sleeve hole were sealed.

The Result

Mainmark worked with the design engineers and chose the specialised resin for its highly compressive tensile strength and water-displacing properties.


Mainmark injected a low-viscosity specialised resin through the basement slab and around the perimeter of the pit to minimise the flow of groundwater into the damaged pit. Mainmark was also required to undertake additional reinforcement replacement work to the pit by welding in new reinforcement steel where it had been damaged by other contractors in the slab coring process. After the slab reinforcing was remediated, Mainmark fully sealed the slab access hatch and sleeve core holes, while managing the reduced flow of groundwater into the pit. Mainmark used a high-strength 80 MPa, rapid setting concrete to fully seal and reinstate the basement slab to prevent any leakage of groundwater.

Mainmark worked with the design engineers and chose the specialised resin for its highly compressive tensile strength and water-displacing properties. The fast-acting waterproofing solution was able to seal the concrete pit slab quickly, addressing the leaks and preventing further issues from occurring; a larger and more serious basement slab failure could have led to long-term structural damage to the basement slab. During the project, Mainmark identified further ongoing water ingress issues relating to the latent ground conditions. When coupled with the existing water table and the original ground water drainage relief to the pit, which was now fully sealed, there was the risk of new leakage points forming as the new ground water levels and pressures re-established themselves. Mainmark was able to identify the source of these leaks and seal them using a specialised resin injection application system as a final part of this solution. The emergency works were successfully completed within 10 days, delivering a quick and cost-efficient solution compared to alternative remediation methods that would have required the damaged pit to be excavated and reconstructed.


Project Resources & Downloads

Specialised Resin and High-Strength Concrete Used to Seal Damaged Groundwater Pit 
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