Tunnelling Under Mosman – Excavation & Demolition
- Client PBS Building
- Location Mosman
- Duration 11 months
By Daniel Green
Recently, Perfect Contracting was contracted to excavate a pair of tunnels under a residential property at Sydney’s north shore. Built to give easy access to the waterline, the 51m long passage would link to an 18m deep vertical shaft that would soon house a lift. Due to the limited property access, anything of bulk had to be moved by barge. This included any machinery and all waste.
Utilising one of our Brokk 200’s, a Perfect Kubota 2.5t excavator and scores of bulker bags, excavation began in earnest in May 2021. Initial works were focused on concrete cutting and demolition in the existing house to accommodate drilling the vertical shaft. Once this was complete, that task was handed over to Rock Breaking Solutions. Perfect began work on the horizontal shaft.
Perfect has completed thousands of residential demos to the point where we could almost do them with one hand tied behind our back. So when PBS Site Manager Bill Brier took Perfect PM and Engineer Ondrej Krpalek through the scope of works, the astute Czech was super enthused to dig in. Literally. You see, not only were there a multitude of propping and engineering considerations to wrap his ample brain around, but the owner had stipulated retrofitting a 19m tall internal lift complete with panic room. If you do the maths, that’s more than six storeys from street to sea. This would tee into an equally fictional, more horizontal shaft that led out to the picturesque Cobbler’s Bay. Except the second tunnel was some fifty metres long.
Due to the proximity of the tunnel to the harbour and the need for constant mist for dust suppression, a false floor was built at the tunnel entrance consisting of multiple layers of geofab. The floor was also ringed with a watertight lip to catch any runoff. Using the speed and efficiency of the Brokk, we began tunnelling toward the vertical shaft. Spoil was loaded into bulker bags with the excavator and stored until the weekly load out onto the barge.
Several additional factors has to be accounted for. The residential location meant a low tolerance for vibration and noise. The tunnel design was considered a confined space and was quite restrictive in terms of machinery. Both the Brokk and the Kubota couldn’t be turned around inside, they had to be tracked to the entrance and re-enter. Exhaust fans extracted dust and waste air with air monitoring remaining in constant check.
It was several months of work until both teams broke through, linking the shafts. Detailed excavation was then completed, with handover occurring soon after.
Perfect would like to thank PBS, Polaris Marine and Rock Breaking Solutions for the opportunity.
We Get It Done.