Mt Eliza Reservoir
Project Summary
Perth, WA
Trenchless Technology
Water Corporation
- Completed
July 2015
November 2015
Project Overview
This project required the relining and modification of the main reservoir outlet of pond No 2 at the Mt Eliza reservoir complex in Kings Park.
This project required the relining and modification of the main reservoir outlet of pond No 2 at the Mt Eliza reservoir complex in Kings Park. DM Civil was awarded the contract to install an HDPE outlet within the existing DN1400 mild steel concrete lined pipework. This was one of the first main reservoir outlet structures to be upgraded within the Perth metropolitan area using this technique.
DM Civil drew on previous experience to propose an alternative to extrusion welding for the installation of the HDPE pipe system. The initial contract scope required the installation of the pipe segments and bends sequentially joining them by extrusion welding from within the confined pipe space. DM Civil proposed an alternative, using a butt fusion welding machine where the weld times could be reduced by half while increasing the level of quality control.
The originally scope was made more complex when taking into account the pipe size of DN1200. Creating a safe and functional welding area 6 metres below ground for a butt welding machine of this size was critical to the success of this methodology.
Meticulous planning was required for multiple shutdowns and isolations to avoid interruption of the water supply to Perth’s western suburbs and CBD area.
Significant achievements and benefits
The cavernous limestone ground conditions and water charged ground required specialised trenchless technology equipment. We used our microtunneling machines with Rockman head to ensure each drill shot would be to the required accuracy specified by the client.
Trenchless technology drastically reduced the amount of dewatering required, which in turn allowed for Department of Water clearance for the project to proceed.
The sewer alignment was in the middle of Fawcett Road, with homes on both sides of the road. If this project was constructed using open cut excavation, deep trenching up to a depth of 11m would have been necessary. However, the use of microtunneling eliminated any residential access issues along the route.
The microtunneled pipe was launched inside 4.6m diameter sealed, reinforced concrete caisson thrust and receival pits, up to 11m deep. The sacrificial caissons were constructed insitu so that access chambers could be constructed inside once microtunneling was complete. This method mitigated the risk of settlement around the caisson.
In total, 515m of DN150 and 294m of DN225 sewer was constructed with microtunneling. This was constructed in cavernous limestone ground conditions with the majority below the water table. By using the sealed concrete caisson thrust pits, the dewatering requirements were kept to an absolute minimum. This limited the environmental impacts of the project when compared to traditional open excavation.
By offering trenchless techniques for the deep sewer installation, the client was able to achieve project approvals and minimise environmental impact along the sensitive construction corridor. Residential access was also maintained.
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