Rottnest Island Wastewater Pumping Station, Sewer Replacement and Pressurised Fire Main Upgrade

September 2019

DM Civil was engaged in April 2019 by Rottnest Island Authority in a contract to construct a new wastewater pumping station (WWPS) and sewer reticulation on Bedford Avenue in the main settlement. The requirement for this infrastructure upgrade was driven by the plan to expand the existing Hotel Rottnest by adding an 80 room accommodation wing overlooking Thomson Bay. This work is currently underway by the builder and provided a critical date for commissioning of the WWPS.

The existing settlement is a congested mix of holiday accommodation units, commercial premises and a road network carrying countless tourists within the main hub and to and from the island’s other attractions. The existing underground infrastructure is an aging matrix of gas, electrical, bore water, potable water and communications pipework and wiring – the alignments and depths vary and do not necessarily conform to the rigid standards of utilities installed on the mainland. This is typical of some of the older areas of Perth and Fremantle where standards evolved since the early 1900’s and presents a challenge when installing new services.

A key factor in the decision to award the contract to DM Civil was the demonstration of our capability to work in such an environment without interrupting the supply of these services. Rottnest Island is an important and growing tourist attraction for Western Australia. Even in the winter months, tourist numbers have proven to be high, partly because of the historic as well as natural attractions but also because our relatively mild winters are no barrier to international tourists. A unique natural feature of Rottnest Island is the Quokka population. These curious marsupials are gaining international fame since Roger Federer and several other famous identities posted ‘Quokka Selfies’ to the delight of their fans who now insist on the same when visiting the island. The proven ability to carry out our works without disturbing the island’s tourist traffic was another factor in the award of the contract to DM Civil. This was largely facilitated by our offer of cost-effective trenchless installation of the sewer main. Protection of the Quokkas by eliminating trenching and isolating drill pits with sand bags on fencelines as well as ‘Quokka Ladders’ as an escape contingency was seen as a big positive by the selection panel.

The wastewater pumping station installation was another challenge. At over 5 metres deep, the base of the excavation would be in groundwater, which was approximately 2 metres below existing ground level. The excavation would need to be dewatered to allow for installation of the concrete structure and a strict rate of extraction of groundwater needed to be adhered to for compliance with Department of Water regulations. To limit the groundwater extraction rate, DM Civil proposed the use of a sacrificial steel caisson and a concrete plug as a contingency against base heave. The actual installation proved to be favourable in terms of groundwater extraction rates and only metal bedding was required, providing a programme shortcut. The pumping station was commissioned without incident and is now operating.

A further scope of works awarded to DM Civil was the upgrade of the pressurised fire main. The existing main is limited in coverage and was originally a simple gravity feed, unsuitable for the growing resort. The new fire main configuration will increase coverage and reliability of the firefighting network. Installation of this network was also carried using trenchless techniques, thereby virtually eliminating any interface with tourist visitors and bypassing existing services.

DM Civil has succeeded in delivering this infrastructure upgrade in complying with the project quality scope and as importantly, in minimising disturbance to tourism, businesses and existing infrastructure. We look forward to successful completion in the coming weeks and a satisfied client.