As significant construction occurs throughout the Pimpama Coomera region, in the northern Gold Coast, trenchless methods are being used to deliver the key water infrastructure required under the international award-winning Pimpama Coomera Waterfuture Master Plan.

The Master Plan is an innovative integrated urban water cycle management program that aims to reduce the use of potable water in the Pimpama Coomera region by up to 84 per cent. Under the Master Plan, all new houses in the region will be plumbed to receive three sources of water: potable water, rainwater and Class A+ recycled water. Stormwater and wastewater management is also addressed under the Master Plan.

Spanning a 7,000 hectare region in the city’s north, the population of the Pimpama Coomera region is expected to grow from 15,000 in 2007 to more than 120,000 in 2056. With such a rapid population boom expected, Gold Coast City Council recognised the opportunity to establish a community with positive attitudes and behaviours towards recycled water and other water conservation initiatives.

Construction of the Master Plan infrastructure began in October 2006. Currently under construction are the Pimpama Wastewater and Recycled Water Treatment Plants, which will begin sending Class A+ recycled water to dual reticulated homes in Pimpama Coomera by the end of 2008. Construction of additional potable water, wastewater and recycled water mains, reservoirs and pump stations will continue until end 2009.

The Pimpama Coomera Alliance, an alliance between Gold Coast City Council, Gold Coast Water and MWH, is program managing the delivery of the critical infrastructure, including the treatment plants and networks that will carry wastewater, Class A+ recycled water and potable water throughout the region.

To deliver the $190 million of infrastructure required for this project, four works packages have been established. These packages and infrastructure delivery partners include:

Package A – Pimpama Wastewater and Recycled Water Treatment Plants: WaterCycle Alliance (United Group, Australian Water Services and John Wilson and Partners)

Package B – Trunk Wastewater Main: BMP Alliance (Tenix/GHD)

Package C – Outgoing Recycled Water Pipeline: Pumps and Pipes Project, Managing Contractor (Abigroup/SKM)

Package D – Water and wastewater network mains, pumping stations and reservoirs: Pimpama Coomera Waterfuture Master Plan Implementation Package D Managing Contractor (Abigroup/KBR)

Construction Packages B and C are both utilising trenchless methods, including directional drilling and microtunnelling, to install sections of pipeline.

Infrastructure Delivery Package B: Trunk Wastewater Main

The Beenleigh Merrimac Pimpama Alliance (BMP Alliance), consisting of Gold Coast Water, Tenix and GHD, is installing a 4,820 metre trunk wastewater main. This main will carry the region’s wastewater to a gravity sewer at Cunningham Drive that connects to the Pimpama Wastewater Treatment Plant. In total, the BMP Alliance is tunnelling 1,490 metres, using microtunnelling and directional drilling to install a new gravity main and a rising main in this region.

A 1,231 metre long, 900 mm ID gravity sewer main is currently being installed along the northern end of Dreamworld Parkway, Coomera. This gravity sewer will connect into an existing major pumping station (PA09) at Foxwell Road.

Space restrictions, the location of existing optic fibre, water and telephone services and the need to minimise above ground disturbances, were major factors that influenced the decision to use thrust boring techniques to install the gravity sewer in this area. By installing the pipe below these services, this method was effective in preventing any additional costs and further disturbances and delays of relocating these facilities.

Commencing works in March 2007, the BMP Alliance used an Iseki Microtunnelling Boring Machine to install 900 mm diameter Watani polycrete jacking pipe for this trunk gravity sewer. The microtunnelling was performed from ten thrust and receiving pits of up to 9 metres in depth.

Ground conditions along Dreamworld Parkway and Service Road were predominantly clay and soft rock. Bentonite slurry was injected at the cutting face of the tunnelling machine to lubricate the drill head and to assist with stabilising the open face of the tunnel. The slurry and excavated earth were removed from the tunnel via a return line and the bentonite and earth slurry separated to allow the bentonite to be reused. A vacuum truck removed the waste drilling slurry from the site. An experienced operator with a sound knowledge of the ground conditions was on hand to closely monitor the progress.

The BMP Alliance is also installing a 500metre rising main, 277 metres south of Dreamworld that will connect to the gravity sewer. The OD 160 PN16 rising main is being installed via directional drilling along the southern section of Dreamworld Parkway in close proximity to the local primary school. The decision to use directional drilling methods in this area was made to minimise the construction impacts on local residents, tourists and traffic from Dreamworld and local schools.

Infrastructure Delivery Package C: Recycled Water Pipeline

Pumps and Pipes Project is responsible for the design and construction of the program’s major recycled water pipeline. The 9.5 km Pimpama to Coombabah recycled water pipeline will connect the Pimpama Wastewater Treatment Plant to the Coombabah Wastewater Treatment Plant to release any recycled water not used within the community, via the existing Coombabah system.

Commencing in early 2007, these works have predominantly consisted of open trenching through green field development sites and existing road verges. However, in order to connect the pipeline from Coomera to Coombabah, directional drilling methods were used to tunnel under the Coomera River. The drilling took place along an easement on private property and consultation and engagement with the property owner started seven months prior to drilling.

Special challenges

Site specific problems included the fact that boring under the Coomera River was necessary, and equated to 75 per cent of the bore length; the prevalence of saltwater in the ground; very soft ground with occasional gravels, and the use of large diameter pipe.

Construction of the project

Abigroup Contractors engaged UEA to undertake the Coomera River bore project. To assist in working to the required tolerances, a surveyor was engaged to peg out the route and verify the profile. Once this was completed the information was entered into the bore planner software and a profile was created.

Pilot bore

UEA utilised its 25 tonne Vermeer D50 x 100 machine to undertake the pilot bore. The ground conditions varied between high plasticity clay, sand, silts and gravels. The geotechnical information supplied by the client was very accurate and enabled the drill muds to be managed effectively through the continually changing ground conditions. With the varying ground conditions, Baroids’ (mud doctor) experience was utilised to develop a successful mud plan prior to the project commencing, which proved to be invaluable during the entire pilot, reaming and pipe installation process.

Due to the saltwater, conventional walkover locating systems could not be used. As a result of this, UEA utilised its own Digital Control Incorporated “steering system” to undertake the pilot bore. All information from the bore was recorded onto a software program to verify the position of the bore and provide data for the client. The initial bore was used not only as an exploratory tool, but also to provide a 100 mm return line which would later be used to return drill fluid from the pipe side of the bore back to the drill.

Once the drill fluid return line was completed, a second pilot bore was bored parallel to and approximately 2 m from the return line in preparation for installation of the 630 mm pipe. Following its completion, the rods were left in place until the larger Vermeer D300 x 500 (136 tonne) HDD arrived at site.

Pre- ream

The D300 x 500 utilised a cross over to connect to the D50 (3 ½ inch) rods remaining in the hole and the smaller rods were pushed out and replaced by the larger 5 inch rods. The D50 rods were run out over the ground as they were to be used as a dead string during the pre-reaming process where an 850 mm flow through reamer was attached and pulled into place. The D50 rods were then attached to the back of the reamer via a swivel to ensure the hole would not be lost at any point during the pre-reaming process. The rod time for the pre-reaming process was approximately 40 minutes per rod with the “bottom up” (time it took for the mud to reach the surface) taking approximately 15 minutes per rod. The pre-reaming process was completed in two days with good returns at all times.

The D300 rods were pushed back out with the help of the dead string attached to the D50 on the opposite side in preparation for pull in.

Pipe installation

The pipe to be installed was a 630mm SDR 9 and weighed approximately 40 tonne. In addition to this, the pipe had to be filled with water to create neutral buoyancy and prevent the pipe lifting in the hole and cutting through the surface. Approx. 55,000 litres of water had to be added which increased the total weight to nearly 90 tonne. To aid with pullback, three 30 tonne excavators were utilised. The pullback process took 14 hours, after which the pipe was installed whilst ensuring that pullback pressures did not exceed 20 tonne at any time during the installation process.

Outcome

The project was a complete success for all parties concerned, with no major incidents occurring. The project also highlighted the advantage of the client supplying reliable and accurate geotechnical information prior to the commencement of works, and working the solution through with the drilling contractor prior to work commencing on site. The ground conditions were found to be very accurate when referring back to the geotechnical information provided at tender stage, enabling UEA to make the correct decisions with regards to the mud program, tooling and the type of machine required to undertake the project. The pipe was installed ahead of time in preparation for the connections to the open trenched sections either side of the river.

UEA received excellent feedback from the client with regards to the quality of the work undertaken, the project management and timing.