The project is being excavated in sections, one of which is termed the Eastern Pipeline. With over 40 trenchless crossings required, the section of pipeline requires the full gamut of Trenchless Technology, including auger boring machines, microtunnelling machines, tunnel boring machines, horizontal directional drills and a type of hard rock cutting head called the Robbins Small Boring Unit (SBU-A). The SBU-A is currently being utilised on the Eastern Pipeline by contractor Winslow Constructors to bore up to ten road and rail crossings.
The WCRW project will be the largest recycled water project in the Southern Hemisphere when complete, involving a 200 km long network of pipelines linking six existing wastewater treatment plants and three new advanced water treatment plants.
Along the way, the pipeline must traverse about 80 roads and railways – with installation often requiring trenchless methods.
The project is owned by Western Corridor Recycled Water Pty Ltd, which is in turn owned by the Queensland Government. By the end of 2008, the scheme will have the capacity to provide up to 232 million litres/day of recycled water, with a potential future capacity of 310 million litres/day. The WCRW pipeline is being constructed in two main phases – termed the Eastern Pipeline and the Western Pipeline.
Eastern Pipeline
When complete, this portion of the project will bring wastewater from the Oxley, Wacol, and Goodna sites to a main water treatment plant at Bundamba. Purified recycled water will also be pumped from Luggage Point and Gibson Island into the same Bundamba treatment plant. From Goodna to Bundamba, the pipeline will convey three types of water in parallel pipes – one line will carry treated wastewater, one purified recycled water, and one will discharge treated reverse osmosis concentrate. This concentrate, a by-product of the water treatment process, will be filtered to remove excess nutrients and metals before disposal. Contracted to the Eastern Pipeline Alliance (a Project Alliance of AJ Lucas, Transfield Services, GHD, SunWater, and McCalls), the pipeline and associated structures should be complete by September 2008.
Western Pipeline
The Western Pipeline comprises of 81km of pipe that will transport purified recycled water to Caboonbah for off-take at the Tarong power station. Work began on this section in April 2007 and is scheduled for completion by April 2008.
Trenchless Technology
The selection of the trenchless method to be utilised depends upon several factors including geology and crossing length.
“Crossings range from 30 to 330 m in length,” explained Andrew Buckley, Community Relations Manager for the Eastern Pipeline Alliance.
“Auger boring machines are being used for crossings around 100 m in length, while a tunnel boring machine and microtunnelling machine will be used for longer crossings.”
Microtunnelling machines tend to work better on long crossings in soft ground and on tunnels below the water table, such as river crossings. Microtunnel pit sizes are generally 12 m in length and 5 to 12 m wide. Diameter of the tunnels is 1.5 m to allow final installation of 1,086 mm recycled water pipe.
Auger boring machines can be utilised with tungsten carbide drilling bits such as propeller and roller cone style cutting heads in soft ground and soil crossings. However, the bullet bits on these heads tend to break off in rock above 75 MPa UCS.
The Small Boring Unit: hard rock crossings
For auger boring machine crossings located in hard rock and mixed ground conditions above the water table, the SBU-A is used as a boring attachment.
A series of crossings (up to 30) require the use of two 1.5 m diameter SBU-A machines – one with a hard rock cutterhead consisting of single disc cutters and one with a mixed ground cutterhead. The machines are welded to the lead casing while an auger boring machine provides both torque and forward thrust to the cutting head.
Disc cutters mounted on the cutting head allow the machine to excavate medium to hard rock from 25 to over 170 MPa UCS. In mixed ground conditions, another type of cutterhead can be used. This cutterhead uses disc cutters, two-row carbide cutters, and carbide cutter bits with larger muck openings to accommodate soft ground and boulders.
The SBU-A, in diameters from 600 mm to 1.8 m, is typically used on drives less than 100 m in length. The machine, manufactured by The Robbins Company, utilises a full-face auger to excavate hard rock and mixed ground crossings for utility installations.
Crossing Excavation
By October, Winslow had completed three crossings using an SBU-A. The machine averages about 1 m per hour in clay and medium-strength sandstone. Each crossing, ranging from 60 to 80 m in length, takes approximately 1 week to complete, with work crews operating in 10-hour shifts per day.
“The SBU machines have performed very well thus far. They are an integral part of our fleet now and we plan to use them on future projects,” said Geoff Brewster, Manager of Winslow Infrastructure, a division of Winslow Constructors.
Each crossing must be completed to a contractual 200 mm horizontal tolerance, and a 50 to 75 mm vertical tolerance. Work crews measure the heading of the SBU as it bores using a Dutch-Level. Manually adjustable stabiliser pads located on the sides of the SBU-A allow it to be steered without pulling the auger for the first 6 to 12 m of the bore. If the SBU-A is too far from the heading then the auger is pulled from the bore and reset to keep the machine on track. To date all three of the crossings have finished within the line and grade requirements and the machines have emerged with minimal cutter wear.
The crossings excavated so far by Winslow involved three separate crossings side by side in the same bore pit, for the triple pipeline that will carry treated water, raw water, and runoff. These crossings were excavated in one larger bore pit of 12 m x 8 m. The pipelines are 1.5 m apart centerline to centerline and were installed using 1.5 m diameter steel casing in 6 m lengths. Crews used a double-continuous-feed welding machine to weld pipe lengths together, cutting the welding time from 3 to 4 hours per pipe to just 1 hour. All of Winslow’s hard rock crossings should be completed by late 2008. Once the casing is installed, a final pipeline of 1 m diameter cement-lined steel water main is placed inside.
By early October, portions of pipeline had been completed including the triple pipelines from Goodna to Bundamba. The vast majority of the pipeline being installed uses conventional cut and cover methods in alluvial and residual soils, as well as weathered rock. A trench has been dug and the pipe is buried approximately 900mm under the soil after welding and internal grouting of the pipe lengths.
Two pumping stations currently under construction will drive water through the pipeline, while balancing tanks at high points will allow water to flow into the water treatment plant by gravity. The finished system will be automated and will regulate the flow of water 24 hours a day.