From the magazine, Microtunnelling, Pipe jacking, Tunnelling

Microtunnelling for the environment

The project was selected unanimously as the Tunnelling Project of the Year by a panel of industry experts from across the globe. The project overcame considerable technical challenges and risks associated with both ground conditions and geographical location, as it required both onshore and offshore engineering works.

Tunnel materials

The Southern Seawater Joint Venture contracted ZÌ_blin to manufacture and install two intake tunnels and one outfall tunnel for the new Southern Seawater Desalination Plant, which is part of a significant Water Corporation initiative to ensure Western Australia’s future water supplies.

The intake tunnels had a length of 850 m each and an internal diameter of 2.4 m, while the outfall tunnel was 950 m with an internal diameter of 2 m. At the completion of each shore-crossing tunnel the pipe jacking tunnel boring machine (TBM) was recovered from the seabed.

ZÌ_blin produced the HDPE lined concrete jacking pipes from its factory in Southeast Asia and shipped them to Australia to be used for the tunnel construction. The tunnels constructed formed a shore crossing beneath pristine sand dunes and the ocean in an environmentally sensitive area of south-west Western Australia. The tunnels were designed with alignments that traversed beneath the earth, across the shore and beneath the seabed for between 850 and 950 m, in a curved “÷S’ shaped spiral, requiring the use of leading edge equipment and a highly skilled workforce. All tunnels were completed within a few millimetres of the designed completion point.

Upon completion of the tunnel drives the TBMs were recovered from their final position, towed behind a barge to Bunbury Port, removed from the ocean and transported back to site where they were fully refurbished for use again on other ZÌ_blin projects.

A fragile environment

The tunnels were an essential component of the desalination plant for the supply and discharge of seawater in order to produce much-needed potable water. The site of the Southern Seawater Desalination Plant was identified as environmentally fragile and subject to very close scrutiny by local residents, environmentalists and the Department of Environment and Conservation.

The areas of greatest potential environmental impact were the onshore sections, including sand dunes, native vegetation and the ocean floor, approximately 500 m into the sea. The impact caused by excavating 500 m of seabed would have been significant, including the destruction of habitat for reef growth, reef dwelling animals, seaweed, fish and other sea-life such as turtles and dolphins.

For this reason, tunnel boring and pipe jacking technologies were selected for the tunnel construction, as these technologies would leave a considerably smaller footprint on the environment than alternative methods.

Pipe jacking for the construction of the three tunnels was completed in a non-invasive manner, without dust, plumes in the ocean, noise pollution, air pollution or disturbance to the local community “Ó and most importantly, the land surface was left undisturbed.

Another substantial environmental benefit gained by the use of pipe jacking techniques, when compared with open-cut approaches, was that no incoming tipping of spoil and quarrying of imported stone fill occurred, which led to a 90 per cent reduction in heavy vehicle movements.

Minimising community impacts

The impact to the local community was minimised with regular recreational activities continuing in the area throughout the construction period, with a short beach closure during tunnel construction under the beach area as a safety precaution only.

Trenchless Technology meant the area was not impacted by lengthy stretches of open trenches during tunnel construction. With pipe jacking, there was no surface scarring; no re-instatement of the landscape/oceanscape was required, again providing a more cost-effective option.

Using new technologies

The implementation of new industry technologies was key to the successful completion of the project. The use of a dual mode pipe jacking machine allowed the TBM to be operated in slurry mode or earth pressure balance mode to cope with the variations in ground conditions, thereby providing greater flexibility, safety and efficiency during the tunnelling operations. Furthermore, ZÌ_blin employed an orbital welder which provided a 360è_ HDPE weld around the pipe joints that provides protection against marine growth; a two-way lubrication system to reduce the effect of friction; and designed and produced the jacking pipes to provide the tunnels with a 100-year life span.

Corkscrew complications

Successful completion of the project relied on ZÌ_blin facing several major obstacles, including variable ground conditions, design alignment and developing solutions to achieve successful, on-time construction.

The complex tunnel alignment for the three tunnels provided a challenge that had not been undertaken before in Western Australia. Each tunnel had a different configuration that not only deviated on the vertical plane, but also on the horizontal plane in a corkscrew fashion.

Precision in the guidance of the TBM was guaranteed through a universal navigation system that immediately processed all the parameters on the current line of drive and displayed the progress of the tunnel being driven. Remarkably, given the complex design, the universal navigation system ensured that each tunnel was completed to within a few millimetres of the designed final tunnel position.

The ground conditions were highly variable, ranging from soft sand to hard rock, with voids in the alignment path. Countering this problem meant ensuring the use of the latest technology, dual-mode pipe jacking machine that could handle such conditions and a well-trained, experienced tunnelling team who could monitor and adjust the operations to suit any conditions encountered.

Recovery of the TBM

Recovering the tunnel boring machine from an offshore and underwater location had not been attempted before in Western Australia. The process was facilitated by introducing a recovery can and bulkhead, enabling safe recovery from beneath the seabed, leaving the tunnel dry and enabling completion works to continue inside the tunnel after the recovery. This also allowed the off-shore team to take advantage of breaks in the weather to achieve the recovery safely.

Successful completion

It was the culmination of the use of new industry technologies, developed between ZÌ_blin and the TBM manufacturers over many years, and a well-informed, well trained and loyal workforce that ensured the difficult project was completed successfully, with the end result being three high-quality, lined tunnels that left no environmental footprint outside the plant construction fence. The environment along the tunnel alignment has not been disturbed and the project was finished ahead of time and hit budget targets.

The trenchless tunnelling approach adopted by ZÌ_blin on this project meant that Water Corporation incurred no project delays due to inclement weather or high seas. The Southern Seawater Joint Venture expressed their appreciation to ZÌ_blin for their operational, safety and environmental excellence throughout the construction period.

The International Tunnelling Award was the second major award for this project, after winning a Civil Contractors Federation (WA) Earth Award in 2011.

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