From the magazine, HDD, Microtunnelling, Pipe jacking, Tunnelling

Extreme accuracy in changing ground conditions

Major contractor John Holland has been undertaking various civil works as part of the project, and when extreme accuracy was called for the jacking pipe installation, they were able to rely on
Edge Underground.

Woolloongabba, home to the famous “÷Gabba’ cricket ground, is an inner suburb to the south east of the Brisbane CBD. Changing demographics mean the population is expected to increase by 80,000 in the next few years as high-density apartment living becomes increasingly popular. To ensure infrastructure in the area keeps pace with the population, Queensland Urban Utilities has undertaken a two-phase program, with the first phase being completed in 2011, and the second underway from July 2012 to June 2014.

The program includes the addition of three trunk sewer mains, constructed by a small diameter tunnel boring machine, as well as various branch lines. The mains each have a diameter of 1.4 m and will have a capacity of 5,000 litres per second, with an average of 1,000 litres per second anticipated. The pipes are built on a gradual decline, dropping 1 m for every 800 m of pipeline.

The project required installation in some particularly tight spots, including installing HOBAS 525 mm jacking pipe under a major roadway and in high density housing. Extreme accuracy was called for, so John Holland called in Edge Underground.

Edge Underground specialises in “÷keyhole’ pipeline installation, using highly developed technology to precisely install a pipeline in tight conditions and avoiding the trauma of open-cut installation. Edge uses the Vermeer AXIS laser guided boring system, a technology actually developed by Edge Underground founder Stuart Harrison.

The team set up on-site in July 2013 and were faced with a very tight working site, requiring them to keep their footprint to an absolute minimum. John Holland diverted the live sewer pipes during the procedure, increasing the need for a fast and accurate performance.

The challenge of maintaining accuracy was magnified by the changing ground conditions, which shifted from wet mud to high plasticity clay. This required Edge Underground to use a closed-faced cutter to manage the softer unsupporting ground, which then made for slow progress through the clay.

But the end result showed that the judgement was right. Edge Underground’s wide experience on similar projects in a large variety of ground conditions has well prepared them to understand the technology and approach required in
any situation.

In all, Edge Underground completed three bores, of 70 m, 103 m and 97 m. Detailed tests, including as-built survey, vacuum and infiltration tests and CCTV inspection, were all carried out and all passed showing accuracy of +/- 10 mm for each drive, allowing John Holland to have this component signed off by their client shortly after construction was completed.

With these drives completed to everyone’s satisfaction, the final completion of the sewer upgrade is
well on track.

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