The Woolloongabba Sewer Capacity Upgrade is a major project currently being undertaken by Queensland Urban Utilities to increase capacity to cater for rapid population growth. With works taking place in a highly urbanised area, the need for accuracy in construction is non-negotiable. The challenges of such an installation are compounded when ground conditions vary. Different types of earth offer varying levels of support and often require different tools and methods for successful tunnelling.

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.

The upgrade is a two-phase program, with the first phase having been completed in 2011, and the second underway from July 2012 to June 2014.

The sewerage upgrade includes the construction 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 metres 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 one metre for every 800 metres of pipeline.

Major contractor John Holland (JHG) has been undertaking various civil works for the project and has also worked with specialist subcontractor Edge Underground to ensure accuracy for a number of on-grade drives. Existing infrastructure meant that installation had to take place in some particularly tight spots, including installing HOBAS 525mm jacking pipe under a major roadway and in high density housing.

Edge Underground, who specialise in ‘keyhole’ pipeline installation, use their laser guided boring system, Vermeer AXIS, in order to precisely install pipeline in tight conditions.

They set up on site in July 2013 in a very restricted working site which required them to keep their footprint to an absolute minimum. As JHG diverted live sewer pipes during the procedure, the installation had to be completed quickly and accurately.

The challenge of maintaining the required level of precision was magnified by changing ground conditions in the area, which shifted from wet mud to high plasticity clay. This required Edge to use a closed faced cutter to manage the softer un-supporting ground. However, this meant slowed progress through the clay.

Nevertheless, three bores were completed, one of 70 metres, one of 103 metres and one of 97 metres. Detailed tests, including as-built survey, vacuum and infiltration tests, and CCTV inspection were carried out. The installation passed all of these, showing accuracy of +/- 10mm for each drive, allowing JHG to have this component signed off by the client shortly after construction was completed.

With these drives completed to everyone’s satisfaction, the sewer upgrade is on track for completion in June 2014.

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