Client name: Coe Drilling

Duration: 2008

Location: Nerang River, Queensland, Australia


When the Nerang River project experienced unexpected challenges, Coffey helped get things back on track.

Situation

Coe Drilling constructed a wastewater transfer pipeline using microtunnelling beneath a river in south-east Queensland.

A third party had completed an initial geotechnical borehole drilling and logging investigation to support the concept design for the 100m long river crossing, to be installed at least 8m below the river bed. Temporary access shafts were also planned on both river banks.

The investigation found varying subsurface conditions across the five boreholes used in the testing.

During construction of the west bank access shaft, excessive water inflow from the gravel/cobble layer caused the immediate suspension of microtunneling works. The client needed to confirm the subsurface conditions – and where they were different to what had been expected.

Solution

We quickly mobilised our geophysics team to complete a geophysical survey along the line of boreholes. We used a combination of underwater and land seismic refraction with bottom-laid hydrophone cables and an airgun energy source.

The results revealed the initial geotechnical baseline report did not adequately identify the base of the alluvial gravel/cobble deposits which caused the water inflow problems during tunnelling. After deepening the west bank shaft and modifying the under-bore trajectory, the issue was addressed and the project completed.

Result

Our detailed and cost-effective geophysical testing clearly identified the source of the problem and provided advice to address it. Our ability to respond quickly and achieve results efficiently ensured the project could progress without further delay, saving the client from incurring additional costs. As a result, the project was successfully completed.