Illawarra Coal plants fibre optics in Hume Highway

BHP Billiton's Illawarra Coal has embedded three kilometres of fibre optic cables in the Hume Highway to track subsidence caused by a longwall mine that runs under the road.

Arrows show fibre optic cable paths in the Hume Highway outside of Sydney. (Photo courtesy BHP Billiton)

The "world-first" use of fibre optics was installed in mid-2009 in readiness for Longwall 703 of the Appin Mine, southwest of Sydney.

The project was first revealed publicly at a NSW Minerals Council conference late last year (pdf).

Illawarra Coal uses fibre Bragg grating sensors to measure temperature and strain at ten-metre intervals along the road's pavement to detect any forces that could damage the road.

Pavement typically comprises sub-grade, sub-base, base and surface layers. The sub-base is a "strengthening layer" that sits between the base and natural soil the road is built on.

In its presentation, Illawarra Coal noted the potential for "compressive stresses in the stiff sub-base pavement layer" caused by longwall mining, which is an underground method of mining blocks of coal in 'panels' typically up to one metre thick.

It noted the risk of "a sudden release by abrupt shearing and stepping of that layer", causing damage to the road and potentially jeopardising road user safety.

Illawarra Coal's in-pavement monitoring system is connected to a site-based bank of interrogators that analyse the raw data on a real time basis.

"All data is transferred via wireless network link and is maintained on a web server which is managed by one of the key stakeholders," a BHP Billiton spokeswoman told iTnews.

"The captured data is compared against pre-determined triggers and has the capability to initiate mobile phone SMS-generated alarms if required for appropriate response as determined by the trigger."

The data is also transmitted to the Appin Mine control centre for redundancy purposes.

"For example, in the event of certain triggers being initiated, the control centre is also notified and if necessary can ensure that the nominated personnel have received the alarm notification and are responding accordingly," the spokeswoman said.

BHP Billiton's spokeswoman said that real-time, in-pavement monitoring was "only a component" of its "management measures" along the Hume Highway.

"The key objective of the management measures for the Hume Highway is to ensure the safety and serviceability of the Hume Highway and in so-doing ensure continued mining operations," she said.

"Experience to date has demonstrated that the system has been successful in delivering key objectives."

The fibre optic installation addresses long-running concerns over the impact of longwall mining on the highway.

In 2005, the NSW Roads and Traffic Authority (RTA) commissioned a geotechnical report that warned of the potential for pavement cracking and movement of the Douglas Park twin bridges, which bring traffic over the Nepean River.

The RTA undertook $9 million of bridge works in late 2007 to enable them to "cope with any movements" from longwall mining, according to a Sydney Morning Herald report. The work included installation of "devices to monitor movement".

The Hume Highway carries over 39,000 vehicles a day with no viable alternative routes for much of the traffic in and out of Sydney.