Subsidence monitoring at Cataract Tunnel portal: Lessons learnt

The Cataract Tunnel began operation in 1886 as part of Sydney Catchment Authority's (SCA) Upper Nepean water supply scheme. The unlined tunnel carries water for 3 km from Broughton's Pass Weir pondage to the start of the open section of the Upper Canal. The tunnel has a maximum capacity of 700 Ml/day, however it normally operates at a flow rate of approximately 250Ml/day. The tunnel has an average height of 2.6 m and a width of 3.2 m. Longwall coal mining of the Bulli Seam at the Appin Colliery, owned by BHPBilliton Illawarra Coal, has been carried out beneath the tunnel and part of the canal. During extraction a range of monitoring techniques were employed to monitor the impact of subsidence upon the tunnel and canal. These observations were compared with predictions to anticipate unforseen impacts. This paper provides an assessment of the efficacy of the monitoring systems employed to safeguard the downstream tunnel portal where movements were concentrated on a low angle fault. These systems included traditional as well as novel methods including 3D survey, stress cells, piezometers, extensometers, inclinometers, open observation borehole and convergence monitoring. The success and shortcomings of these techniques is discussed. The paper highlights the multifaceted issues of stability of man-made infrastructures in a dynamic mining environment.