Cataract Tunnel Instrumentation

Cataract Tunnel was constructed through the Hawkesbury Sandstone over 100 years ago to allow water from the large catchment area to be sent to Sydney. Mining activity by BHP in the area was due to progress under the Cataract Tunnel and concerns had been raised as to the effect that this would have on the tunnels stability. Conventional surveying methods could not be used to continually monitor the tunnel, as they required periodical draining to allow access. An automated instrumentation package was selected to allow this important tunnel to be monitored throughout the period of mining activity. The program involved two contracts running back to back. The first contract was to rock bolt and mesh the crown of the tunnel, then drill the extensometer holes. This was carried out by Shaft and Tunnel Pty Ltd.

Geotechnical Systems Australia was awarded the second contract to design and construct the instrumentation package. As a subcontract we also supervised the installation of Hollow Inclusion Cells. The extensometers were installed in fan arrays of up holes at several critical locations. They were grouted in place and cabling run back to a series of access holes at the surface. From here each array was connected to a dedicated telemetry box. The central connection point was the Sydney Water Corporation Offices at the start of the Cataract Tunnel. Dedicated software was written to allow the extensometer displacements to be monitored in real time and alarm outputs to be triggered.

The contract was completed successfully and Sydney Water Corporation is now operating the system.