Rock Stress in Tasmania

Hills, Peter1

1Pitt & Sherry Operations Pty Ltd, Hobart, Australia

Rock stress can have a profound impact on underground excavations in mining and civil space. Rock stress measurement was first undertaken in Tasmania to inform the design of excavations for the Poatina Power Station in 1960 and led to an innovative trapezoidal section to the crown of station cavern in near horizontal Permian sediments. Since that time stress measurements have been undertaken at 15 sites across western and northern Tasmania and this has provided a database, which in addition to assisting in the management of individual assets, has allowed a number of more general observations to be made[1].

Rock stress is the result of tectonic forces and overburden pressures. The former creates a background stress condition that is widely evident within a given geological terrane. All Tasmania’s stress measurements have been undertaken in the Western Tasmanian Terrane and indicate a major principal stress (σ1) orientation striking WSW-ENE which is an anticlockwise rotation from that observed in Victoria where measurements have largely been undertaken in rocks of the Lachlan Fold Belt. This is thought to reflect upon the key location of Tasmania during the accretion of Gondwanaland and the eventual separation from Antarctica. The latter typically reflects depth below surface with the magnitude of the stress generally increasing with depth. In most cases in Australia, and it is the case in Tasmania, that the orientation of the minor principal stress (σ3) steep and consequently the intermediate principal stress (σ2) is relatively flat. The general stress condition in Tasmania can be described in terms of orientation and magnitude as follows:

  • σ1; 23°/261°         (0.039 x depth) + 10.7 MPa
  • σ2; 15°/164°         035 x depth MPa
  • σ3; 62°/043°         023 x depth MPa

Local geological and geomorphological setting can significantly impact the stress locally. At Cethana Power Station the stress measured in the crown of the station prior to excavation of the cavern was found to be four times the expected magnitude. However it is readily explained by its location in the steep sided Forth River Gorge which was rapidly cut through the Fossey Mountains during relatively recent glaciation. At Hellyer Mine the magnitude of the measured stress is more-or-less consistent with the depth of the measurements beneath the Waratah Plateau, but the orientation of the shallower measurements is strongly influenced by the orientation of the nearby Southwell River Gorge. Renison Mine provides a further example where the orientation of the stress field is strongly influenced by the orientation of the Federal-Bassett Fault and the associated Pine Hill Horst.

Stress measurement in Tasmania has been undertaken for direct engineering construction as in the case of the John Butters Tunnel where is was specifically directed at determination of the length of tunnel lining required. It has also been used extensively to understand the orientation and magnitude of the stress condition in deep mines at Mount Lyell, Renison, Rosebery and Beaconsfield and assist with excavation, and particularly, stope design. Stress change due to extraction has also been monitored at those mines as well as Dolphin and Cleveland.

[1] Hills, P B, 2020. Tasmanian rock stress, Australian Geomechanics, 55(1), 77-111.


Biography

Originally graduating as a geologist at UTAS in the early 1980’s, Peter undertook further studies in rock mechanics and Commenced transitioning to mining geomechanics over the following 10-15 years. He now consults in the field geomechanics after a 30 year career in underground mines in Tasmania and Papua New Guinea.

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