Structural Integrity and Liesegang Rings

Hills, Peter1

1Pitt & Sherry Operations Pty Ltd, Hobart, Australia

Many will be familiar with natures abstract paintings generated by the formation of Liesegang rings.

The often spectacular features illustrated in vibrant yellows, oranges and browns cross cutting the bedded fabric of the strata are a feature of the Tasmanian landscape in many areas, particularly, but not exclusively, in outcrops of the Ross Sandstone. Those near the former settlement of Darlington on Maria Island, the Painted Cliffs, are a frequent feature in tourism brochures, with good reason.

The railway tunnel at Rhyndaston, 45 km due north of Hobart passes a distance of 943 m through the same strata as that hosting the Painted Cliffs; the Liesegang rings and other solution features displayed in the largely unlined tunnel are no less spectacular. However, they have also played a role in preserving the structural integrity of the civil asset.

The Rhyndaston Tunnel was originally excavated in the 1870’s when the Main Line between Hobart and Launceston was established. A small tunnel, it was inadequate to meet the late 1950’s watershed development in global freight that was heralded by the creation of the standard shipping container. Consequently, the tunnel was enlarged in 1964/65 using a tunnel boring machine (TBM). That TBM was the first to have been used in Australia, boring the tailrace and sections of the headrace tunnels for the Poatina Power Station development over the previous three years. Rhyndaston became the third bored tunnel in Australia by several years[1].

The profile of the tunnel is circular affording a clear opportunity to monitor dilapidation of the exposed rockmass. At an elevation of 400 m A.S.L. the environment in the tunnel is often cold and the air moving through it is frequently moist. The rockmass presents in three facies; a competent coarse grained yellow-brown sandstone displaying strong current bedding, a white silty sandstone usually displaying laminar bedding, and occasional thin micaceous shale bands. The yellow-brown sandstone which is the dominant facies, shows little sign of dilapidation, with the radial marks of the TBM cutters still clearly visible after 56 years. The TBM head was 4.93 m diameter and the average tunnel diameter in the yellow-brown sandstone is less the 4.95 m. Much of the 20 mm difference is due to the gauge of the TBM cutters. However, in the white silty sandstone, the tunnel diameter is as much as 5.01 m, some 60 mm more. In comparison to tunnels in similar sandstones in Sydney where erosion rates of <2 mm/yr have been recorded[2], the effective erosion rate of ~0.5-0.9 mm/yr is favourable. However, Liesegang rings within that sandstone are no more eroded than the yellow-brown sandstone and the TBM cutter marks remain clearly visible. It is posited that the Liesegang rings are serving to protect the silty white sandstone from greater erosion than might otherwise have been the case.

The shale bands, which are confined within the yellow-brown sandstone are of insignificant dimension, and while they are prone to severe slaking, they do not pose a significant threat to tunnel integrity.

[1] Stack, B, 1982. Handbook of Mining and Tunnelling Machinery, 742 p., John Wylie & Sons: Chichester.

[2] Nash, T, Bertuzzi, R and de Ambrosis, A, 2019. Deterioration of sandstones and shales in Sydney tunnels, in Proceedings of the 13th Australia New Zealand Conference on Geomechanics, pp. 877-882, Australian Geomechanics Society: Sydney.


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.

About the GSA

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