Real Stress Distributions in Sedimentary Strata

Gray, Ian, Jeff Wood and Yulia Shelukhina (2013). Real Stress Distributions in Sedimentary Strata. 6th International Symposium on In-situ Rock Stress (RS2013), 20-22 August. Sendai, Japan.


This paper examines the stress fields in two coal mines and in a coal seam gas field in Eastern Australia at depths of up to 600 m. These stresses have been measured using multiple overcore measurements (50 to 100 per site) undertaken from surface at each site and supported by the examination of borehole breakout trends. The paper also comments on the appropriateness of these and other stress measurement techniques.

The stress distributions have been found to range from situations where they can be modelled very well using simple tectonic strain theory to those which are far more complex. Those that can be readily understood are cases where the stresses are produced by a combination of lithostatic effects and constant tectonic strains through the rocks of varying stiffness in the sedimentary sequence.

More complex cases may be found where the tectonic strain theory generally applies but is modified locally by the effects of faults which invariably serve to relieve stress. A highly complex situation is presented in which the initial principal stress distribution was clearly NE-SW in orientation and this was changed through reverse and slip strike faulting so that in most cases the principal stress is now at perpendicular to that that previously existed. There are, however, locations, generally close to faults, where the principal stresses rotate through the sequence.

In addition to the stress measurements that have been made in virgin ground the results of stress measurements have been made over areas where the coal seam has been mined by longwall mining. While the stress field has been changed the level of stress in these rocks has been found to be remarkably consistent.

Key words: Stress, Strain, Coal, Goaf, Gob, Mining, Longwalls

Author: Ian Gray
Ian initially worked in Australia supervising the installation of the first gas drainage system at Central Colliery and then went on to work as Senior Geotechnical engineer building a mining wing to the consulting company DJ Douglas and Partners. In 1990 he became Principal Engineer, Mining Research with the Safety In Mines Testing and Resarch Station of the Department of Resource Industries of the Queensland Government. In this role he worked on frictional ignitions, mine explosions, windblasts, gas drainage and directional drilling including the first surface to in-seam operation in the country in 1991. In 1994 he started Sigra as a one man business. Since then the company has grown under his guidance to span mining, gas, civil, geotechnical work and a number of mechanical and electronic product developments.

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