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Measuring the Height of Fracturing above Extracted Longwall Panels to Improve Reliability of Groundwater Impact Predictions

Underground » Strata Control and Windblasts

Published: June 20Project Number: C28026

Get ReportAuthor: Ken Mills | SCT Operations

Longwall mining is a mining method widely used in the Australian underground coal industry. The disturbance to the strata overlying the mining horizon caused by longwall mining is recognised to impact the groundwater system and, in some cases, surface water systems as well. This project aimed to determine the height of fracturing above extracted longwall panels by measuring the residual horizontal stress acting through the overburden strata above these panels based on the concept that failed rock strata would be expected to carry lower horizontal stresses than intact rock strata. This report presents the results of seven overcore measurements conducted between two extracted longwall panels and discusses the concepts relating to height of fracturing and groundwater impacts.

All seven measurements were successful and provide high to very high confidence indications of the residual in situ stresses in three dimensions at each point of measurement. This set of overcore measurements is the first time in Australia, and to our knowledge anywhere in the world, that measurements aimed to measure the three dimensional in situ stresses between extracted longwall panels have been conducted.

The overburden strata at the site where the measurements were made includes two 150m thick sandstone units separated by one or more horizons of lower shear strength. The profile of residual horizontal stresses indicated by the overcore measurements in the upper sandstone is consistent with the concept of a zone of rock failure extending to a height above the mining horizon equal to one panel width. The residual horizontal stresses in the lower sandstone indicates residual stress can be resisted by another mechanism that is, as yet, not well understood.

Residual horizontal stresses are elevated above pre-mining levels at two depth horizons, 70m and 199m deep. These horizons correspond with the upper parts of the two sandstone units. Residual horizontal stresses are less than pre-mining levels at the base of each of the two sandstone units.

The relationship between mining induced fracturing, height of fracturing and impacts to groundwater systems are investigated, discussed and clarified. The terminology is found to be confusing and misleading.  The report recommends that the term “height of fracturing”, an essentially geotechnical terminology, is not used to describe the zone of groundwater depressurisation that develops above extracted longwall panels. Mining induced fracturing extends well above the zone of depressurisation. The height of depressurisation represents the height to which groundwater inflows can occur under the action of gravity alone. It does not represent the edge of groundwater impacts, strata desaturation or complete drainage. It should be recognised that changes in hydraulic conductivity and groundwater impacts extend beyond both the height of fracturing and the height of depressurisation.

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