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Regional Ground Movement on Bedding Planes and Potential Impacts to Groundwater

Underground » Environment - Subsidence and Mine Water

Published: October 23Project Number: C33015

Get ReportAuthor: Ken Mills | SCT Operations

Bedding plane shears are horizons within rock strata along which sliding movements occur. They develop as a result of natural processes but can be remobilised, or even created, by differential ground movements associated with subsidence from mining. Their low strength in shear and lateral persistence has a profound effect on ground movements around extracted longwall panels and give rise to a broad range of phenomena.

The outcome of this project is a desktop study characterising the four recognised types of bedding plane shears, their nature, mechanical properties, impacts on underground mining and potential to influence groundwater and surface water systems.

Bedding plane shears that form at or near the mined coal seam contribute to far-field horizontal movements, horizontal stress relief effects, mining induced seismic events, and challenging mining conditions at monoclines and geological faults. These types of shears are inferred to have friction angles as low as 5° leading to horizontal stress relief and low-level movements extending up to 2-3 km from the edge of longwall mining.

Bedding plane shears associated with topography are remobilised by mining to cause valley closure impacts to river channels, enhanced flow paths for groundwater movement and a range of surface impacts such as ripples, overrides and shears. These shears form naturally at or near the base of valleys as part of natural erosion processes and are remobilised by mining subsidence processes to cause sideways movements and a range of phenomena including valley closure. Friction angles in the range 8° to 11° are inferred on these features.

Bedding plane shears that occur close to the edge of longwall panels are caused by shear distortion in the overburden strata as this strata sags downward into the extracted longwall panel void. These features are limited to within half depth of the edge of extracted longwall panels and develop at regular spacing throughout the overburden strata.

Combination bedding plane shears develop in a range of circumstances including at the start of longwall panels. At longwall start-up, shear planes develop within the overburden strata on low strength horizons at or near the top of the upward progressing caving zone. Horizontal stress relief towards the cave zone and the buttressing effect of the strata above the cave zone leads to the development of a bedding plane shear horizon. As the longwall advances, the top of the caving zone moves upward through the strata leading to the development of multiple bedding plane shears. Movement on these planes has been observed to influence caving in massive strata.

Groundwater systems outside the area directly above longwall panels are influenced by horizontal stress relief effects and by increases in hydraulic conductivity on bedding plane shears. Outside the panel, stress relief effects cause a general increase in hydraulic conductivity of the overburden strata at the edge of mining, from double at half depth from the edge of mining and diminishing to normal over several kilometres. Bedding planes shears associated with surface topography have potential to create pathways with increased hydraulic conductivity. Increases in hydraulic conductivity to 1x10-6 m/s over a 6 m test interval are observed, but this hydraulic conductivity remains within the range of hydraulic conductivities observed more generally at equivalent depth.

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