ABSTRACT

ABSTRACT

This paper presents an analysis of measurements of inter-burden deformations above six longwall faces. An attempt is made to demonstrate some correlation between the movements at the various sites, and to examine their importance with respect to predicting caving height, disruption of overlying seams, and disruption of aquifers. This analysis demonstrates some significant differences between predicted surface subsidence, and inter-burden deformation. In addition it is shown that the caving height above a longwall face is equal to 8 to 12 times the extraction height, with a zone of fractured rock extending to approximately 50 times the extraction height above the seam.

INTRODUCTION

As the easily accessible seams are exhausted, mining companies will be forced into extracting deeper, underlying seams. Due to increased depth, lower seams could well be extracted by the longwall method. To ensure the optimum longwall layout, and to minimize interaction effects, a better understanding of strata deformation above longwall faces is required. This is essential in areas of high extraction, so as to minimize surface subsidence, aquifer disruption, and interaction between superincumbent workings. Over the past twenty years this problem has attracted the attention of many investigators, however, due to the high cost of inter-burden instrumentation programs, the majority of these investigations were confined to surface and in-mine measurement. This paper presents an analysis of measurements of inter-burden deformations above 6 longwall faces, 5 in the United Kingdom, and 1 in the U.S.A. Details of the various sites are summarized in table 1, along with a summary of the instrumentation utilized. A previous paper by Styler and Dunham described the inter-strata movements, and arrived at the following conclusions:

- No significant vertical movements were measured in boreholes located above, and near to the ribside, indicating that gateroad deformation is mainly caused by localized effects.

- There was a significant difference between strata behavior in the British and American sites. This was due to the behavior of the strata in the American case histories being dominated by strong/competent beds, whilst in the British case histories, there were no strong beds to have a noticeable effect on the movements above the face.

- Empirical methods of subsidence prediction, such as the Subsidence Engineers Handbook (S.E.H.), cannot be used for both countries without a more complete understanding of the influence of geology.

- In order to model deformations above a longwall face, an area of fractured rock must be included, the extent of which is dependent upon the presence of any competent beds in the immediate roof.

MAXIMUM SUBSIDENCE/HEIGHT ABOVE THE SEAM

The absolute maximum anchor displacements have been plotted as a percentage of the extraction thickness against height above the seam, figure 1. It was only possible to plot the results from the British case histories, since only relative, and not absolute movements were reported for the American investigation. The plotted results were all obtained from boreholes near to the face centerline, and are therefore, comparable. As would be expected subsidence decreased with height above the seam, and the rate of reduction in subsidence also reduces with increasing height above the seam.

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