ABSTRACT: The objective of this U.S. Bureau of Mines investigation was to characterize the change in the overburden due to mining of a 270 m wide longwall panel. The overburden thickness ranged from 95 to 105 m. Five-anchor, multipoint borehole extensometers (MPBX) were installed in each of five coreholes to measure subsurface displacement. Survey data of an array coincident with the boreholes show that surface movement began when the longwall face was about 45 m in front of any point. Over 90% of the total subsidence was achieved when the face was one overburden thickness past a point. Overburden failure was detected by the MPBX units when a borehole was undermined. Overburden failure occurred first in the center of the panel and then migrated outward toward the panel margins. The overburden appeared to collapse vertically as a unit and was laterally restricted to the mined-out area of the panel.

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