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Post-failure behavior of two mine pillars confined with backfill.

Tesarik-DR; Seymour-JB; Yanske-TR
Int J Rock Mech Min Sci 2003 Feb; 40(2):221-232
Researchers from the National Institute for Occupational Safety and Health used a series of instruments (borehole extensometers, earth pressure cells, and embedment strain gauges) to study the post-failure behavior of two pillars confined by backfill in a test section at the Buick Mine near Boss, Missouri, USA. Evaluation of these pillars was part of a research project to assess the safety of the test section when high-grade support pillars were mined. Data from borehole extensometers installed in several backfill-confined pillars and numerical modeling indicated that these pillars failed during extraction of the support pillars. Failure was corroborated by the post-yield pillar strain response in which the immediate elastic strain was negligible compared to the time-dependent strain component measured between blasting rounds. A three-dimensional, finite-element program with an elastic perfectly plastic material model was calibrated using extensometer data to estimate rock mass modulus and unconfined compressive strength. The resulting rock mass modulus was 45-60% of the average deformation modulus obtained from laboratory tests, and the calibrated compressive strength was 40% of average laboratory values. A rock mass modulus equal to 52% of the average laboratory deformation modulus was calculated using the rock mass rating (RMR) system. Rock mass strength was calculated with the generalized Hoek-Brown failure criterion for jointed rock and indicated that in situ strength was 33% of laboratory strength. Post-failure stresses calculated by the finite-element model were larger for confined pillars than post-failure stresses in unconfined pillars calculated using empirical plots. Data from the calibrated model provided a strain-hardening stress-versus-strain relationship. This knowledge is critical for the design of mines that use partially failed pillars to carry overburden load.
Mining-industry; Ground-control; Ground-stability; Occupational-hazards; Hazards; Models; Laboratory-testing; Rock-bursts; Computer-models
NIOSH-Spokane Research Laboratory, 315 E. Montgomery Avenue, Spokane, WA 99207, USA
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International Journal of Rock Mechanics and Mining Sciences