Long-term stability of a backfilled room-and-pillar test section at the Buick Mine, Missouri, USA

Rock mechanics instruments have been providing data in a backfilled room-and-pillar test section of the Buick Mine near Boss, Missouri, USA, for nearly 16 years. Host rock instruments include borehole extensometers installed in the mine roof and pillars, and biaxial stressmeters used in pillars and abutments. Embedment strain gauges, extensometers, and earth pressure cells were installed in the cemented backfill. The instruments monitored stability of the test section for two years while the pillars were extracted, and 14 years after pillar extraction to monitor long-term stability. Of the transducers that were not mined out when the pillars were extracted, 68% still function. Data from these instruments demonstrate that backfill improves long-term underground safety by supporting the mine roof and maintaining the strength of support pillars. For example, backfill significantly limited the dilation of a remaining support pillar by providing confinement on one side of the pillar. Post-mining stress and strain in the backfill account for 35% and 28% of the total stress and strain that was measured, respectively. Earth pressure cell stress measurements confirmed visual observations that the backfill remained stable. Post-mining stress measurements from the earth pressure cells fit natural log equations as a function of time with r-squared values ranging from 0.76 to 0.98. Natural log equations also described post-mining strain behavior of the backfill with r-squared values ranging from 0.30 to 0.99. Stresses calculated for the backfill by a three-dimensional numerical model of the test area were consistent with those that were measured by earth pressure cells.