Thompson, BD, Simon, D, Grabinsky, MW, Counter, DB & Bawden, WF 2014, 'Constrained thermal expansion as a causal mechanism for in situ pressure in cemented paste and hydraulic backfilled stopes', in Y Potvin & T Grice (eds), Proceedings of the Eleventh International Symposium on Mining with Backfill
, Australian Centre for Geomechanics, Perth, pp. 365-378, https://doi.org/10.36487/ACG_rep/1404_29_Thompson
In situ pressures are compared within cemented paste backfill (CPB) and cemented hydraulic backfill (CHB) at five mines. Pressures are initially controlled by backfill rise rates, and subsequently moderated as the backfill transitions from a fluid to a soil-like material and gains shear strength. At Kidd Mine, significant pressure increases were measured within the CPB during backfilling shutdowns. A positive correlation is found between pressure and temperature, with pressure increasing by ~30 kPa/°C. In one location, temperature increased by 11°C, corresponding to a 300 kPa pressure increase. Extrapolating the rates of pressure increase to the end of backfilling suggests up to 50% of the vertical pressure measured was thermally-induced. Similar magnitude pressures that are apparently thermally-induced were measured in CHB. As noted by others, a component of pressure increase measured by a total earth pressure cell (TEPC) due to differential thermal expansion of the TEPC constituent materials and the stiffening surrounding medium should be expected. However our calibrations (physical and theoretical) suggest the instrument calibration to be significantly smaller than the measured, thermally-induced pressure increase in the various backfills. Constrained thermal expansion of CPB/CHB within the stope is the suggested causal mechanism for the measured thermally correlated pressure changes. This interpretation is supported by laboratory tests that demonstrate CPB expands, and can induce pressure increases of ~200 kPa when the backfill materials are heated to mimic the in-stope thermal conditions. Increased sample air content minimises thermally−induced pressure increase, making it difficult to accurately replicate in situ conditions in the laboratory. Limited in situ pressure changes at Cayeli Mine also appear to correlate with temperate increases during downtimes in backfilling. This was not observed at Williams Mine, possibly due to low binder contents. Therefore, constrained thermal expansion potential as a mechanism for pressure generation in CPB is not common to all mines but should be considered when interpreting field data. The interpretation of thermal expansion of backfill emphasises the requirement to better understand the response of TEPCs to changes in temperature while under load, in order to isolate the effects of instrumentation and physical backfill behaviour.
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