Bamford, WE & Potdar, RS 2015, 'Impact on stability of boreholes in brown coal over time and changes to in situ stress', in PM Dight (ed.), FMGM 2015: Proceedings of the Ninth Symposium on Field Measurements in Geomechanics, Australian Centre for Geomechanics, Perth, pp. 705-714, https://doi.org/10.36487/ACG_rep/1508_51_Bamford
(https://papers.acg.uwa.edu.au/p/1508_51_Bamford/)
Abstract: Brown coal is a complex geo material that has significantly different behaviour to rocks with regard to how it changes over time and with changes to in situ stress and environmental conditions. Boreholes in brown coal formations are susceptible to in situ radial deformation of the borehole and these changes may have a negative impact on instrumentation installed in monitoring boreholes as the inner walls squeeze the instruments in the radial direction. The data collected by sensors affected by borehole deformations over time may not be completely consistent with the actual long-term in situ properties of the coal. Mining operations near boreholes in brown coal tend to reduce the in situ stresses and this change may contribute to accelerated deformation or collapse of the borehole.
Major time dependent changes can also occur in brown coal in the form of mineral creep and creeping from dissipation of micro-pore gas, resulting into movements of joints/micro fractures in the coal matrix. Internal micro-cracking is promoted by the high pressure of entrapped gas present in the micro pores in brown coal matrix at different stages of loading and unloading. Such additional creeping in conjunction with the mechanical creep, leads to unusually high and unpredictable time dependent deformation patterns in the coal mass. Prolonged exposure to oxygen and slow release of internal gas from exposed coal surfaces (for example the inner walls of a borehole) also alter the chemical as well as engineering properties of brown coal, causing further problems in predicting its geotechnical behaviour. These critical time dependent characteristics of brown coal have received minimal attention in the past. The presented paper attempts to highlight these issues of brown coal behaviour and demonstrates the impact of changes in stress regime due to an approaching mine batter near a borehole in brown coal.