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.), 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
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.
Amitrano, D & Helmstetter, A 2006, ‘Brittle creep, damage, and time to failure in rocks’, Journal of Geophysical Research: Solid Earth, vol. 111, no. B11, p. 111.
Backers, T 2013, ‘Borehole geomechanics and well design’, in IGA Service GmbH (comp.), IGA Academy Report, vol. 1, IGA Service GmbH, Bochum, viewed 3 March 2015,
Brantut, N, Heap, MP, Meredith, P & Baud, P 2013, ‘Time-dependent cracking and brittle creep in crustal rocks: a review’, Journal of Structural Geology, vol. 52, pp. 17-43.
Couling, C, Tolooiyan, A, Mackay, R & Xue, J 2014, ‘Measurement of pore water pressure properties of unsaturated brown coal using triaxial test’, in N Khalili, AR Russell & A Khoshghalb (eds), Unsaturated Soils: Research and Applications, Proceedings of the 6th International Conference on Unsaturated Soils, CRC Press/Balkema, Netherlands, vol. 2, pp. 1531-1535.
Freudenstein, U, Crowley, D & Welch, F 2000, ‘Chemical incident management: gaseous emissions from a stockpile of coal’, Public Health, vol. 114, no. 1, pp. 41-44.
George, AM 1982, ‘Latrobe Valley brown coal — lithotypes: macerals: coal properties’, in CW Mallett (ed.), Proceedings of the Symposium on Coal Resources: Origin, Exploration and Utilisation in Australia, Geological Society of Australia, Melbourne, pp. 111-130.
Guenot, A 1989, ‘Borehole breakouts and stress fields’, International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, vol. 26, no. 3, pp. 185-195.
Haimson, B 2007, ‘Micromechanisms of borehole instability leading to breakouts in rocks’, International Journal of Rock Mechanics and Mining Sciences, vol. 44, no. 2, pp. 157-173.
Heap, MJ, Baud, P, Meredith, PG, Bell, AF & Main, IG 2009, ‘Time-dependent brittle creep in Darley Dale sandstone’, Journal of Geophysical Research, vol. 114, no. B7, p. 203.
Heap, MJ, Faulkner, DR, Meredith, PG & Vinciguerra, S 2010, ‘Elastic moduli evolution and accompanying stress changes with increasing crack damage: implications for stress changes around fault zones and volcanoes during deformation’, Geophysical Journal International, vol. 183, no. 1, pp. 225-236.
Hsieh, A, Dyskin, AV & Dight, P 2014, ‘The increase in Young's modulus of rocks under uniaxial compression’, International Journal of Rock Mechanics & Mining Sciences, vol. 70, pp. 425-434.
Hutton, AC 2009, ‘Geological setting of Australasian coal deposits’, in R Kininmonth & E Baafi (eds), Australasian coal mining practice, The Australasian Institute of Mining and Metallurgy, Victoria, pp. 40-84.
Lockner, DA 1993, ‘Room temperature creep in saturated granite’, Journal of Geophysical Research, vol. 98, no. B1, pp. 475-487.
Pomeroy, CD 1956, ‘Creep in coal at room temperature’, Nature, vol. 178, pp. 279-280.
Ramamurthy, T (ed.) 2014, Engineering in Rocks for Slopes, Foundations and Tunnels, PHI Learning Private Ltd, Delhi.
Schoenball, M, Sahara, DP & Kohl, T 2014, ‘Time-dependent brittle creep as a mechanism for time-delayed wellbore failure’, International Journal of Rock Mechanics and Mining Sciences, vol. 70, pp. 400-406.
Skotnicki, AL & Raisbeck, D (1985), ‘Report on Yallourn Open Cut, earthmovement analyses for the western and northern batters. SECV Design Engineering and Environment Department, Report No. GD32.
Tessler, J & Jehlicka, O 1989, Deterioration of brown coal during open-air storage’, Bulk Solids Handling, vol. 9, no. 4, pp. 427-428.
Trollope, DH, Rosengren, KJ & Brown, ET 1965, ‘The mechanics of brown coal’, Geotechnique, vol. 15, no. 4, pp. 363-386.
Zoback, MD, Moos, D, Mastin, L & Anderson, RN 1985, ‘Well bore breakouts and in situ stress’, Journal of Geophysical Research, vol. 90, no. B7, pp. 5523-5530.