Minerals 2022, 12, 463. https://doi.org/10.3390/min12040463 www.mdpi.com/journal/minerals Article Rock Mass and Pore Fluid Response in Deep Mining: A Field Monitoring Study at Inclined Longwalls Qingdong Qu 1, *, Hua Guo 1 , Liang Yuan 2 , Baotang Shen 1 , Guofeng Yu 3 and Johnny Qin 1 1 Commonwealth Scientific and Industrial Research Organisation (CSIRO), Brisbane, QLD 4069, Australia; hua.guo@csiro.au (H.G.); baotang.shen@csiro.au (B.S.); johnny.qin@csiro.au (J.Q.) 2 School of Safety Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China; yuanl_1960@sina.com 3 National Research Institute for Coal Mining Engineering and Technology, Huainan Mining Group, Huainan 232000, China; yuguofeng16@163.com * Correspondence: qingdong.qu@csiro.au Abstract: The dynamics of stress, deformation and pore pressure in the surrounding strata of un- derground mines are of fundamental importance to groundwater and gas emission management. Compared to numerous studies on the overlying strata, there are significantly fewer investigations concerning the underlying strata, particularly involving large-scale field monitoring. This paper presents a comprehensive field monitoring study conducted at two longwall panels with a depth of around 800 m and an inclination angle of 21°. The monitoring program focused on the floor strata within 50 m below the mining operation, but also covered the roof strata close to the longwall road- way. The purpose was to characterise the favourable zone for gas extraction from the floor coal seams. A combination of stressmeters, extensometers and piezometers were deployed and installed underground. The monitored results demonstrated that the stress change exhibited a three-stage variation—increase, decrease and recovery—during which the strata deformed and the pore pres- sure changed correspondingly. Strata expansion in the floor occurred primarily in the region 0–35 m behind the longwall face and vertically to at least 42 m below. Some predictive methods of the depth of the failure zone used in shallow mines were analysed to determine if they were still appli- cable for mining at a depth of around 800 m. Keywords: deep mining; inclined longwalls; stress change; pore pressure; field monitoring 1. Introduction Underground longwall mining induces deformation and breakage in the surround- ing strata and the potential discharge of pore fluids (e.g., groundwater and methane gas) trapped in the pores and fractures of the strata. Several environmental and operational issues are directly related to these mining disturbances, including water inflow and in- rush, excessive methane emissions and gas outbursts. Some of these issues can lead to fatal accidents, causing significant consequences. A clear understanding of the dynamic responses of strata and pore fluids is of fundamental importance to effective engineering designs that improve the environmental and safety performances of mining. The general patterns of mining-induced stress distribution and strata deformation and fractures have been well appreciated and described in the literature. As mining ad- vances, the stress is redistributed to the solid coal ahead and along the sides of the mining panel (the so-called front and side abutments). The front abutment stress increases from the faceline, attains a peak value at a certain distance into the solid strata and gradually reduces to the in-situ value at a distance as far as 300 m [1–4]. In the goaf region, stresses are initially reduced in the disturbed strata, then gradually recover as the overburden strata subside and eventually stabilise at a certain distance into the goaf [1,5]. In terms of Citation: Qu, Q.; Guo, H.; Yuan, L.; Shen, B.; Yu, G.; Qin, J. Rock Mass and Pore Fluid Response in Deep Mining: A Field Monitoring Study at Inclined Longwalls. Minerals 2022, 12, 463. https://doi.org/10.3390/ min12040463 Academic Editors: Dongsheng Zhang, Gangwei Fan and Simon Dominy Received: 6 March 2022 Accepted: 5 April 2022 Published: 10 April 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).