深部红页岩地质特性及其巷道掘进地压控制技术(1).pdf

J. Cent. South Univ. 2018 25 2979−2991 DOI https//doi.org/10.1007/s11771-018-3968-4 Geotechnical characterization of red shale and its indication for ground control in deep underground mining WANG Dong-yi王栋毅1, LI Xi-bing李夕兵1, PENG Kang彭康2, 3, MA Chun-de马春德1, ZHANG Zhen-yu张振宇2, 3, LIU Xiao-qian刘晓茜2 1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China; 2. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China; 3. College of Resources and Environmental Science, Chongqing University, Chongqing 400044, China Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Geotechnical properties of red shale encountered in deep underground mining were characterized on both laboratory and field scale to reveal its unfavorably in geoenvironment. Its constituents, microstructure, strength properties and water-weakening properties were investigated. In situ stress environment and mining-induced fractured damage zone after excavation were studied to reveal the instability mechanism. The results show that red shale contains swelling and loose clayey minerals as interstitial filling material, producing low shear strength of microstructure and making it vulnerable to water. Macroscopically, a U-shaped curve of uniaxial compressive strength UCS exists with the increase of the angle between macro weakness plane and the horizon. However, its tensile strength reduced monotonically with this angle. While immersed in water for 72 h, its UCS reduced by 91.9 comparing to the natural state. Field sonic tests reveal that an asymmetrical geometrical profile of fractured damage zone of gateroad was identified due to geological bedding plane and detailed gateroad layout with regards to the direction of major principle stress. Therefore, red shale is a kind of engineering soft rock. For ground control in underground mining or similar applications, water inflow within several hours of excavation must strictly be prevented and energy adsorbing rock bolt is recommended, especially in large deation part of gateroad. Key words red shale; soft rock; deep mining; geotechnical characterization; ground control Cite this article as WANG Dong-yi, LI Xi-bing, PENG Kang, MA Chun-de, ZHANG Zhen-yu, LIU Xiao-qian. Geotechnical characterization of red shale and its indication for ground control in deep underground mining [J]. Journal of Central South University, 2018, 2512 2979–2991. DOI https//doi.org/10.1007/s11771-018-3968-4. 1 Introduction Soft rock usually tends to behave unfavorably in geoenvironment for engineering stability, such as low strength, excessive deation and vulnerable to weathering, and it has been a major concern for stability of underground mining and geotechnical engineering under dynamic disturbance [1–3]. The soft rock strength generally falls between soil and hard rock, and is weathering-dependent, resulting in the difficulties of sampling and testing with conventional techniques of rock mechanics for some specific soft rocks [4, 5]. Moreover, due to insufficient geotechnical characterization, the empirical design based on rock mechanics usually ignores the nature of soft rock but focuses on defects similar to conventional jointed rock mass, Foundation item Projects51774058, 51674047 supported by the National Natural Science Foundation of China; Projectscstc2016jcyjA1861, cstc2018jcyjA3320 supported by Chongqing Basic Science and Cutting-edge Technology Special Projects, China; Project2015M570607 supported by Postdoctoral Science Foundation of China Received date 2017-09-30; Accepted date 2018-03-13 Corresponding author PENG Kang, PhD, Associate Professor; Tel 86-15974269965; E-mail pengkang; ORCID 0000- 0002-1405-3272 J. Cent. South Univ. 2018 25 2979–2991 2980 while the conservative design based on soil mechanics usually leads to high cost during construction and service periods [6]. Therefore, it is of great significance to conduct geotechnical uation of soft rock considering site-specific geoenvironment. Since 1950s, several definitions of soft rock were proposed. Qualitatively, loose, weak, highly weathering and swelling rocks are generally sorted as the soft rock. However, such conceptual recognition is hard to be implemented into engineering design. Quantitatively, the ISRM and ISSMFE Technical Committee distinguished the soft rock using the intact uniaxial compressive strength UCS. In the er classification, soft rock refers to the rock of intact UCS ranging 0.25– 25 MPa labelled as extremely weak, very weak and weak rock sequentially, where the overlap exists between extremely weak rock and very stiff and hard clays in category of soil mechanics [7, 8], while in the latter, the soft rock denotes geomaterial of intact UCS ranging 0.5–25 MPa [9]. Many other definitions of soft rock using similar indicators can be found in Ref. [6]. HE et al [10] proposed the concepts of geological soft rock and engineering soft rock, where the geological soft rock has the natural characteristics of low strength, large porosity and vulnerable to weathering, and the engineering soft rock refers to the rock of excessive plastic deation under engineering disturbance, suggesting the significance of site-specific geotechnical and geological conditions when uating rock mass classification. Comprehensively considering mineral constituents, rock structure, plastic deation mechanism, HE [4] further divided the soft rock into swelling soft rock, high-stress soft rock, jointed soft rock and combined-typed soft rock. These qualitative and quantitative concepts of soft rock have significantly advanced the understanding of soft rock and promoted the soft rock technology in geoengineering. The intact mechanical behavior of soft rock on the laboratory scale, such as strength, deability, swelling and weathering characteristics, is mainly influenced by mineral constituents. CIANTIA et al [11, 12] experimentally revealed the water-induced weathering process of soft carbonate rocks using scanning electron microscope, X-ray Micro- Computer-Tomography and Mercury Intrusion Porosimetry. Macroscopically, YOSHINAKA et al [13] characterized the mechanical behavior of soft rocks subjected to triaxial cyclic loading and found that their mechanical properties are dependent on plastic straining, following some simple exponential equations with coefficients dependent on the confinement. In sampling and geotechnical testing, COVIELLO et al [14] conducted direct pull test, Brazilian test, three point bending tests, four point bending tests and Luong tests to uate tensile strength of soft rock, and found that the measured tensile strength of soft rock varies with the specific testing s. This may beyond the scope of soft rock mechanics and the rationale is possibly due to schemes of different testing s. ULUSAY et al [15] and PENG et al [16] used needle penetrometer NP developed in Japan to uate UCS of soft and weak-to-very weak rocks, showing that the UCS of soft rock can be expressed as a power function of needle penetration resistance at a confidence level of 95. Hence, the NP can be used to uate UCS of soft and weak-to-very weak rocks up to 40 MPa excluding the soft rock of coarse grains. Furthermore, AYDAN et al [17] experimentally demonstrated that the needle penetrometer can also be used to uate the effects of water content, weathering state, number of cycles of freezing-thawing and time-dependent properties of soft rocks. For core recovery of soft rock from field, swivel type double barrels and sometimes triple barrels are encouraged to minimize drilling disturbance, and alternatively a new sampling system for regular and swelling samples is developed by State Key Laboratory of Geomechanics and Deep Underground Engineering, China, which consists of a compressed air driven drill, a portable sample cutting tool, and a portable sample box [5]. In order to improve core recovery of soft rock for the purpose of geotechnical uation, it is believed that further development for better sampling is essential for some critical soft rock. The mechanical behavior of soft rock on field scale is also influenced by site-specific geological conditions. CORTHSY et al [18] modified doorstopper technique to measure in situ stress in soft rocks. TAHERI et al [19] developed a down- hole in situ triaxial testing system to measure the shear strength and deability of soft rock at J. Cent. South Univ. 2018 25 2979–2991 2981 engineering sites. Except research efforts on field characterization, ground control for the underground mining and tunneling in soft rocks has been challenging and also been addressed. CHANG et al [20] used hydraulic expansion bolts to reduce floor heave in soft rock roadway, which can not only reinforce floor rock but also provide circumfluence constraint to plastic flow of floor rock. Starting from in situ stress measurement, SHEN [21] attributed the large deation and instability of a longwall entry, China, to inappropriate entry layout to principle stress direction and unfavorable geotechnical properties of coal and surrounding weak rock, suggesting full length grouting and high pretension rock bolts and cables together with optical cable/bolt arrangement. KANG et al [22] improved the rock bolting system to control excessive deation of a longwall mining entry by using competent load-bearing plates, steel-welded screens and high-strength rock bolts and cables with high pretension. ZHOU et al [23] conducted a series of geotechnical tests to uate the mechanical behavior of chlorite soft rocks and addressed the importance of sufficient pre- support in time in deep tunneling with sequential excavation s. This is in good accordance with the principles of New Austrian Tunneling . Red shale can be encountered in underground mining. For example, at Kaiyang phosphorus mine group, Guizhou Province, and Xingcun coal mine, Shandong Province in China, main gateroads were developed in red shale strata. Different from panel entris of coal mines in soft rocks mentioned above, the targeted main gateroads are designed to serve through the whole mine service period, typical more than 50 years, instead of only serving a specific mining panel in short term. Taking the red shale of Shabatu mine, Kaiyang County, Guizhou Province, China as the example, geotechnical characterization was conducted considering site-specific geological conditions. Firstly, the geotechnical properties of red shale encountered in underground mining environment were investigated. Subsequently, in situ stress measurement and field geotechnical characterization were conducted to uate the instability mechanism of targeted gateroad. Later on, suggestions for ground control improvement were presented. 2 Geological ination and in situ stress measurement Shabatu mine is located at the Kaiyang county of Guizhou Province in western China, producing high quality phosphate for fertilizer and causing serious environmental consequences at the same time [24, 25]. It was founded in 1950s and the annual production has been 2 million tons after half a century of mining. With the mining depth increasing, gateroads have suffered different degree of deation and failure due to high earth pressure. Currently, the preparation of mine development has extended to the level of 600–800 m and the mining activities has reached the depth of 500– 600 m underground. During the mining process, many challenges were encountered in ground control of main gateroads and existing support failed. Many induced seismicities have been monitored and located through the microseismic monitoring technology [26, 27]. In particular, collapse and large cracks appeared in roof corner, rib wall and floor corner on the unilateral side of gateroad, and severe squeeze occurred on the ditch side, destroying the ditch completely as shown in Figure 1. As a result, multiple maintenance and reinforcement are required to ensure normal production and the safety of miners, leading to heavy workload and high cost of support. The problems of ground control mentioned above occurred in the gateroad that was developed in red shale strata. In order to explain the instability mechanism of gateroad, especially the severe asymmetrical deation and damage phenomenon of gateroad, the macrostructure of red shale strata was visually investigated at mine site as shown in Figure 2. It can be seen that axial direction of the developed gateroad is parallel to the strike of the bedding plane and the dip of the bedding plane of red shale is inclined at 32 to the horizon. The in situ stress environment was uated using the LUT overcoring gauge. The LUT-gauge overcoring technique for stress measurement is in the category of stress relief , where the stresses are inferred from strains developed by the relief process and measured on isolated rock samples. In LUT-gauge, twelve stress gages were J. Cent. South Univ. 2018 25 2979–2991 2982 Figure 1 Failure patterns of main gateroad of Shabatu mine in red shale strata a Failure at roof corner; b Rib failure; c Roof collapse; d Failure at floor corner; e Floor heave; f Rock bolt retraction Figure 2 Bedding plane exposure at advance head of gateroad of Shabatu mine mounted in the probe to monitor the strain of borehole while the elastic modulus E and Poisson ratio υ can be obtained by conducting laboratory biaxial tests over the cylindrical rock sample comprising the overcore. The six stress components can then be derived based on the elasticity theory and finally transed to principal stresses. The stress measurement in Shabatu mine was conducted in the measurement chamber of the main gateroad location A at 700 m deep level as shown in Figure 3. The in situ stress measurement shows that the major and intermediate principal stresses are horizontal stresses with a magnitude of 24.64 and 13.20 MPa, respectively, and the minor principal stress is vertical stress with a magnitude of 6.02 MPa. The ratio of the maximum horizontal stress to vertical stress is approximately 4.09, indicating that the mine region has a strong tectonic J. Cent. South Univ. 2018 25 2979–2991 2983 Figure 3 Locations of in situ stress measurement and horizontal stress distribution Unit m stress field. Figure 3 shows that the axial direction of main gateroad is not parallel with the direction of the maximum horizontal stress and hence much more severe damage occurred on the unilateral side of the developed gateroad due to stress concentration on this unilateral side after exca