minerals Article Research on Supporting Method for High Stressed Soft Rock Roadway in Gentle Dipping Strata of Red Shale Chunde Ma 1,2, Jiaqing Xu 1,* , Guanshuang Tan 1, Weibin Xie 1 and Zhihai Lv 1 1 School of Resources and Safety Engineering, Central South University, Changsha 410083, China; [email protected] (C.M.); [email protected] (G.T.); [email protected] (W.X.); [email protected] (Z.L.) 2 Advanced Research Center, Central South University, Changsha 410083, China * Correspondence: [email protected]; Tel.: +86-15904920851 Abstract: Red shale is widely distributed among the deep mine areas of Kaiyang Phosphate Mine, which is the biggest underground phosphate mine of China. Because of the effect of various factors, such as high stress, ground water and so on, trackless transport roadways in deep mine areas were difficult to effectively support for a long time by using traditional supporting design methods. To deal with this problem, some innovative works were carried out in this paper. First, mineral composition and microstructure, anisotropic, hydraulic mechanical properties and other mechanical parameters of red shale were tested in a laboratory to reveal its deformation and failure characteristics from the aspect of lithology. Then, some numerical simulation about the failure process of the roadways in layered red shale strata was implemented to investigate the change regulation of stress and strain in the surrounding rock, according to the real rock mechanical parameters and in-situ stress data. Therefore, based on the composite failure law and existing support problems of red shale roadways, some effective methods and techniques were adopted, especially a kind of new wave-type bolt that was used to relieve rock expansion and plastic energy to prevent concentration of stress and excess Citation: Ma, C.; Xu, J.; Tan, G.; Xie, deformation. The field experiment shows the superiorities in new techniques have been verified and W.; Lv, Z. Research on Supporting successfully applied to safeguard roadway stability. Method for High Stressed Soft Rock Roadway in Gentle Dipping Strata of Keywords: high stressed soft rock roadway; wave-type bolt; support technology Red Shale. Minerals 2021, 11, 423. https://doi.org/10.3390/min11040423 Academic Editor: Abbas Taheri 1. Introduction As the mining depth increases, the in-situ stress increases rapidly, the original support Received: 15 March 2021 technology and measures are invalid, the roadway repair rate is high and there are frequent Accepted: 13 April 2021 Published: 16 April 2021 roof falls, slabs, bottom heaves, etc. [1–3] after permanent roadway support, which requires multiple maintenances. With reinforcement, maintenance workload and high support cost Publisher’s Note: MDPI stays neutral have a serious impact on the normal and safe mining of the mine in Kaiyang Phosphate with regard to jurisdictional claims in Mine. The large deformation of the roadway severely restricts the production of soft rock published maps and institutional affil- mines [4–6]. Roadway support is a crucial step in the mining process, and it is also the iations. direction that many scholars have been studying [7–9]. The traditional support method of bolts, cables and steel supports do not distinguish between soft and hard rock. At the same time, the current internationally popular New Austrian Tunnelling Method (NATM) does not clearly answer the question of what the support object is [10,11]. Therefore, it is particularly critical to ensure the safety of roadways in deep inclined rock formations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Before supporting, it is necessary to clarify the failure mechanism of surrounding This article is an open access article rock. The failure mechanism of the surrounding rock of roadway has been extensively distributed under the terms and investigated in recent years. Wu et al. [12] studied the deformation mechanism of soft conditions of the Creative Commons rock roadway through in-situ monitoring technology, and the study pointed out that high Attribution (CC BY) license (https:// ground pressure and low rock strength are the main factors that cause large roadway creativecommons.org/licenses/by/ deformation. Li et al. [13] used an exploratory physical model, numerical simulation and 4.0/). field measurements to evaluate the deformation and failure modes of soft rock roadways. Minerals 2021, 11, 423. https://doi.org/10.3390/min11040423 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 423 2 of 17 He et al. [14] believe that deep underground support engineering should mainly con- sider how to counteract dynamic impact and retain the suitable rock mass deformation. Li et al. [15] discovered that the surrounding rock stress gradually decreases, meanwhile the deformation and failure mainly occur to the center of the roadway roof. Yang et al. [16] conducted a study on the failure mechanism of deep soft rock roadways in Xin’an Coal Mine, Gansu, China and found that the failure was first caused by high deviator stress caused by excavation, and then the damage expanded upon the deep part. At the same time, various damage phenomena in the shallow part were analyzed as tensile. Guo et al. [17] observed that the deformation of the soft rock roadway started from the roof and then led to the destruction of the entire support system. They believed that the cause of the failure was the uncoupling of the support mechanism and the surrounding rock. In addition, many scholars have discovered different failure mechanisms of surrounding rocks and analyzed the reasons for a targeted manner [18–20]. In response to the above problems, researchers have developed some theories and techniques for soft rock support. In order to solve the serious initial deformation of the soft rock roadway in Xiaokang Coal Mine, Wang et al. [21] designed a set of circular fully enclosed combined support systems, including high-strength prestressed bolts, a “W”-type strong anchor cable and U-shaped yielding steel. Chang et al. [22] used hydraulic expansion bolts to solve the problem of the floor heave of the soft rock roadway and overcome the adverse effects of residual rock powder in the bolt holes. Wang et al. [23] developed a confined concrete support system, using high-strength arch supports to ensure safe mining in Liangjia Coal Mine. Shreedharan et al. [24] used discrete element simulation software to analyze the stability of tunnels with different cross-sections under high stress, which provided a reference for the shape and support of the tunnel. Yu et al. [25] investigated a combined support methodology and optimized the support parameters by analysis of the field structure of weak surrounding rock to ensure the safety of the roadway. Although scholars work together to attempt to ensure safe production of deep stopes, there are still hidden safety hazards in some soft roadways. The support technique for highly stressed soft rock roadways still needs further development and study [26,27], especially when the roadway lays in a gently dipped layered soft rock mass with the typical bedding structure, which is a challenge to the control of soft rock with great deformation failure and rheological property [28]. In this paper, the failure mechanisms of the typical soft rock (red shale) have been investigated by testing for mineral composition and microstructure, anisotropic and hy- draulic mechanical properties and ground stress and analyzing of evaluation on existing support and numerical simulation on layered strata. Based on the failure mechanism, the approved version of the shotcrete rock bolt mesh technique is proposed to new-designed wave-type bolt layout installed at different angles for controlling the bedding structure. Besides, other helpful engineering suggestions were proposed. Finally, in-site industrial experiments results have validated the new support technique’s reasonableness. 2. Mechanical Properties of Red Shale In Kaiyang Phosphate Mine, most of the main development roadways are arranged in the footwall (red shale rock strata), which presents an inclination angle close to 31 degrees as shown in Figure1. The effect of rock mechanical properties on roadway stability is obvious, and the mineral components of rock have a decisive influence on rock mechanical properties. To investigate the failure mechanisms of the deep red shale rock roadways, a series of laboratory tests have been completed to learn the special physical and mechan- ical characteristics of red shale, including mineral composition and microstructure test, hydraulic mechanical properties, creep characteristic and anisotropic test. 2.1. Mineral Composition and Microstructure Test The red shale sample was taken from a new heading face of main haulage roadway in No.640 middle level in Kaiyang Phosphate Mine, then cut into slices and tested using Minerals 2021, 11, x 3 of 18 Minerals 2021, 11, 423 3 of 17 a microscope. The main mineral compositions were figured out as shown in Table1: its components of debris are quartz, orthoclase, sericite, muscovite and little zircon. And Minerals 2021, 11, x 3 of 18 the interstitial materials are composed of calcite, quartz, sericite, clay-type mud and ferruginous cementation. Clay-type mud accounted for 35% of mineral composition. Figure 1. The red shale inclined bedding structure around roadway cross section. 2.1. Mineral Composition and Microstructure Test The red shale sample was taken from a new heading face of main haulage roadway in No.640 middle level in Kaiyang Phosphate Mine, then cut into slices and tested using a microscope. The main mineral compositions were figured out as shown in Table 1: its components of debris are quartz, orthoclase, sericite, muscovite and little zircon. And the interstitial materials are composed of calcite, quartz, sericite, clay-type mud and ferrugi- nous cementation. Clay-type mud accounted for 35% of mineral composition. FigureFigure 1. 1.The The red red shale shale inclined inclined bedding bedding structure structure around around roadway roadway cross section.