**1. Introduction**

China's proven reserves of steeply inclined coal seams account for about 20% of its total coal resources, but its annual mining volume only accounts for 4% of the total coal mining volume, showing huge mining potential of steeply inclined coal seams in the future [1]. Notably, in provinces such as Yunnan, Guizhou, and Sichuan in Southwest China, more than 50% of the coal seams are steeply inclined coal seams, and the mining of steeply inclined coal seams is crucial to the region's economy. Compared with flat coal seams or gently inclined coal seams, it is more difficult to control roadways in steeply inclined coal seams. Characterized by complex geological structures and unpredictable coal seam occurrences, such coal seams are prone to large deformations, destruction, and even roof collapses. A survey [2,3] shows that the repair rate of steeply inclined coal seam roadways exceeds 80%, and many roadways are scrapped due to improper maintenance. The main reason for this is that unreasonable roadway support parameters cause large deformations of surrounding rocks, severely hindering sustainable production. Therefore, the safety maintenance and control of roadways in steeply inclined coal seams have attracted more and more attention from experts and scholars.

**Citation:** Xie, Z.; Wang, J.; Zhang, N.; Guo, F.; He, Z.; Xiang, Z.; Zhang, C. Study on Time-Dependent Failure Mechanisms and CBAG Differential Support Technology of Roadway in Steeply Inclined Coal Seam. *Processes* **2023**, *11*, 866. https://doi.org/ 10.3390/pr11030866

Academic Editors: Feng Du, Aitao Zhou and Bo Li

Received: 27 February 2023 Revised: 11 March 2023 Accepted: 12 March 2023 Published: 14 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The difficulties in controlling roadways in steeply inclined coal seams are that the self-stable performance of surrounding rocks is poor, and it is not easy to build a stable and reliable anchorage structure [4–6]. Three factors contribute to this situation: first, all of the occurrence angles of coal seams exceed 45◦, and a coal–rock mass with a large dip angle will naturally be subject to slipping and dislocation deformations under the action of self-weight stress; second, most steeply inclined coal–rock masses have experienced multiple geological tectonic movements and have poor lithology as well as low strength; third, the thickness of a coal–rock seam varies greatly, the coal–rock interface is widely distributed, and the development of joints is obvious. However, the damage process and failure mechanisms of roadways in steeply inclined coal seams are still unclear, so the existing support system does not have a good control effect, and deformations as well as damage continue to occur.

Most roadways in underground coal mines are characterized by time-dependent deformations; that is, the deformations of surrounding rocks are due to the cumulative damage of coal–rock masses, which does not happen suddenly. Scholars study the timedependent deformations of surrounding rocks mainly through numerical simulations, theoretical calculations, and field measurements [7,8]. The deformations and damage of roadway in steeply inclined coal seams are closely related to time; large deformations and damage of roadways often occur in later periods. Yang's analysis [9] shows that the gradual adjustment of surrounding rock stress is the main reason for large deformations of soft rock roadways with discrete elements. Sun [10] simulated the time-dependent deformation characteristics of roadways with FLAC 3D, and reduced the deformation speed of roadways by using jet grouting to impose constraints on surrounding rocks. Zhang [11] adopted concrete-filled steel tubular supports to passively control deep soft rock roadways and obtained deformation parameters of steel tubes via theoretical calculations to invert the deformation law of surrounding rocks. Tao [12] used a physical model to test the failure process of inclined layered soft rock roadways; through the real-time monitoring of the internal strain of the model during the failure process the failure mechanisms of inclined layered soft rock roadways are explored. On-site observation methods, such as borehole peeping, are the most intuitive means by which to study time-dependent failure of roadways, allowing clear observations of the number, angles, and openings of cracks in surrounding rocks for analyses of the evolution regularity of cracks in surrounding rocks of roadways over time [13–15].

In order to study the dynamic failure law and new control technologies of surrounding rocks of roadways in steeply inclined coal seams, this work studied the typical steeply inclined coal seam roadway of the Zhaojiaba Coal Mine in Sichuan Province, China. Methods such as numerical simulations, theoretical calculations, and field observations were employed to comprehensively analyze the time-dependent failure mechanisms of the roadway in the steeply inclined coal seam. A targeted CBAG differential support technology that has achieved a good control effect through field verification is proposed.
