Comparative study on the mechanical mechanism of confined concrete supporting arches in underground engineering

In order to solve the supporting problem in underground engineering with high stress, square steel confined concrete (SQCC) supporting method is adopted to enhance the control on surrounding rocks, and the control effect is remarkable. The commonly used cross section shapes of confined concrete arch are square and circular. At present, designers have no consensus on which kind is more proper. To search for the answer, this paper makes an analysis on the mechanical properties of the two shapes of the cross-sections. A full-scale indoor comparative test was carried out on the commonly used straight-wall semi-circular SQCC arch and circular steel confined concrete arch (CCC arch). This test is based on self-developed full-scale test system for confined concrete arch. Our research, combining with the numerical analysis, shows: (1) SQCC arch is consistent with CCC arch in the deformation and failure mode. The largest damages parts are at the legs of both of them. (2) The SQCC arch’s bearing capability is 1286.9 kN, and the CCC arch’s ultimate bearing capability is 1072.4kN. Thus, the SQCC arch’s bearing capability is 1.2 times that of the CCC arch. (3) The arches are subjected to combined compression and bending, bending moment is the main reason for the arch failure. The section moment of inertia of SQCC arch is 1.26 times of that of CCC arch, and the former is better than the latter in bending performance. The ultimate bearing capacity is positively correlated with the size of the moment of inertia. Based on the above research, the engineering suggestions are as follows: (1) To improve the bearing capacity of the arch, the cross-sectional shape of the chamber should be optimized and the arch bearing mode changed accordingly. (2) The key damaged positions, such as the arch leg, should be reinforced, optimizing the state of force on the arch. SQCC arches should be used for supporting in underground engineering, which is under stronger influence of the bending moment and non-uniform load on the supporting arches. The research results could provide a theoretical basis for the design of confined concrete support in underground engineering.

Funding: This work was supported by the Natural Science Foundation of China (Nos. 51674154, 51704125), and the China Postdoctoral Science Foundation (Nos. 2017T100491 and 2016M602144). The Erlintu Mining is the work unit of the first author, which provided support in the form of salaries to the first author, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section.

Copyright: © 2018 Lv et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

In recent years, underground engineering is continuous to develop. So, high stress has become one of the key scientific problems of rock mechanics and engineering geology in engineering construction [1, 2, 3]. Many experts have carried out various researches on deformation control of surrounding rocks under high stress [4, 5, and 6]. The commonly used steel arch is insufficient in strength and often causes fracturing. And the instability of the arch leads to buckling and unstable failure, as shown in Fig 1. So the arch could not play its full bearing capacity [7, 8]. In addition, other scholars have discussed the bifurcation and stability behaviors in ground engineering [9].

Confined concrete component is made of steel with concrete inside, so that the two produce "force of symbiosis", and give full play to the advantages of both. The confined concrete structure has many advantages, such as high bearing capacity, good plasticity, convenience in construction and obvious economic, so it has been widely adopted in ground engineering [10, 11, 12, 13, 14, 15, 16, 17]. In recent years, researchers, including the authors’ team, have applied the confined concrete supporting method to underground engineering with complex conditions [18, 19].

Based on the commonly used U-shaped steel arch, Wang et al. [20] designed a confined concrete supporting structure. The open side of the U-shaped steel is closed by a sealing plate and the steel is filled with concrete. Its control effect is obviously superior to that of the conventional U-shaped steel arch. Wang et al. [21] proposed a new type of supporting method of square steel confined concrete (SQCC) for the deep underground roadway with fault fracture zones, and they conducted researches on the main supporting component—SQCC arch. Gao et al. [22] applied the circular steel confined concrete arch in the roadway with soft rocks and dynamic pressure, and they tested the mechanical properties of the concrete filled steel tubes and optimized the arch structure. These studies all show that the strength of confined concrete arch is 2 to 3 times higher than that of the conventional I-beam or U-shaped steel arch [23], and the confined concrete arch has good control effect in underground engineering applications.

The cross-section shape of the confined concrete arch is square or circular. At present, designers have no consensus on the cross-section shapes, so further study is necessary on the mechanical properties of the arch to make a reasonable choice on the cross-section shapes. For this purpose, the authors have conducted a comparative laboratory test on full-scale arches of both SQQC and CCC. The test is conducted with a self-developed full-scale test system for confined concrete arches. With the numerical calculation, studies are conducted on the ultimate bearing capacity and the deformation damage mechanism of the arch. Some suggestions are made on the arch shape selection.