Study on mechanical properties and influencing factors of confined concrete arch in underground engineering with complex conditions

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Confined concrete (CC) is a new form of high-strength support. To verify it, the authors have developed a large-scale mechanical test system. With the test system, a full-scale laboratory experiment is conducted in this paper to compare the mechanical properties of CC and traditional U-shaped steel arches in a straight-wall semicircle. Both of the CC and the U-shaped steel contain the same amount of steel. The experiment is also developed to study the deformation characteristics and the bearing mechanism of various arch types. Based on the numerical calculation, the variation law of bearing capacity of the CC arch is studied under different influencing factors. The factors include wall thickness of steel tube, concrete strength, lateral pressure coefficient, and others. The results show that (1) The U-shaped steel arch is prone to local buckling instability, resulting in failure of the entire arch. In contrast, the CC arch has an ultimate bearing capacity of 1072.4 kN, which is more than 2 times that of the U-shaped steel arch; and it also has high later bearing capacity. (2) Under the action of compression and bending, the arch is damaged with the key failure position on its legs. The bending moment is the main factor for the arch failure, and its action effect is enhanced with the increase in the lateral pressure coefficient. In field application, arch legs should be reinforced to reduce the peak of the bending moment and increase the overall strength of the arch. (3) Compared with the strength of core concrete, the wall thickness of the steel tube has a more significant effect on the mechanical properties and economy of CC arches. In the arch design, the field geological conditions and construction convenience should be considered. So, the steel tube should be used with the minimum wall thickness which meets the requirements of the bearing capacity of arches; and the common brand concrete should be used. Based on the above research, CC arches are designed for support in underground engineering with complex geological conditions. The control effect is remarkable with an average deformation of the surrounding rock controlled within only 72.2 mm.