Mechanical Properties of Basalt Fibre Reinforced Polymer (BFRP) Bars Reinforced Concrete‐Filled BFRP Circular Tubes and Their Application in Adjacent Tunnel Construction

ABSTRACT Basalt fibre reinforced polymer (BFRP) bars reinforced concrete‐filled BFRP circular tubes have high strength and excellent ductility. During the construction of an adjacent tunnel, the excavation of a new tunnel may cause stress redistribution in part of the surrounding rock, which in turn may cause damage to the lining structure of the existing tunnel. Therefore, effective reinforcement measures must be taken to reduce the construction risk and prevent casualties during the construction process of the adjacent tunnelling. Taking the Yanan No.1 tunnel as a case study, this paper first analysed the mechanical evolution law of micropiles and determined the optimal micropile structure through an axial pressure test. Then, numerical simulation was used to analyse the stress characteristics of the existing tunnel lining structure after excavating the new tunnel. Finally, the optimal spacing and length of micropiles were determined. The results showed that BFRP bars reinforced concrete‐filled BFRP circular tubes were the optimum form of micropile structure, with an ultimate stress of 51.80 MPa, and its stress‐strain curve had a bonding stage, elasticity stage, elastoplasticity stage and stress reduction stage. During the construction of the new tunnel, the lining structure of the existing tunnel was only damaged in tension at the arched roof. The maximum tensile and compressive stresses occurred at the arched roof and right wall, respectively. Compared with the length, the spacing of the BFRP bars reinforced concrete‐filled BFRP circular tubes had a more significant effect on the stress state of the existing tunnel lining structure, and the optimal spacing and length were 0.5 m and 1.0 B, respectively.