Evolution mechanism of axial force of super-long pipe roof

Pipe roofs are widely used as an effective proactive support measure in the construction of tunnel entrances, shallow-buried and underground excavated tunnels, underground stations, and large-section soft and weak soil structures. However, the stress variation characteristics of pipe roofs exceeding 40 m in length are not yet clear. This paper utilizes numerical simulation methods to conduct a comprehensive analysis of the deformation characteristics of three excavation methods: center cross-diaphragm method (CRD), both-side heading method, and the three-bench excavation method with super-long pipe roofs combined with temporary inverted arches. It specifically compares the deformation control effectiveness and stress variation patterns of pipe roofs of different lengths. The results indicate that the deformation control effectiveness of 40 m and 20 m long pipe roofs is inferior to that of super-long pipe roofs. Within a range of 30 m in front of the tunnel face and 20 m behind it, significant stress variations of the pipe roof are observed. The most influential range is within 10 m in front of the tunnel face and 5 m behind it. It is evident that the overall load-bearing capacity of the super-long pipe roof is higher than that of pipe roofs below 40 m. Furthermore, in this study, a novel approach is adopted by utilizing fiber optic grating testing technology to achieve comprehensive monitoring of the axial forces in super-long large pipe roofs. The measured data strongly corroborate the accuracy of the numerical calculations.