Mechanical behaviors and supporting effect evaluation of pipe roof in tunneling engineering considering micro-arch effects

Considering the micro-arch effects between the adjacent pipes, the delay effects of the initial support, the spatio-temporal effects of stress release of the surrounding rock and the coefficient change of subgrade reaction, the pipe roof is longitudinally divided into different sections. The corresponding mechanical model for the pipe roof is established both in the deep- and shallow-buried tunnels. Then, the mechanical response laws of the pipe roof are revealed through the parameter analysis. It is found that: (1) In the transverse direction, the pipe roof shows the micro-arch effects to bear the loads. The external loads on the pipe roof are the rock pressures transferred by the micro arches. (2) In the longitudinal direction, the pipe roof shows the beam effects to transfer the loads from the forward rock to the initial support. (3) Based on the Pasternak theory of elastic-plastic foundation beam, the analytical solutions for the deflection and the internal force of pipe roofs are derived, and the proposed theoretical solutions are more accurate than the existing theoretical ones. (4) The bearing ratio of the surrounding rock to the primary support and the pipe deflection are proposed to evaluate the supporting effects of the pipe proof quantitatively. To reliably control the deformation of the pipe roof and effectively transfer the loads of the surrounding rock, the excavation length of large-span tunnel should be limited in 2 m, and the distance between pipe roofs within 30° near the tunnel vault should be 5~10 cm shorter compared with that of other parts.