Abstract: Pipe-shed support is a commonly used auxiliary construction method for tunnel engineering in complex geological environments, and it mainly relies on the micro-arch effect between pipes to play a supporting role. The formation and failure of micro arches are closely related to the spacing between pipe-sheds. After considering the lateral earth pressure and the non-uniform distribution of micro-arch section stress, the shape of the rational arch axis is determined and its standard equation of micro-arch is derived. By combining the failure conditions of micro-arch, a calculation method of rational pipe-shed spacing is obtained. Several engineering cases are adopted to verify the proposed method and the rationality of the method is validated by comparing with existing calculation methods of pipe-shed spacing. On this basis, the main parameters affecting the pipe-shed spacing are analyzed. The results show that the pipe-shed spacing is negatively correlated with the vault load. The pipe-shed spacing increases linearly with the increase of pipe-shed diameter and soil cohesion, and increases in nonlinear with the internal friction angle of soil. Significantly, the increase of internal friction angle of soil has an increasing impact on the pipe-shed spacing. However, the influence of the above parameters on the pipe-shed spacing is weakened with the increase of load.

Key words: tunnel entrance, micro-arch effect, lateral earth pressure, arch axis, pipe-shed spacing