Three-dimensional stability analysis of shied tunnel face adjacent to existing tunnels

Aiming at the stability of the shield tunnel face adjacent to an existing tunnel, the numerical simulation and three-dimensional theoretical analysis methods are used for in-depth research. The failure modes of the shield tunnel face are obtained through numerical simulation. The results show that the failure mode of shield tunnel face adjacent to the existing tunnel is mainly affected by the distance between the tunnel face and the existing tunnel (s/D), and it looks like a logarithmic spirochete with elliptical section when -1 < s/D < 0.5. The proposed three-dimensional failure model which is composed of logarithmic spiral body and elliptic column can reflect the non-uniformity of the instability expansion along the longitudinal and transverse directions, respectively, hence, the analytical results of the tunnel face stability may be more accurate. The parameter analysis indicates that the limit support pressure of the shield tunnel face is the most sensitive to the strength parameters of soils, followed by the buried depth and adjacent distance. The limit support pressure of the tunnel face increases first and then decreases with the increase of the internal friction angle when the cohesion of soils remains unchanged within a certain buried depth and adjacent distance (C/D = 2.0 ~ 3.0 and d/D = 0.5 ~ 1.0), and the critical value of the internal friction angle is about 11°~13°.