Physical model tests on stability of shield tunnel face in saturated gravel stratum

To detect the stability of tunnel face with different depth in saturated gravel stratum, three reduced scale model tests were conducted, and the progressive failure process under insufficient support pressure was explored. According to the particle analysis, loosens of particle contact and the initiation and closure of crack were investigated, the results indicate that delayed deformation occurs and it can be weakened by decreasing the tunnel depth. Based on the PIV analysis, the failure mode of tunnel face is obtained, it shows a combination mode of a sliding wedge and an overlying silo, and the inclination of sliding wedge is about 45°+φ/2. Due to the delayed deformation of soil in silo, a sudden collapse mode of tunnel face may occur. In order to produce a precise prediction of limit support pressure, a modified wedge model is proposed, and the obtained results compare well with test results.