Uplift resistance and progressive failure mechanisms of metro shield tunnel in soft clay

•Transparent soil and PIV technique are applied to observe the soil movement.•Two different resistance-displacement response curves are presented.•The resistance on an uplifting tunnel is smaller than that on a pipe.•Progressive failure mechanism of clay soil are revealed and presented. In soft clay strata, a reliable design for disposing longitudinal uneven settlement of shield tunnels with grouting uplift technique requires the prediction of the uplift behavior. In this study, a laboratory model test system was developed to simulate the tunnel uplifting process by considering different burial depths of the tunnel. Transparent soils were prepared to model the typical clay, particle image velocimetry techniques were adopted to capture the internal continuous displacement field, and the load system was used to obtain the uplift resistance. During the uplifting process, the failure mechanism and response characteristics of the overlying soil were analyzed with different embedment ratios. According to the test results, two different resistance-displacement relationship curves are presented and described by four flag values. The pressure on the model tunnel is smaller than that on the pipe because there is no downward suction force underneath the tunnel during the uplift. Four different states are presented to describe the progressive failure mechanism of clay soil. First, the shear slip appears in the soil at the haunch of tunnel. Then, with the development of shear surface, the uplift resistance reaches the elastic limit. Soon afterwards, the uplift resistance reaches the peak; however, the complete shear band has not been formed. Finally, a complete shear band and an approximate vertical crack on the surface is observed, the overburden pressure reaches the residual uplift resistance, and the angle made by shear surface of soil wedge with the vertical plane are correlated with the embedment ratio of tunnel and the angle of internal friction of soils. Based on velocity fields, especially on the velocity direction, four-part failure mechanisms has been obtained.