Transparent soil model test and numerical study on the effect of adjacent spring on the stability of tunnel face in composite strata

To study the instability and failure mechanism of tunnel face in composite stratum and the evolution law of supporting pressure in the areas with spring, this paper used two different types of transparent soil and a self-designed 3D model test system. Six large transparent soil model tests were carried out by considering different confined water heads of spring and tunnel burial depth. The optical laser and high-speed camera were controlled to move on a high-precision linear platform. CT scanning was performed to obtain the failure model under different conditions. The finite element method considering a two-way fluid–structure coupling was used to validate the model test. The research results indicate under spring, the support pressure curves can be divided into three stages: rapid decline, rebound-rise, and constant. There is no rebound-rise stage under the condition of no spring. With increased confined water heads or reduced tunnel burial depth, limit support pressure shows an increasing trend. 2D and 3D damage models for different working conditions were obtained by PIV technology and 3D reconstruction technology. If there is a spring, the maximum displacement moves to the top of the tunnel with the increase of water head and the failure mode is a combination of “silo shape” and “inverted prism”. When there is no spring, the maximum displacement appears at the interface of the soil layer, and the failure mode is a combination of “silo shape” and “wedge shape”. The presence or absence of springs and the change of the confined water head have no significant effect on the height of the loosening area. With the tunnel burial depth ratio of 0.5 to 2.0, the height of the loose area increases from 0.17 to 0.83 D, and the soil arch area develops outward.