3D visual model tests on stability of tunnel excavation surface based on transparent soil

To study the supporting pressure and instability failure modes of shield tunnel face in composite soil strata under seepage and confined water, a three-dimensional visualized model test system for tunnel face stability is invented to freely exert multiple seepage and confining effects. For the first time, the system uses two different transparent soil materials to simulate a clay-gravel composite stratum, which can eliminate the particle size effect and demonstrate the internal deformation of the composite stratum clearly. Based on the digital image measurement technology and a self-written 3D reconstruction program, the 3D failure mode view of the excavation surface can be obtained accurately. A series of model tests are carried out to investigate the failure modes of the tunnel face with three different cover-to-diameter ratios of 0.5, 1.0 and 2.0 under seepage and confined water in clay-gravel composite strata. The test results show that the curve of the supporting pressure can be divided into three phases. In the critical failure state, the shape of the failure modes is related to the tunnel diameter ratio C/D obtained by the model tests. The shape of the failure modes is different when the tunnel covered depth is relatively small or large. In addition, the shape of the failure modes is also related to the type of soil strata. Compared to that under no seepage, the soil failure zone of the excavation surface under seepage confined water has a wider influence range. At the same time, the test system can vividly reproduce the evolution process and development law of the failure zone on the tunnel face. It has the advantages of high sensitivity, good stability and strong operability. It can be used to study the stability of the tunnel face under various complex conditions.