Study on the stability of shield tunnel face in clay and clay-gravel stratum through large-scale physical model tests with transparent soil

The soil layers that are traversed by urban tunnel are usually complex. The failure mode of the tunnel face under complex soil conditions with different layers plays a key role for the construction and design of the tunnel. In order to investigate the response of the excavation face under different soil conditions and buried depth of the tunnels, two different types of transparent soils are used in the present paper to carry out six large-scale physical model tests by self-designed model device. Two high-precision motoring linear control platforms are utilized to simultaneously control the optical laser and a Charge Coupled Device (CCD) high speed camera aiming to perform "CT scan". The failure model of the excavation face is obtained through a three-dimensional program created by the team of this research. Based on the physical tests, it can be observed that the displacement and support pressure curve of the excavation face with respect to the withdrawal of the rigid plate can be divided into two stages in the clay stratum and three stages in the clay-gravel composite stratum. By conducting image analysis on the pictures captured by the CCD high speed camera, the two-dimensional failure zone of tunnel face is obtained. In the clay stratum, the failure zone forms a funnel that reaches the surface for small buried depths. Nevertheless, for larger depths, it consists of a sliding triangle and an overlying loosen rectangle without reaching the surface. In the clay-gravel stratum, the failure zone consists of a triangle and a funnel for small depths, while for larger depths, it consists of a triangle and a rectangle. However, the three-dimensional reconstruction of the failure mode provides more detailed information about its spatial distribution, constituting a useful guideline for the design and construction of tunnels.