The concept of composite arching forclosely-spaced tunnels in soils
The conventional arching effect (such as active and passive arching) cannot reasonably explain the vertical load transfer between twin closely-spaced tunnels. In this study, a new type of arching called composite arching and corresponding models in soils are proposed to illustrate the arching phenomenon of the first tunnel, in which both sides of the soil column above an existing tunnel are acted on by shearing resistance with the upward and downward direction respectively. Model tests of double trapdoors indicate that the distribution of tunnel pressure in composite arching is highly asymmetric and approximately trapezoidal. In case the first tunnel reaches the limit state of active arching during excavation, the transfer path of negative friction is nearly interrupted by the outer sliding surface of the first tunnel during the construction of the second tunnel. The composite arching is the third type of arching and lays a foundation for theoretical research on closely-spaced tunnels in soils.