The stability of tunnels constructed in clay with overlying layers of coarse grained soil is examined using data from centrifuge model tests. Results show that the stability of a tunnel in the lower clay layermay be influenced by the presence of an upper layer depending on the type, state and thickness of the overlying material, and the amount of clay cover above the tunnel crown.
In many parts of the world tunnels are excavated through clay with overlying layers of coarse grained sands and gravels. Although the weights of these overlying materials are currently taken into account when assessing tunnel stability, the contribution of their strength and stiffness on tunnel stability is difficult to quantify. Centrifuge model testing has proven to be a very powerful tool for investigating the stability of geotechnical structures as it allows simulation of stress paths from full-scale (prototype) geotechnical events using small-scale models. Recently, a large number of plane strain centrifuge model tests have been conducted at City University, London, to investigate the effect of overlying coarse grained soil layers on the ground movements around tunnels in clay (Grant, 1998; Hagiwara et al., 1999). In addition to providing high quality data on the distribution of ground movements, the tests also produced a considerable quantity of data concerning the stability of the tunnels. The results are commonly used in engineering practice, but strictly speaking are only applicable for tunnels through ground profiles comprising clay alone. As far as the authors are aware there is currently no standard procedure for taking into account the contribution to tunnel stability of overlying layers of different materials.
The centrifuge model tests were conducted under conditions of plane strain. A schematic diagram of a typical model is shown in Figure 3.
