From the viewpoint of tunnel stability, two proposed models are presented in this paper for estimating the minimum rock cover of a sub-water tunnel in the cases of homogeneous rock mass and inhomogeneous rock mass, which are based on the concept that minimum rock cover is obtained when support pressure at crown reaches to the minimum. The first model is based on the assumption that rock mass is homogeneous, where rock mass strength complies with Mohr-Coulomb failure criterion, while the latter is for inhomogeneous rock mass. Furthermore, comparison is made among the proposed models, Norwegian empirical curve and Japanese minimum water inflow model to estimate the thickness of the minimum rock cover.
Alignment of a sub-water tunnel is determined by geological and topographical conditions, as well as the tunnel's maximum gradient requirements, and minimum rock cover above tunnel crown. The minimum rock cover, defined as the minimum distance between the crown of a tunnel and the surface of bed rock under water (sea or lake), is a crucial dimension for the layout of a sub-water tunnel.
Currently, two methods are popularly used in determining the minimum rock cover for planning a sub-water tunnel, one is Norwegian empirical curve which is founded on the basis of Nordic subsea tunnel construction practice (Dahle & Nilsen 1994, Palmstrom 1994 & 2002). The other is Japanese minimum water inflow model which has been applied in most of subsea tunnels in Japan and also in China (Li et al. 2007).
In this paper, main works are emphasized on the following three subjects.
Brief introduction of two popularly used determination methods;
Basing on the concept of minimizing support pressure at crown, two models are proposed to be used in estimating the minimum rock cover for homogeneous rock mass and inhomogeneous rock mass, respectively;
Comparison is made among these three determination methods.