Influences of tunnel radius on minimum overburden thickness for underwater shield tunnels crossing structural planes

Abstract

To ensure the stability and cost-effectiveness of underwater shield tunnels, reasonable design of the minimum overburden thickness is required. Nevertheless, how to rationally determine this thickness remains unclear when the coupled effects of rock structural planes and tunnel radius are considered, and existing design methods provide little guidance on this critical issue. This study used discrete element software to establish a three-dimensional mechanical model of an underwater shield tunnel that comprehensively considers the structural planes and tunnel radius. Initially, triaxial and permeability tests were conducted to obtain the basic mechanical and permeability parameters of the surrounding rock and structural planes, which were used as input parameters for the numerical model. Then, discrete element numerical simulations were performed to investigate the influence of tunnel radius and overburden thickness on the distribution and evolution of stress, displacement, pore water pressure, and water inflow in the surrounding rock. The results indicated that when the radius increases from 3.3 m to 5.0 m, stress release gradually intensifies, particularly at the structural planes, where it is most pronounced. At a radius of 5 m, tensile stress develops in the surrounding rock, accompanied by considerable abrupt displacement in both the rock mass and the crown lining. In contrast, water inflow increases gradually with increasing radius, without any abrupt jump. As the overburden thickness increases from 3 m to 30 m, the crown lining displacement exhibits a trend of first decreasing and then increasing. Based on the analysis, a calculation formula for the minimum overburden thickness of underwater tunnels crossing structural planes was proposed, which can provide a reference for engineering design and construction.