Mechanical models have been established for four-lane and two-lane highway bias tunnels with different cover depths by using finite differential code. The mechanical behavior of loose rock masses is studied. The displacement and convergence ratio of surrounding rock are analyzed when the impact of shear dilation to the stability of surrounding rock is considered. From the numerical results

for the same conditions of dilation angle

rock property and cover depth

some conclusions are drawn that the shear fracture band of four-lane highway tunnel tends to be expanded to the ground than that of two-lane highway tunnel

that the displacement vector and crown settlement decreases with the increase in dilation angle

but the horizontal displacement of abutment increases

and that the influence of dilation angle on the large span tunnel is greater than that of small span tunnel.