Tunnel portal section mostly faces unfavorable geologic and topographical conditions

such as broken surrounding rock

shallow and unsymmetrical loadings. The current Chinese code for design of road tunnel provides the load calculation method of tunnel under unsymmetrical. However

according to the mechanic and deformation characteristics of the tunnel lining on site

the assumption is not suitable for the shallow tunnel under unsymmetrical loadings in broken surrounding rock. Therefore

through engineering case analysis and results of three-dimensional numerical analysis

the failure mechanism of shallow tunnel under unsymmetrical loadings in broken surrounding rock is presented firstly

that is

deep buried side broken surrounding rocks slump down after excavation and squeeze the lining

which makes the lining deform outward and bear passive earth pressure. Through the failure mechanism

the surrounding rock mainly includes slumping area and passive area. Then the corresponding formulas are deduced based on limit equilibrium theory. Finally

a case study is adopted to verify the feasibility of the new method. It is show that the new method is more reasonable than the method of current code.