In accordance with the problems of existing methods for surrounding rock pressure of shallow large-span tunnel in up-soft/ low-hard rock stratum

by using DEM (distinct element method) and orthogonal array testing method

the law and sensibilities of the failure mechanism of loose zone and the thickness of overlying weak rock stratum and the joint occurrence influencing on shape and scope of the loose zone are analyzed. In sequence

the sensibilities of factors are as follows in the magnitude order: thickness of overlying weak rock stratum

tunnel depth

angle and distance of continuous inclined joint

distance of horizontal joints in weak rock stratum. Through comparative analysis two failure modes of arch collapse and caving collapse are obtained; the boundary curves are analyzed and fit with linear and non-linear models by using the software ORIGIN

the test results show their boundary curves are power function curve and parabola curve. In accordance with caving collapse loose zone

new formulae for the length of earth surface crack and the soil strip width above the vault are formulated by using the liner least squares method. The errors of the empirical formulae are analyzed. According to the stress transfer principle

a differential trapezoid geotechnical strip is chosen as research object

the formula of surrounding rock pressure of loose zone which considering the joint occurrence and thickness of overlying weak rock stratum is deduced. Case studies show that the results of proposed method are smaller than the results of the existing representative methods.