Based on the structural plane group (a total of four

named J1－J4) in Qingfeng Ancient Quarry located in Shepan Island of Zhejiang Province of China

the values of long-term shear strength parameters

cohesion and internal frictional angle

of the corresponding structural planes of the tuff were carried out using back analysis method. The results show that the cohesion is 0.078 MPa and the internal frictional angle is 15.76°

which are lower considerably than that of the rock mass with cohesion of 16.55 MPa and the internal frictional angle is 48.2°. The steps of the back analysis method are as follows. Firstly

it is assumed that structural planes of J1-J4 are consistent with Mohr-Coulomb strength criterion; and the rock mass is homogeneous elastic and isotropic. Secondly

the equilibrium equations with the cohesion and internal frictional angle for the unknown parameters are given based on that the structural planes are under the state of the limit equilibrium. Thirdly

carry on the 3D numerical calculation using the software of FLAC3D

extract the data needed for solving the above equations from the calculation results

substituting the above extracting data into the corresponding equation

and then four linear equations of two unknowns will be attained. Fourthly

the values of the long-term shear strength parameters

cohesion and internal frictional angle

of the structural planes can be obtained by solving this contradictory equations.