Considering the limitation of the unified strength theory (UST) in nonlinear fitting and nonlinear features of geotechnical materials

a brittle-shear strength analysis method is proposed; and a transition bilinear formula is derived. Nonlinear strength feature of Hoek-Brown criterion is explained based on the double failure mechanism of fracture and shear; and an associated formula with highly correlation is obtained. Mine-by tunnel and the strength of Beishan granite data results analysis show that coefficients of correlation of Hoek-Brown rock mass strength

experimental data and brittle-shear strength values are all above 0.98. Twin formulas of Hoek-Brown criterion in the form of UST are derived based on the brittle-shear nonlinear fitting. Application of UST to fitting nonlinear strength of geotechnical materials is realized.