The crack isolating theory was proposed. On the basis of the crack isolating theory and the near crack line analysis method, the elasticplastic stress field near the crack line was analyzed, the laws that the length of the plastic zone along the crack line is varied with external loads were established by the matching condition of the elastic-plastic fields on the boundary, and the strength properties of rock masses were determined. The solution is a function of the geometry of the crack array. The results show that the strength of rock masses depends on the crack spacing, orientation of the cracks, the crack interface friction coefficient and the crack half-length. The conclusions are of important significance for rock masses engineering.
