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Based on the taking sample by geological drilling, combined with the fractal principle, analysis on the cracked backfill particle size of its fractal features and strength correlation. Even each backfill sand specimen particle size is difference, but calculation data shows that the particle size of each sand specimen has preferable fractal feature, the sand specimen particle size distribution has remarkable fractal structure by the linear fitted results of the sand specimens. The fractal relationship of strength and particle size distribution shows that with the increased of fractal dimension, the strength of backfill is decreased, that is to say there is negative correlation, the main cause is that the higher parameter D of the fractal dimension, the higher fine-grained content and more non-uniform of the particle size distribution, especially for the thinner full tailings, if properly increasing the content of slightly crude particles, the strength of backfill will be certainly improved to some extent.

Hard rock generally performs ideal plastic character after the stress reaches its peak value with high confining pressure. Once the plastic strain satisfies certain condition, the brittle failure occurs, and the rock mass located in the residual region. Based on the mechanical model of hard rock under high confining pressure, we studied the surrounding rock stress and deformation evolution law of circular openings. The results shows that there is a significant end constraint effect at the opening face, and the deformation induced by disturbance is about one sixth of its total value. The results can provide theoritical foundation for deep underground engineer lining design.