Study on the Initiation and Propagation Laws for Cracks in Drilled Holes of Mechanically Expanded Rocks

Taking green medium sandstone as the research object, indoor experiment, numerical simulation and theoretical analysis methods were comprehensively used to study the initiation and propagation laws of single-hole and multi-hole prefabricated cracks in rocks under mechanical expansion cracking. The results show that: The crack initiation pressure is reduced by more than 53% compared with that of the unprepared crack specimens, and the theoretical model coincides well with the results of numerical simulation and indoor tests; the penetration pressure of cracks under multi-hole conditions increases with the increase of hole spacing and peripheral pressure, and its reduction rate gradually decreases with the increase of peripheral pressure while it is not affected by the hole spacing; the crack initiation pressure gradually rises with the increase of peripheral pressure, and the hole spacing has no effect on it. The multi-hole mechanical expansion crack extension process can be divided into three basic stages: crack initiation, crack propagation and penetration, and the above expansion process is caused by mutual interference of peak tensile stresses generated by neighboring boreholes. The results of the study are of guiding significance to the engineering of hydraulic fracturing, expansion agent fracturing and directional blasting fracturing of hard roof slabs in coal mines.