Abstract: The application of static expansion rock fracturing technology has shown that drilling holes with sharp corners significantly enhances the fracturing effect due to the pronounced stress concentration effect. Therefore, it is of great significance to study the directional fracturing mechanism of static rock fracturing using quadrilateral holes with multiple sharp corners and to propose criteria for quantitatively analyzing the fracturing effectiveness. To this end, soft rock specimens with varying fractured rock areas, numbers, and shapes of drill holes were designed to conduct static expansion tests. The fracture processes and dynamic fracture mechanics behaviors of the specimens were monitored and analyzed using particle image velocimetry (PIV) technology and strain rosettes. Secondly, to quantitatively assess the fracturing effectiveness of the specimens, dimensionless parameters such as the average energy utilization rate, fracture time efficiency ratio, stress ratio, and fracture area ratio coefficient were introduced. The fracture coefficient was proposed as a comprehensive evaluation criterion. The results indicate that each specimen developed two main cracks along the angular bisectors of the quadrilateral’s sharp corners. As the cracks propagated, the peak stress and total energy decreased sequentially, while the fracture time lagged. For single-hole fracturing, the stress concentration effect of trapezoidal holes was 13.75% higher than that of rhombic holes, resulting in a better fracturing effect. For dual-hole interaction fracturing, the stress concentration effect of rhombic holes was 16.67% higher than that of trapezoidal holes, and the stress decay was slower. Among the combinations, the square-rhombic hole configuration had the highest average energy utilization rate at 62.66%. The fracturing effectiveness of the rhombic-trapezoidal hole combination was not significantly improved, with a fracture coefficient only 6.67% higher than that of a single trapezoidal hole. The square-trapezoidal hole combination exhibited the best fracturing effect, with a fracture coefficient of 1.15.

Key words: static expansion, quadrangular holes, directional cracking, PIV technology, energy utilization rate, fracture coefficient