Shankun Zhao

Multi-Factor Coupling Simulation and Mechanism Investigation of Hydraulic Fracture Propagation in Hard Roof

The propagation behavior of hydraulic fractures in deep hard roofs is governed by the multi-factor coupling of injection rate, in situ stress, and fluid viscosity. Taking the Cuimu coal mine as the engineering background, this study systematically investigates the effects of injection rate, lateral pressure coefficient, and fluid viscosity on fracture propagation through numerical simulation. Theoretical derivations based on the KGD (Kristianovich-Geertsma-de Klerk) model are further integrated and validated by field tests. The results indicate a critical injection rate of 6 × 10−8 m3/s, above which the marginal increase in acoustic emission events declines significantly. Increasing the lateral pressure coefficient from 1.0 to 3.5 shifts the fracture pattern from relatively simple to increasingly complex and interwoven, accompanied by a logarithmic increase in fractal dimension from 1.38 to 1.80. The total acoustic emission count rises to a peak of 125,910 as viscosity increases from 0.001 Pa·s to 0.5 Pa·s, then drops to 44,061 at 1.0 Pa·s, showing a unimodal trend. Theoretical analysis shows that during the propagation stage, the fracture length follows L∝Q1/2, and the maximum fracture opening follows wmax∝Q1/3. The lateral pressure coefficient controls the complexity of the fracture network through the directional distribution of stress intensity factors. Field tests at the Cuimu coal mine adopted a combination of stepwise injection rate and low-viscosity fluid, together with borehole densification and interval-skipping fracturing sequences. The effective fracturing radius reached 25~30 m, roof convergence was reduced by 31%, and the proportion of high-energy microseismic events decreased from 12% to 4%. This study establishes a complete theoretical framework from initiation theory to propagation dynamics and then to multi-crack competition, providing both a theoretical basis and engineering example for optimizing fracturing parameters in hard roofs under high stress anisotropy.

Loading-angle-dependent rate effects on dynamic mixed-mode I/II fracture behavior of coal-measure sandstone

Sandstone roof strata in deep coal mining are frequently subjected to impact disturbances, and their dynamic mixed-mode fracture response is closely related to crack initiation, propagation, and instability of surrounding rock. Previous studies have addressed rate-dependent dynamic fracture and angle-dependent mixed-mode fracture largely in isolation, but how loading rate and loading angle interact to govern the transition from opening-dominated to shear-dominated cracking, and how this coupled behavior manifests across different length scales has not been fully investigated. Therefore, this study conducted dynamic mixed-mode fracture tests on cracked straight-through Brazilian disc sandstone specimens with loading angles of 0°–90° and loading rates of 20–100 GPa/s using a split Hopkinson pressure bar system. High-speed photography, scanning electron microscopy, and three-dimensional fracture-surface fractal analysis were combined to characterize crack propagation, Mode I/II stress intensity factors, effective fracture toughness, and fracture morphology. The results indicate that the dynamic Mode I stress intensity factor decreases monotonically with increasing loading angle and changes sign near 45°, indicating a transition from opening-dominated to shear-dominated fracture. The dynamic Mode II stress intensity factor first increases and then decreases with loading angle, reaching its peak within approximately 45°. The dynamic effective fracture toughness follows a logarithmic relationship with loading rate within the tested range (R 2  = 0.94–0.99), with the strongest rate sensitivity coefficient a ranges from 0.59 to 3.08 across all loading angles and peaks at 3.08 near 45°, indicating the strongest rate sensitivity at the mixed-mode transition. The fracture-surface fractal dimension increases with loading rate and reaches its maximum within the 45–60° interval, where crack branching, grain pull-out, and local fragmentation are most pronounced. These findings demonstrate how loading rate and loading angle jointly govern dynamic mixed-mode I/II fracture in sandstone and provide laboratory-scale evidence for understanding rate-dependent fracture of sandstone roof strata under impact loading.

Experimental study on uniaxial compression mechanical properties and fracture fractal characteristics of dense sandstone under different temperature conditions