Showing papers by "Jian Zhao published in 2021"

Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

Abstract: Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

Numerical investigation of blast-induced fractures in granite: insights from a hybrid LS-DYNA and UDEC grain-based discrete element method

Abstract: The micro heterogeneities have a significant effect on the mechanical behaviour and failure mode of brittle rocks. In order to study the influence of microstructure and micro-mechanical properties on rock fracturing under blast loadings, a hybrid LS-DYNA and UDEC grain-based discrete element method (UDEC-GBM) is developed in this study. The LS-DYNA code is used to simulate the explosive detonation process, while the UDEC code focuses on the simulation of the rock fracturing process induced by the shock wave. First, the fracturing process of Barre granite under blasting is reproduced by the developed method. The fracture patterns simulated by the coupled LS-DYNA/UDEC-GBM method show a good agreement with experimental results. Then, using the coupled method, the effects of the grain size distribution, the average grain size, the mineral composition, and the contact tensile strength between minerals on the blast-induced fractures are systematically investigated. Numerical results show that intergranular tensile cracks dominate the rock fracturing under blast loadings. The number of microcracks is affected by all these four factors. However, only the mineral composition has a significant influence on the proportion of transgranular cracks to intergranular cracks. In addition, compared with the grain size distribution and mineral composition, the average grain size and contact tensile strength have a more significant influence on fracture patterns.

Laboratory-scale investigation of response characteristics of liquid-filled rock joints with different joint inclinations under dynamic loading

Abstract: In underground rock engineering, water-bearing faults may be subjected to dynamic loading, resulting in the coupling of hydraulic and dynamic hazards. Understanding the interaction mechanism between the stress waves induced by dynamic loadings and liquid-filled rock joints is therefore crucial. In this study, an auxiliary device for simulating the liquid-filled layer was developed to analyze the dynamic response characteristics of liquid-filled rock joints in laboratory. Granite and polymethyl methacrylate (PMMA) specimens were chosen for testing, and high-amplitude shock waves induced by a split Hopkinson pressure bar (SHPB) were used to produce dynamic loadings. Impact loading tests were conducted on liquid-filled rock joints with different joint inclinations. The energy propagation coefficient and peak liquid pressure were proposed to investigate the energy propagation and attenuation of waves propagating across the joints, as well as the dynamic response characteristics of the liquid in the liquid-filled rock joints. For the inclination angle range considered herein, the experimental results showed that the energy propagation coefficient gently diminished with increasing joint inclination, and smaller coefficient values were obtained for granite specimens compared with PMMA specimens. The peak liquid pressure exhibited a gradually decreasing trend with increasing joint inclination, and the peak pressure for granite specimens was slightly higher than that for PMMA specimens. Overall, this paper may provide a considerably better method for studying liquid-filled rock joints at the laboratory scale, and serves as a guide for interpreting the underlying mechanisms for interactions between stress waves and liquid-filled rock joints.

Reclamation of coal mining subsidence based on people’s esthetic preference

Abstract: Underground coal mining causes subsidence. How to use this subsidence is a big challenge for planners and policy makers. The subsidence in urban area is typically used as green space in China. However, is the green space transformed from subsidence preferred by the general public? Also, is there another better way to use subsidence? To answer these questions a subsidence in Xuzhou, eastern China was selected to explore participants’ esthetic perception for three reclamation modes: urban park (called designed landscape), natural succession of ecosystem (called natural landscape) and the scenario formed by adding a path to natural landscape using photomontage simulation (called combination of design and nature) across four seasons. Statistical analysis showed that designed landscape was preferred over natural landscape. Although there were no significant differences in preference between four seasons for both natural and designed landscape, designed landscape had a smaller preference fluctuation across four seasons than natural landscape. And the combination of design and nature gained higher preference score than both designed landscape and natural landscape. The findings in this study provide reliable evidence to guide the reclamation of coal mining subsidence.