Yaodong Jiang

Experimental and numerical modelling investigation on fracturing in coal under impact loads

In this paper, fracturing in coal under impact loads was studied using experimental and numerical approaches. Three-point beam bending tests were carried out on coal samples under impact loads. During the testing, cracking velocity in the samples was captured using a multi-spark high-speed photography system. Characteristics of the fracture surface were investigated using the scanning electron microscopy, 3D laser surface topography scanner and X-ray micro Computed Tomography (X-ray micro-CT). Differences between the fracture surface under impact loads and that in quasi-static test were analysed. Moreover, discrete numerical modelling was conducted to assess the influence of impact velocity, heterogeneity, and grain size on dynamic fracturing in coal. Based on observations from the testing and numerical simulation, it was concluded that the influence of heterogeneity and grain size was more pronounced in dynamic fracturing comparing to that under quasi-static loading.

Dynamic Tensile Strength of Coal under Dry and Saturated Conditions

The tensile failure characterization of dry and saturated coals under different impact loading conditions was experimentally investigated using a Split Hopkinson pressure bar. Indirect dynamic Brazilian disc tension tests for coals were carried out. The indirect tensile strengths for different bedding angles under different impact velocities, strain rates and loading rates are analyzed and discussed. A high-speed high-resolution digital camera was employed to capture and record the dynamic failure process of coal specimens. Based on the experimental results, it was found that the saturated specimens have stronger loading rate dependence than the dry specimens. The bedding angle has a smaller effect on the dynamic indirect tensile strength compared to the impact velocity. Both shear and tensile failures were observed in the tested coal specimens. Saturated coal specimens have higher indirect tensile strength than dry ones.

Failure mechanisms in coal: Dependence on strain rate and microstructure

The brittle coal failure behavior under various axial strain rates from 10−3 to 10−2 s−1 is experimentally and numerically studied. The numerical microscale finite difference model is built on the accurate X-ray microcomputed tomography images, which provides a ground-breaking and bottom-up approach to investigate the effects of microstructure on coal failure under various strain rates. Experimentally, prior to loading, the coal sample is scanned, and the three-dimensional coal structure model is constructed. The microheterogeneous structures are incorporated in the model, which facilitates the deformation and failure mechanism analysis under different loading conditions. The results reveal that the microheterogeneous structures significantly affect the evolution of stress concentrations and deformation behaviors in the sample. The coal tends to fail in the shear mode before the peak strength, since the shear zone is created with high displacements. However, tensile failure ultimately controls the failure process after the peak strength. Notably, the strain rate dependence of coal strength is observed, and an empirical relationship is proposed to describe the dynamic strength of the coal under various loading strain rates. Importantly, the coal strengthens with an increase in strain rate. For brittle material, such as coal, the strength and failure mechanism are strain rate and microstructure dependent. The strain rate-dependent coal strength index (n) is found to be a dynamic parameter in the range of strain rate from 10−3 to 10−2 s−1, and this finding may extend the concept of strain rate dependence over a broader range of loading conditions.

Investigation of Intrinsic and External Factors Contributing to the Occurrence of Coal Bumps in the Mining Area of Western Beijing, China

An investigation has been made to relate the occurrence of coal bumps to specific geological and mining conditions to the mining area of western Beijing. This investigation demonstrates that the high frequency of coal bumps in this area is due to four localized conditions, namely intrinsic coal properties, the presence of overturned strata and thrust faults, high in situ stress and the extraction of coal from island mining faces. Laboratory tests of coal samples indicated that the coals have a short duration of dynamic fracture, high bursting energy and high elastic strain energy, indicating that the coal is intrinsically prone to the occurrence of coal bumps. This investigation has also revealed that there are overturned strata and well-developed large- and medium-scale thrust faults in this area, and the presence of these structures results in plastic flow, severe discontinuities, rapid changes in overburden thickness and dipping of the coal seams. Well-developed secondary fold structures are also present in the axes and limbs of the primary folds. The instability of thrust faults, in combination with large-scale intrusion of igneous rocks, is closely associated with sudden roof breaking and induces sharp variations in electromagnetic radiation (EMR) and micro-seismic signals, which could be used to help predict coal bumps. In situ stress tests in the mining area demonstrate that the maximum and minimum principal stresses are nearly horizontal and that the intermediate principal stress is approximately vertical. The in situ stress level in the area is higher than the average in the Beijing area, North China and mainland China. In addition to the presence of overturned strata and thrust faults and high in situ stress levels, another external factor contributing to the frequency of coal bumps is coal extraction from island mining faces in this area. Island mining faces experience intermittent mining-induced abutment stress when a fault exists at one side of the island mining face due to reactivation of the fault, and this stress redistribution increases the likelihood of coal bumps during coal extraction from island mining faces.

Field Investigation of a Roof Fall Accident and Large Roadway Deformation Under Geologically Complex Conditions in an Underground Coal Mine

An investigation was undertaken to study the characteristics of large roadway deformation and driving force of roof fall in a geologically complex zone at Huangyanhui underground coal mine, Shanxi Province, China, and to determine the main factors contributing to a roof fall accident that occurred in this mine. A series of field tests were conducted in the mine to study the geological structures, in situ stress, excavation-damaged zones of the roadway, roof-to-floor and sidewall convergences, roof separation, bolts loading and island coal pillar stress. The results of these tests have revealed that the driving force of the large roadway deformation and roof fall was not the activation of the karst collapsed pillars or concentration stress in island coal pillar, but high levels of horizontal tectonic stress and fault slip were induced by mining activities.

Investigation on Strain Localization of Coal Using Micro-finite Difference Modelling

The behavior of saturated coal under compressive loads is very important for evaluating the coal mass stability and mining hazards control. This paper presents a numerical study of localized deformation process in saturated coal sample. Our approach combines X-ray micro-computed tomography (microCT) imaging with micro-finite difference analysis (microFDA). The three-dimensional images of a coal sample before compression were acquired by microCT scanner. Then the images were used to create finite difference model. The material properties of different groups in saturated coal were assumed based on previous experiments results. Finally, the FLAC3D code was employed calculate the strain localization features. The model was used to examine the material’s structural response to compressive loading by studying the stress distributions and material deformation. Results indicate that the nature of internal material distribution in coal sample affect failure features greatly. It also shows that a diffuse, wide shear strain localization band progressively thinning during the failure process.