Lianguo Wang

Semi-analytical and Numerical Studies on the Flattened Brazilian Splitting Test Used for Measuring the Indirect Tensile Strength of Rocks

Based on the two-dimensional elasticity theory, this study established a mechanical model under chordally opposing distributed compressive loads, in order to perfect the theoretical foundation of the flattened Brazilian splitting test used for measuring the indirect tensile strength of rocks. The stress superposition method was used to obtain the approximate analytic solutions of stress components inside the flattened Brazilian disk. These analytic solutions were then verified through a comparison with the numerical results of the finite element method (FEM). Based on the theoretical derivation, this research carried out a contrastive study on the effect of the flattened loading angles on the stress value and stress concentration degree inside the disk. The results showed that the stress concentration degree near the loading point and the ratio of compressive/tensile stress inside the disk dramatically decreased as the flattened loading angle increased, avoiding the crushing failure near-loading point of Brazilian disk specimens. However, only the tensile stress value and the tensile region were slightly reduced with the increase of the flattened loading angle. Furthermore, this study found that the optimal flattened loading angle was 20°–30°; flattened load angles that were too large or too small made it difficult to guarantee the central tensile splitting failure principle of the Brazilian splitting test. According to the Griffith strength failure criterion, the calculative formula of the indirect tensile strength of rocks was derived theoretically. This study obtained a theoretical indirect tensile strength that closely coincided with existing and experimental results. Finally, this paper simulated the fracture evolution process of rocks under different loading angles through the use of the finite element numerical software ANSYS. The modeling results showed that the Flattened Brazilian Splitting Test using the optimal loading angle could guarantee the tensile splitting failure initiated by a central crack.

Combined early warning method for rockburst in a Deep Island, fully mechanized caving face

Rockbursts annually cause hundreds of casualties and huge economic losses in China. The possibility of rockbursts occurring in a deep island, fully mechanized caving coal face is much higher than in a normal face. To predict and prevent rockbursts in this type of coal face, a comprehensive approach based on multi-instrument monitoring, including microseismic (MS) monitoring, electromagnetic emission (EME) monitoring, and the drilling cutting method (DCM), has been proposed. The case of working face 1304 in the Yangcheng coal mine in Shandong Province, China, was analyzed to test the combined early warning method. In situ investigations showed many early warning precursors before a rockburst, such as a period of silence in MS activity, persistent rising of EME intensity values and pulse numbers, and a large jolt of EME. Based on the quantitative early warning indexes, the combined early warning method for rockbursts in coal face 1304 was established, which mainly includes three rockburst risk judgment conditions and one rockburst risk identification condition. This method has been applied to rockburst forecasting in the Yangcheng coal mine. The application showed that this comprehensive approach could accurately identify potential rockburst risks and trigger early warnings. The case study demonstrated that the likelihood of a rockburst occurring can be forecasted by the combined early warning method based on multi-instrument monitoring.

Failure Mechanism Analysis and Support Design for Deep Composite Soft Rock Roadway: A Case Study of the Yangcheng Coal Mine in China

This paper presented a case study of the failure mechanisms and support design for deep composite soft rock roadway in the Yangcheng Coal Mine of China. Many experiments and field tests were performed to reveal the failure mechanisms of the roadway. It was found that the surrounding rock of the roadway was HJS complex soft rock that was characterized by poor rock quality, widespread development of joint fissures, and an unstable creep property. The major horizontal stress, which was almost perpendicular to the roadway, was 1.59 times larger than the vertical stress. The weak surrounding rock and high tectonic stress were the main internal causes of roadway instabilities, and the inadequate support was the external cause. Based on the failure mechanism, a new support design was proposed that consisted of bolting, cable, metal mesh, shotcrete, and grouting. A field experiment using the new design was performed in a roadway section approximately 100 m long. Detailed deformation monitoring was conducted in the experimental roadway sections and sections of the previous roadway. The monitoring results showed that deformations of the roadway with the new support design were reduced by 85–90% compared with those of the old design. This successful case provides an important reference for similar soft rock roadway projects.