Study on failure mechanism of room and pillar with different shapes and configurations under uniaxial compression using experimental test and numerical simulation
Experimental and discrete element methods were used to investigate the failure behavior of the room and pillar with different configurations under uniaxial loading. Concrete samples with a dimension of 15 cm × 15 cm × 5 cm were prepared. Within the specimens, rooms and pillars with different configurations were provided. The room dimension was 1 cm × 1 cm, and the pillar dimension was according to the room configuration. Twelve different configurations were chosen for rooms and pillars. The axial load was applied to the model at a rate of 0.05 mm/min. The results show that the failure process was mostly governed by both the non-persistent joint angle and joint number. The compressive strength of the specimens was related to the fracture pattern and failure mechanism of the pillars. It was shown that the shear behavior of pillars was related to the number of induced tensile cracks, which increased by increasing the room angle. The compressive strength of samples increased with the increase of the room angle. The failure pattern and failure strength are similar in both methods, i.e., the experimental testing and the numerical simulation.
When the room and pillar mining method is employed to extract the mineral resources, pillars are always retained to support the overburden. Although a significant amount of research has been conducted to understand the loading and failure characteristics of pillars, it is still difficult to assess the stability of pillars or a pillar and roof support system, which is affected by many factors, such as in situ stress and geological conditions and pillar failure with roof collapse continues to occur. In particular, when remnant pillars in the upper seam are disturbed by mining activity in the lower seam, pillars can progressively fail and additional subsidence can be induced. Thus, it is critical to study the interaction between coal seams that can lead to the failure of pillars and the unsteady movement of the roof. In past decades, researchers have focused on the mechanical behavior of a single pillar. Modeling of the room and pillar method is aimed at developing optimal methods of selecting a deposit while maintaining a high safety factor and obtaining minimal operational losses. Many empirical formulas have been established to estimate the strength of a pillar and the average pillar stress was calculated based on the tributary area or pressure arch method. Then, a factor of safety (defined as the ratio of pillar strength to average pillar stress) is utilized to design the size of a pillar or assess the stability of a pillar. However, some pillars with a high factor of safety also fail without any precursor. Cascading pillar failure is usually caused when one pillar fails and the additional load is transferred to overload the adjacent pillars. In some cases, only a few pillars fail; in the worst scenario, hundreds of pillars will fail and produce a large area of surface subsidence. A post-failure stability criterion was developed to assess whether the failure process occurred in a stable, nonviolent manner or unstable manner.
The criterion has been applied to optimize the pillar layout to prevent the occurrence of catastrophic domino-type pillar failure stands abolished a dynamic analysis method to investigate the cascading pillar collapse phenomenon using the Voronoi graph method, which considered the stress redistribution. A load transfer method was also developed to investigate the bearing characteristics of a pillar and roof system and has been utilized with the consideration of pillar interactions to successfully assess the subsidence risk of a decommissioned bord-and-pillar colliery in Australia performed a series of experimental and numerical tests to analyze the stress redistribution and progressive failure characteristics of multiple pillars–roof system. Generally, new mining panels are designed below an existing mine to increase the recovery rate of the mineral resources as shallow resources are exhausted. The effect of multiple seam interactions has caused roof caving, rib spalling, and bumps. Especially in northeast China, some parts of shallow coal seams had been previously extracted by small mines using the room and pillar mining method, which generated a considerable number of coal pillars. Some of the coal pillars have collapsed, causing significant surface subsidence. When longwall mining was employed underneath the room and pillar mine in the Shengyu coal mine region to satisfy market demand, sudden roof-caving events occurred. These accidents pose a serious risk to underground projects and the environment with additional strata movement. In the previous works, there is an absence of the study of the behavior of multiple rooms with different shapes and configurations. In this paper, the failure behavior of multiple rooms with different shapes and configurations has been investigated under uniaxial compression loading.
Uniaxial compression testing of rock-like samples containing rooms and pillars:
To simulate room and pillar, rock-like materials were used in the experiments. The materials blended well, and a weight ratio of 2 to 1 cement to water was prepared. There were no additives in the concrete mixture. The uniaxial compression strength of the sample was 7.4 MPa. The specimen size (length × width × height) was 15 cm × 15 cm × 5 cm. This dimension was chosen due to the limitation of machine space. To create the room and the pillar, cubic steel molds with dimensions of 1 cm × 1 cm were pre-inserted into the material and removed after the initial hardening of the specimen (Fig. 1). The dimension of the holes was 1 cm × 1 cm × 5 cm. To increase the reliability of the test and remove accidental errors, three similar prefabricated crack test blocks were provided for every set. Each model consisted of four rooms. The room dimensions were 1 cm × 1 cm × 5 cm. Twelve different configurations were chosen for rooms and pillars (Fig. 2). The samples were preserved in a cool and ventilated condition for 28 days. The electrohydraulic universal testing machine was employed to perform the uniaxial compression test on samples. The experimental system includes the test bed, control system, and data logger. In the experiment, the rate of loading displacement was set to 0.05 mm/min (Fig. 3). In this condition, Rock mass was subjected to uniaxial compression.
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Journal Reference: science direct
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