This paper presents an innovation work on the development of a novel energy-absorbing bolt characterized by an extraordinarily large elongation and high constant resistance. The bolt has a compound ...structure consisting of a cone-like piston sliding inside an elastically-deformable sleeve pipe. The frictional resistance generated by the sliding of the cone body relative to the internal surface of the sleeve pipe was mathematically formulated which is dependent on the elastic property of the sleeve pipe, the geometry of the cone and the frictional properties of the sliding interface, and independent of the external loads under the static loading conditions. A dashpot element for the cone-sleeve relative motion, termed “stick-slip element”, was proposed in construction of the lumped-mass model of the bolt for development of the constitutive equations that exhibits a frequency-dependent frictional behavior and a stick-slip oscillating response. The results from the static pull tests compared very well with the predicted working resistances, energy-absorbing capacity and elongations. The time-marching scheme of the bolt׳s impact load from the weight-dropping tests evolves with the pulsation response in the initial phase, stick-slip oscillation in the subsequent regime over which the dynamic energy is consumed, and a quasi-linear attenuation in the later phase. It demonstrates the fact that this bolt is robust in damping the dynamic load. The analytical work in this study provides solutions in the assessment of the large deformation and establishment of the forewarning precursors associated with deep mines.
•A novel energy-absorbing bolt was developed with unique working resistance generation mechanism.•Bolt can yield at high strength with large elongation in response to the loading.•Stick-slip model was established to model the load–elongation behavior of the bolt.•Static pull tests and dynamic impact tests were conducted to validate the analytical solutions.
Gob-side entry retaining (GER) is a popular non-pillar mining technique regarding how to reserve a gateroad for the use of next panel mining. When used in thick coal seams, the conventional entry ...retaining method requires a huge amount of filling materials and may cause entry (gateroad) accidents. Thus, an innovative non-pillar longwall mining approach is introduced. First, structural and mechanical models were built to explore the mechanism of the new approach. The modeling results indicate that effective bulking of the gob roof and reasonable support of the entry roof were key governing factors in improving entry stabilities and reducing roof deformations. Accordingly, a directional roof fracturing technique was proposed to contribute to gob roof caving, and a constant resistance and large deformation anchor (CRLDA) cable was used to stabilize the entry roof. Subsequently, the evolutionary laws of the roof structure and stresses were explored using numerical simulation. It was found that the structure of the surrounding rocks around the retained entry changed significantly after roof fracturing. The stress-bearing center was transferred to the gob area, and the entry roof was in a low stress environment after adopting the approach. Finally, the approach was tested on a thick coal seam longwall mining panel. Field monitoring indicates that the retained entry was in a stable state and the index of the retained entry met the requirement of the next mining panel. This work provides an effective and economical approach to non-pillar longwall mining in thick coal seams.
Due to strain energy releases sharply, fault slip is often accompanied with sudden stress change around fault structures, especially on the fault surface. In this study, the slippage of a reverse ...fault structure induced by coal mining in Yima mining area, Henan province, China is studied by employing physical and numerical simulation. The stress evolution around this fault is analyzed to further understand the mechanism of coal bursts around the fault structures. The results show that the normal stress is relatively greater than shear stress on the fault surface, and the normal and shear stress on the fault surface near the coal seam are greater than that far away from the coal seam. It is interesting to find that the variation of normal stress and shear stress on the middle of fault is opposite in the physical model. To in-depth understand this phenomenon, the numerical simulation method is employed to investigate the stress variation and dynamic energy evolution on the fault surface before, during and after fault slip by applied different horizontal stress on the lateral surface of model. The results show that the opposite variation of normal and shear stress is induced by the greater horizontal stress. This phenomenon can be considered as a precursory information for major fault slip. The horizontal stress is the main driving force of the fault slip induced by the coal seam mining. In this study, it is also suggested that the existence of extra-thick conglomerate rock and reverse fault in the high horizontal stress environment could cause large area of roof movement and provide the dynamic pressure source for the occurrence of coal bursts in Yima mining area.
•The reason why the horizontal loading is the main driving force of the fault slip has been found out.•The fault slip precursory information in terms of fault stress is found out.•The dynamic environment of coal bursts from combined effect of extra-thick rock and F16 fault is studied.•The reason why the coal burst accidents occur so frequently in Yima mining area is analyzed.
•A Constant Resistance Large Deformation (CRLD) bolt was introduced.•Experiments on a Split-Hopkinson Tension Bar for CRLD bolts were performed.•Numerical simulations were carried out by using ...different codes.•Improved understanding of dynamic behavior of the CRLD bolt was achieved.
Rockbursts frequently occur in deep underground excavations in a sudden or violent ejection of blocks of rocks from excavation walls. In addition to understanding rockburst mechanisms, rockburst control is an important issue for the safety of mining operations. Bolts and anchors are efficient measures to control rockbursts and permit the efficient exploration of underground excavations in mines. As mining depths increase, anchor and bolts with larger extension and higher loading capacity are needed. This paper introduces a Constant Resistant Large Deformation (CRLD) bolt which has been developed at the State Key Laboratory for Geomechanics and Deep Underground Engineering (GDUE), in Beijing, China. Static and dynamic tests were developed for this type of bolt. Advancements in Hopkinson tests were recently developed for CRLD bolts and Split-Hopkinson Tension Bar (SHTB) experiments for these bolts were performed. This paper presents the SHTB equipment and the results obtained with one and two CRLD bolts, as well a deep analysis of the obtained results. To better understand the complex nature of the problem, numerical simulations using several softwares were done for the case of one bolt, which had the dual goal of verifying the experimental and numerical results. In addition, the numerical predictions permit one conducting analysis the behavior of the bolt while targeting the development of large deformations. Finally some relevant conclusions are drawn.
Important task for acoustic emission (AE) monitoring involves detecting frequency shift phenomenon and intense periodic components. In the present research, we investigate time dynamics embedded in ...AE signal acquired in the laboratory rock burst experiment on limestone sample. By applying the Singular Spectrum Analysis (SSA)-based algorithm developed in this research, we reconstruct the decomposed components and then select the main component with a decision-making process based on the criterion that it should be significant both in the eigenvector space and spectral domain, termed eigenfrequency. The frequency shift phenomenon is represented by the eigenfrequencies of the first main component consistently. Precursory waves of the first main component represents time dynamics of the rock burst process by elastic wave over the low-level loading phase, high-frequency wave with self-oscillating envelopes at unloading, low-frequency quasi-shock waves during the rheological delay phase and low-frequency shock wave at complete rock burst failure.
•SSA-based algorithm was developed for analyzing AE data from rock burst test.•Main part of AE data were reconstructed with the decomposed components.•Selected main components are significant in eigenvector and spectral spaces.•Main eigenfrequecies well represent frequency shift in rock burst process.•Main component waves represent dynamic precursors in rock burst process.
•The generation and development of eddy in fracture was observed using PIV.•The relationship between macroscopic nonlinearity and eddies was investigated.•The shrinking of effective aperture and ...losing of kinetic energy make flow nonlinear.
The understanding of nonlinear flow in rough-walled rock fractures is critical for underground engineering and geological sciences. The macroscopic behavior and microfluidic field of flow in rough-walled artificial fracture models was investigated through microscopic visualization of hydraulic experiment. The results indicate that there are three states of flow regime in fracture, namely linear, weak inertial and strong inertial. In linear flow regime, the relationship between pressure drops and flow rate is linear, and all the streamlines in the microfluidic field are parallel. In weak inertial flow regime, pressure drop and flow become nonlinear, and the inertial pressure loss is less than the viscous pressure loss. Moreover, backflow occurs near the tortuous wall in the microfluidic field. In strong inertial flow regime, the inertial pressure loss is greater than viscous pressure loss, and the backflow begins to detach from the wall and eventually develop into a significantly large eddy far away from the wall. However, the flow in the mainstream remains laminar rather than turbulent in this experiment. This result indicates that the main cause of the nonlinearity of flow in fracture is eddies due to the increase of the inertia term and the variation of aperture. The shrinking of effective channel and losing of kinetic energy caused by eddies make fluid flow nonlinear. Moreover, the area and number of eddies depend on the fracture roughness and the Reynolds number.
The energy-absorbing bolt is designed to counteract both burst-prone and squeezing rock conditions in rock excavation. In this paper, the mechanical contribution of energy-absorbing bolts installed ...systematically around tunnels excavated in rock masses was investigated based on the assumptions of the reinforced rock unit (RRU) model. The time-dependent behavior of rock with a large deformation problem was simulated using a Nishihara model, while that of an energy-absorbing bolt was simplified by a rheological model connecting a Hookean elastic element and a Saint-Venant element in series. The mechanical model of RRU anchored by an energy-absorbing bolt was established by connecting the rock mass model and the energy-absorbing bolt model in a parallel combination. The closed-form solution presented the time-dependent behavior of RRU under an instantaneously applied tension stress and an linearly increasing load, respectively. The results showed the reinforcement could reduce the time-dependent deformation of the reinforced region of tunnels significantly. A preliminary estimate of NPR-bolt reinforcement for large-deformation controls in the Muzhailing tunnel was carried out using the proposed model. The results showed that the NPR cables could have critical effects in controlling the large deformation of the tunnel.
•Time-dependent behavior of the reinforcement rock unit (RRU) model was proposed.•Analytical investigations were carried out to understand the RRU model.•A case study was carried out to illustrate the application of proposed model.
A physical simulation model is constructed to study the characteristics of fault displacement evolution at Yima mining area, Henan Province, China. The geological condition at Yima mining area is ...complicated with high in situ stress, reverse faults and extremely thick conglomerate rock strata. In this study, the fault movement process during extraction of a coal seam is simulated and the precursory information on the fault slip is investigated. The fault displacement and shear stress, as well as the fault slip location are monitored in the simulation using digital speckle image correlation method, shear stress sensors, and acoustic emission monitoring. The investigations reveal the following: a relatively long suspension prior to the sharp increase in acoustic emission events might be used as the precursory information on an impending fault slip; the combined effect of the fault slip and extremely thick conglomerate rock strata may induce coal bursts; the interaction zone of thick conglomerate rock strata and fault structure is a high-risk region that can induce fault slips.
This paper attempts to present the findings involving rockbursts classification, rockburst failure criteria, and related control measures. Experimental investigations were performed using the ...strainburst testing machine and impact-induced rockburst testing machine. According to the stress paths and experimental methods, rockbursts were classified into two major groups, i.e. the strainbursts and impact-induced bursts. The mechanisms and criteria of rockburst obtained from experimental investigations were discussed. Then, the developments of constant-resistance and large-deformation bolt （CRLDB）, which can adapt itself to the external loading at a constant resistance by elongating continually, were introduced. The deformation energy Of country rocks with large deformation can be absorbed by CRLDBs. Finally, the principles and the experimental results for control and prevention of rockburst using the CRLDBs were presented.
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