Search

Underground coal mining can cause the deformation, failure, and collapse of the overlying rock mass of a coal seam. If the mining design, monitoring, early warning, or emergency disposal are improper, in that case, it can often lead to mining disasters such as roof falls, water inrush, surface collapse, and ground fissures, seriously threatening the safety of mine engineering and the geological environment protection in mining areas. To ensure the intrinsic security of the entire coal mining process, aspace–time continuous sensing system of overburden deformation and failure was developed, which breaks through the limitations of traditional monitoring methods that characterize the evolution process of overlying rock deformation and ground subsidence. This paper summarizes the classification of typical overburden deformation and failure modes. It researches the space–time continuous sensing of rock–soil mass above the coal seam based on Distributed Fiber Optic Sensing (DFOS). A multi-range strain optical fiber sensing neural series from micron to meter was developed to achieve synchronous sensing of overburden separation, internal micro–cracks, and large rock mass deformation. The sensing cable–rock mass coupling test verified the reliability of the optical fiber monitoring data. The sensing neural network of overburden deformation was constructed using integrated optical fiber layout technology on the ground and underground. Different sensing nerves' performance and application effects in overburden deformation and failure monitoring were compared and analyzed with field monitoring examples. A physical model was used to carry out the experimental study on the overburden subsidence prediction during coal mining. The results showed that the optical fiber monitoring data were reliable and could be used to predict overburden subsidence. The reliability of the calculation model for overlying rock subsidence based on space–time continuous optical fiber sensing data was verified in the application of mining subsidence evaluation. A systematic review of the shortcomings of current overburden deformation observation technology during coal mining was conducted, and a space–time continuous sensing system for overburden deformation and failure was proposed. This system integrated sensing, transmission, processing, early warning, decision-making, and emergency response. Based on the fusion of multi-parameter sensing, multi-method transmission, multi-algorithm processing, and multi-threshold early warning, the system realized the real-time acquisition of space–time continuous information for the overburden above coal seams. This system utilizes long-term historical monitoring data from the research area for data mining and modeling, realizing the prediction and evaluation of the evolution process of overburden deformation as well as the potential for mining subsidence. This work provides a theoretical reference for the prevention and control of mining disasters and the environmental carrying capacity evaluation of coal development. [ABSTRACT FROM AUTHOR]

Large compressive deformation of tunnels is a phenomenon involving plastic deformation and failure of surrounding rocks and often refers to the weak surrounding rock self-bearing capacity loss or partial loss. This research discusses the formation and evolution of large compressive deformation and effectiveness of the combined support of high in situ stress tunnel. From the new perspective of large deformation disaster caused by the structural failure of high in situ stress surrounding rock to clarify it, this paper illustrates the mechanism of progressive cracking and large deformation of high in situ stress soft rock tunnel from the aspects of the formation of self-bearing system, deformation evolution of the surrounding rock, mechanical properties of the surrounding rock, and failure characteristics. Accordingly, the continuous and discontinuous numerical simulation methods are used. The following conclusions are drawn by comparing the simulation results of surrounding rock under combined support with no support. (1) The supporting structure constitutes the self-supporting system with the surrounding rock and plays the roles of codeformation and load-bearing. (2) The support structure has evident reinforcing effect on the rock mass in the relaxation zone, thereby leading to the phenomenon of weakened rock mass failure. Moreover, the shear area develops to the compaction zone. (3) The supporting structure improves the bearing capacity of rock mass in the relaxation zone. It also increases the surrounding rock stress and reduces the range of the compaction zone. Simulation results verify that the combined support measures have a good suppression effect on the large compressive deformation, thereby providing a reference for similar projects and research on the large compressive deformation of soft rock. [ABSTRACT FROM AUTHOR]

The conventional bolts used in surrounding rock tunnels with large deformation often fail. As a solution to this problem, we developed an extensible bolt with energy-absorbing and constant-friction-force (EACF) characteristics. The EACF bolt mainly comprises a damping device, a hollow threaded bolt, a tightening nut, and a face plate. To reveal its working mechanism, the bolt was tested in terms of its friction, displacement, and energy absorption through a modified tensile test device in a laboratory. The static pull-out test results showed that the axial force-displacement curve of the bolt can be mainly divided into three stages: a conical extrusion stage, an elongation stage, and an elastic failure stage. The EACF bolts exhibited stable energy absorption behaviors when subjected to static loading. The maximum constant friction force could be adjusted by increasing the size and diameter of the straight section of the damping block, and the maximum elongation could be adjusted by increasing the length of the damping cylinder. When the properties of the bolt materials are kept constant, increasing the diameter of the damping block can help achieve a high constant resistance. The proposed EACF bolt has reliable deformation and energy-absorption properties, which ensure its stability when employed in tunnels under the combined action of support and surrounding rocks. [ABSTRACT FROM AUTHOR]

Recent research shows that the brittleness of rock is closely related to the initiation and propagation of internal microcracks, but there are few brittleness evaluation indices considering the characteristics of rock initiation. Based on the theoretical analysis of brittleness and the characteristics of rock initiation, this study proposes an evaluation method of rock brittleness based on the prepeak crack initiation and postpeak stress drop characteristics. First, based on the description and definition of brittleness by George Tarasov and Potvin et al., the feasibility of an evaluation method based on the prepeak crack initiation and postpeak stress drop is theoretically analyzed. Second, the component Bi representing the prepeak brittleness of rock and component Bii representing the prepeak brittleness of rock are constructed, and the product of the two is the brittleness index BI, representing the prepeak crack initiation and postpeak stress drop. Finally, experimental tests of granite and marble were conducted to evaluate the new index, and the brittleness indices of different methods are calculated and compared. The results show that, like other brittleness indices (B1∼B5), the brittleness index BI can effectively reflect the effects of different confining pressures and loading modes on rock brittleness. The brittleness of marble decreases with increasing confining pressure from 5 MPa to 35 MPa. At a confining pressure of 5 MPa, the brittleness of granite during a triaxial unloading test is greater than that during a triaxial compression test. The calculated results are consistent with the experimental results. By tests and comparison results, the reliability of this evaluation method was verified, which provides a way to evaluate rock brittleness from the perspective of crack initiation and is helpful to enrich the analysis and evaluation of rock brittleness in the laboratory. [ABSTRACT FROM AUTHOR]

As an effective technique for detecting microfractures in rock mass, microseismic monitoring (MS) plays an important role in obtaining and interpreting early warning signs of rock engineering hazards. Although microseismic events provide abundant source information (e.g., source parameters and mechanisms) of rock mass fracture, the validity of interpreting rock fracture behavior is restricted due to the discreteness of the temporal-spatial distribution of microseismic events. Feeding back the microseismic information to numeric simulation and calibrating a rock fracture model can improve the simulation accuracy and can effectively interpret the progressive fracture effect in the cluster of microseismic events. Based on this, a microseismic-based method and its implementation steps for numerical simulation and the interpretation of hard rock fracture is proposed in this paper. Indexes resembling the source parameters of the fracture source are introduced in the method to ensure its capability in simulating microseismicity. In the engineering case study of a rockburst hazard, the source mechanism information is used to improve the synthetic rock mass model with a more accurate rock-mass structure. Therefore, the simulated microseismicity is matched well with the measured microseismicity, and the progressive fracturing effect of the rockburst hazard in the cluster of microseismic events is interpreted well by the evolution of indexes that characterize the continuous mechanical effect (stress components, volumetric strain and kinetic energy). This method provides a new way to interpret early warning signs of rock engineering hazards and provides an alternative approach that combines rock mechanics and microseismicity. • Microseismic information are analyzed to characterize rock mass fracture. • The simulation of microseismicity is realized by the indexes to reflect the source dynamic properties. • A simulation method of rock mass fracture based on microseismic information is established. • An actual rockburst and the progressive fracture effect is interpreted by this method. [ABSTRACT FROM AUTHOR]

Multiple slides of rock slopes with dip weak layers have occasionally occurred in Chinese roadway engineering, representing a very complex problem associated with roadway excavation. Unfortunately, the failure mechanisms of such slopes are not clearly investigated prior to excavation, thus leading to expensive remedial work installed after the significant deformative phenomena occurred. Furthermore, pre-reinforced piles have not received enough attention to be used to stabilize the slopes during the excavation process, which enables the slope movements. This paper, therefore, aims to explore the failure mechanisms of a basalt rock slide with dip weak layers and analyse the impacts of pre-reinforced piles on the slope stability. Field investigations, laboratory tests, and in situ deformation monitoring are conducted. Slope stability before and after excavation is analyzed using the finite element and limit equilibrium analysis methods, respectively. The outcomes of the analyses reveal that excavation and the presence of dip weak layers are the main contributory factors to the rock slide. The failure surface of the slope varies with increases in excavation rather than always occurring along the revealed weak layers. The instability appears to be multiple translational slides. Additionally, a finite element analysis for the slope pre-reinforced with row piles during different stages of excavation is conducted. Based on the study results, the pre-reinforced pile is crucial for the slope stability, and the impact of pile position should be prudently considered in design. [ABSTRACT FROM AUTHOR]

High ground temperatures lead to the risk of rockburst in hard rock tunnels located in high-stress environments. However, existing rockburst studies have not considered the influence of temperature. Therefore, the laboratory experiments were conducted to measure the brittleness of granite samples under different temperature scenarios, and the tendency of rockbursts were evaluated at different temperatures in this paper. The brittle failure mechanism of granite was explored by using uniaxial and triaxial rock tests under the influence of thermo-mechanical coupling. The results indicated that the post-peak deformation of the rock samples changes from ductile to brittle with an increase in temperature to 100 °C. The frequency of shear failure occurred in this test increases with increasing temperature, and the brittle failure of the surrounding rock is enhanced with an increase in temperature. The results indicated that rockbursts occurring in deep granite tunnels at high ground temperatures will be more intense than those occurring at room temperature. The results of this study can be applied to predict the intensity of rockbursts before the tunnels are excavated. [ABSTRACT FROM AUTHOR]

A total of 81 mountain tunnels that were damaged in 10 strong earthquakes are studied. They are classified into six typical damage characteristics: lining cracks, shear failure of lining, tunnel collapse caused by slope failure, portal cracking, leaking, and deformation of sidewall/invert damage. Further study and discussion are carried out on influencing factors for mountain tunnels, including seismic parameters, structural information, and rock conditions. Suggestions are also made regarding seismic resistance and reduction. [ABSTRACT FROM AUTHOR]

Abstract: The Mounigou tunnel crosses complex geological strata, mostly with surrounding rock of poor quality with an IV or V classification. Failure in the form of collapses or squeezing deformation occurred many times during the tunnel construction. Especially, in the soft and hard rock contact zone of phyllite and meta-sandstone, the deformation failure was most obvious. This paper uses the FLAC-3D modeling analytical software to establish a 3D-numerical model, on the basis of a field tracing survey and laboratory tests to analyze the stress and strain changes before and after excavation of the tunnel. After excavation there are obvious stress concentrations and differentiations near the interface of the soft-and-hard rock stratum. Stress transfers and concentrates towards the hard rock stratum while in the soft rock zone stress release could be observed to some degree. The range of plastic deformation is obviously larger in the soft rock zone than in the hard rock zone. Both differences in principal stress and principal stress ratio increase after excavation. The amplification of the difference in principal stress is larger than the augmentation of the principal stress ratio. Moreover these principal stress amplifications are smaller in the soft rock zone than in the hard rock zone. The difference in principal stress amplifications between the soft and hard rock zones, together with the fact that soft rock strength is far lower than that of hard rock, causes local “relatively high geostress” effects in the soft rock. It is the main controlling factor of the large deformation of soft rock in the tunnel. It is concluded that the large deformation in the soft and hard rock contact zone in combination with the surrounding rock deformation failure in the Mounigou tunnel and the damage of the supporting structure, belongs to the type of soft rock plastic-squeezing failure. [Copyright &y& Elsevier]

• Compressive-shear tests were conducted on double-notched sandstone rock specimens with different freeze–thaw cycles. • The influence of F-T treatment on the mode II fracture toughness are analyzed. • The 3D surface topography and fractal dimension value of fracture surface was obtained. • The 3D JRC value for 3D fracture surfaces was calculated. For jointed rock masses in cold regions, freeze–thaw cycles cause damage to the rock matrix and degradation of the strength parameters. To further understand the deterioration mechanism of freeze–thaw cycles, substantial efforts have been devoted to the investigation of the failure mechanism of rock after freeze–thaw treatments. Until now, little attention has been given to the degradation of fracture toughness and the change in the fracture surface morphology of rocks after cyclic freeze–thaw treatments, especially for planar shear fracture characteristics. In this research, experiments on double-notched sandstone specimens after different freeze–thaw cycles subjected to compression-shear tests were conducted to further investigate the effect of the freeze–thaw treatment on the mode II fracture toughness and fracture surface morphological characteristics. The results show that freeze–thaw cycles have an important role in the mode II fracture toughness and that the fracture toughness shows an obvious decreasing trend with an increase in freeze–thaw cycles (N). Moreover, the 3D surface topography was reconstructed by using 3D laser scanner scanning. Parameters such as fractal dimension, asperity height and slope angle show that the fracture surface becomes rougher with increasing N. In addition, based on the 2D contour lines extracted from fracture planes, the 3D joint roughness coefficient (JRC) values of 3D fracture surfaces were calculated. The results indicate that more freeze–thaw cycles will produce rougher fracture surfaces. [ABSTRACT FROM AUTHOR]

Most back analysis methods for geotechnical engineering are based on the measured displacement. However, before the monitoring sections are assembled, the displacement—termed the displacement loss—has already been induced; this displacement is difficult to determine, and thus, it is not considered in the back analysis. In the present study, a novel displacement back analysis method considering the displacement loss is developed, that can obtain not only the reasonable mechanical parameters of rock masses but also the displacement loss. To reduce the computational cost of back analysis, a new hybrid optimization algorithm based on the Gaussian process (GP) and particle swarm optimization (PSO) is presented. The GP is used as an inexpensive fitness evaluation surrogate to predict the global optimum solution and accelerate the local search of PSO, which is employed to determine the best mechanical parameters for the model. Combined with FLAC3D numerical analysis, a novel back analysis method called GP-PSO-FLAC3D is proposed. The results of a case study demonstrate that this method can effectively predict more reasonable mechanical parameters and displacement loss using the monitored displacement. An engineering application in the auxiliary tunnel of the Jinping II hydropower station indicates that the elastic deformation of the surrounding rock increases rapidly after excavation, especially for deep tunnels, thereby resulting in a large displacement loss. The back analysis results for the main powerhouse of the Taian pumped storage power station indicate that the displacement loss also exists in engineering processes involving ordinary geostress conditions. Therefore, the displacement loss of a surrounding rock mass cannot be ignored in the stability evaluation or back analysis of underground engineering, especially for deep underground rock engineering. [ABSTRACT FROM AUTHOR]