Guoqing Chen

Temperature effect of rock burst for hard rock in deep-buried tunnel

Much research has been conducted on the influence of rock burst mechanisms and temperature on the mechanical properties of hard rock while research on the effect of temperature on rock bursts is scarce. Therefore, this paper focuses on Rock Burst Proneness Index tests and acoustic emission (AE) tests under the action of high temperature. It was found that the Rock Burst Proneness Index and the AE energy will rise as the temperature rises. It means that the degree of rock burst is increasing instead of decreasing with rising temperature. The research results revealed the temperature effect of rock burst in long deep tunnels under a certain thermal stress condition, which is helpful for explaining the rock burst disaster in tunnels at high ground temperature.

Progressive modelling of the gravity-induced landslide using the local dynamic strength reduction method

The failure of slope is a progressive process, and the whole sliding surface is caused by the gradual softening of soil strength of the potential sliding surface. From this viewpoint, a local dynamic strength reduction method is proposed to capture the progressive failure of slope. This method can calculate the warning deformation of landslide in this study. Only strength parameters of the yielded zone of landslide will be reduced by using the method. Through continuous local reduction of the strength parameters of the yielded zone, the potential sliding surface developed gradually and evolved to breakthrough finally. The result shows that the proposed method can simulate the progressive failure of slope truly. The yielded zone and deformation of landslide obtained by the method are smaller than those of overall strength reduction method. The warning deformation of landslide can be obtained by using the local dynamic strength reduction method which is based on the softening characteristics of the sliding surface.

Brittle mechanical characteristics of hard rock exposed to moisture

Rich groundwater content can produce a complex geological environment for underground tunnels. Therefore, the brittle mechanical characteristics of hard rock exposed by tunnelling in a moist environment are of great significance. Uniaxial tests and acoustic emission (AE) analysis of sandstone with respect to different moisture conditions are described in this paper. Moisture-controlled trends of the stress–strain relationship, mechanical parameter variation, maximum instantaneous AE energy, cumulative AE energy and macro crack behaviour were analysed in detail. In addition, X-ray diffraction and scanning electron microscopy were performed to analyse the fracture surfaces of rock at the microscale. Sandstone particles were found to be more likely to slip past each other at high moisture contents, resulting in increased plastic deformation and dissipation of internal energy. Increasing moisture reduces the brittleness of hard rock and the probability of rock burst. The results reveal the mechanical mechanisms cause moisture to affect brittle hard rock and can contribute to the improvement of the design and construction of deep tunnels.

Brittle Rock Modeling Approach and its Validation Using Excavation-Induced Micro-Seismicity

With improvements to the bonded-particle model, a custom indicator of crack intensity is introduced to grade rock fractures accurately. Brittle fracturing of rock mass is studied using the bonded-particle model; here, “brittle” refers to the process where more energy is released towards making particles collide and disperse, and hence results in the quick emergence of “chain cracks”. Certain principles concerning how to construct brittle rock are then proposed. Furthermore, a modeling approach for brittle rocks based on the adaptive continuum/discontinuum (AC/DC) method is proposed to aid the construction of large-scale models of tunnel excavations. To connect with actual tunneling conditions, fundamental mechanical properties, the mechanism for brittle fracturing, the joint distribution, and the initial stress field are considered in the modeling approach. Results from micro-seismic monitoring of a tunnel excavation confirmed the suitability of this modeling approach to simulate crack behavior, and results show that simulated cracking exhibit similar trends (evolution, location, and intensity) with micro-seismic cracking.

Experimental study on the brittle failure of the locking section in a large-scale rock slide

A locking section is a conceptual model employed to analyze large-scale rock slides, of which the role was commonly found governing the mechanism of such mass movements. The physical experimental study presented herein is designed to study the failure mechanism of the locking section by measuring its basic characteristics. Dataset on the magnitude of the energy release, stress state, and displacement near and within the locking section are obtained by acoustic emission sensor, strain gauge, and micrometer gauge. The study captured the continued stress and energy accumulation at the locking section under increasing loads and the final brittle shear failure, which depicts the comprehensive deformation and failure processes occurred at the locking section. The concurrence of the locking section failure is the abrupt intensive energy release that results in the occurrence of the high-speed rock sliding. In conclusion, the experiment aims at investigating the initiation mechanism of large-scale rock slides governed by a locking section.

Failure Mechanism of Rock Bridge Based on Acoustic Emission Technique

Acoustic emission (AE) technique is widely used in various fields as a reliable nondestructive examination technology. Two experimental tests were carried out in a rock mechanics laboratory, which include (1) small scale direct shear tests of rock bridge with different lengths and (2) large scale landslide model with locked section. The relationship of AE event count and record time was analyzed during the tests. The AE source location technology and comparative analysis with its actual failure model were done. It can be found that whether it is small scale test or large scale landslide model test, AE technique accurately located the AE source point, which reflected the failure generation and expansion of internal cracks in rock samples. Large scale landslide model with locked section test showed that rock bridge in rocky slope has typical brittle failure behavior. The two tests based on AE technique well revealed the rock failure mechanism in rocky slope and clarified the cause of high speed and long distance sliding of rocky slope.

Weakening effects of the presence of water on the brittleness of hard sandstone

The weakening effects of increasing the water content in rocks have been studied extensively in a wide variety of rock types over the past few decades. High energy release is typically characteristic of rock burst hazards. However, few studies have investigated the mechanism of water involved in rock bursts from the perspective of energy distribution based on the water content. Therefore, in this study we investigated the evolution of microcracks in hard sandstone determined from triaxial tests combined with an acoustic emission analysis for different water contents and confining pressures. Then, the weakening effects of water on the brittleness of the rock were studied from an energy viewpoint. The study shows that the capacity of the storage energy of hard rock decreases due to the dissipation of internal elastic energy in the compression stage in the presence of water. These results revealed the mechanical and energetic mechanisms that caused the weakening effects induced by water on rock burst and can contribute to design and construction improvements in tunnel engineering.

Distribution and mechanism of gently dipping fractures subjected to river incision: A case study from Nujiang River, China

Gently dipping fractures subjected to river incision are widely distributed on rock slopes. In this paper, a rock slope on the Nujiang River (China) is investigated to study the role of gently dipping fractures in the rock slopes evolution. Detailed field surveys indicate that gentle fractures are concentrated in four main zones. Moreover, the kinematics of the fracture system suggest that the genesis of these fractures can be synthesized into a progressive evolution model. This model indicates that the joints begin with the formation of an array of en echelon cracks that are subjected to continued crack elongation and shearing before ultimately approaching one another and interacting to form a complex joint system. Geomechanical analysis is performed to reveal the mechanisms of this genesis, and three main fracture patterns are identified based on the slope stress and are classified with respect to the slope evolution. Based on the detail field investigations and the evolutionary history of the river valley, we propose that intermittent incision by the river was the main factor contributing to the concentrated distribution of gently dipping joints.

Evaluation of the possible slip surface of a highly heterogeneous rock slope using dynamic reduction method

A new method, the dynamic reduction method (DRM) combined with the strain-softening method, was applied to evaluate the possible slip surface of a highly heterogeneous rock slope of the Dagangshan hydropower station in Southwest China. In DRM, only the strength of the failure elements is reduced and the softening reduction factor K is adopted to calculate the strength parameters. The simulation results calculated by DRM show that the further slip surface on the right slope of the Dagangshan hydropower station is limited in the middle part of the slope, while both SRM (strength reduction method) and LEM (limit equilibrium method) predict a failure surface which extends upper and longer. The observations and analysis from the three recorded sliding events indicate that the failure mode predicted by DRM is more likely the scenario. The results in this study illustrate that for highly heterogeneous slopes with geological discontinuities in different length scales, the proposed DRM can provide a reliable prediction of the location of the slip surface.

Influence of temperature on the brittle failure of granite in deep tunnels determined from triaxial unloading tests

Abstract 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.