Stability Of Surrounding Rock Mass Research Articles

Classification and Visualization of Surrounding Rock Mass Stability Based on Multi-Dimensional Cloud Model

The classification of the stability of surrounding rock is an uncertain system with multiple indices. The Multi-dimensional Cloud Model provides an advanced solution through the use of an improved model of One-dimensional Cloud Model. Setting each index as a one-dimensional attribute, the Multi-dimensional Cloud Model can set the digital characteristics of each index according to the cloud theory. The Multi-dimensional cloud generator can calculate the certainty of each grade, and then determine the stability levels of the surrounding rock according to the principle of maximum certainty. Using this model to 5 coal mine roadway surrounding rock examples and comparing the results with those of One-dimensional and Two-dimensional Cloud Models, we find that the Multi-dimensional Cloud Model can provide a more accurate solution. Since the classification results of the Multi-dimensional Cloud Model are difficult to be presented intuitively and visually, we reduce the Multi-dimensional Cloud Model to One-dimensional and Two-dimensional Cloud Models in order to visualize the results achieved by the Multi-dimensional Cloud Model. This approach provides a more accurate and intuitive method for the classification of the surrounding rock stability, and it can also be applied to other types of classification problems.

Distance effects of the fault on the surrounding rock mass stability of the main powerhouse at the Huanggou pumped-storage power station

The distance between the fault and the underground cavern affects the surrounding rock mass stability, which is of great importance to the design and construction of the large-scale underground cavern especially the underground powerhouse. This study focuses on the distance effects of the fault on the rock mass stability of the main powerhouse at the Huanggou pumped-storage power station. Firstly, a microseismic monitoring system was constructed to investigate the distribution and mechanism of distance-dependent damage of the surrounding rock masses of the main powerhouse. Then, the rock failure process analysis and strength reduction method (RFPA-SRM) was adopted to reproduce the failure processes of the surrounding rock masses of the main powerhouse at different distance to the fault f34. Finally, by comparing the numerical simulation results with microseismic monitoring results, the distance effects of the fault f34 on the damage mechanism and failure modes were discussed. It can be concluded that when the distance is low, the damage of the surrounding rock masses subjected to excavation evolved from the spandrel toward the fault, eventually causing a V-shaped collapse if the strength reduction continues. As the distance increases, the effects of the fault on the damage mechanism and failure modes are reduced and the safety factor of the underground cavern is improved. The findings are expected to provide references for evaluating the surrounding rock mass stability of an underground powerhouse under complex geological conditions.

Comprehensive analysis of the surrounding rock mass stability in the underground caverns of Jinping I hydropower station in Southwest China

The geological conditions of the underground caverns in the Jinping I hydropower station are particularly complicated, with high ground stress (the extreme value of the measuring point is 35.7 MPa), complicated deformation and failure mechanisms, and large unloading depth. The large deformation of the surrounding rock mass was beyond current engineering and academic understanding. During the construction period, the deformation and failure mechanisms were complex, and the unloading depth was large. The maximum deformation of the sidewall of the main powerhouse reached 246 mm. The monitoring results of multi-point extensometers basically converged two years after the completion of cavern excavation. Regarding the deformation and failure mechanism of the large underground caverns, the energy dissipation theory that can better describe the complex mechanical response analysis was proposed. The deformation failure mechanism, mechanical behaviour, supporting effect and stability evaluation of the large underground caverns were investigated from the perspectives of excavation unloading and energy dissipation and release. The excavation unloading-induced relaxation characteristics of the surrounding rock mass were analysed using a comprehensive approach incorporating the results of borehole television, acoustic wave testing, multipoint extensometer and numerical simulations. A criterion of the surrounding rock mass delineation incorporating the energy dissipation ratio, acoustic wave testing, wave velocity variation and elastic modulus variation was also proposed. Furthermore, the phenomena and law of deformation and failure of surrounding rock mass in the large underground caverns were revealed. The stability of surrounding rock mass in the underground caverns was evaluated. The related achievements can provide some references for similar deep underground rock engineering.

Key Factors Affecting the Deformation and Failure of Surrounding Rock Masses in Large‐Scale Underground Powerhouses

The stability of the surrounding rock masses of underground powerhouses is always emphasized during the construction period. With the general trends toward large‐scale, complex geological conditions and the rapid construction progress of underground powerhouses, deformation and failure issues of the surrounding rock mass can emerge, putting the safety of construction and operation in jeopardy and causing enormous economic loss. To solve these problems, an understanding of the origins and key affecting factors is required. Based on domestic large‐scale underground powerhouse cases in the past two decades, key factors affecting the deformation and failure of the surrounding rock mass are summarized in this paper. Among these factors, the two most fundamental factors are the rock mass properties and in situ stress, which impart tremendous impacts on surrounding rock mass stability in a number of cases. Excavation is a prerequisite of surrounding rock mass failure and support that is classified as part of the construction process and plays a pivotal role in preventing and arresting deformation and failure. Additionally, the layout and structure of the powerhouse are consequential. The interrelation and interaction of these factors are discussed at the end of this paper. The results can hopefully advance the understanding of the deformation and failure of surrounding rock masses and provide a reference for design and construction with respect to hydroelectric underground powerhouses.

Large Deformation Mechanics of Gob‐Side Roadway and Its Controlling Methods in Deep Coal Mining: A Case Study

Maintaining surrounding rock mass stability of roadways is essential to the safety of deep coal mining. In this study, the No. 2‐2092 roadway of the No. 2‐209 mining face in Ganhe coal was taken as the target roadway for field analysis. The selected region can be considered a typical area with dominating geological tectonic stress, based on the geological survey and in situ stress results. A mechanical model of roadway overburdens was developed to analyse the large deformation and stress field distribution. It is found that the large deformation is caused by the combined superposed stress field including laterally transferred stress formed in structures at overlying strata, mining‐induced advanced abutment pressure, and the regional in situ stress. Thus, a Two‐Direction Hydrofracturing Technique (TDHT) was proposed to reduce the pressure of the No. 2‐2092 roadway by altering the roof structure in the influenced zones. Compared with the original roadway without fracturing, it is found that the roof to floor convergence has dropped by nearly 47% after fracturing; the displacement of sidewalls has reduced by almost 31%, demonstrating the effectiveness of the proposed method in pressure relief. Results from this study can provide guidance on controlling the large deformation of roadways in deep underground mines.

Strength degradation of a natural thin-bedded rock mass subjected to water immersion and its impact on tunnel stability

Strength anisotropy is a typical feature of thin-bedded rock masses and their strength will be degraded subjected to water immersion effect. Such effect is crucial for the operation of hydropower plant because the impoundment lifts the water level of upstream reservoir and causes the rock mass of nearby slopes saturated. So far, researches regarding mechanical property of natural thin-bedded rock masses and their strength variation under water immersion based on field test method are rarely reported. This paper focuses on a thin-bedded stratified rock mass and carries out field test to investigate the mechanical property and strength variation characteristics. The field test is highlighted by samples which have a large shear dimension of 0.5 m*0.5 m, representing a more realistic in-situ situation than small size specimen. The test results confirm the anisotropic nature of the concerned rock mass, whose shear strength of host rocks is significantly larger than that of bedding planes. Further, the comparison of shear strength parameters of the thin-bedded rock mass under natural and saturated conditions show that for both host rocks and bedding planes, the decreasing extent of cohesion values are larger than friction values. The quantitative results are then adopted to analyze the influence of reservoir impoundment of a hydropower plant on the surrounding rock mass stability of diversion tunnels which are located in the nearby slope bank. It is evaluated that after reservoir impoundment, the strength degradation induced incremental deformations of surrounding rock mass of diversion tunnels are small and the stresses in lining structure are acceptable. It is therefore concluded that the influences of impoundment are small and the stability of diversion tunnels can be still achieved. The finings regarding field test method and its results, as well as the numerical evaluation conclusions are hoped to provide references for rock projects with similar concerns.

Excavation Sequence and Surrounding Rock Mass Stability of Large‐Scale Underground Engineering with 8 Tunnels

In some large‐scale hydroelectric power projects, there are more than 3 tunnels that are too close to each other to eliminate the mutual influence during the excavation period, especially for large‐scale tunnel groups. In this paper, aimed at analyzing the Bukun hydropower station consisting of 8 tunnels in Malaysia, the displacement, stress, and plastic zone of the surrounding rock mass are analyzed to study the effect of the excavation sequence on the stability of the surrounding rock mass for large‐scale tunnel groups. On the one hand, the in situ monitoring of the surface displacement of the rock mass surrounding the tunnel using extensometers is performed to obtain the deformation characteristics on the excavation limit under the typical excavation sequence. On the other hand, a series of elastic‐plastic 3D numerical experiments are carried out to explore the displacement characteristics, stability of the large‐scale tunnel groups, and safety of the initial supporting system. The results show that the tendencies of the displacement increase corresponding to the tunnel face movement are similar for the three excavation sequences. The displacement under initial excavation sequence 2 (IES2) is the smallest among the three sequences; the area of the plastic zone under IES2 is the smallest among them; and the stresses in the shotcrete layer and axial forces in the rock bolt under the three excavation sequences are within the safety limitation. Initial excavation sequence 2 is an optimized excavation sequence, in which tunnels #1 and #5 are excavated first; after an advance of 3 times the diameter of the tunnel, tunnels #3 and #7 are excavated; tunnels #2 and #6 are excavated after an advance of 3 times diameters; and tunnels #4 and #8 are excavated after an advance of 3 times diameters.

Rockburst prediction and stability analysis of the access tunnel in the main powerhouse of a hydropower station based on microseismic monitoring

The access tunnel in the main powerhouse of the Shuangjiangkou hydropower station in China has complex geological conditions and is subject to high in situ stress in deep buried sections. Microseismic activity in the surrounding rock mass of the tunnel was monitored by a microseismic monitoring system, and rockburst was effectively predicted. Based on abundant data obtained from the microseismic monitoring, statistical parameters, which include cumulative apparent volume, the energy index, cumulative released energy and the Es/Ep value, were used to analyze the microseismic activity before and after rockburst to determine a more accurate early warning period and construction safety period. A sharp decrease in the energy index and a rapid increase in the cumulative apparent volume indicated a deterioration of the surrounding rock mass stability. The change characteristics of Es/Ep values revealed that the rockburst process underwent a transformation of compression-shear damage, tension-shear mixed damage and tension damage. Finally, based on the number of daily events N and the b value of the microseismic events, lgN/b was first established to evaluate the rockburst risk of tunnels. When the value of lgN/b was greater than 1, rockburst was more likely to occur; the larger the lgN/b value, the more severe the rockburst was. The research results provide an important reference value for the prediction of rockbursts in deep tunnels and the regulation of site construction progress.

Jointed Surrounding Rock Mass Stability Analysis on an Underground Cavern in a Hydropower Station Based on the Extended Key Block Theory

The jointed surrounding rock mass stability is of utmost importance to integral stability during the construction and long-term safety operation of the underground caverns in hydropower stations. The key blocks play a significant role in the integral stability of the jointed surrounding rock mass, therefore it is critical to determine the location, size, and failure mode of random key blocks. This paper proposes an improved method combining the traditional key block theory (KBT) and the force transfer algorithm to accurately calculate the safety factors of probabilistic key blocks in the surrounding rock mass. The force transfer algorithm can consider the interactions between the internal blocks. After the probabilistic characteristics of the joint fissures are obtained, the stereographic projection method is employed to determine the locations of dangerous joints. Then the vector analysis method is used to search the random blocks, determine the sliding directions of random blocks, and calculate the block sizes and safety factors near the free surface of the underground cavern, which can be used to comprehensively evaluate the surrounding rock mass stability. The above numerical results have provided powerful guidance for developing a reinforcement system for the surrounding rock mass.

Impact of In Situ Stress Distribution Characteristics on Jointed Surrounding Rock Mass Stability of an Underground Cavern near a Hillslope Surface

In this paper, a series of numerical simulations are performed to analyze the in situ stress distribution characteristics of the rock mass near different slope angles hillslope surfaces, which are subjected to the vertical gravity stress and different horizontal lateral stresses and the influence which the in situ stress distribution characteristics of 45° hillslope to the integral stability of surrounding rock mass when an underground cavern is excavated considering three different horizontal distances from the underground cavern to the slope surface. It can be concluded from the numerical results that different slope angles and horizontal lateral stresses have a strong impact on the in situ stress distribution and the integral surrounding rock mass stability of the underground cavern when the horizontal distance from the underground cavern to the slope surface is approximately 100 m to 200 m. The relevant results would provide some important constructive suggestions to the engineering site selection and optimization of large-scale underground caverns in hydropower stations.

Engineering rock mechanics practices in the underground powerhouse at Jinping I hydropower station

Based on the analyses of data obtained from the underground powerhouse at Jinping I hydropower station, a comprehensive review of engineering rock mechanics practice in the underground powerhouse is first conducted. The distribution of strata, lithology, and initial geo-stress, the excavation process and corresponding rock mass support measures, the deformation and failure characteristics of the surrounding rock mass, the stress characteristics of anchorage structures in the cavern complex, and numerical simulations of surrounding rock mass stability and anchor support performance are presented. The results indicate that the underground powerhouse of Jinping I hydropower station is characterized by high to extremely high geo-stresses during rock excavation. Excessive surrounding rock mass deformation and high stress of anchorage structures, surrounding rock mass unloading damage, and local cracking failure of surrounding rock masses, etc., are mainly caused by rock mass excavation. Deformations of surrounding rock masses and stresses in anchorage structures here are larger than those found elsewhere: 20% of extensometers in the main powerhouse record more than 50 mm with the maximum at around 250 mm observed in the downstream sidewall of the transformer hall. There are about 25% of the anchor bolts having recorded stresses of more than 200 MPa. Jinping I hydropower plant is the first to have an underground powerhouse construction conducted in host rocks under extremely high geo-stress conditions, with the ratio of rock mass strength to geo-stress of less than 2.0. The results can provide a reference to underground powerhouse construction in similar geological conditions.

Stability Evaluation on Surrounding Rocks of Underground Powerhouse Based on Microseismic Monitoring

To study the stability of underground powerhouse at Houziyan hydropower station during excavation, a microseismic monitoring system is adopted. Based on the space-time distribution characteristics of microseismic events during excavation of the main powerhouse, the correlation between microseismic events and blasting construction is established; and the microseismic clustering areas of the underground powerhouse are identified and delineated. The FLAC3D code is used to simulate the deformation of main powerhouse. The simulated deformation characteristics are consistent with that recorded by microseismic monitoring. Finally, the correlation between the macroscopic deformation of surrounding rock mass and microseismic activities is also revealed. The results show that multiple faults between 1# and 3# bus tunnels are activated during excavation of floors V and VI of the main powerhouse. The comprehensive method combining microseismic monitoring with numerical simulation as well as routine monitoring can provide an effective way to evaluate the surrounding rock mass stability of underground caverns.

Analysis of Surrounding Rockmass Stability and Supporting Effect of Underground Caverns of a Power Station

Stability analysis of surrounding rock mass of Hongping underground caverns are studied by means of numerical simulation using Fast Lagrangian explicit finite difference method. The process of excavation and support are simulated. The pattern of deformation, stress state, and the distribution of plastic areas are analyzed. Finally, the whole stability of surrounding rock mass of underground caverns was evaluated.

Hydro-Mechanical Coupled Analysis of the Stability of Surrounding Rock Mass of Underground Water-Sealed Oil Storage

Based on the fluid-solid coupling theory, we study the stability of surrounding rock mass around underground oil storage in Huangdao, Shandong province, analyze the stress of the surrounding rock mass around three chambers and the displacement change of several key monitoring points after excavation and evaluate the stability of surrounding rock mass using COMSOL Multiphysics software. Research results show that the stress at both sides of the straight wall of cavern increases, especially obvious stress concentration forms at the corners of the cavern, and the surrounding rock mass moves towards the cavern after excavation. The stress and displacement of the surrounding rock mass will increase accordingly after setting the water curtains, but the change does not have a substantive impact on the stability of surrounding rock mass.

Study on the Rock Mass Quality Influences on the Surrounding Rock Mass Stability of the Underground Cavities

In this paper, taking the underground cavities of Nuozadu Hydro-electrical Power Station located in Lancang River as an example, three-dimensional finite element numerical modeling technique is used to analyze systematically the distribution features secondary stress field, displacement field and failure zone in the surrounding rock mass of the underground cavities after the excavation when the surrounding rock mass quality rank change fromⅡ to IV. The influences of rock quality on excavation effects are discussed in detail.

An Index for Estimating the Stability of Brittle Surrounding Rock Mass: FAI and its Engineering Application

Based on the geometric analysis of the relationship between the stress state at a point and the yield surface defined in the principal stress space, a coefficient ω is set up as an estimation index to describe the stress-induced yield risk. After yield, the equivalent plastic shear strains is usually used to characterize the failure degree (FD) of the material and adopted here as an index of the damage degree for the surrounding rock masses. Then, a unified variable combining ω and FD, named failure approaching index (FAI), is constructed to estimate the stability of rock mass which may be at different deformation stages. The formulas of FAI are derived for some popular yield criteria in geomechanics. Details for such development are addressed in the paper. Its rationality is verified by numerical simulation and comparative analysis of the conventional triaxial compression tests and typical tunnel projects. In addition, the method for applying FAI to the stability estimation of surrounding rock mass is proposed. As examples, the stability of the underground powerhouse, access tunnels and headrace tunnels at the Jinping II hydropower station are estimated by making use of the method we presented. The results indicate that not only is the index rational in mechanics, but the theory also has good expansibility, and the estimation methods are simple and practical as well. It is easier for field engineers to analyze and understand the numerical results.

Finite Element Analysis of the Stability of Tunnel Surrounding Rock with Weak Rock Layer

The evaluation of local stability of the tunnel surrounding rock mass with weak rock layer is very important for the economy and security of the tunnel construction. Based on the rock tunnel geological conditions in the construction, the finite element model of the tunnel surrounding rock mass stability analysis with weak rock layer is established in the article by selecting the ABAQUS program linear D-P materials model as the gneiss yield criterion. The computation result indicates that the explicit dynamic analysis program could simulate the whole process of the instabilization and failure of the weak rock layer, and avoid the convergence existing in the general finite element computation, and the analysis model and program could offer effective measure for the analysis of local stability of the tunnel surrounding rock mass with weak rock layer.

Stability Analysis of Diversion Tunnels in Jinping II Hydropower Station Project

Surrounding rock mass stability is one of the key technical problems in the design of the diversion tunnels in Jinping II Hydropower Plant Project. The major difficulties lie in three facts: (1) high stress induced by the great depth; (2) the brittle failure characters of marble; (3) the interaction of these four tunnels. A systemic procedure is introduced in this paper. Firstly, the integration of the code FLAC3D and the multivariate linear regression method is adopted to back analyze the in situ stress field. Then the mechanical parameters of the surrounding rock mass are back analyzed based on the PSO (Particle Swarm Optimization) algorithm and the code FLAC3D. The stress release method is adopted in the numerical analysis of the excavating and supporting process of tunnels by the code FLAC3D. Finally, the multi indexes integration method is presented to analyze and evaluate the stability of the tunnels and to validate the rationality of the design scheme.

3-D FLAC Numerical Analysis of Rheological Character and Stability of Underground Caverns Surrounding Rock Mass of Xiao Langdi Key Water Control Project

A brief description is given to the visco-plastic constitutive model of 3-D FLAC software and some treating methods for the numerical analysis. The excavation steps and the exerting time of excavation loads are determined in the light of the real construction process. The rockmass mechanical parameters are determined according to back analysis of in-situ measured displacement data. The analysis results of displacement and stability of the opening complex surrounding rocks in the construction process and in a long period of time after excavation completion are given.