Integrated approach of predicting rock stability in high mountain valley underground caverns

High mountain valleys are characterized by the development of intricate ground stress fields due to geological processes such as tectonic stress,river erosion,and rock weathering.These processes introduce considerable stability concerns in the surrounding rock formations for underground engineering projects in these regions,highlighting the imperative need for rigorous stability assessments during the design phase to ensure construction safety.This paper introduces an innovative approach for the pre-evaluation of the stability of surrounding rocks in underground caverns situated within high mountain valleys.The methodology comprises several pivotal steps.Initially,we conduct inverse calculations of the ground stress field in complex geological terrains,combining field monitoring and numerical simulations.Subsequently,we ascertain stress-strength ratios of the surrounding rocks using various rock strength criteria.To assess the stability characteristics of the surrounding rocks in the 1^(#)spillway cave within our project area,we employ numerical simulations to compute stress-strength ratios based on different rock strength criteria.Furthermore,we undertake a comparative analysis,utilizing data from the 5^(#)Underground Laboratory(Lab 5)of Jinping II Hydropower Station,aligning the chosen rock strength criterion with the damage characteristics of Lab 50s surrounding rocks.This analysis serves as the cornerstone for evaluating other mechanical responses of the surrounding rocks,thereby validating the pre-evaluation methodology.Our pre-evaluation method takes into account the intricate geological evolution processes specific to high mountain valleys.It also considers the influence of the initial geostress field within the geological range of underground caverns.This comprehensive approach provides a robust foundation for the analysis and assessment of the stability of surrounding rocks,especially in high mountain valley areas,during the design phase of underground engineering projects.The insights derived from this analysis hold substantial practical significance for the effective guidance of such projects.

Automatic identification of rock discontinuity and stability analysis of tunnel rock blocks using terrestrial laser scanning

Local geometric information and discontinuity features are key aspects of the analysis of the evolution and failure mechanisms of unstable rock blocks in rock tunnels.This study demonstrates the integration of terrestrial laser scanning(TLS)with distinct element method for rock mass characterization and stability analysis in tunnels.TLS records detailed geometric information of the surrounding rock mass by scanning and collecting the positions of millions of rock surface points without contact.By conducting a fuzzy K-means method,a discontinuity automatic identification algorithm was developed,and a method for obtaining the geometric parameters of discontinuities was proposed.This method permits the user to visually identify each discontinuity and acquire its spatial distribution features(e.g.occurrences,spac-ings,trace lengths)in great detail.Compared with hand mapping in conventional geotechnical surveys,the geometric information of discontinuities obtained by this approach is more accurate and the iden-tification is more efficient.Then,a discrete fracture network with the same statistical characteristics as the actual discontinuities was generated with the distinct element method,and a representative nu-merical model of the jointed surrounding rock mass was established.By means of numerical simulation,potential unstable rock blocks were assessed,and failure mechanisms were analyzed.This method was applied to detection and assessment of unstable rock blocks in the spillway and sand flushing tunnel of the Hongshiyan hydropower project after a collapse.The results show that the noncontact detection of blocks was more labor-saving with lower safety risks compared with manual surveys,and the stability assessment was more reliable since the numerical model built by this method was more consistent with the distribution characteristics of actual joints.This study can provide a reference for geological survey and unstable rock block hazard mitigation in tunnels subjected to complex geology and active rockfalls.

Comprehensive analysis of multiple machine learning techniques for rock slope failure prediction

In this study,twelve machine learning(ML)techniques are used to accurately estimate the safety factor of rock slopes(SFRS).The dataset used for developing these models consists of 344 rock slopes from various open-pit mines around Iran,evenly distributed between the training(80%)and testing(20%)datasets.The models are evaluated for accuracy using Janbu's limit equilibrium method(LEM)and commercial tool GeoStudio methods.Statistical assessment metrics show that the random forest model is the most accurate in estimating the SFRS(MSE=0.0182,R2=0.8319)and shows high agreement with the results from the LEM method.The results from the long-short-term memory(LSTM)model are the least accurate(MSE=0.037,R2=0.6618)of all the models tested.However,only the null space support vector regression(NuSVR)model performs accurately compared to the practice mode by altering the value of one parameter while maintaining the other parameters constant.It is suggested that this model would be the best one to use to calculate the SFRS.A graphical user interface for the proposed models is developed to further assist in the calculation of the SFRS for engineering difficulties.In this study,we attempt to bridge the gap between modern slope stability evaluation techniques and more conventional analysis methods.

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 Multidimensional 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 Multidimensional 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.

Application of rock mass classification systems to rock slope stability assessment:A case study

The stability of rock slopes is considered crucial to public safety in highways passing through rock cuts, as well as to personnel and equipment safety in open pit mines. Slope instability and failures occur due to many factors such as adverse slope geometries, geological discontinuities, weak or weathered slope materials as well as severe weather conditions. External loads like heavy precipitation and seismicity could play a significant role in slope failure. In this paper, several rock mass classification systems developed for rock slope stability assessment are evaluated against known rock slope conditions in a region of Saudi Arabia, where slopes located in rugged terrains with complex geometry serve as highway road cuts. Selected empirical methods have been applied to 22 rock cuts that are selected based on their failure mechanisms and slope materials. The stability conditions are identified, and the results of each rock slope classification system are compared. The paper also highlights the limitations of the empirical classification methods used in the study and proposes future research directions.

Analysis of mechanical behavior of soft rocks and stability control in deep tunnels

Due to the weakness in mechanical properties of chlorite schist and the high in situ stress in Jinping II hydropower station, the rock mass surrounding the diversion tunnels located in chlorite schist was observed with extremely large deformations. This may significantly increase the risk of tunnel instability during excavation. In order to assess the stability of the diversion tunnels laboratory tests were carried out in association with the petrophysical properties, mechanical behaviors and waterlweakening properties of chlorite schist. The continuous deformation of surrounding rock mass, the destruction of the support structure and a large-scale collapse induced by the weak chlorite schist and high in situ stress were analyzed. The distributions of compressive deformation in the excavation zone with large deformations were also studied. In this regard, two reinforcement schemes for the excavation of diversion tunnel bottom section were proposed accordingly. This study could offer theoretical basis for deed tunnel construction in similar geological condition~

Stability and reinforcement analysis of rock slope based on elasto-plastic finite element method

The rigid body limit equilibrium method(RBLEM) and finite element method(FEM) are two widely used approaches for rock slope's stability analysis currently. RBLEM introduced plethoric assumptions; while traditional FEM relied on artificial factors when determining factor of safety(FOS) and sliding surfaces. Based on the definition of structure instability that an elasto-plastic structure is not stable if it is unable to satisfy simultaneously equilibrium condition, kinematical admissibility and constitutive equations under given external loads, deformation reinforcement theory(DRT) is developed. With this theory, plastic complementary energy(PCE) can be used to evaluate the overall stability of rock slope, and the unbalanced force beyond the yield surface could be the identification of local failure. Compared with traditional slope stability analysis approaches, the PCE norm curve to strength reduced factor is introduced and the unbalanced force is applied to the determination of key sliding surfaces and required reinforcement. Typical and important issues in rock slope stability are tested in TFINE(a three-dimensional nonlinear finite element program), which is further applied to several representatives of high rock slope's stability evaluation and reinforcement engineering practice in southwest of China.

Research on the stability analysis and design of soil tunnel surrounding rock

The paper first analyzes the failure mechanism and mode of tunnel according to model experiments and mechanical calculation and then discusses the deficiency of taking the limit value of displacement around the tunnel and the size of the plastic zone of surrounding rock as the criterion of stability. So the writers put forward the idea that the safety factor of surrounding rock calculated through strength reduction FEM(finit element method) should be regarded as the criterion of stability,which has strict mechanical basis and unified standard and would not be influenced by other factors. The paper also studies the safety factors of tunnel surrounding rock (safety factors of shear and tension failure) and lining and some methods of designing and calculating tunnels. At last,the writers take the loess tunnel for instance and show the design and calculation results of two-lane railway tunnel.

Study on engineering geological stability of rock mass at Shanmen silver deposit

The natural balance conditions will be disturbed and produce a series of problems when mineral deposit has mined.This paper has researched the engineering rock masses have been researched in this study,structural planes,the distribution characteristics of tectonic geological factors and the stability of engineering structures according to the theory and research methods of rock mechanics,it will provide the engineering geological evidence for mining area exploited,meanwhile pledge the safety production.Shanmen silver deposit is a large epithermal deposit,it is controlled by NE to NNE strike faults.The stability of rock mass is acted on the tectonic movement and hot metalliferous brine in long-term.Especially,strength of rock mass becomes softened,muddy and loosed under the action of water,so the lower stability of rock mass is,the easier it can take place for harm of disaster threatening production safe of mining.For this reason,it is very important that drawing up a plan to lower harm for mine and protect.

Prediction on rock stratum stability using numerical simulation

Numerical simulation, which is one of the important methods for tectonic simulation, can be successfully applied into the stability analysis of rock stratum in mining engineering. With numerical simulation, the characteristics of stress deformation field of the area study can be well discovered, the stress concentration regions can be clearly located and the mechanism and effect of the stress concentration can be analyzed. The results of these studies offer fundamental data for evaluation of the rock stability and prediction of the tunnel wall stability in the working area.

Stability analysis and determination of rock pillar between two adjacent caverns in different regions of Asmari formation in Iran

Large underground caverns are commonly used in variety of applications. In many cases, because of the geomechanical limitations of dimensions and requirement of high volume, several parallel caverns are used. Plastic zone integration requires a larger rock pillar distance of theses adjacent caverns while eco- nomic and access reasons require smaller distance. In lran many underground projects are located in West and South West, Asmari formation covers a large part of these regions. The stability of underground spaces that are constructed or will be constructed in this formation has been investigated. A proper cross section based on plastic analysis and a stability criterion has been proposed for each region. Finally, in each case, allowable rock pillar between adjacent caverns with similar dimension was determined with two methods （numerical analysis and fire service law）. Results show that Fire Service Law uses a very con- servative safety factor and it was proposed to use a correction factor for allowable distance based on application of underground space.

Dynamic Mechanical Behavior and Numerical Simulation of an Ancient Underground Rock Mass under Impact Loading

To study the dynamic mechanical properties of tuff under different environmental conditions,the tuff from an ancient quarry in Shepan Island was prepared.The impact damage to the rock was tested using a triaxial dynamic impact mechanical testing system(TDIMTS)with different ground stresses,temperatures,and groundwater pressures.The time-strain relationship,dynamic stress-strain relationship,energy dissipation law,energy-peak strain relationship,and the impact damage pattern of the tuff specimens under impact air pressures were investigated.The TDIMTS experiment on ancient underground rock mass under impact loading was also simulated using the finite element analysis software LS-DYNA based on the Holmquist-Johnson-Cook(HJC)material model.The dynamic failure process,failure pattern and peak stress of tuff specimen were calculated.The simulation results obtained using the above methods were in good agreement with the experimental results.The results of the dynamic experiment show that with the same local stress,groundwater pressure,and temperature,the damage to the tuff specimens caused by blasting and quarrying disturbances gradually increases as the impact pressure increases.Under the same local stress,groundwater pressure,and temperature,the energy required to rupture the tuffs in ancient underground caverns is relatively small if the impact pressure is low accordingly,but as the impact pressure increases,the damage to the tuff caused by quarrying disturbance gradually increases.The damage gradually increases and the degree of damage to the tuff and the strain energy exhibit asymptotic growth when the tuff specimens are subjected to the greater strain energy,increasing the degree of rupturing of the tuff.In addition,the average crushing size decreases with increasing strain energy.By comparing the simulation results with the experimental results,it was found that the HJC model reflected the dynamic impact performance of tuff specimen,and the simulation results showed an evident strain rate effect.These results of this study can offer some guidance and theoretical support for the stability evaluation,protection,and safe operation of the ancient underground caverns in future.

Numerical analysis of deformation and failure characteristics of deep roadway surrounding rock under static-dynamic coupling stress

In actual production,deep coal mine roadways are often under typical static-dynamic coupling stress(SDCS)conditions with high ground stress and strong dynamic disturbances.With the increasing number of disasters and accidents induced by SDCS conditions,the safe and efficient production of coal mines is seriously threatened.Therefore,it is of great practical significance to study the deformation and failure characteristics of the roadway surrounding rock under SDCS.In this paper,the effects of different in-situ stress fields and dynamic load conditions on the surrounding rock are studied by numerical simulations,and the deformation and failure characteristics are obtained.According to the simulation results,the horizontal stress,vertical stress and dynamic disturbance have a positive correlation with the plastic failure of the surrounding rock.Among these factors,the influence of the dynamic disturbance is the most substantial.Under the same stress conditions,the extents of deformation and plastic failure of the roof and ribs are always greater than those of the floor.The effect of horizontal stresses on the roadway deformation is more notable than that of vertical stresses.The results indicate that for the roadway under high-stress conditions,the in-situ stress test must be strengthened first.After determining the magnitude of the in-situ stress,the location of the roadway should be reasonably arranged in the design to optimize the mining sequence.For roadways that are strongly disturbed by dynamic loads,rock supports(rebar/cable bolts,steel set etc.)that are capable of maintaining their effectiveness without failure after certain dynamic loads are required.The results of this study contribute to understanding the characteristics of the roadway deformation and failure under SDCS,and can be used to provide a basis for the support design and optimization under similar geological and geotechnical circumstances.

Cap rock blast caving of cavity under open pit bench

A laser technique based scanning system was employed to make a comprehensive scanning through borehole forunmapped cavity under open pit bench,then the three-dimensional data will be obtained,and these data were used for theoreticalanalysis and numerical simulation to analyze the stability of cap rock.Acoustic emission techniques were also adopted to carry outlong term real time rupture monitoring in cap rock.Therefore,a complete safety evaluation system for the cap rock was establishedto ensure safe operation of subsequent blasting processes.The ideal way of eliminating collapse hazard of such cavity is cap rockcaving through deep-hole blasting,thus,two deep-hole blasting schemes named as vertical deep-hole blasting scheme and one-timeraise driving integrated with deep-hole bench blasting scheme were proposed.The vertical deep-hole blasting scheme has moreexplosive consumption,but the relatively simple blasting net work structure can greatly reduce workloads.However,the one-timeraise driving integrated with deep-hole bench blasting scheme can obviously reduce explosive consumption,but the higher technicalrequirements on drilling,explosive charging and blasting network will increase workloads.

Coal Rock Condition Detection Model Using Acoustic Emission and Light Gradient Boosting Machine

Coal rock mass instability fracture may result in serious hazards to underground coal mining.Acoustic emissions(AE)stimulated by internal structure fracture should carry lots of favorable information about health condition of rock mass.AE as a sensitive non-destructive test method is gradually utilized to detect anomaly conditions of coal rock.This paper proposes an improved multi-resolution feature to extract AE waveform at different frequency resolutions using Coilflet Wavelet Transform method(CWT).It is further adopt an efficient Light Gradient Boosting Machine(LightGBM)by several cascaded sub weak classifier models to merge AE features at different views of frequency for coal rock anomaly damage recognition.The results denote that the proposed method achieves excellent recognition performance on anomaly damage levels of coal rock.It is an effective method to detect the critical stability further to predict the rock mass bursting in time.

Stability of coal pillar in gob-side entry driving under unstable overlying strata and its coupling support control technique

Considering the situation that it is difficult to control the stability of narrow coal pillar in gob-side entry driving under unstable overlying strata, the finite difference numerical simulation method was adopted to analyze the inner stress distribution and its evolution regularity, as well as the deformation characteristics of narrow coal pillar in gob-side entry driving, in the whole process from entry driving of last working face to the present working face mining. A new method of narrow coal pillar control based on the triune coupling support technique (TCST), which includes that high-strength prestressed thread steel bolt is used to strain the coal on the goaf side, and that short bolt to control the integrity of global displacement zone in coal pillar on the entry side, and that long grouting cable to fix anchor point to constrain the bed separation between global displacement zone and fixed zone, is thereby generated and applied to the field production. The result indicates that after entry excavating along the gob under unstable overlying strata, the supporting structure left on the gob side of narrow coal pillar is basically invalid to maintain the coal-pillar stability, and the large deformation of the pillar on the gob side is evident. Except for the significant dynamic pressure appearing in the coal mining of last working face and overlying strata stabilizing process, the stress variation inside the coal pillar in other stages are rather steady, however, the stress expansion is obvious and the coal pillar continues to deform. Once the gob-side entry driving is completed, a global displacement zone on the entry side appears in the shallow part of the pillar, whereas, a relatively steady fixed zone staying almost still in gob-side entry driving and present working face mining is found in the deep part of the pillar. The application of TCST can not only avoid the failure of pillar supporting structure, but exert the supporting capacity of the bolting structure left in the pillar of last sublevel entry, thus to jointly maintain the stability of coal pillar.

Effects of confining pressure on deformation failure behavior of jointed rock

For a deeper understanding of the deformation failure behavior of jointed rock, numerical compression simulations are carried out on a rock specimen containing non-persistent joints under confining pressure with the bondedparticle model. The microscopic parameters which can reflect the macroscopic mechanical properties and failure behavior of artificial jointed specimens are firstly calibrated. Then, the influence of joint inclination and confining pressure on stress-strain curves, crack patterns, and contact force distributions of jointed rock are investigated. The simulation results show that both the compressive strength and elastic modulus of the specimens increase with increasing confining pressure, and these two mechanical parameters decrease first and then increase with the increase of joints inclination. The sensitivity of strength and elastic modulus to confining pressure is not the same in different joints inclinations, which has the least impact on specimens with α=90°. Under low confining pressure, the failure modes are controlled by the joint inclination. As the confining pressure increased, the initiation and propagation of tensile crack are gradually inhibited, and the failure mode is transferred from tensile failure to shear-compression failure. Finally, the reinforcement effect of prestressed bolt support on engineering fractured rock mass is discussed.

Engineering rock mechanics practices in the underground powerhouse at JinpingⅠhydropower 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.

Deep-seated large-scale toppling failure in metamorphic rocks：a case study of the Erguxi slope in southwest China

Deep-seated large-scale toppling failure presents unique challenges in the study of natural slope deformation process in mountainous regions.An active deep-seated toppling process was identified in the Erguxi slope located in southwest China,which affected a large area and damaged critical transportation infrastructure with the volume of the deforming rock mass exceeding 24×10~6 m^3.It poses significant risks to the downstream Shiziping Hydropower Station by damming the Zagunao River.Field investigation and monitoring results indicate that the deformation of the Erguxi slope is in the advanced stage of deep-seated toppling process,with the formation of a disturbed belt but no identifiable master failure surface.It was postulated that the alternating tensile and shear strength associated with the hard/soft laminated rock strata of metasandstone and phyllite layers preclude the development of either a tensile or shear failure surface,which resulted in the continuous deformation and displacement without a catastrophic mass movement.The slope movement is in close association with the unfavorable geological conditions of the study area in addition to the construction of transportation infrastructure and the increase of the reservoir level.On the basis of the mechanism and intensity of the ongoing toppling deformation,a qualitative grading system was proposed to describe the toppling process and toevaluate the slope stability.This paper summarized the field observation and monitoring data on the toppling deformation for better characterizing its effect on the stability of the Erguxi slope.The qualitative grading system intends to provide a basis for quantitative study of large-scale deep-seated toppling process in metamorphic rocks.

Cyclic load testing of pre-stressed rock anchors for slope stabilization

The objective of this research was to assess the characteristics of seismic induced damage and the deformation patterns of pre-stressed cement-grouted cables that are used for rock slope stabilization projects subjected to quasi-static cyclic loading.The experimental configuration includes the installation of 15 pre-stressed cables in a slope model made of concrete blocks(theoretically rigid rock mass) on top of a pre-existing sliding surface.The study showed that:(i) The pre-stressed cables exhibited great seismic performance.Rapid displacement of the model blocks was observed after the complete loss of the initial pre-stress load under continued applied cyclic loads and exceedance of the state of equilibrium,which implies the higher the initial pre-stress load,the better the seismic performance of the rock anchor;(ii) The failure of the pre-stressed cables was due to fracture at the connection of the tendons and cable heads under cyclic loading.The sequence of failure had a distinct pattern.Failure was first observed at the upper row of cables,which experienced the most severe damage,including the ejection of cable heads.No evidence of de-bonding was observed during the cyclic loading;(iii) The stress distribution of the bond length for pre-stressed cables was highly non-uniform.High stress concentrations were observed at both the fixed end and the free end of the bond length both before and immediately after the state of equilibrium is exceeded.The results obtained can be used to evaluate the overall performance of pre-stressed rock anchors subject to seismic loading and their potential as rockfall prevention and stabilization measures.