Safety risk management of underground engineering in China:Progress,challenges and strategies

Underground construction in China is featured by large scale, high speed, long construction period,complex operation and frustrating situations regarding project safety. Various accidents have been reported from time to time, resulting in serious social impact and huge economic loss. This paper presents the main progress in the safety risk management of underground engineering in China over the last decade, i.e.(1) establishment of laws and regulations for safety risk management of underground engineering,(2) implementation of the safety risk management plan,(3) establishment of decision support system for risk management and early-warning based on information technology, and(4) strengthening the study on safety risk management, prediction and prevention. Based on the analysis of the typical accidents in China in the last decade, the new challenges in the safety risk management for underground engineering are identified as follows:(1) control of unsafe human behaviors;(2) technological innovation in safety risk management; and(3) design of safety risk management regulations. Finally, the strategies for safety risk management of underground engineering in China are proposed in six aspects, i.e. the safety risk management system and policy, law, administration, economy, education and technology.     

Measures for controlling large deformations of underground caverns under high in-situ stress condition——A case study of Jinping Ⅰ hydropower station

The Jinping Ⅰ hydropower station is a huge water conservancy project consisting of the highest concrete arch dam to date in the world and a highly complex and large underground powerhouse cavern.It is located on the right bank with extremely high in-situ stress and a few discontinuities observed in surrounding rock masses.The problems of rock mass deformation and failure result in considerable challenges related to project design and construction and have raised a wide range of concerns in the fields of rock mechanics and engineering.During the excavation of underground caverns,high in-situ stress and relatively low rock mass strength in combination with large excavation dimensions lead to large deformation of the surrounding rock mass and support.Existing experiences in excavation and support cannot deal with the large deformation of rock mass effectively,and further studies are needed.In this paper,the geological conditions,layout of caverns,and design of excavation and support are first introduced,and then detailed analyses of deformation and failure characteristics of rocks are presented.Based on this,the mechanisms of deformation and failure are discussed,and the support adjustments for controlling rock large deformation and subsequent excavation procedures are proposed.Finally,the effectiveness of support and excavation adjustments to maintain the stability of the rock mass is verified.The measures for controlling the large deformation of surrounding rocks enrich the practical experiences related to the design and construction of large underground openings,and the construction of caverns in the Jinping Ⅰ hydropower station provides a good case study of large-scale excavation in highly stressed ground with complex geological structures,as well as a reference case for research on rock mechanics.     

Preliminary engineering application of microseismic monitoring technique to rockburst prediction in tunneling of Jinping Ⅱ project

Monitoring and prediction of rockburst remain to be worldwide challenges in geotechnical engineering.In hydropower,transportation and other engineering fields in China,more deep,long and large tunnels have been under construction in recent years and underground caverns are more evidently featured by ＂long,large,deep and in group＂,which bring in many problems associated with rock mechanics problems at great depth,especially rockburst.Rockbursts lead to damages to not only underground structures and equipments but also personnel safety.It has been a major technical bottleneck in future deep underground engineering in China.In this paper,compared with earthquake prediction,the feasibility in principle of monitoring and prediction of rockbursts is discussed,considering the source zones,development cycle and scale.The authors think the feasibility of rockburst prediction can be understood in three aspects：（1） the heterogeneity of rock is the main reason for the existence of rockburst precursors;（2） deformation localization is the intrinsic cause of rockburst;and（3） the interaction between target rock mass and its surrounding rock mass is the external cause of rockburst.As an engineering practice,the application of microseismic monitoring techniques during tunnel construction of Jinping II Hydropower Station was reported.It is found that precursory microcracking exists prior to most rockbursts,which could be captured by the microseismic monitoring system.The stress concentration is evident near structural discontinuities（such as faults or joints）,which shall be the focus of rockburst monitoring.It is concluded that,by integrating the microseismic monitoring and the rock failure process simulation,the feasibility of rockburst prediction is expected to be enhanced.     

Rock brittleness indices and their applications to different fields of rock engineering:A review

Brittleness is an important parameter controlling the mechanical behavior and failure characteristics of rocks under loading and unloading conditions,such as fracability,cutability,drillability and rockburst proneness.As such,it is of high practical value to correctly evaluate rock brittleness.However,the definition and measurement method of rock brittleness have been very diverse and not yet been standardized.In this paper,the definitions of rock brittleness are firstly reviewed,and several representative definitions of rock brittleness are identified and briefly discussed.The development and role of rock brittleness in different fields of rock engineering are also studied.Eighty brittleness indices publicly available in rock mechanics literature are compiled,and the measurement method,applicability and limitations of some indices are discussed.The results show that(1)the large number of brittleness indices and brittleness definitions is attributed to the different foci on the rock behavior when it breaks;(2)indices developed in one field usually are not directly applicable to other fields;and(3)the term“brittleness”is sometimes misused,and many empirically-obtained brittleness indices,which lack theoretical basis,fail to truly reflect rock brittleness.On the basis of this review,three measurement methods are identified,i.e.(1)elastic deformation before fracture,(2)shape of post-peak stressestrain curves,and(3)methods based on fracture mechanics theory,which have the potential to be further refined and unified to become the standard measurement methods of rock brittleness.It is highly beneficial for the rock mechanics community to develop a robust definition of rock brittleness.This study will undoubtedly provide a comprehensive timely reference for selecting an appropriate brittleness index for their applications,and will also pave the way for the development of a standard definition and measurement method of rock brittleness in the long term.     

大型地下洞室地震灾变过程三维动力有限元模拟

 The earthquake impact is a major external factor influencing the long term stability of underground caverns. Time-history analysis method is able to simulate the earthquake response of surrounding rock in underground caverns during earthquake disaster process. It has been proved to be an effective method to analyze the aseismic issue of underground caverns. The numerical simulation system using three-dimensional dynamic finite element method is developed to study the earthquake disaster in underground caverns. Central difference method is employed in this system to solve the problem. To enhance the solving speed,parallel procedures and hybrid Gauss point integration method are proposed in programming. Strengthening features of materials under high strain rate and damage features of surrounding rock under cyclic loading are considered. The dynamic constitutive model for rock masses is presented. It is suitable for the aseismic analysis of underground caverns. Visco-elastic boundary is adopted in artificial boundary. The spatial oblique incident method which is suitable for the aseismic analysis of underground caverns is proposed. It is able to reflect the specific incident direction,the multi-incident surfaces and the inconsistency of seismic wave. An example is given to verify the correctness of the developed system by comparing its calculation results with common software?s calculation results. A case study is then conducted to simulate the earthquake disaster process of the cave-typed underground caverns at Lujichang hydropower plant. The results derived from time-history analysis indicate the reliability and practicability of the developed system.     

Application of artificial intelligence to rock mechanics: An overview

Different artificial intelligence(AI)methods have been applied to various aspects of rock mechanics,but the fact that none of these methods have been used as a standard implies that doubt as to their generality and validity still exists.For this,a literature review of application of AI to the field of rock mechanics is presented.Comprehensive studies of the researches published in the top journals relative to the fields of rock mechanics,computer applications in engineering,and the textbooks were conducted.The performances of the AI methods that have been used in rock mechanics applications were evaluated.The literature review shows that AI methods have successfully been used to solve various problems in the rock mechanics field and they performed better than the traditional empirical,mathematical or statistical methods.However,their practical applicability is still an issue of concern as many of the existing AI models require some level of expertise before they can be used,because they are not in the form of tractable mathematical equations.Thus some advanced AI methods are still yet to be explored.The limited availability of dataset for the AI simulations is also identified as a major problem.The solutions to the identified problems and the possible future research focus were proposed in the study subsequently.     

ON USING DEM FOR SIMULATING RESPONSE OF JOINTED ROCK TO UNDERGROUND EXPLOSION

To verify the capability of DEM in modeling the dynamic response of rock masses to blasting loads, two underground explosion tests are numerically Investigated, Results show that rock joints can significantly affect the transmission and the attenuation of shock waves, and can therefore influence the stability of the adjacent underground structures. Rock joints act as a kind of filters through which only low-frequency components of the shock waves are allowed to pass, and the high-frequency components of the shock waves do no harm to far-field tunneis if a predominant joint set exists in between. The spacing of joint set can also remarkably affect the propagation process, It Indicates that as a discontinuum-based numerical approach,DEM is good at simulating the propagation and attenuation of blasting wave in jointed rock masses, and in modeling the stability of underground structures subjected to blasting loads.     

Latest progress of soft rock mechanics and engineering in China

The progress of soft rock mechanics and associated technology in China is basically accompanied by the development of mining engineering and the increasing disasters of large rock deformation during construction of underground engineering.In this regard,Chinese scholars proposed various concepts and classification methods for soft rocks in terms of engineering practices.The large deformation mechanism of engineering soft rocks is to be understood through numerous experiments;and thus a coupled support theory for soft rock roadways is established,followed by the development of a new support material,i.e.the constant resistance and large deformation bolt/anchor with negative Poisson’s ratio effect,and associated control technology.Field results show that large deformation problems related to numbers of engineering cases can be well addressed with this new technology,an effective way for similar soft rock deformation control.     

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.     

Dynamic compression characteristics of layered rock mass of significant strength changes in adjacent layers

Layered rock mass of significant strength changes for adjacent layers is frequently observed in underground excavation,and dynamic loading is a prevalent scenario generated during excavation.In order to improve the driving efficiency and reduce engineering accidents,dynamic compression characteristics of this kind of rock mass should be understood.The dynamic properties of a layered composite rock mass are investigated through a series of rock tests and numerical simulations.The rock mass is artificially made of various proportions of sand,cement and water to control the distinct strength variations at various composite layers separated by parallel bedding planes.All rock specimens are prefabricated in a specially designed mould and then cut into 50 mm in diameter and 50 mm in height for split Hopkinson pressure bar(SHPB)dynamic compression testing.The test results reveal that increasing strain rate causes the increases of peak strength,σ_p,and the corresponding failure strain,ε_p,while the dynamic elastic modulus,E_d,remains almost unchanged.Interestingly,under the same strain rates,Ed of the composite rock specimen is found to decline first and then increase as the dip angle of bedding plane increases.The obtained rock failure patterns due to various dip angles lead to failure modes that could be classified into four categories from our dynamic tests.Also,a series of counterpart numerical simulations has been undertaken,showing that dynamic responses are in good agreement with those obtained from the SHPB tests.The numerical analysis enables us to Iook into the dynamic characteristics of the composite rock mass subjected to a broader range of strain rates and dip angles than these being tested.     

深基坑施工环境影响的模糊风险分析

 Based on fuzzy mathematics theory,a risk analysis model was established for the influence of deep excavation construction on surrounding environment. By using this model,reasonable risk loss evaluation index,risk grade division and risk loss formula were proposed through analytical and quantitative analysis of a concrete project example. After studying the evaluation index and relative weight of building damage,pavement damage and underground pipeline damage around the deep excavation,risk analysis and assessment of the influence on surrounding environment caused by excavation of foundation pit were achieved. The analysis and assessment is conducted through the membership degree which is given from the membership function structured for each evaluation factor. Then through the comprehensive risk evaluation,risk level can be finally determined. This result based on project example can be used as a reference for engineering decision-making.     

Modelling of blast-induced damage in tunnels using a hybridfinite-discrete numerical approach

This paper presents the application of a hybrid finite-discrete element method to study blast-induceddamage in circular tunnels. An extensive database of field tests of underground explosions above tunnelsis used for calibrating and validating the proposed numerical method; the numerical results areshown to be in good agreement with published data for large-scale physical experiments. The method isthen used to investigate the influence of rock strength properties on tunnel durability to withstand blastloads. The presented analysis considers blast damage in tunnels excavated through relatively weak（sandstone） and strong （granite） rock materials. It was found that higher rock strength will increase thetunnel resistance to the load on one hand, but decrease attenuation on the other hand. Thus, undercertain conditions, results for weak and strong rock masses are similar.
2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.     

An operational approach to ground control in deep mines

As mines go deeper and get larger,mine designs become more fragile largely due to the response of the rock mass to mining.Ground control and rock support become important levers in the mine construction schedule,production performance,and excavation health.For example,in cave mines,the production footprint together with associated mine infrastructure are significant investments in a modern caving operation.This investment must be protected and maintained to reduce the risk of ground-related production disruptions.It is necessary to preserve the health of these excavations and their maintenance through an effective rock support design.Rock support thus becomes a strategic element in asset management.This article focuses on support design for brittle ground when displacements induced by stress-fracturing consume much of the support’s capacity.It deals with the functionality of the support in deforming ground.Several interlinked concepts are important when assessing excavation health.Designs must not only account for load equilibrium but also for deformation compatibility and capacity consumption.Most importantly,the support’s displacement capacity is being consumed when the rock mass is deformed after support installation.Hence,it is necessary to design for the support capacity remaining at the time when the support is needed.If support capacity can be consumed,it can also be restored by means of preventive support maintenance(PSM).This concept for cost-effective ground control is introduced and illustrated on operational evidence.Furthermore,how design can impact construction costs and schedule are discussed.Support is installed to provide a safe environment and preserve an operationally functional excavation.It also must assure senior management that investments in high quality support and its maintenance will substantially reduce delays and with it,costs.It is demonstrated that the use of‘gabion-like’support systems can achieve these goals.A technical summary of the‘gabion panel’support system design is presented.     

Overhanging rock slope by design: An integrated approach using rock mass strength characterisation,large-scale numerical modelling and limit equilibrium methods

Overhanging rock slopes(steeper than 90°) are typically avoided in rock engineering design, particularly where the scale of the slope exceeds the scale of fracturing present in the rock mass. This paper highlights an integrated approach of designing overhanging rock slopes where the relative dimensions of the slope exceed the scale of fracturing and the rock mass failure needs to be considered rather than kinematic release of individual blocks. The key to the method is a simplified limit equilibrium(LE) tool that was used for the support design and analysis of a multi-faceted overhanging rock slope. The overhanging slopes required complex geometries with constantly changing orientations. The overhanging rock varied in height from 30 m to 66 m. Geomechanical modelling combined with discrete fracture network(DFN)representation of the rock mass was used to validate the rock mass strength assumptions and the failure mechanism assumed in the LE model. The advantage of the simplified LE method is that buttress and support design iterations(along with sensitivity analysis of design parameters) can be completed for various cross-sections along the proposed overhanging rock sections in an efficient manner, compared to the more time-intensive, sophisticated methods that were used for the initial validation. The method described presents the development of this design tool and assumptions made for a specific overhanging rock slope design. Other locations will have different geological conditions that can control the potential behaviour of rock slopes, however, the approach presented can be applied as a general guiding design principle for overhanging rock cut slope.     

GPGPU-parallelised hybrid finite-discrete element modelling of rock chipping and fragmentation process in mechanical cutting

Mechanical cutting provides one of the most flexible and environmentally friendly excavation methods.It has attracted numerous efforts to model the rock chipping and fragmentation process,especially using the explicit finite element method(FEM) and bonded particle model(BPM),in order to improve cutting efficiency.This study investigates the application of a general-purpose graphic-processing-unit parallelised hybrid finite-discrete element method(FDEM) which enjoys the advantages of both explicit FEM and BPM,in modelling the rock chipping and fragmentation process in the rock scratch test of mechanical rock cutting.The input parameters of FDEM are determined through a calibration procedure of modelling conventional Brazilian tensile and uniaxial compressive tests of limestone,A series of scratch tests with various cutting velocities,cutter rake angles and cutting depths is then modelled using FDEM with calibrated input parameters.A few cycles of cutter/rock interactions,including their engagement and detachment process,are modelled for each case,which is conducted for the first time to the best knowledge of the authors,thanks to the general purpose graphic processing units(GPGPU) parallelisation.The failure mechanism,cutting force,chipping morphology and effect of various factors on them are discussed on the basis of the modelled results.Finally,it is concluded that GPGPU-parallelised FDEM provides a powerful tool to further study rock cutting and improve cutting efficiencies since it can explicitly capture different fracture mechanisms contributing to the rock chipping as well as chip formation and the separation process in mechanical cutting.Moreover,it is concluded that chipping is mostly owed to the mix-mode Ⅰ-Ⅱ fracture in all cases although mode Ⅱ cracks and mode Ⅰ cracks are the dominant failures in rock cutting with shallow and deep cutting depths,respectively.The chip morphology is found to be a function of cutter velocdty,cutting depth and cutter rake angle.     

Time-sensitivity mechanism of rock stress memory properties under tensile stress

In deep underground engineering,understanding of time-related stress memory properties is critical to evaluate the in situ stress conditions of a rock mass.In this study,the time-sensitivity mechanism of the rock stress memory properties under tensile stress was investigated.It was found that the material property(Poisson's ratio) and crack angle were the controlling factors of the Kaiser effect(KE) under tensile stress.In particular,the time-sensitivity of the stress memory properties was closely related to the crack growth path.When the failure of the rock specimen was dominated by tensile microcracks and the crack development direction was deflected by up to 30° in the successive loading process,the stress memory capacity was likely to be time-independent for a sandstone specimen.The distribution of the Felicity ratio in a Brazilian test was more discrete than that in a three-point bending test It also showed that the changes in the crack path,rather than the time interval between successive loading cycles,led to inaccuracy of the detected KE.This study provides insights into stress memory-related issues under uniaxial or more complex stress conditions and thus facilitates development of methods for testing in situ mechanical behaviors of rocks with acoustic emission(AE) technology.     

Key issues in rock mechanics of the Three Gorges Project in China

The Three Gorges Project is one of the essential key projects for flood controlling and water resources regulation in the Yangtze River.The project includes a river-crossing dam,underground powerhouses,and navigation structures.Because of the huge size and complicated construction technologies,the project faced a series of challenging engineering issues.In terms of rock mechanics,there are many key technical issues,including the sliding resistance and stability of the dam section along the foundations of powerhouses No.1-5,the slope stability of the double-line five-stage shiplock,excavation of large-scale underground powerhouses,and curtain grouting under the dam.With decades of scientific research and 16 years of practical construction experiences and reservoir operations,these key technical issues in construction of the Three Gorges Project are successfully resolved,which will attribute to the development of hydropower technology.On the basis of the monitoring data during construction and normal operation periods of the Three Gorges Project,this paper presents a systematic analysis of these key rock mechanical issues in terms of behaviors,solutions,dynamic controlling,monitoring arrangement and integrated assessment.     

Augmented Reality Applied to Building Assessment with Building Information Model Visualization

AR （augmented reality） is a technology that adds information to the real world adding virtual elements to its visualization in real time. AR used in AECO （architectural, engineering, construction and operations） can contribute in augmenting visualization during design, construction and operation of the buildings. This article presents a study that applies AR to building assessment with BIM （building information） model visualization. The use of AR on existing applications for smart phones and tablets is validated. AR proposed an adaptation of the method of POE （post-occupancy evaluation） subsidized. Traditional POE process model involves three phases： planning, conducting and applying. In order to incorporate AR, it is proposed a total restructuring of the planning phase, developing the research instruments in three steps： 3D modeling, model treatment and AR application development. It was observed that for POE studies, the 3D models are in large scale and need to be detailed for precise comparison. BIM models for facility management, representing building use situation, are of the highest level of detail. A balanced point between simplicity and representativeness was the solution adopted in this experiment for uploading and downloading performance issues. This article presents and discusses findings for the new proposition for the activity of research instruments development for the planning phase of POE with AR as well as initial tests with first results and difficulties faced.     

A two-dimensional limit equilibrium computer code for analysis of complex toppling slope failures

Evaluation of blocky or layered rock slopes against toppling failures has remained of great concern for engineers in various rock mechanics projects.Several step-by-step analytical solutions have been developed for analyzing these types of slope failures.However,manual application of these analytical solutions for real case studies can be time-consuming,complicated,and in certain cases even impossible.This study will first examine existing methods for toppling failure analyses that are reviewed,modified and generalized to consider the effects of a wide range of external and dead loads on slope stability.Next,based on the generalized presented formulae,a Windows form computer code is programmed using Visual C#for analysis of common types of toppling failures.Input parameters,including slope geometry,joint sets parameters,rock and soil properties,ground water level,dynamic loads,support anchor loads as well as magnitudes and forms of external forces,are first loaded into the code.The input data are then saved and used to graphically draw the slope model.This is followed by automatic identification of the toppling failure mode and a deterministic analysis of the slope stability against this failure mode.The results are presented using a graphical approach.The developed code allows probabilistic introduction of the input parameters via probability distribution functions(PDFs)and thus a probabilistic analysis of the toppling failure modes using Monte-Carlo simulation technique.This allows calculation of the probability of slope failure.Finally,several published case studies and typical examples are analyzed with the developed code.The outcomes are compared with those of the main references to assess the performance and robustness of the developed computer code.The comparisons demonstrate good agreement between the results.