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The study of rock fracturing is a fundamental research topic in rock mechanics and engineering. In this paper, based on complete stress–strain curves obtained under TTC and the corresponding AE and strain energy results, the mechanical properties and fracturing process of Beishan granite are studied. A representative test result (σ 2 = 75 MPa, σ 3 = 20 MPa) is selected to analyse and illustrate the three-dimensional fracturing process of the Beishan granite. The results show that the variations in the characteristics of the brittle fracture behaviour, AE and strain energy of Beishan granite change with σ 2 or σ 3 following certain relationships, and the mechanisms driving these various relationships are very different. The AE and strain energy characteristics of Beishan granite are also quite different among the five fracturing evolution stages. The variations in the increments of AE count, AE event and total elastic strain energy per unit time (ΔS c / Δt , ΔS e / Δt and ΔU e / Δt) are studied at different times and under different stresses during rock failure. The rock fracturing evolution can be illustrated through the variation in these parameters, providing deep insight. This research provides a perspective for studying deep underground fracture-forming processes, such as earthquakes, tunnel sudden failure, spalling, splitting and rockburst. [ABSTRACT FROM AUTHOR]

To investigate the deformation and failure mechanism of rock influenced by initial stress and unloading rates, this study presents minimum principal stress unloading tests of Jinping marble under true triaxial compression conditions under various initial stresses and unloading rates based on the in situ stresses measured in the China Jinping Underground Laboratory Phase II (CJPL-II). The effects of initial stresses and unloading rates on the bearing capacity and the deformation and failure characteristics of Jinping marble were analysed. The relationship between the strains in the three principal stress directions and the unloading extent of σ 3 is defined using parameters such as the strain-unloading extent ratio and the unloading strain rate. With an increase in the initial σ 1 , the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential earlier, and sample failure occurred with a smaller unloading extent. Meanwhile, the number of acoustic emission rings and the amount of cumulative energy increased, and the sample failure changed from tensile failure to a mixture of tensile and shear failure that showed more transgranular cracks in the microscopic images. With an increase in the initial σ 2 or σ 3 , the overall trend of the failure characteristics was similar to that with an increase in the initial σ 1 , but the sample failed with a larger unloading extent, and the increase in strain-unloading extent ratio and unloading strain rate changed from linear to exponential later. The influence of unloading strain rate shows that as unloading strain rate increased, the sample failed with a larger unloading extent and changed from a mixture of tensile and shear to tensile failure, and more transgranular cracks were observed in the microscopic images. • σ 3 unloading tests with different unloading rates and initial stress levels under true triaxial stresses were carried out. • The bearing capacity, deformation process, and failure mode of Jinping marble under σ 3 unloading condition were examined. • The relationships between strains along principal stress directions and unloading extent of σ 3 (Δσ 3) were established. • Failure mechanisms of Jinping marble were identified based on AE data and SEM pictures of fractures. [ABSTRACT FROM AUTHOR]

Abstract The time-dependent deformation, creep rate, and long-term strength of Jinping marble under different 3-D stress states were studied by using true triaxial creep experiment. A 3-D visco-elastic-plastic creep model, which can describe the accelerated creep， was developed according to the true triaxial time-dependent deformation curve. The results show that the time-dependent behaviour of Jinping marble is affected by the stress level. When the stress is less than the long-term strength, not obvious time-dependent behaviour of the rock sample is found, and the strain remains unchanged after primary creep. When the stress exceeds the long-term strength, the time-dependent behaviour of the rock sample gradually increases with increasing σ 1 and a clear steady state creep stage is seen in the strain-time curve. With constant σ 1 and σ 3 , the time-dependent deformation and creep rate in the three main stress directions gradually decrease with increasing σ 2. Compared with conventional triaxial test data, a large intermediate principal stress increases the time-dependent dilatation in the σ 3 direction. In addition, a 3-D visco-elastic-plastic creep model considering damage is proposed to describe the creep behaviour of hard rock under different 3-D stress states. The model results fit the experimental results, and the selection of model parameters is closely related to the intermediate principal stress. The model results also verify the applicability of the 3-D element model. [ABSTRACT FROM AUTHOR]

Large-scale columnar jointed basalt (CJB) developments have had serious adverse effects on the stability of the project in progress to create the diversion tunnels at the Baihetan hydropower station in China. An in situ microseismic (MS) monitoring experiment was carried out to help understand the fracturing process in the CJB rock mass as a result of tunnel excavation. MS monitoring performance (accuracy of location estimation and system sensitivity) is analyzed first. Then, the spatiotemporal evolution of the MS activity associated with the Baihetan CJB rock mass fractures along the tunnel axis and at the tunnel sidewalls during the entire excavation unloading process is obtained. Finally, the forms and opportunities to deploy appropriate support are suggested according to the characteristics of the CJB microseismicity. Meanwhile, the MS monitoring methods required for CJB or jointed hard rock engineering (e.g. sensor types, sensor spacing, sensor array forms, system protection, and selection of analysis regions) resulting from this exploration experiment are summarized. [ABSTRACT FROM AUTHOR]

Layered rock masses exhibit significant anisotropy in both the deformability and strength. Properties of this rock mass are largely affected by its bedding structure and also by the stress state change. For the purpose of characterizing these salient features using the equivalent continuum concept, an enhanced model is proposed in this paper based on the ubiquitous-joint model. In the enhanced model, the intact bedded rock is assumed to behave as a transversely isotropic elastic body. A modified anisotropic strength criterion is adopted to describe the direction dependence of the strength of intact bedded rock. In addition, stress-sensitive parameters are utilized for both the intact rock and bedding plane, among which the dilation angle and strength parameters of intact rock are influenced by confining pressure and loading history, whereas the stiffnesses of bedding planes are closely related to the current normal stress. The effect of layer thickness on the mechanical behavior of rock mass is reflected by continuously updating the global stiffness matrix after failure of bedding plane, and by conditions for conducting bedding plane related calculations determined by the relative scales between element size and layer thickness. Preliminary validation of this model is conducted by comparing with the closed-form solutions and with laboratory tests. Generally good agreements can be achieved between numerical simulations and theoretical/experimental results, which indicate that applicability of the enhanced model on underground engineering issues related to layered rock mass is promising. [ABSTRACT FROM AUTHOR]

In the process of underground engineering excavation, the adjustment of the stress field includes not only the magnitude of stress but also the direction of stress. The failure characteristics of Portuguese granite caused by interchange of the σ 2 and σ 1 directions are studied using a true triaxial testing system to investigate the effects of the interchange of principal stress directions on the failure characteristics of rock. After the interchange of the σ 2 and σ 1 directions, the bearing capacity of the rock increases, the anisotropic characteristics of deformation and fracture are interchanged, the macroscopic failure surface is deflected, and the brittleness of the rock is enhanced. When the rock failure degree increases upon the interchange of the σ 2 and σ 1 directions, the amplitude of the rock bearing capacity increases, the deformation difference decreases, and the number of deflection failure surfaces decreases. Based on the anisotropic characteristics of true three-dimensional stress-induced fractures, the reason for the change in failure mode induced by the interchange between the σ 2 and σ 1 directions is explained. By measuring acoustic emission parameters and performing scanning electron microscopy (SEM) observations, the tensile and shear fracture mechanism and fracture mode with interchange of the σ 2 and σ 1 directions are investigated. The effect of stress rotation on the rock strength and the reason for the change in the failure mode of deep hard surrounding rock excavated in underground engineering projects are discussed. [ABSTRACT FROM AUTHOR]

Microwave-assisted mechanical mining is a new mining method with great potential. A self-developed true triaxial system for microwave-induced fracturing and cutting of hard rocks was used to study the microwave cracking process, cracking mechanism, and its effects on subsequent cutting, grinding and mineral dissociation by taking Sishanling banded iron ore as the research object. The results show that compared with microwave irradiation alone, the failure mode and mechanism of ore samples were more complex under the combined action of true triaxial stress and microwaves. Large cracks that were parallel to the bedding plane appeared in the sample under microwave irradiation without triaxial stress. When true triaxial stress and microwave irradiation were used simultaneously, the surface of the sample was subject to spalling or spalling and cracking. Additionally, the correlation (parallel or vertical) between the direction of σ 1 and the direction of the bedding plane affected the fracture characteristics. Microwave irradiation could promote subsequent mechanical mining, grinding, and beneficiation of samples. Compared with no microwave treatment, under the action of stress and microwaves, the total amount of rock breakage of samples was increased by 55.1%, the powder content of the product after grinding of cut particles was increased by 21.9%, and the degrees of dissociation of cutting products (<0.1 mm and <0.2 mm) were increased by 11.6 and 12.7%, respectively. The influence of the strip width of magnetite on the microwave cracking effect of ore was estimated. At a given magnetite content, the microwave absorption was worse for wider strips, and the reflection performance was similar for strips of various widths. [ABSTRACT FROM AUTHOR]

Microseismic (MS) monitoring techniques have been used to warn of rockbursts in tunnels to reduce rockburst risks and ensure safe construction with different degrees of success. In order to reveal the characteristics of MS activity in tunnel rockburst development processes, and lay a foundation for tunnel rockburst warning, MS sequences accompanying rockburst development processes in tunnels are studied in this paper. The study is based on tunnels (with a maximum burial depth of 2525 m) of the Jinping II hydropower station, China. Firstly, two MS parameters are put forward to express the comprehensive microseismicity and intensity of the MS events in the rockburst development process. Secondly, the laws governing the evolution of the two parameters are analyzed based on 44 moderate and intense immediate rockburst cases in the tunnels of Jinping II. Then, the MS sequences associated with immediate rockburst development processes in tunnels are divided into two types: fore-main and swarm shock MS sequences. The characteristics of the two types of MS sequences are different with each other. Finally, based on a theoretical analysis and verification using engineering examples, the mechanism of formation of the two MS sequences is analyzed. The fore-main MS sequences are mainly controlled by excavation unloading in the tunnel. The swarm type is mainly controlled by the combined effects of excavation unloading and the presence of structure planes. [ABSTRACT FROM AUTHOR]

After being excavated for a certain time, deeply buried rock mass is still likely to suffer time-dependent failure. Long-term time-dependent failure may lead to the stability failure of engineering structures. As a special case of time-dependent behaviour, the stress relaxation of hard rock be of great research significance. In this paper, true triaxial relaxation tests were carried out. The test results showed that Jinping marble presented obvious stress relaxation behaviours under true triaxial stresses. Before reaching the peak strength, the stress relaxation degree increased monotonically with the increase of axial strain. Under true triaxial unequal stresses, the ε 2 and ε 3 are different during the stress relaxation test. With the increase of σ 2 , the difference between ε 2 and ε 3 increased. Interestingly, the time-dependent deformation of ε 3 was always in a state of dilation while that of ε 2 changed from dilation to compression with the increase of σ 2. The reason for this is that the crack growth was inhibited in the σ 2 direction under high σ 2. During relaxation test, the decrease of axial stress led to rebound of elastic deformation in the σ 2 direction. This rebound of elastic deformation exceeded the irreversible viscoplastic deformation in the σ 2 direction induced by time-dependent cracking, thus finally causing the time-dependent deformation of ε 2 to exhibit compressive characteristics. Under high axial stresses, the dilation deformation triggered by the crack growth in the σ 3 direction exceeded the compressive deformation induced by the decrease of axial stress. Thus, the deformation in the σ 3 direction was always dilated. [ABSTRACT FROM AUTHOR]

Abstract: Aiming at forecasting techniques of ground pressure hazards in deep metal mines, a microseismic monitoring system was established at a deep stope in Hongtoushan copper mine in China. Based on theories of geophysics and rock mechanics, a forecasting approach was introduced using microseismic multi-parameters including cumulative apparent volume, energy index, spatial correlation length, fractal dimension and b value. The approach was used to predict large scale fractures and ground pressure hazards in rock masses. The results showed that microseismic activities were very weak before mining commenced. However, activities became significant when mining started. When mining finished, microseismic activities remained at a higher level for about one week then became weakened. Before large scale fractures occurred, all microseismic multi-parameters behaved differently. The apparent volume and spatial correlation length were continually increasing, while energy index, fractal dimension and b value were gradually dropping to their minimum values. Therefore, these parameters could be used as precursory indicators for great ground pressure hazards. Comprehensive analysis on microseismic multi-parameters changes could improve reliability of forecasting, which is of great significance to deep metal mines in managing ground pressure failure risks. [Copyright &y& Elsevier]

Abstract: This paper presents results of in situ measurements of fracture evolution within the excavation damaged zone (EDZ) surrounding the tunnel boring machine (TBM) excavated headrace tunnel No. 3 of Jinping II hydropower station, China. Fractures were measured by borehole images obtained with a digital panoramic borehole camera throughout the processes of tunnel excavation and support installation. Boreholes for the digital camera were pre-drilled perpendicular to the tunnel axis from a pre-excavated branch tunnel, and a series of digital images of the borehole walls were made as the TBM passed. Digital processing revealed the lithology, pre-existing fractures and characters of rock core discing of the surrounding rock mass. Processes of fracture initiation, propagation, and closure were observed, and width, occurrence and zoning of EDZ were measured. The relationship between fracture evolution and TBM construction progress is described in detail. Based on true triaxial tests and theoretical analysis of unloading conditions of deep rock masses, the occurrence of tunnelling induced new fractures at the EDZ is discussed with respect to in situ measurement results. The results provide valuable data for EDZ analysis, future designs and the construction of deeply buried tunnels using TBM. [Copyright &y& Elsevier]

Abstract: The headrace tunnels of the Jinping II Hydropower Station are 16.7km in length and have a maximum depth of 2525m. Many difficulties have been encountered in estimation of virgin rock stresses along the tunnels. One method was presented including the macroscopic analysis of the region geological structures and topography and the integrated analysis of in situ stresses based on the multi-source information in local tunnel sections. The characteristics of geological structures and tectonic movement were studied to determine the orientations of the tectonic stresses within the Jinping II tunnel project region. The influences of the topography on the distribution of in situ stress were analyzed using numerical methods. The virgin stress regimes were preliminarily estimated based on the geological and topographical analysis and the World Stress Map. Under the guidance of these results, multi-source information was collected regarding rock brittle failures, EDZs, core disking and large-scale deformation of soft rock. Several techniques were adopted, including multivariate regression, numerical simulation, and stereographic projection. An iterative method for the orientations and magnitudes of in situ stresses is presented. The in situ stress regimes, orientations, and magnitudes in tunnel sections of different depths were obtained and verified. They can provide the basic conditions for analysis of the stability of surrounding rock mass during excavation. [Copyright &y& Elsevier]

Abstract: We present a procedure for technically auditing rock mechanics modelling and rock engineering design. As rock mechanics modelling becomes more sophisticated, with the ability to include more parameters and to be coupled with other disciplines such as hydrogeology, and as the requirements on rock engineering projects become ever more challenging, it is prudent, if not essential, to have some form of procedure for checking that the modelling and design are suitable within the limits of current knowledge. Accordingly, we present here an appropriate auditing procedure, and demonstrate its use with the examples of measurement of rock stress (as a soft audit) and modelling for the design of hydropower caverns in China (as a semi-hard audit), the auditing questions being tailored to the nature of the work being audited. The types of procedures demonstrated can be used from the initial project concept, through site investigation and modelling, to construction, monitoring and back analysis, allowing the whole process to be concurrently checked providing a transparent and traceable audit trail of results and associated decisions made. [ABSTRACT FROM AUTHOR]

Abstract: Following two previous papers in which we discussed the ways ahead for rock engineering design methodologies, and presented two updated flow charts, this paper discusses how to establish the necessary quality and quantity of information required for rock engineering modelling and design. We demonstrate how different types of information are required by the different modelling and design methods, via their required inputs and constraints (a corollary being that a ‘standard site investigation’ is not appropriate) and that the degree of co-operation between the site investigation personnel and the modelling and design personnel is a crucial component. A five step procedure for establishing the required information is recommended. Two illustrative rock slope case examples (one ‘simple’, one ‘complicated’) demonstrate the different modelling methods used, the type of information that was obtained for the modelling and design, with the results and conclusions. The paper illustrates the importance of understanding and establishing the necessary information requirements and ensuring that this information is in fact obtainable and obtained. [Copyright &y& Elsevier]

Abstract: A numerical model for simulating the rock fracturing process has been developed and implemented in the three-dimensional code EPCA3D, which is based on the earlier EPCA2D (elasto-plastic cellular automaton). EPCA3D has the ability to simulate the initiation, propagation and coalescence of cracks in the failure processes of rocks. Using this code, we studied the failure processes of simulated rock specimens with different sizes and shapes in three-dimensional space. It is concluded that the ‘scale effect’ is not directly related to the heterogeneity, if the rock specimens have the same homogeneity. If the platen-rock elastic mismatch effect in physical testing is not considered in the numerical simulations, there is almost no scale effect for rock specimens with different sizes and shapes. However, by considering the platen-rock interaction in the simulation of the rock failure process with the EPCA3D code, the modeling results reproduced the well-known phenomenon of scale effects found in physical experiments. [Copyright &y& Elsevier]

Abstract: This paper presented a number of experimental results on investigating mechanism of multi-crack interaction in limestone specimens with two or three flaws of different arrangements under the coupled uniaxial compressive stress and chemical solutions NaCl, CaCl2 and NaHCO3 with different ionic concentrations and pH values. The experiments were conducted on a testing system with a microscope and a CCD camera. The process of initiation, propagation and coalescence of cracks under the influence of the chemical corrosion were observed and digitally recorded. Cracking velocity, strength, cracking patterns, initial angle of new cracks and multi-crack interaction mechanism affected by chemical corrosion were also analyzed and compared with air and distilled water cases. The results indicated that the effect of chemical corrosion was quite complicated, depending on chemical ions and their concentrations and pH values, mineral components of rock, geometry and the number of flaws. The results are helpful guidance to establishing a coupling model for mechanical-hydro-chemical behavior of rocks. [Copyright &y& Elsevier]

Abstract: Class I and Class II behavior of the rock failure process under uniaxial compression has been numerically simulated using an elasto-plastic cellular automaton (EPCA). A linear combination of stress and strain was used as the feedback signal to control the loading process in the numerical model. Uniaxial compressive simulations of heterogeneous rock failure were made using this control method and by considering different homogeneity indices and different linear combinations of parameters, as well as different yield criteria. The results indicate that Class II behavior can be numerically simulated in a complementary way to the laboratory testing method utilized in a servo-controlled testing machine. Whether the rock behaves in a Class I or Class II manner depends on the homogeneity of the rock sample, the linear combination parameters and the yield criterion selected. [Copyright &y& Elsevier]

Abstract: In this paper, an elasto-plastic cellular automaton (EPCA) with the associated code was developed to simulate the non-linear fracturing process of rocks under uniaxial compression. It is a useful method for simulating the process of self-organization of the complex system using simple rules. It has the advantages of localization, parallelization, and being able to consider the heterogeneity of rocks. By this means, the fracturing processes, stress–strain curves, acoustic emissions, and compressive strength of 2-D heterogeneous rock specimens under uniaxial compression were numerically investigated using the EPCA code for different cases such as: (1) the influence of the cyclic load; (2) the influence of inhomogeneity; (3) the effect of different softening coefficients; (4) the effect of specimen sizes and height/width ratios; (5) the effect of different spatial distributions of material parameters; (6) the influence of different yield criteria; and (7) the influence of the residual strength coefficient. The results indicate that the numerical simulation reproduced some of the well-known phenomena observed by previous researchers in uniaxial compression tests. [Copyright &y& Elsevier]

Abstract: The response of rocks to stress can be highly non-linear, so sometimes it is difficult to establish a suitable constitutive model using traditional mechanics methods. It is appropriate, therefore, to consider modeling methods developed in other fields in order to provide adequate models for rock behavior, and this particularly applies to the time-dependent behavior of rock. Accordingly, a new system identification method, based on a hybrid genetic programming with the improved particle swarm optimization (PSO) algorithm, for the simultaneous establishment of a visco-elastic rock material model structure and the related parameters is proposed. The method searches for the optimal model, not among several known models as in previous methods proposed in the literatures, but in the whole model space made up of elastic and viscous elementary components. Genetic programming is used for exploring the model''s structure and the modified PSO is used to identify parameters (coefficients) in the provisional model. The evolution of the provisional models (individuals) is driven by the fitness based on the residual sum of squares of the behavior predicted by the model and the actual behavior of the rock given by a set of mechanical experiments. Using this proposed algorithm, visco-elastic models for the celadon argillaceous rock and fuchsia argillaceous rock in the Goupitan hydroelectric power station, China, are identified. The results show that the algorithm is feasible for rock mechanics use and has a useful ability in finding potential models. The algorithm enables the identification of models and parameters simultaneously and provides a new method for studying the mechanical characteristics of visco-elastic rocks. [Copyright &y& Elsevier]

Evaluation of the non-linear deformation behavior of geo-materials is an important aspect of the safety assessment for geotechnical engineering in complex conditions. This paper presents a novel machine learning method, termed support vector machine (SVM), to obtain a global optimization model in conditions of large project dimensions, small sample sizes and non-linearity. A new idea is put forward to combine the SVM with a genetic algorithm. The method has been used in the analysis of the high rock slope of the permanent shiplock of the Three Gorges Project and the horizontal deformation at depth in the Bachimen landslide in Fujian Province, China. The 92 non-linear SVMs in total were constructed with their kernel functions and the parameters were recognized using a genetic algorithm. The results indicate that the established SVMs can appropriately describe the evolutionary law of deformation of geo-materials at depth and provide predictions for the future 6–10 time steps with acceptable accuracy and confidence. [Copyright &y& Elsevier]

Optimization of a soft rock replacement scheme for a large cavern excavated in alternating hard and soft rock strata is a complicated non-linear mechanical problem having a large parameter search space. Obtaining a global optimum solution is the key to the problem. A hybrid intelligent method is proposed for this purpose. It is an integration of an evolutionary neural network and finite element analysis using a genetic algorithm. The non-linear relation of the soft rock replacement scheme with the displacement and damage zone of the cavern due to excavation in the given geological conditions is learnt and represented by a forward neural network whose structure and connection weights are global optimally recognized by using the genetic algorithm. The learning samples are obtained from finite element calculations. The optimal soft rock replacement scheme, having the minimal displacement and damage volume induced by cavern excavation, is searched in a global space using the genetic algorithm. The new methodology is used to evaluate soft rock replacement schemes for the Shuibuya cavern in China excavated in strata consisting of alternating soft and hard rocks. The results indicate that the new methodology can recognize the optimal soft rock replacement scheme for a large cavern in such complicated geological conditions and the neural network model can provide a solution which is close to the finite element analysis for the same geological and construction conditions. [Copyright &y& Elsevier]

In this paper we discuss the future of rock engineering design methodologies within the framework of the eight current and potential basic design approaches. We note that many rock engineering projects are on the limit of precedent practice, involving large scales, increasing costs, enhanced environmental awareness and the need to understand and predict the consequences of coupled mechanisms involving stress, water, heat and chemical reactions. At the same time, rock mechanics modeling is also becoming more complex and many more component elements and mechanisms can now be included in the numerical codes. Thus, because both the projects and the modeling are becoming more sophisticated, yet it may not be possible to obtain the necessary site information to support all the required input data for modeling, there is a need to consider how a design approach can be developed that integrates the current methods and includes those methods that are not based on exact 1:1 mapping between the problem geometry/mechanisms and the model. Our emphasis, therefore, is on development of the integrated design approach and the ‘intelligent’ methods. We also discuss how a virtual laboratory and the Internet can support these developments. An illustrative design case history of the shiplock slopes at the Threee Gorges Dam project in China is included. The paper concludes with nine specific points. [Copyright &y& Elsevier]

The computerized tomography (CT) images and CT values for the process of compression, micro-cracking, and dilation up to the failure of sandstone specimens under different loading levels have been obtained using the real-time CT technique for triaxial loading of sandstone subjected to chemical corrosion. Clear CT images and CT value of the stages from compression of micro-cavities→emergence of micro-cracks→bifurcation→development→fracture→collapse→unloading can be observed. The CT value, equivalent to rock density at the CT scan layer, is the most important parameter describing the damage evolution process of rock. The paper also presents results of the corrosive influence of chemical solutions with different pH values and ionic concentrations on the sandstone strength. Stronger acidity (pH<7) or causticity (pH>7) has a stronger effect on the rock micro-fracturing evolution. The mechanism of damage evolution of sandstone is analyzed and a damage model based on the chemical corrosive influence and CT values is proposed. [Copyright &y& Elsevier]

Sliding in a dam foundation along potential sliding paths is generally caused by two kinds of external factors: one is the overloading of the designed upstream hydrostatic load due to flooding; and the other is the gradual degradation of the shear strength of joints due to seepage, deformation, damage, and geochemical reactions between water and joint surface minerals. Based on the conceptualized geomechanical model of the Three-Gorges Dam, described in the Part I paper, in this Part II paper the limit equilibrium method and finite element method are used to study the effects of gradual degradation of the shear strength of joints on the stability of the dam foundation. The numerical modeling focuses on the stability conditions of the no. 3 powerhouse-dam section which are estimated to be the most critical. The constraint influences from the adjacent no. 2 and no. 4 powerhouse-dam sections are also included. The failure mechanisms, factors of safety and critical displacements of these dam sections are derived numerically as the measures for stability evaluation. The factor of safety is defined as the ratio between the combined shear strength of joints and intact rock bridges, and the mean shear stress along potential sliding path required for limit equilibrium under the designed external loads. All the results obtained from these different numerical models, together with the results of physical model tests as presented in Part I, are compared in this paper. The comparisons show that both the numerical modeling and physical modeling results support each other and demonstrate the stability of the Three-Gorges Dam foundation as designed. Nevertheless, considering the overall engineering and social–economical importance of the Three-Gorges Dam complex, some additional treatment and reinforcement measures are recommended in this paper. [Copyright &y& Elsevier]

Foundation stability is one of the most important factors influencing the safety of a concrete dam and has been one of the key technical problems in the design of the Three-Gorges Project. The major difficulties lie in two facts. The first one is that the dam foundation consists of rock blocks, with joints and so-called ‘rock bridges’ and the gently dipping joints play a critical role in the foundation stability against sliding. The second one is that, even in the regions where the gently dipping fractures are most developed, there are no through-going sliding paths in the rock mass due to the existence of the rock bridges; so the dam could slide only if some of the rock bridges fail, so as to create at least one through-going sliding path. To date, due to unavoidable shortcomings in physical and numerical modeling techniques, there is not a single satisfactory method to solve the problem completely. For this reason, the integration of multiple methods was adopted in this study and proved to be an effective and reliable approach.This Part I paper describes work based on the results of geological investigations and mechanical tests, relating to the geological and geomechanical models of the Three-Gorges Dam, and then a systematic study procedure was developed to carry out the stability assessment project. Then, 2D and 3D physical model tests for some critical dam sections were performed. In the physical tests, based on similarity theory, various testing materials were selected to simulate the rock, concrete, fracture and rock bridge. The loading and boundary conditions were also modeled to meet the similarity requirements. The failure mechanism was derived through a progressive overloading that simulated the upstream hydrostatic pressure applied to the dam, and the factor of safety was defined as the ratio between the maximum external load inducing the start of sliding instability of the dam foundation and the upstream hydrostatic load. The experimental results indicated that the stability of the Three-Gorges Dam foundation satisfies the safety requirements. Nevertheless, further discussions demonstrated that because of the incomplete definition of factor of safety adopted in the physical model tests, it is also essential to study the stability of the Three-Gorges Dam foundation using numerical modeling, which will be presented in the companion Part II paper. [Copyright &y& Elsevier]

The conventional (σ 1 > σ 2 = σ 3) multi-step creep test and true triaxial (σ 1 > σ 2 > σ 3) multi-step creep test of Jinping marble with rectangular shape were conducted. In this way, the effects of σ 1 , σ 2 , σ 3 and time on creep behaviour were compared and analysed. Unlike conventional creep tests, Jinping marble showed differential creep deformation and failure characteristics in σ 2 and σ 3 -directions under true triaxial stress. The crack time-dependent evolution was-directional, and the crack tip growth was nearly parallel to the σ 1 -direction with high stress and opened along the σ 2 and σ 3 -directions with low stress. Moreover, when σ 2 > σ 3 , the crack growth in the σ 2 -direction was inhibited, and the crack evolution mainly occurred along the σ 3 -direction. This difference eventually led to the macroscopic failure plane of Jinping marble generally acting parallel to the σ 2 -direction and along the σ 3 -direction. The larger the value of σ 2 , the steeper the macroscopic failure surface, and the failure mode of the rock samples changed from shear to near-splitting failure: however, increasing σ 2 can only cause closure of initial cracks opening along the σ 2 -direction, but did not change the number of initial cracks opening along the σ 3 -direction. The experimental data also showed that the differential creep deformation in the σ 2 and σ 3 -directions caused by true triaxial stress did not change with time and σ 1 , but was only affected by σ 2 and σ 3. The differential creep deformation in σ 2 and σ 3 -directions can be described by the formula DI = a (1-exp(b ((σ 2 -σ 3)/σ 3))). [ABSTRACT FROM AUTHOR]

Rock fracture process is associated with the degradation and mobilization of the mechanical and strength properties of rock. A number of true triaxial cyclic loading and unloading tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) to investigate the evolution of the rock properties as a function of accumulated damage. In addition to stress and strain measurements, acoustic emission (AE) signals were monitored and used to associate different stages of crack development with different stages of the mechanical and strength parameter evolution. Young's modulus and five strain increment ratios were calculated from the measured increments of stresses and strains in each cycle. It was found that hard rocks exhibited profound deformation anisotropy and directional dilation. A general trend of decreasing volumetric strain reversal stress and peak stress with increasing plastic strains was observed for all the tested specimens. The evolution of the strength properties of rock under true triaxial compression was investigated using a linear 3D hard rock failure criterion and was found to be generally consistent with the cohesion weakening and frictional strengthening concept. The influence of the intermediate principal stress on the evolution of the rock properties was also discussed. [ABSTRACT FROM AUTHOR]

Abstract: We improve the original numerical manifold method (NMM) capability to correctly model the stability of underground openings embedded in discontinuous rock masses under high in situ stress conditions by implementing an algorithm which models the excavation sequence during NMM simulations, starting with a domain with no opening at all and progressively adding openings according to the planned construction phases. The significance of this improvement is demonstrated using the case of Zedekiah cave underneath the old city of Jerusalem, excavated 2000 years ago at a shallow depth in a highly discontinuous rock mass and still stands unsupported. The results clearly show that modeling underground openings in discontinuous rocks without consideration of the excavation sequence is overly conservative. We proceed with developing a new procedure to impose high initial stresses in NMM for accurate deformation modeling of deep underground excavations, and verify our suggested approach using the analytical Kirsch solution. Finally, we apply our enhanced NMM code to the Jinping hydroelectric project tunnels in Sichuan Province, China. Using very accurate sliding micrometer data obtained during the excavation of a research tunnel within the Jinping tunnel complex we constrain the in situ stress field at depth by inversion of the measured displacement data using the modified NMM code. The results provide a quantitative assessment of the in situ stress field in Jinping tunnels at a depth of 2525m below ground surface, where the execution of in situ stress measurements by conventional procedures proves an extremely challenging task. [Copyright &y& Elsevier]

Underground excavation designs in mining and civil engineering projects are associated with a different geometric, complex ground structure, field–stresses, and groundwater conditions. Due to diverse influence factors and various requirements for each project, the design procedure should be customised based on the site-specific factors. The design procedure is only a direct and triggering process, while in a complex ground condition, mixed failure mechanisms may occur. The objective of this research is to present a new methodology "Comprehensive Underground Excavation Design" called CUED method with emphasis on diagnosis of ground behaviour and failure mechanism(s) in deep and hard rock conditions for long-term life expectations. The CUED method proposed in six steps including ground characterisation, diagnosis of ground behaviour, identifying failure mechanism, design analysis to manage ground behaviour, construction, field measurements/monitoring and design update. A procedure has been defined for each step by determination of input data, processing data and output data, so-called IPO approach. IPO is applied to determine parameters of the CUED method in each step. Based on the proposed method, rock mass composition provides sufficient information to the diagnosis of ground behaviour. Then, different types of failure modes and related mechanisms are evaluated following three main factors: rock mass structures, stress concentration and construction conditions/key features of the projects. In some cases, only one failure mechanism is dominant, but in many cases, may failure starts with one mechanism and then followed by other mechanism or combination of mechanisms. It makes the design more complicated and should be adjusted during operational stages. Design analysis to manage ground behaviour is carried out to determine appropriate strategy in an unstable ground such as selecting suitable excavation method, excavation sequences, and ground stabilisation methods regarding time-cost model of the projects. Meanwhile, the design parameters are implemented in the construction stage by excavation, loosening, depressurisation/stress management, quality control of materials and installing ground support systems. Optimisation of design parameters is performed through new acquired data in field measurements to reduce the risk of rock failures. The proposed design procedure was verified through several case studies from mining and underground excavation projects, and some typical cases were presented. The purpose of the proposed design method is to attempt to increase productivity, cost optimisation, safety improvement, and consequently diminish instability in underground excavations. The research results indicated that the proposed method efficiently increase safety and optimise the project's cost and time. The presented CUED method could be used as engineering tools for underground excavation design. • Proposed a new comprehensive underground excavation design (CUED) methodology in deep and hard rock condition. • A practical model for diagnosis of ground behaviour and failure mechanism in complex ground condition. • Recommended appropriate static – dynamic design methods, excavation, and ground management strategy. • Modification of design and operation adjustment based on filed observations. • Improvement of safety, enhancement of reliability of design and cost – optimisation due to instability reduction. [ABSTRACT FROM AUTHOR]

In this work, thermal damage effect on the thermal conductivity inhomogeneity of granite was experimentally investigated. To study the influence of different cooling methods, granite samples were heated at different temperature levels (from 100°C to 1000 °C), then cooled in air and water, respectively. Distributed thermal conductivity of granite samples were measured using optical scanning method before and after thermal damage treatment. Besides, some non-destructive tests (open porosity and P-wave velocity) were performed. The results indicate that thermal damage will lead to the decrease of thermal conductivity and the increase of thermal inhomogeneity degree. The decrease degree of thermal conductivity values increases with the treatment temperature. The maximum decrease can be 54.0% (after heated at 1000 °C). When the treatment temperature is higher than the phase transition temperature of quartz (573 °C), the thermal inhomogeneity degree of the damaged samples increased obviously. After heated at each temperature level, the water cooled samples exhibit greater thermal conductivity decrease and greater thermal inhomogeneity factor increase than air cooled samples. This may be attributed to the fact that rapid cooling has extra effects on the damage evolution of rock, which is induced by the high temperature gradient generated when temperature changes occur in a very short time. A thermal damage model was proposed to describe the evolution characteristics of thermal conductivity with treatment temperature. Research results in this work can provide better knowledge to the evolution mechanism of distributed thermal conductivity of engineering rock masses experienced high temperature environment, and the influence of different cooling methods. [ABSTRACT FROM AUTHOR]

Both coal and uranium resources are found in the Dongsheng area which is located in the northern part of Ordos Basin, China with the coal seam below the uranium deposit. In this paper, we attempt to study the feasibility of co-exploitation of the coal seam and the uranium deposit. The permeability evolution in strata around the coal seam are selected as the key parameter indexing the separate and safe concurrent recovery of two deposits and a novel strain-dependent permeability model is proposed covering the full deformation range of rock. The proposed model is first verified with experimental data with good fitting of the results. Then an upscaled numerical model is established considering the geology conditions of Dongsheng area. This is implemented using COMSOL Multiphysics; and the interface of COMSOL with MATLAB is used to study the damage to the rock mass. We show that (a) compared with the stress-dependent permeability model, the strain-dependent permeability model proposed in this work has the advantage that each strain value is associated with a unique permeability value; (b) The feasibility of co-exploitation of the coal seam and the uranium deposit depends on the relative distance between two resources. When the caved and fractured zone encroaches on the uranium deposit the coal seam should be abandoned. Conversely, when the uranium deposit lays in the constrained zone, the recovery of the coal seam would benefit recovery of the uranium deposit; (c) The feasibility of the co-recovery of the two resources can be optimized by the choice of mining approach. The top-caving mining approach and room-and-pillar mining approach can reduce the height of the caved and fractured zones and make the co-exploitation of the uranium and coal feasible. This work provides a new approach to investigate the feasibility of co-exploitation of uranium and coal. [ABSTRACT FROM AUTHOR]