Search

The anisotropy combined with the time-dependency of rock mass is commonly observed in tunnel construction. In this study, a comprehensive analytical investigation is performed on analysing time-dependent ground responses induced by tunnel excavation in an anisotropic rheological rock mass. An innovative analytical solution for the anisotropic viscoelastic problems is first proposed, which can be applied to the general cases that the rheological behaviours in anisotropic principal directions are independent and arbitrary. Using the generalized corresponding principle, the analytical model, which can precisely and rapidly address the problem, is then presented for the deformation and stresses around tunnels in transversely isotropic viscoelastic rock mass, considering different viscoelastic characteristics, different anisotropic angles, and circular/elliptical tunnel shapes. The analytical solutions agree very well with the numerical predictions for the consistent models, and can also be reduced to the isotropic viscoelastic case. Furthermore, the application of analytical solutions in practical engineering is validated by the good agreements between analytical solutions and field measurements. A parametric analysis is then performed to investigate the time-dependency of stress, and the effects of anisotropy ratios and anisotropy angles on displacements and stresses. The proposed analytical solution has a valuable contribution to the field of rock rheological mechanics, as it offers a benchmark for anisotropic viscoelastic problems, insights into the complex behaviour of anisotropic rock masses and provides a more accurate prediction of the ground response that may be useful to optimize the design of tunnel excavation in anisotropic rock masses. [ABSTRACT FROM AUTHOR]

Stress cycle disturbances induced by underground excavation can trigger fault activation and slip, leading to earthquakes and other dynamic hazards. This study investigates shear slip induced by cyclic unloading in sawed faults through laboratory experiments. We examine the effects of unloading rate, amplitude, and initial stress on fault slip behavior. Results suggest that cyclic unloading induces fault slip and modifies fault failure characteristics, leading to a gradual rise in confinement pressure during activation. Fault displacement correlates positively with higher unloading rates, amplitudes, and initial stress levels on the fault. Fault slip rate is mainly influenced by unloading rate, whereas initial stress primarily impacts the initial slip rate. Due to fault roughness, higher unloading amplitudes can induce several acceleration events on the fault. Moreover, data from all sub-slipping events demonstrate a positive correlation between the magnitude of stress drop and maximum slip rate. This study offers a theoretical framework for the rational selection and optimization of underground development strategies, aimed at effectively mitigating the dynamic instability of faults. • Cyclic unloading drive fault slip and alters activation conditions. • Increasing unloading rate enhances fault slip rate. • Increasing unloading amplitude drives fault multiple accelerations. • Lower initial stress enhances the both the displacement and rate of initial fault slip. • A positive correlation exists between fault stress drop and maximum slip rate under cyclic unloading driving. [ABSTRACT FROM AUTHOR]

• A Burgers-viscoplastic strain-softening (BVSS) model has been implemented into CODE_BRIGHT. • The BVSS is capable to simulate the time-dependent plastic behavior of geomaterials. • A coupled solid-fluid model is used to simulate the interaction between solid deformations and fluid flow. • The coupled solid-fluid response of tunnel excavation in saturated time-dependent plastic rock masses has been analyzed. This article provides a general numerical approach for modelling the response of deep tunnels excavated in saturated time-dependent plastic rock masses, considering a coupled solid-fluid interaction and time-dependent plastic behaviour. In order to do that, a Burgers-viscoplastic strain-softening model has been developed and implemented into the finite element method software CODE_BRIGHT, and a coupled solid-fluid model is used to simulate the interaction between solid deformations and fluid flow. Parametric analyses are then performed to analyse the influence on the tunnelling response of different time-dependent models, different standstill times and different excavation rates. It has been observed that the time-dependent model selection is crucial to simulate the response of underground excavations. Additionally, the coupled solid-fluid results are significantly different from the purely mechanical ones. The liquid pressure build-up in the vicinity of the tunnel face and the overpressure dissipation with time due consolidation can be accounted for. Moreover, the higher the excavation rate, the larger build-up of liquid pressures occurs. [ABSTRACT FROM AUTHOR]

When the drill and blast technique is used to excavate a roadway in underground coal mines, the relationship between the carriage and roadway space affects the automated hole positioning of a computer-controlled drill jumbo. This leads to low efficiency in hole positioning and high safety risks. This study analyzes the effects of deviations in carriage positioning, left-stacked stones, and inappropriate hole-position design and proposes solutions. This method builds a functional relationship between the size of the drill-bit coverage area and the hole-position parameters based on the drill-arm workspace and its projection. Based on the real-time carriage positioning results, the program checks the alignment between the hole positions and drill-bit coverage area. The crucial angle parameters were adjusted based on the extent to which the blast holes exceeded the coverage area. The analysis showed that the carriage deviation from the midline of the roadway and the height of the remaining stacked stones should not exceed 35–45 cm. Hole drilling and blasting tests demonstrate that this strategy ensures continuous high operational efficiency of the jumbo, thereby safeguarding the blasting efficiency. [ABSTRACT FROM AUTHOR]

As more and more TBM tunnels are constructed under complex conditions such as high stress and fault fracture zones, large deformation disasters of squeezing soft surrounding rocks occur frequently. However, TBM method shows poor adaptability to major engineering challenges. Equipment damage, track deviation and support structures failure are widely observed. It is important to propose an effective support scheme and to fully clarifying the mechanical properties of the support structures in TBM tunnel. In this case, this paper aims to conduct laboratory and numerical tests considering steel–concrete interface bond-slip behavior so as to verify the effectiveness of the proposed support scheme. Thus, the quantitative relationship between arch type, arch spacing, longitudinal connection strength, concrete strength, steel fiber content, and tunnel surrounding rock deformation can be obtained. The research results show that the support scheme proposed in this paper shows good applicability for deformation control of TBM tunnels under squeezing soft rock conditions, which provides a reference for TBM tunnel support under squeezing soft rock conditions. [ABSTRACT FROM AUTHOR]

Highlights • The relationship between Young's modulus and confining pressure was described as a non-linear function according the test results. • Considering the effect of confining pressure on Young's modulus, the stress and deformation of rock mass around tunnel was calculated by both analytical and numerical methods. • Tawara saka Tunnel in Japan was taken as an example to illustrate the influence of confining pressure-dependent Young's modulus. Abstract The actual convergence of an excavation located in fractured rock mass or the soft rock is largely different with the theoretical result in many cases. Experimental results showed that the influence of confining pressure on Young's modulus is very significant. This paper attempted to illustrate the influence of the confining pressure-dependent Young's modulus in the ground reaction analyses of mountain tunnel. Firstly, the relationship between Young's modulus and confining pressure was described as a non-linear function according to the test results. Based on the plane strain axial symmetry assumption and the incremental theory of plasticity, equilibrium equations and compatibility equations of rock mass around a circular tunnel were deduced theoretically. Based on fourth Runge-Kutta method, a semi-analytical solution was achieved. Considering the effect of confining pressure on Young's modulus, the stress and deformation of rock mass around tunnel was calculated by both analytical and numerical methods. The influence of confining pressure-dependent Young's modulus in surrounding rock was estimated quantitatively. Finally, Tawara saka Tunnel in Japan was taken as an example to explain the influence of confining pressure-dependent Young's modulus. The results showed that the error with respect to the monitoring data was largely reduced with the confining pressure-dependent Young's modulus model, which indicated the necessity of considering the non-uniform distribution of Young's modulus. [ABSTRACT FROM AUTHOR]

• Hole-shape effects on mechanical behavior of sandstone under uniaxial compression are revealed. • The initiation and growth of crack around holes are analyzed using DIC technique. • Fracturing mechanism of pre-holed sample is revealed by analytic stress solution around the hole. • The localization of compressive stresses dominates the macroscopic failure of pre-holed rock. Rock failure commonly initiates near the wall of openings with various geometries in underground excavation. In this study, to elucidate the hole-shape influences on the mechanical response and fracturing behavior on rock sample subjected to uniaxial compression, a thorough experimental–numerical-theoretical investigation is conducted on the intact and pre-holed rock samples with different hole shapes, including circular, elliptical, rectangular, and inverted-U shapes. Digital image correlation technology is employed to monitor the crack initiation and development around the hole during the loading process. The results show that the presence of different hole shapes leads to varying degrees of mechanical property deterioration in rock samples. The load capacity and stiffness of the tested samples follow the same order: intact sample > circular-holed sample > elliptical-holed sample > inverted-U holed sample > square-holed sample. Irrespective of the hole shape, the primary tensile crack initiates at the top and bottom of the hole, but its development is trapped. The growth and coalescence of shear cracks near the sidewalls eventually give rise to the sample failure. Stress analysis around the holes reveals a positive dependence of the reduction in load capacity on the maximum compressive stress concentration which is controlled by the shape of the hole. This indicates that the amplification of compressive stress induced by the hole governs the rock collapse. These findings offer valuable insights for the design and construction of underground engineering projects. [ABSTRACT FROM AUTHOR]

• A method for evaluating the effectiveness of tunnel support is proposed. • Arch instability and steel-shotcrete bond slip is considered in the research. • Field monitoring of a typical tunnel are carried out to validate the results. Multi-layer composite support structure is widely applied in tunnel excavation but shows complicated failure behavior under rock pressure. Local joint broken, out-of-plane instability and arch-shotcrete interface slip widely exist and highly weaken the overall strength of support structure, which is inconsistent with the ideal assumption, leading to sever engineering disaster. Based on above situation, this paper adopts laboratory and numerical methods to investigate the bearing mechanism and failure behavior of arch joint, arch frame and arch-shotcrete composite lining. Then, a modified safety evaluation method of arch is proposed based on convergence-confinement method. The research results proved that, component strength and arch critical instability strength check are necessary in arch design. On the other hand, compared with thin-walled steel arch, the concrete-filled steel tube arch shows higher critical instability strength and better ductility, effective for controlling large deformation of soft rock. The research conclusions can provide reference for related tunnel design and construction. [ABSTRACT FROM AUTHOR]

The Gongbei tunnel, as a critical part of the Zhuhai connection highway of the Hong Kong-Zhuhai-Macao Bridge project, is being constructed in Zhuhai, China. The tunnel, close to the Lingdingyang bay, is located in a very environmentally sensitive location with poor geological conditions and is extremely difficult to be constructed. To ensure no interruptions to customs clearance at Gongbei Port, an innovative long-distance curved pipe roof with soil freezing method is adopted to allow a super-shallow-buried large sectional double-decked six-lane tunnel to traverse under the Port. The area of excavation cross-section is up to 337 m 2 , however the burial depth is only 5–6 m and only 0.5 m away from the pile foundations of buildings at the Port. The pipe roof around the tunnel is made up of 36 steel pipes with a length of 257 m and diameter of 1620 mm at intervals of about 356 mm, and the soil freezing method is applied for sealing. There are daunting technical challenges such as controlling the jacking precision of the long pipe roof group, guaranteeing the precise thickness of long horizontal soil freezing, controlling the ground surface deformation of the underground excavation section, etc. The paper will elaborate on the selection of the construction method for traversing under Gongbei Port, on the basic principles of the pipe-roof and soil freezing underground excavation method, on the design of a long-distance and large sectional curved pipe roof, and the design of long-distance horizontal precise soil freezing curtain. [ABSTRACT FROM AUTHOR]

Arching effect has been attributed as a factor of roof stability of underground rock excavations, which affects excavation geometry and rock support. In this paper, a series of numerical investigations were performed using the 2D finite element method (FEM) to study the formation and features of pressure arches. Based on horizontal and vertical stress distributions, three characteristic lines of the arching area were defined, i.e., the outer boundary line, the inner boundary line and the centroid line. It is found that the height of the inner boundary of the arching area is an indicator of the roof stability of underground excavations. Furthermore, detailed parametric studies including the Geological Strength Index (GSI from 20 to 80), the overburden depth ( H from 40 m to 420 m), in-situ stress ratio ( k 0 from 0.8 to 3) and excavation roof rise-to-span ratio ( h /B ratio from 0.1 to 0.5) were conducted to discuss their influences on the arching area. The results show that the initial stress state ( H and k 0 ) has more significant effects on the roof stability of underground rock caverns than the GSI and cavern geometry ( h /B). A semi-circular roof is optimal design scheme for underground excavation in surrounding mass if only considering the cavern roof stability. [ABSTRACT FROM AUTHOR]

The paper investigates the mode I fracture propagation in brittle rocks using numerical simulations based on the discrete element method. The rock material is represented as an assembly of bonded particles interacting according to an elastic-brittle contact law. The formulation of the method includes the ability for near neighbour interactions to occur so as to control the density of interparticle bonds in the medium, and thus ensures the non-dependence of the model's predictions with respect to its discretization. The adopted model was calibrated to reproduce the behaviour of a granitic rock with the aim to focus more specifically on brittle failure. Numerical samples with different discretizations were generated and subjected to different loading paths. Firstly, the response of laboratory scale pre-cracked specimens subjected to direct tension and three points bending tests was studied. Then, the fracture process zone was characterized by evaluating the volume of the damaged zone as well as the energy dissipated by cracks for different discretizations. It is shown that the proposed approach ensures the fracture process zone characteristics to be independent on the discretization of the model. In addition, large scale simulations of a well-documented underground excavation were performed in order to assess the model's capability to describe brittle failure mechanisms at the structure scale. The results confirm the independence of the model with respect to its discretization and highlight the pertinence of the proposed discrete approach for the study of large scale boundary value problems. [ABSTRACT FROM AUTHOR]

• The theoretical solutions of stress variations for rock mass subjected to multi perforation hydraulic fracturing (MPHF) under excavation condition were obtained. • A numerical model to simulate the MPHF under excavation was established using a flow-coupled discrete element method (DEM), and it is verified by the theoretical solutions. • Stress and fracture characteristics during MPHF under excavation condition were presented by considering different MPHF parameters. Performing multi-perforations hydraulic fracturing (MPHF) on the rock mass is a highly effective method for increasing the fracture network, in this paper, MPHF technology was considered to assist mechanical excavation for hard rock mass in underground engineering. To end this, the influences of MPHF on the stress and fracture characteristics of hard rock mass under excavation conditions were investigated theoretically and numerically by considering different perforation spacings, perforation heights, and injection pressures. Theoretical analyses were performed based on the stress field theory of hydraulic fracturing and analytical solutions for excavation stress redistribution. A numerical model was established using a flow-coupled discrete element method, and it was verified using theoretical solutions. The results show that the local maximum principal stresses are in the horizontal direction under different MPHF parameters. The local stress differences decrease as the increasing of perforation spacing and the decreasing of perforation height and injection pressure. With an increase in perforation spacing, fewer horizontal fracture trajectories were generated between the perforation zones, and the number of microcracks initially increased and then decreased, indicating there is an optimal perforation spacing in MPHF under excavation. Moreover, the increase in perforation height, injection pressure, and injection time promote the propagation of crack trajectories and increase the number of microcracks. However, in practice, they should not increase without limits owing to efficiency and economic considerations. Therefore, when using MPHF for preconditioning the rock mass to assist mechanical excavation in the hard rock mass, the project economics and performance can be enhanced by careful selection of appropriate MPHF parameters. [ABSTRACT FROM AUTHOR]

A 3D numerical model with strong discontinuities implemented within the Enhanced Finite Element Method (E-FEM) is developed to address multi-cracking problems. Two failure criteria are proposed for two (tensile and shear) fracture kinematics; namely an anisotropic Mohr–Coulomb criterion with sliding and an anisotropic principal strain criterion with pure opening. This model is used to reproduce induced fracture networks around drifts after an underground excavation. A transversely isotropic behaviour is considered for the host rock. The influence of the anisotropy of rock properties and the in situ stress field on the induced fractures and the convergence of drifts are also studied. [ABSTRACT FROM AUTHOR]

This study develops a coupled elastoplastic damage model for brittle rocks within the framework of irreversible thermodynamics. In the model, degradations in the deformation and strength of rocks induced by damage are considered in the elastic stiffness matrix and plastic yield criterion. The thermodynamic force associated with damage is expressed as a state function of elastic strain and the internal plastic variable. The damage potential and criterion, as functions of the thermodynamic force, are constructed to describe the damage evolution of rocks, and to determine whether they are damaged, respectively. Constitutive relationships, and their numerical formulations of a return mapping algorithm for finite element method implementation, are deduced under three different loading conditions: damage, plastic and coupled plastic damage. For engineering application, the developed model is implemented in the finite element programme Abaqus as a user-defined material model (UMAT). The coupled model is then used for simulating the re-distributed stress fields, displacement and EDZs of the sidewall rock mass around the underground powerhouse of the Wudongde hydropower plant. [ABSTRACT FROM AUTHOR]

Responses and damage to the structures subjected to blast-induced vibration are highly dependent on vibration frequency. Understanding the vibration frequency characteristics and its formation mechanism is essential for the safe and economic design of mining and construction blasts. When millisecond delay blasting is used in excavation, a global frequency value of all of the delays of vibrations fails to present a true picture of the vibration frequency characteristics. In the present study, comparisons of frequency characteristics for the blast-induced vibration per delay were first conducted via an underground opening case excavated by the full-face millisecond delay blasting sequence. The results show that if the blasthole geometry and charge structure are kept the same in each blast delay, the frequencies of single-delay vibration signals will decrease as the dimensions of the equivalent blasting vibration sources increase. However, the cutting blast used for the headmost holes inside is an exception, and its vibration frequency is lower than that of the breaking blast outside. It is the authors’ belief that this counter-intuitive phenomenon is caused by the free faces created by the cutting blast for the breaking blast. A small-scale blasting test and the related numerical simulation were subsequently performed to demonstrate this belief. It is found that the vibration frequency from the holes in the free-face blast is higher than that in the confined blast. In terms vibration frequency, therefore, the vibration from the breaking blast is less harmful to structures compared to the cutting blast for a certain velocity, and decreasing the burden of breaking blastholes is beneficial to reduce the vibrational damage of structures. [ABSTRACT FROM AUTHOR]

A new analytical model that represents the behavior of reinforced ground near a circular underground opening in a homogeneous, uniform stress field has been developed by considering the interaction behaviors between the grouted rock bolt and ground. Axial force distribution along the rock bolt has been analyzed at first. The coupling and decoupling behaviors of the interface between the ground and rock bolt are taken into account in the model. Thereafter, the mechanical behaviors of the reinforced mass are also evaluated according to the elasto-plasticity concept and the strain softening characteristic of ground. It highlights the influence of the different bolting patterns on the extent yield zone and tunnel deformation. The derivation of the analytical model and the influence of bolts on the stress and displacement field near an opening are illustrated in this paper. The theoretical predictions are verified with the measurement data in field, e.g. a Shinkansen tunnel named as Tawara Zaga Tunnel, which is planned to be open in 2022. Moreover, the applications of the proposed approach to tunnel design are discussed further according to the rock bolting effects of the standard supporting patterns of Japan Highway Public Corporation (JH) in different ground conditions. [ABSTRACT FROM AUTHOR]

Designing a rock bolt reinforcement system for underground excavation involves determining bolt pattern, spacing, and size. In this paper, a topology optimisation technique is presented and employed to simultaneously optimise these design variables. To improve rock bolt design, the proposed technique minimises a displacement based function around the opening after bolt installation. This optimisation technique is independent of the material model and can be easily applied to any material model for rock and bolts. It is also extremely flexible in that it can be applied to any mechanical analysis method. To illustrate the capabilities of this method, numerical examples with non-linear material models and discontinuities in the host rock are presented. It is shown that the complexity of systems optimised using this approach is only restricted by limitations of the method used to analyse mechanical system responses. [ABSTRACT FROM AUTHOR]

Intersecting discontinuities are often encountered in rock engineering and sometimes associated with damaging geohazards. Our understanding of intersecting discontinuities instability remains vastly insufficient due to difficulties in comprehensively monitoring the failure process. Here we use microseismic (MS) monitoring to virtualize the MS events in the rock masses surrounding a powerhouse crown and investigate the effect of geological features on the occurrence of MS events. We subsequently build a three-dimensional numerical model and validate this model using the in-situ measurements by multipoint displacement meters. The numerical results demonstrate how the displacements of surrounding rock masses near the fault and the dyke increase and reveal possible causes, such as stress condition as well as geometry and orientation of rock discontinuities. We also discuss the correlation between the tempo-spatial distribution of MS events and the failure pattern of rock masses and confirm the weakened dyke as the main cause of the rock collapse. This study highlights that the stability of intersecting discontinuities can be controlled by both the geometrical and mechanical properties of individual discontinuities, and attentions should be paid to key properties favorable for rock instability. [ABSTRACT FROM AUTHOR]

Abstract: In the context of nuclear waste disposals, this paper deals with hydromechanical modelling in saturated conditions in deep geological formation, using a specific elastoviscoplastic model hereafter called the L&K model. While classical Biot''s framework is followed for the hydromechanical coupling, the mechanical L&K model offers a coupling between instantaneous and delayed behaviour and a variation of dilation often related to softening. These volumetric strains are especially highlighted in coupled hydromechanical conditions. In order to avoid mesh dependency and numerical localized solutions, this type of modelling needs the use of a regularization method which is here referred to as the second gradient dilation model. After describing the numeric tools, we use them for simulating a gallery of the underground research laboratory of Bure. The approach is validated by the good general agreement found between numeric results and in situ measures for both hydraulic pressure and displacement. [Copyright &y& Elsevier]

Abstract: A methodology to model and simulate joint zone for stability analysis of the underground excavation engineering is proposed. Firstly, the jointed rockmass is regarded as a layered anisotropic rockmass zone and a mechanical model of the layered anisotropic rockmass is put forward based on the finite element method (FEM). Secondly, a set of indexes are introduced as the stability assessment indicators to analyze the stability of the surrounding rock using FEM calculation results. Finally, a case study is further illustrated with excavation of an underground pumping house using the proposed methodology to analyze the effect of the steep joints on the surrounding rock stability. Compared to the isotropic condition, when considering the layered anisotropic property with different horizontal and normal elastic modulus, surrounding rock experiences more stress disturbance that resulting in increased plastic zones, dissipation energy and deformation around opening, which is unfavorable for the stability of surrounding rock of the underground caverns. The mechanical model of layered anisotropic rockmass is proved rational and suitable to simulate the joint zone for practical engineering application and of reference. [Copyright &y& Elsevier]

Abstract: The demand for representative rock property parameters related to planning of underground excavations is increasing, as these parameters constitute fundamental input for obtaining the most reliable cost and time estimates. The Brittleness Value (S 20), Sievers’ J-Value (SJ), Abrasion Value (AV) and Abrasion Value Cutter Steel (AVS) have been used extensively at NTNU/SINTEF since the 1960s in connection with drillability testing of rock samples. Nearly 3200 samples originating from projects in 50 countries have so far been tested, and the method and associated prognosis model are internationally recognised for giving reliable estimates of time and cost for tunnelling. A classification of the NTNU/SINTEF drillability indices Drilling Rate Index™ (DRI), Bit Wear Index™ (BWI) and Cutter Life Index™ (CLI) has been available since 1998, but until now no official classification has been available for the individual tests used to calculate these indices. In this paper, classifications of the NTNU/SINTEF drillability test methods Brittleness Value (S 20), Sievers’ J-Value (SJ), Abrasion Value (AV) and Abrasion Value Cutter Steel (AVS) tests will be described in detail. The presented classifications of the individual tests are based on statistical analysis and evaluations of the existing test results recorded in the NTNU/SINTEF database. [Copyright &y& Elsevier]

Abstract: Probabilistic analysis of underground rock excavations is performed using response surface method and SORM, in which the quadratic polynomial with cross terms is used to approximate the implicit limit state surface at the design point. The response surface is found using an iterative algorithm and the probability of failure is evaluated using the first-order and the second-order reliability method (FORM/SORM). Independent standard normal variables in U-space are chosen as basic random variables and transformed into correlated non-normal variables in the original space of random variables for constructing the response surface. The proposed method is first illustrated for a circular tunnel with analytical solutions considering Mohr–Coulomb (M–C) and Hoek–Brown (H–B) yield criteria separately. The failure probability with respect to the plastic zone criterion and the tunnel convergence criterion are estimated from FORM/SORM and compared to those obtained from Monte Carlo Simulations. The results show that the support pressure has great influence on the failure probability of the two failure modes. For the M–C model, the hypothesis of uncorrelated friction angle and cohesion will generate higher non-performance probability in comparison to the case of negatively correlated shear strength parameters. Reliability analyses involving non-normal distributions are also investigated. Finally, an example of a horseshoe-shaped highway tunnel is presented to illustrate the feasibility and validity of the proposed method for practical applications where numerical procedures are needed to calculate the performance function values. [Copyright &y& Elsevier]

Abstract: A zone with significant irreversible deformations and significant changes in flow and transport properties is expected to be formed around underground excavations in indurated clay. The stress perturbation around the excavation could lead to a significant increase of the permeability, related to diffuse and/or localised crack propagation in the material. The main objective of this study is to model these processes at large scale in order to assess their impacts on the performance of radioactive waste geological repositories. This paper concerns particularly the hydro-mechanical modelling of a long-term dilatometer experiment performed in Mont Terri Rock Laboratory in Switzerland. The proposed model defines the permeability as a function of the aperture of the cracks that are generated during the excavation. With this model, the permeability tensor becomes anisotropic. Advantages and drawbacks of this approach are described using the results of the Selfrac long-term dilatometer experiment. [Copyright &y& Elsevier]

Abstract: The occurrence of geologic planes is a major feature of rock mass. In underground excavation, the intersection of geologic planes and excavation surfaces generates blocks. A new methodology for block identification is presented. This methodology employs finite element (FE) modeling technique to identify blocks so it is different from the traditional topology-based block identification algorithms. As complex spatial excavation surfaces are precisely simulated by FE model, this methodology can identify blocks considering complex profile and layout of underground caverns. By considering the finiteness of geologic planes, convex and concave blocks can be both identified. As is defined on FE model, the identified block system can be visualized directly by employing FE graphic system. As a result, there is no need to specially develop graphic system for block visualization. A case study is done on a large scale hydropower underground cavern complex. The block identification results prove the effectiveness of the proposed methodology. The results are further compared with the observation data derived from in situ investigation. The comparison indicates that the proposed block identification methodology is reliable. Therefore, it provides a new approach for block analysis. [Copyright &y& Elsevier]

Abstract: The primary objective of the following paper is to determine the value of strata elastic parameters in a selected Polish mining area using a new method based on finite element analysis, GSI geotechnical classification and “back analysis”. The individual rock layer strain parameters were estimated by means of comparing the measured subsidence trough with the result of numerical analysis carried out for the same geological and mining conditions. As a result of such an analysis, the values of the elastic modulus of the anisotropic model of strata were obtained in the region of underground mining influence. [Copyright &y& Elsevier]

Abstract: Artificial neural networks have been used to analyze a number of engineering problems, including settlement caused by different tunneling methods in various types of ground mass. This paper focuses on settlement over shotcrete-supported tunnels on São Paulo subway line 2 (West Extension) that were excavated in Tertiary sediments using the sequential excavation method. The adjusted network is a good tool for predicting settlement above new tunnels to be excavated in similar conditions. The influence of network training parameters on the quality of results is also discussed. [Copyright &y& Elsevier]

Abstract: The Sievers’ J-miniature drill test (SJ) has been used extensively at NTNU/SINTEF since the 1960s in connection with drillability testing of rock samples. Nearly 3000 samples have been tested, and the method is at present internationally recognised as a reliable method for determination of surface hardness of rock samples. A new test method for estimation of cutter life has evolved as a result of a recent update of the SJ test. This update was made possible by the development of instrumentation of the test apparatus. The new test method analyses the penetration depth and speed during the testing. The point in time where an interception occurs between the tangent lines for what could be defined as the “effective drilling” part and the “worn out” part of the penetration curve is defined as the Sievers’ J interception point or SJIP. Testing of 212 rock samples, representing different rock types, and correlations of the results to some of the most widely used methods for estimation of cutter life, show that it is possible to obtain a direct cutter life estimate by use of the SJIP value. The demand for representative parameters for rock properties in connection with underground excavations is increasing, as this constitutes the fundamental input for obtaining the most reliable cost estimates. The new test method provides information that was previously unavailable. This will improve the understanding on how different rock types and properties affect the drilling process and the wear on cutter tools. This test method is not meant to act as a substitute for well established and currently used test methods, but rather constitute an additional tool that can contribute to “bridge the gap” between laboratory testing, modelling and the actual excavation process estimates. [Copyright &y& Elsevier]

Determination of in situ soil or rock geotechnical properties is a difficult task for a design engineer. Back analysis is a helpful technique for evaluation of soil properties by considering and measuring the convergence of an underground opening. In this article, the results of numerical back analysis, performed for Resalat tunnel in Tehran, Iran, are reported. The main purpose of this study was to estimate the soil cohesion and the in situ horizontal stress. Back analysis was performed by matching numerical modeling results with the measured tunnel convergence. In addition, the results of in situ direct shear, plate load and pressure meter tests are reported and compared with those from back analysis. It is shown that the back analysis presented is capable of determining the horizontal in situ stress and soil cohesion with good accuracy. [Copyright &y& Elsevier]

Keyblock has been received a lot of attention in underground engineering because it loses its stability first after excavation. Block theory originally proposed by Shi could not build the geometric characteristics of keyblock and explain how to determine the precise locations of keyblock. Therefore, the Unwedge program based on keyblock theory could not predict complicated polyhedral block except tetrahedral block and determine its precise location. In this paper, the stability of keyblock of a large underground excavation is analyzed in detail. First, the Unwedge program (Ver.2.35) is used to identify possible keyblocks and some limitations are discussed. Then many kinds of geometric models of polyhedral keyblock are built, a new computer program is developed to search keyblocks and determine their precise locations, comprehensively considering the action of gravity and dynamic loadings. Finally, the results of two methods are compared and the differences are analyzed. [Copyright &y& Elsevier]

A closed-form solution is presented in this paper for the prediction of displacements around circular openings in a brittle rock mass subject to a hydrostatic stress field. The rock mass is assumed to be governed by Hoek–Brown yield criterion and a non-associated flow rule is used. For the elastic–brittle–plastic analysis of circular openings in an infinite Hoek–Brown medium, the existing analytical solutions were found to be incorrect. The present closed-form solution is based on a theoretically consistent method and the solution does not require the use of any numerical method.The present closed-form solution was validated by using the finite element method. In the finite element analysis, the infinite boundary was simulated “exactly” by using the newly developed elastic support method. Several cases were analyzed and the present closed-form solutions for stresses and displacements were found to be in an excellent agreement with those obtained by using the finite element method. [Copyright &y& Elsevier]

For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ 3 varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ 3 values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σ i , crack damage stress σ d , and peak stress σ s) and failure mode of the tested specimen under different σ 3 conditions were systematically investigated. The experimental results indicate that with increasing σ 3 , the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ 3 ranging from 0 to 10 MPa is higher than that with σ 3 ranging from 10 to 30 MPa, indicating that low σ 3 impose a greater effect on the characteristic stresses than high σ 3. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ 2 direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ 2 direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ 2 and σ 3 directions) as σ 3 increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σ r encountered near the excavated boundary is much smaller than the axis stress σ a , the surrounding rocks will exhibit a "Ternary failure characteristic", i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σ r encountered near the excavated boundary approximates the original σ a , the surrounding rock masses will exhibit a "Quaternary failure characteristic". Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III. • True triaxial tests with different σ 3 are conducted. • Rock strength under different σ 3 condition are investigated • The influence of σ 3 on the rock failure mode is studied. • Rock failure near the boundary of underground excavations is characterzied. [ABSTRACT FROM AUTHOR]

• A flow-coupled DEM is used to study the HF behaviour under excavation conditions. • Effects of excavation stress redistribution on HF path are analyzed. • The characteristics of HF development under excavation conditions are revealed. • Excavation stress conditions significantly affect the performance of multi-stage HF. The fracture behaviour of single- and multi-stage hydraulic fracturing (HF) under varying excavation stress conditions was studied using a flow-coupled discrete element method (DEM). Conventional HF theories and analytical solutions for excavation stress redistribution are used to verify the numerical model. The numerical simulation results indicated that for the single-stage HF, both the in situ stresses and injection location have major influences on the HF propagation path, caused by the redistributions of the local maximum and minimum principal stresses under excavation conditions. Fractures generally propagate perpendicular to the local minimum principal stress (σ min) and also propagate preferentially towards the area with lower σ min. Hydraulic fractures propagation is dominated by tensile failure, and the generation of fracture branches is disfavored because of the high local compressive stress field induced by the HF process itself. As for multi-stage HF, different excavation stress conditions may cause different fracture development performances because of alterations of the preferred fracture orientation, blunting of propagating fractures, or creating a more complex fracture geometry in the affected zone. Therefore, when using high-pressure HF for preconditioning the rock mass to assist mechanical excavation in hard rock mining, performance can be enhanced by understanding the impact of stress changes and careful selection of appropriate HF methods depending on the in situ stress. [ABSTRACT FROM AUTHOR]

• Excavation-induced fault rupture is related to magnitude and orientation changes of principal stresses. • Principal stress change results in damage transition from matrix tensile failure to fracture shear failure. • Fewer small magnitude AE in rock matrix and more large magnitude AE along fractures cause smaller b value. Fault rupture and associated seismicity have been reported frequently in underground excavation. The mechanism that drives the human-induced geohazards still eludes explanation, and uncovering this mechanism relies heavily on our understanding of the failure characteristics of fractured rock. We carried out the triaxial compression testing and discrete element modelling on intact and fractured granite samples to consider the controllable change of the angle β between the maximum principal stress orientation and pre-existing fracture orientation and to interpret the complicated mechanism of fault rupture adjacent to rock excavation. Our results show that the reduction of the angle β leads to the failure pattern of fractured samples changing from the tensile failure of rock matrix to the shear failure along pre-existing fractures. This transition results in relatively small b value, as small magnitude AE events decrease in the rock matrix and large magnitude AE events concentrate along pre-existing fractures. The change of b value in the pre-peak stage is independent to the angle β , implying the challenge in rock failure prediction. Our study also indicates that fault rupture adjacent to rock excavation is due to the changes of magnitude and orientation of principal stresses, and the associated seismicity is likely dependent on the portions of rock matrix and fractures involved in the failure process. [ABSTRACT FROM AUTHOR]