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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]

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]

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]

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]

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 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 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]

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]