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Abstract: This paper describes the studies and the in situ stress measurements carried out to support the stability analysis of underground ornamental stones quarries. In particular, this paper describes the findings, evidence and analysis carried out at the Ravaccione and Fantiscritti quarries, which are located in the Carrara basin, Italy. The main activities carried out during the study are starting with the characterization of the rock mass which has been done through the interpretation of data obtained from in situ geostructural surveys, combining the geometrical and mechanical characteristics of the discontinuities with the mechanical features of the rock matrix in order to assess the proper rock index in accordance with the more diffused classification systems. Further consideration regarding the tectonic stress orientation are done through the observation of kinematic indicators on brittle structures-such as faults, fractures s.l. (joints s.s., and fractures with no evidence of movement) and systems of conjugated fractures, in order to evaluate the tectonic stress main orientation for the studied area. Then several in situ measurements of actual state of stress on various location inside the Ravaccione and Fantiscritti quarries where carried out using CSIRO cells. This preliminary work lead to the geometric reconstruction of the rock mass, carried out using the computer code Resoblock®. This phase allowed, once the current excavation geometry was given, to define the geometry of a discontinuous rock mass model on which, thereafter, a numerical simulations was carried out using the computer code 3DEC®. At last, the comparison between the numerical model results and the in situ tests allowed the back analysis of the site and lead to a most satisfying model calibration to be used for the provisional analysis of further excavation development. [Copyright &y& Elsevier]

Abstract When a shallow tunnel is excavated, an overall upward movement appears as a result of the weight removal from the excavation. This movement is analogous to buoyancy. When numerical simulation of shallow tunnels is used, the magnitude of the upward movement increases with the depth of the lower boundary, defined as the distance from the tunnel center to the bottom of the mesh. This seems counterintuitive, and yet it is mathematically correct. This paper investigates the influence of the assumptions made to model the buoyancy phenomenon numerically, and specifically the (typical) plane strain and elasticity assumptions. Three-dimensional and two-dimensional plane strain numerical models are carried out using a linear elastic model, where the influence of the ground stiffness increasing with depth is assessed. The results show that the buoyancy effect, i.e. increasing upwards movements with depth of the lower boundary, decreases when the ground stiffness increases with depth. [ABSTRACT FROM AUTHOR]

Abstract Sill mats are important supporting structures commonly applied in underground mines for better recovery. They are used to help recover sill pillars in open stoping or to provide a safer workplace in underhand cut-and-fill mining. To fulfil these functions, the sill mat is typically cast with cemented backfill mainly consisting of mine tailings and binders. To ensure economic and safe mining, a critical issue is to estimate the required strength of the base-exposed sill mat following underneath extraction. So far, a few analytical solutions have been proposed for this purpose by treating the sill mat as an isolated bending beam with two fixed ends. However, previous numerical analyses have indicated that the response of the sill mat can be significantly influenced by the rock wall closures due to underneath extraction. This aspect is ignored by the existing analytical models. In this paper, numerical models are used to evaluate the stability of sill mats upon the underneath excavation. The influence of rock wall closure on the stability and the minimum required strength of sill mats is analyzed. Simulation results indicate that the apparent failure mechanism of the sill mat is caving or rotation, while its actual failure mechanism is crushing or shearing. Accordingly, the required strength of sill mats increases as the variation of studied influencing factors tends to increase the horizontal stresses in the sill mats, including the increase in mine depth, rock pressure coefficients and sill mat stiffness. In contrary, the increase in the span or thickness of sill mat and rock mass stiffness tends to decrease the horizontal stresses in the sill mat, thereby leading to a reduction in the required strength of sill mats. It is also concluded that the stress increase does not always mean an improvement or deterioration of the stability of the sill mats. [ABSTRACT FROM AUTHOR]

Abstract Longwall mining is the most widely used mining method in underground coal mining. The stability of gateways is crucial for longwall mining as they provide many essential services such as transportation of excavated coal, ventilation of longwall face. High in situ stresses, mining induced stresses due to advancing longwall face, and poor rock mass quality leads to large deformation jeopardizing the stability of the gateway in Guobei coal mine. In this paper, the global-local modeling approach is used for analysing the stability of the gateway in the mine. In this approach, a mine scale 3D global model was constructed to extract the stresses acting on the gateway. The extracted stresses from global model were implemented into 2D local model of the gateway for detailed analysis. The used approach was verified by the field measurements taken from the modeled gateway. In an attempt to deal with large mine induced displacements, a support system composed of rock bolt, cable bolt and shotcrete was proposed. The verified model was used to evaluate the performance of proposed support system. It was concluded that the proposed support system can significantly reduce the displacements. The verified model facilitates the performance evolution of other alternative support systems considered by the mine management. The presented approach can be applicable for other deep underground coal mines, experiencing large gateway convergence. [ABSTRACT FROM AUTHOR]

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]

The Discrete Element Method (DEM) is of obvious and inherent advantages in simulating large displacements and discontinuous deformations of rock masses, and thus is suitable to simulate the mining-induced rock movements. However, when the region of the DEM calculation model is large, for example, when both the length and width of the research area are larger than several hundreds of meters, the rock mass in the model range often contains a large number of actual joints. If the calculation model is built according to the distribution of the actual joints, the calculation model of DEM will consist of a large number of blocks, which would lead to the impossibility of numerical calculations. To address this problem, an Equivalent Discontinuous Modeling Method (EDMM) of jointed rock masses for DEM simulation of mining-induced rock movements is proposed in this paper. In this method, an actual geological body with a complex joint system will be simplified into an Equivalent Discontinuous Model (EDM), which is a DEM calculation model with a simple hypothesis joint system. The approach for determining the mechanical parameters, i.e., the cohesive strength ( C ) , the internal friction angle ( ϕ ), the joint normal stiffness (JKN), and the joint shear stiffness (JKS), of the hypothetical joints of the EDM is the core of the proposed EDMM. The main procedure of the proposed EDMM is composed of: (1) for each subarea of an EDM, building a group of Equivalent Discontinuous Cube Models (EDCMs) that are of different joint spacings; (2) determining the joint mechanical parameters of each EDCM based on the mechanical properties of the actual rock masses of the subarea; (3) establishing the relationships between the joint spacings and joint mechanical parameters for each subarea of EDM; and (4) determining the mechanical parameters of the hypothesis joints in each subarea of the EDM according to these relationships. By exploiting the EDM, the DEM numerical simulation is performed to simulate the mining-induced rock movements and failures. To demonstrate the applicability, the proposed EDMM was employed to simulate the rock movements and failures triggered by mining under the western open-pit final slope of Yanqianshan iron mine. [ABSTRACT FROM AUTHOR]

An increasing number of mines in China are being subjected to overall-instability-induced (OII) rockbursts in extracting deep remnant island longwall panels, causing severe damage at far larger scale and intensity compared with common local rockbursts. Using the island longwall panel LW1305 at Zhaolou Coal Mine as a case example, this paper investigates the mechanism of the newly defined OII rockburst by means of stress and microseismic monitoring, and then proposes a straightforward risk assessment method for OII rockbursts based on the actual bearing index in an attempt to provide guidelines for making engineering judgement. The proposed risk assessment method was successfully applied to LW1305, and the assessment result is supported by the occurrence of an OII rockburst in LW1305, and further verified by numerical modeling and microseismic monitoring. [ABSTRACT FROM AUTHOR]

This paper deals with the development of a non-linear constitutive model of a rock mass and its application to predict the extension of the excavation damaged zone (EDZ) around openings. A comparison of the model results with the simulations of triaxial compression tests shows a good agreement between predictions and theoretical values of peak and residual strengths as well as a good reproduction of the transition between brittle failure and ductile response. The relevance of the proposed model to evaluate the potential failure around stopes of the Garpenberg mine was examined. The comparison of the results proved satisfactory and highlighted the interest of considering a more realistic mechanical behavior of hard rock masses in rock engineering compared with the elastic perfectly plastic models generally used in numerical modeling of underground deep mines. Finally, a 3D simulation of the complete mine geometry was carried out in order to model the first phases of stope mining operations. The predicted stresses were compared with the continuous measurements of induced in situ stresses recorded by stress cells at different points around the stope. The predictions of the proposed constitutive model provided a much better agreement with stress measurements than those obtained with elastic perfectly plastic models. [ABSTRACT FROM AUTHOR]

In this paper we present the principles of a methodology for studying the impact of mining subsidence on masonry buildings, using numerical experimental designs, statistical analysis and surface responses, where the subsidence is the input variable, joint tensile strength and cohesion are the control variables and the total length of cracks is the output variable. Numerical experiments are carried out using a distinct element model of a masonry wall where masonry blocks are explicitly taken into account. Calculations are also carried out for two types of subsidence according to the position of the building in the convex curvature or the concave curvature zone of the ground surface subsidence. Statistical analysis of the results shows the dominant influence of the joint tensile strength over the joint cohesion and the absence of interaction between these two parameters. [ABSTRACT FROM AUTHOR]

The objective of the present paper is to propose a methodology to numerically simulate (i.e., initialize) the pre-mining stress field in complex but rather frequent situations in which the classical overburden-weight assumption and existing stress measurements are in disagreement and where the stresses strongly vary from one zone to another, even at constant depth. This methodology is illustrated by the case of a deep mine in France that exploits a 10°-dipping coal seam, in which numerous stress measurements were carried out. Virgin principal stresses in this mine have been shown to be highly heterogeneous and anisotropic. To correctly reproduce such a challenging initial stress state in a numerical model and to be able to later calculate mining-induced stresses, five distinct methods (M#1 to M#5) are successively presented and compared, along with their advantages and drawbacks. All of them are based on “fixed” boundary conditions, with null normal displacements on all lateral boundaries except on the top one, which is considered as a free surface of the Earth coinciding with the natural flat topography. The initial conditions of the model assume that the pre-mining stress components linearly depend on the Cartesian coordinates x , y and z (depth), and the Simplex Method is used to calculate the linear coefficients by minimizing the squared difference between the measured stresses and their simulated values. We show that the use of 3D stress gradients produces more realistic results than a 1D vertical stress gradient. [ABSTRACT FROM AUTHOR]

Ore mining under the open-pit final slope will result in caving and large displacement movement of rock-masses surrounding the goaf. It is difficult for numerical methods that equate jointed rock-masses to a continuum to simulate the aforementioned mechanical behavior of rock-masses. However, the placing of joints in a calculation model according to their actual distribution characteristics will result in an excess of model blocks, which renders the calculation impossible. To solve this problem, a numerical modeling method based on an Equivalent Jointed Rock-mass Model (EJRM) is proposed in this paper. First, the rock-masses in the research area are simplified as an EJRM; then, a back analysis method, combining orthogonal design and numerical simulation with Discrete Element Method (DEM), is used to invert the mechanical parameters of the model; lastly, the DEM is utilized to analyze the movement and failure of the strata and surface induced by mining under open-pit slope. The Yanqianshan iron ore mine was treated as an example in the present study. Mining under the eastern slope of the open pit of the mine was simulated by means of the aforementioned method. The patterns of the failure and movement of the strata and slope at different mining stages were obtained. This numerical modeling method proposed can be used to simulate complex mechanical behaviors of rock-masses induced by underground mining. [ABSTRACT FROM AUTHOR]

This paper provides a comprehensive analytical formulation that entails the features of sand production in gas hydrate-bearing sediments, including grain detachment, migration, sediment deformation and hydrate dissociation. The formulation is thermo-hydro-mechanically coupled such that grain detachment causes stress reduction, sediment shear deformation induces grain detachment and grain flow alters multiphase fluid pressure and temperature profiles. Through a series of analyses, the sensitivity of sand production related parameters on resulting solid volume changes is evaluated. Furthermore, the effect of various operational methods on mitigating sand production in hydrate-bearing sediments during gas production is numerically investigated. It is found that, out of the different operational methods investigated, lowering depressurization rate was the most effective in reducing sand production for a given gas production. [ABSTRACT FROM AUTHOR]

This paper aims to establish a balance between the input parameters used in modeling seismic and aseismic events, and the data that are available in a mine. With regard to the input parameters, their minimal set is suggested. For output seismic parameters, we distinguish three groups of statistical distributions provided by field observations of various quality. The first group includes distributions of the event location . They serve for improving the data on geometrical features of a problem. The second group employs distributions of the event magnitude . They serve for evaluating the risk of strong events. The third group employs distributions of the geometrical parameters of the event source mechanism . They serve for connecting stresses and seismicity. The exposition is illustrated by considering an example of long-wall mining of a coal seam. [ABSTRACT FROM AUTHOR]

Two principal existing analytical methods, which are presented in this paper, can be used to estimate the vertical stress acting on pillars in room-and-pillar mines, based on the concept of extraction ratio. These two methods can adequately predict the pillar load in specific geometrical and mechanical conditions. However, they are not always suitable in the case of irregular geometries with important local variations of the extraction ratio. In fact, one of these methods tends to underestimate the average vertical stresses while the other significantly overestimates the highest values. Therefore, the work undertaken here is focused on the development of a more accurate analytical method for calculating the vertical stress on pillars of irregular size. By using a finite difference code, several hundreds of room-and-pillar mines with random geometries have been simulated in order to obtain numerical estimations of the average vertical stress on pillars under different configurations. A quadratic function of the relative extraction ratio is shown to provide a good fit to the numerical results. It is proposed as a new method for calculating the vertical load on elastic pillars, which offers a better approximation than the existing methods. This is verified for typical ranges of depths and extraction ratios observed in room-and-pillar mines. [ABSTRACT FROM AUTHOR]

The paper addresses the problem of modeling a fracture propagation in linear elastic porous media driven by injection of non-Newtonian power-law fluid. The model involves the lubrication theory equation expressing the mass conservation of fluid and a nonlocal singular integral relation for the fracture aperture as a functional of the fluid pressure. We assume only the viscosity-dominated case in which the fracture toughness is neglected. This allows considering the fracture as an opened part of a preexisting closed fracture of larger length. The numerical method consists in solving equations of the model over the whole length of the preexisting fracture without distinguishing the tip region of the opened part. The problem is solved via the finite element method. The weak formulation of the solution allows pressure singularity with the required asymptotics near the fracture tip. Comparison of the numerical and the exact self-similar solutions in the case of a constant flow rate, zero fluid leakoff, and zero fracture toughness reveals the accuracy of the approximate solution to at least O ( h 1 / 2 ) , where h is the maximal linear size of the grid cells. As an illustration, we also demonstrate the numerical experiments with periodic fluid injection and variable fluid efficiency. [ABSTRACT FROM AUTHOR]

The objective of this paper is to predict the stresses around the tail and main gates of Soma Eynez underground coal mine, located in Western part of Turkey. In the studied mine, the magnitude and the direction of stresses around the gateways frequently change due to a planned mining method, which is a combination of a fully mechanized traditional longwall and longwall top coal caving (LTCC) methods. In the mine, a 30–35 m thick coal seam is going to be produced by three slices. The upper slice will be produced by fully mechanized traditional longwall mining, whereas the remaining two slices will be exploited by the LTCC method. Although both traditional longwall and LTCC methods are widely used around the world, a mine using a combination of both is rare. This unique application presents special challenges for designing a support system, as the magnitude and the orientation of the stresses around the gateways change frequently. Due to the planned mining method, empirical and analytical methods cannot be used to assess the stresses around the gateways. Therefore, a three-dimensional finite difference technique is selected for mine scale stress analysis, and the results presented will be used in further stability analysis of tail and main gates. [ABSTRACT FROM AUTHOR]

The stability of mine developments is of utmost importance during the planned period of production or the life of a mine plan. Many Canadian underground mines use transverse stoping with delayed backfill to extract tabular ore deposits. These methods require access to the orebody through a number of sill drives and cross cuts which link the orezone to the haulage drift hence creating intersections on multiple levels. This paper presents the results of a study on the stability of mine development intersections at Garson Mine of Vale in Sudbury, ON, Canada. Multi-point borehole extensometers (MPBX) are used to monitor the rock deformations of an intersection as mining activities progress. The monitoring results are used to calibrate a multi-level FLAC3D numerical model, which has been developed to assess the stability of the intersection. It is shown that stope extraction causes a lateral shift to the intersection, accompanied by high shear stress in the roof. It is also shown that same-level mining has stronger influence on the stability of the intersection than lower-level mining. [ABSTRACT FROM AUTHOR]

This paper presents both a numerical and an analytical approach for tunnel excavation and support mounting. The three-dimensional aspect of the tunnel face advance and the lining installation is simulated by a two-dimensional study assuming the hypothesis of gradual decompression of the primary stress on the outline of the tunnel. The numerical approach consists of a tunnel axis in an elasto-viscoplastic rock mass and a concrete elastic lining. This study emphasizes some important factors that influence the tunnel calculation, such as tunnel face, history of excavation phases, timing of the lining mounting, lining stiffness, and depth of the tunnel. The analytical approach is based on the determination and integration of the rock-lining interface differential equation. The author presents the analytical solution for (a) constant and non-constant lining pressure, and (b) taking into account the tunnel face influence. The comparison of the numerical results with the analytical solution is performed, and a good agreement is obtained. [ABSTRACT FROM AUTHOR]

Abstract: Fracture of rock involves formation of a localized region of damage or cohesive process zone, which controls size effects on strength and stability. Therefore, any attempt in predicting size of process zone is of prime importance in fracture study of quasi-brittle materials. In this paper, an approximate theoretical relationship between size of process zone and specimen size is proposed. The appropriateness of this relationship is examined by conducting a discrete element analysis of rock fracture. A softening contact bond model is used to study the process zone around a notch tip in three point bending tests. The numerical analysis is utilized to obtain the nominal tensile strength, apparent fracture toughness, and width of process zone. It is shown that the apparent fracture toughness is a function of the specimen size, and that the change in nominal tensile strength with specimen size can be captured by Bazant''s size effect law. In addition, both the theoretical arguments and the numerical results suggest that the inverse of width of the crack tip process zone has a linear relationship with the inverse of specimen size. The numerical results indicate a stronger relationship between width of process zone and specimen size for a material with a larger value of characteristic size. On the other hand, for a brittle material, specimen size has a small or no impact on the size of the process zone. [Copyright &y& Elsevier]

Abstract: Choosing the support system in swelling and squeezing rocks is one of the challenges for rock engineers. There are many factors that need to be considered such as support type, performance of the support, the support element, rock parameters, condition of the rock structure, and creating an appropriate support strategy. TH-type steel supports, which have long been used in deep underground mine headings, have recently started to be used as yield-control support system in tunnels. However, sliding ability of the TH steel support allows cross-section contractions. If the amount of convergence can not be calibrated, re-shaping studies have to be done at the excavated areas due to the occurrence of time-dependent creeps, especially in swelling and squeezing rocks. Also, numerical model studies for highly swellable and/or squeezable rocks must represent the time-dependent creep behavior of the rock. Simple stiffness parameters such as time-dependent creep, swelling index, and compression index should be addressed into the model. It is possible to constitute a high performance support system in this swelling and squeezing types of rocks by choosing appropriate round interval, using a non-deformable and heavier I profile and using sufficient parameters obtained by analysis. The present paper presents in-situ measurements and the analysis of time-dependent numerical model of non-deformable support system at the T-13 tunnel which is applied to one of the 39 tunnels at Eskişehir–Köseköy high-speed railway route. [Copyright &y& Elsevier]

Abstract: The effect of opening size, rock properties and stress conditions on the radial displacements ahead and behind of the advancing face of a circular opening is one of the most important topics in civil and mining engineering. In this paper we develop a prediction system for radial displacements around the face of an opening by combining rock mass rating classification system, a three dimensional finite difference code and multiple regression modeling. Predictive equations and response surfaces for rock mass response are proposed. By using these equations and response surfaces, it is possible to assess radial displacements around the face and to construct the longitudinal deformation profile of a circular opening with different diameters excavated in weak rock mass under different stress conditions. The predictive equations were validated by means of statistical tests and analytical solution. Proposed response surfaces and equations can provide the basis of guiding the design and construction of underground openings. [Copyright &y& Elsevier]

Abstract: A reliable prediction of the caving behavior of strata and support capacity requirement for longwall workings has always been a challenge for rock mechanics researchers and mine planning engineers. It is very important for successful planning and implementation of high capacity longwall projects to have proper selection of site and compatible roof supports, to ensure safe operation of longwall face especially in massive and difficult to cave strata conditions. This paper illustrates a numerical modeling based integrated approach for predicting the progressive caving behavior of strata and optimum capacity requirement of powered support for longwall working in a given geo-mining and strata condition. A set of design criteria for selection of optimal capacity support integrating the field experience and the numerical modeling results of longwall panels, operated in a widely varying strata conditions in different coalfields of India. Case studies of two typical longwall workings are presented to assess the caving behavior and support requirement for safe operation of longwall working under the site specific regular and en masse caving strata conditions. [Copyright &y& Elsevier]

Abstract: Seismic events, i.e., fractures in rock that radiate detectable seismic waves, are a worldwide phenomena associated with mining and reservoir exploitation. Longwall mining in German coal mines often causes seismic events, which can be felt at surface where they can lead to a considerable annoyance of residents and sometimes even lead to minor damages of buildings and infrastructure. Strong seismic events are in a large part responsible for a more and more decreasing community acceptance of mining. The mines therefore have to investigate the local conditions, which cause the generation of seismic events, to find ways of avoiding strong mining induced seismic events. This paper describes the research on mining-induced seismicity around a specific active longwall at 1100m depth under several previously mined longwall panels. Seismic events are recorded by a local network featuring 15 surface and six subsurface stations. Location and fault plane solutions show the occurrence of at least three different event types, which are characterized by distinct modes of failure. Underground core drilling and an extensive rock testing program are used to build up a rock mechanical model featuring a multi-layer strata for use with numerical analyses. The numerical model is able to explain the different failure mechanisms as fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses facilitates the evaluation of the results from seismology approaches, and makes future analysis of longwall-related seismic events possible. [Copyright &y& Elsevier]

Abstract: There has been a debate in the rock mechanics community regarding the mechanisms causing what is known as core discing. This phenomenon occurs when diamond drill cores are retrieved from rock masses in which high in situ stresses relative to rock strength are present. The interest in that phenomenon is to use it to estimate the in situ stresses from the shape and frequency of the failures along the core axis. In the present paper we argue that discing is only an indicator of high stresses, and that estimating in situ stresses from fracture observations is much too inaccurate. As most of the literature found on the subject has tackled the problem using elastic numerical models, it is shown that the stress distribution in the core being formed obtained from such models does not exist once failure has been reached. Numerical analyses using Flac2D with an elasto-plastic cohesion softening friction hardening model show that for a given stress state, discing or core damage may involve tensile failure, a combination of shear and tensile failure, or only shear failure, depending on the stress state and ratio of tensile to shear strength of the rock. The numerical model used is validated by replicating core discing observed under controlled laboratory conditions. Parametric analyses involving changes in mesh density, deformability parameters, dilatancy, drill bit pressure, drilling fluid pressure and applied stress states are also performed. Finally, it is shown that drilling-induced core discing or damage store important residual stresses in the core which may explain why recovered core samples tend to show a deterioration of their mechanical properties with time. [Copyright &y& Elsevier]

Abstract: In this paper, deformation and failure characteristics of the main drift, T-2000, and a roadway, T-3000 excavated for Beypazari trona (natural soda) underground mine, which is located in Ankara, Turkey, were investigated by in situ measurements and numerical analysis. T-2000 was planed as a main drift with a length of 965m, 14° inclined and excavated with an area of 18m2. The T-3000 roadway was driven in the trona ore body for the first stope. Load and convergence measurements were carried out at seven stations installed in main drift (T2000) and roadway (T3000). Measurement results indicate that the total convergences at all the stations were below 1% of the total gallery height and deformation rates were reduced significantly. Three-dimensional numerical modeling was performed to analyze the deformation and failure behavior of the drift for three different models using an elasto-plastic finite element method (FEM) approach considering Drucker–Prager failure criterion. The results of the numerical analyses were found to be comparable with in situ measurements. [Copyright &y& Elsevier]

A rock ridge carrying a major roadway through a quarry in the greater Chicago, IL area sheared along sub-vertical regional joints causing significant distress of the roadway as a result of apparently limited adjacent quarry excavation of a glory hole. The large-scale movement rendered the road unusable. This paper presents a detailed analysis of factors contributing to this response. Post-event field observations of ridge movement revealed that lateral displacements were concentrated along a horizontal thin shaley seam below the bottom of the quarry. These observations show that the sheared ridge slid along the shaley seam and moved as a single block. Three-dimensional numerical analyses of the ridge and the quarry construction activities show that as the quarry was deepened, a buildup of large horizontal stresses occurred in the rock base above the shaley seam. The excavation of the glory hole through this rock base lead to arching and an increase in longitudinal stresses and a reduction in lateral confining stresses along the ridge. The analyses show how the unique combination of large horizontal stresses, the horizontal shaley seam, and stress redistribution due to glory hole construction lead to large shear displacements along the persistent sub-vertical regional joints. [Copyright &y& Elsevier]

Pillar recovery requires stable backfill upon side exposure in open stoping mining methods. Until now, the solutions used to estimate the required strength of side-exposed backfill are mainly based on a wedge model where the effect of rock-wall closure was neglected. In this paper, the influence of mine depth and adjacent extraction on the stability of side-exposed backfill is analyzed through numerical modeling with FLAC 3D . The failure mechanism and cohesion at failure (c f) of backfill are investigated. Results show that the governing failure mechanism can be sliding or crushing, depending on the magnitude of rock-wall closure. When the mine depth and rock-wall closure are small, sliding failure is dominant and rock-wall closure has effect to improve the stability of exposed backfill. The values of c f can become even smaller than those obtained with immobile rock walls solutions. When the sliding failure is dominant, the c f decreases with the increase of mine depth, fill stiffness, rock pressure coefficient, and fill-rock interface strength (friction angle and adhesion). Increasing rock mass stiffness, stope height and width leads to an increase in c f. When mine depth and rock-wall closure are large, the crushing failure is dominant. The c f increases with the increase of mine depth, stope height and length, fill stiffness and rock pressure coefficient. It decreases with the increase of stope width and rock mass stiffness, and is insensitive to the variation of fill-rock interface strength. In all cases, the stability of side-exposed backfill is improved by increasing its internal frictional angle. [ABSTRACT FROM AUTHOR]

Sill mats made of cemented backfill are important structures largely used in underground mines to provide a safer working space, increase ore recovery and improve ground stability conditions. When large span sill mats are exposed at the base, flexure can take place and tension appears at the base center of the sill mats. As cemented backfill and other geomaterials usually have a strength smaller in tension than in compression, a common practice in field is to add reinforcement structures built with bolts and metal grids. The construction of these structures is expensive and time-consuming. In this paper, the use of arched sill mats is for the first time proposed as a new solution to reduce the occurrence of tension and increase the stability. Binder consumption and metal reinforcement can thus be reduced, resulting more economic design of cemented backfill sill mats. A series of numerical simulations have been performed to analyze the efficiency of arched sill mats in reducing the minimum required cohesion. For the stope geometry considered in this study, the numerical results show that arched sill mats exhibit an improved stability compared to traditional rectangular sill mats. The minimum required cohesion of sill mats increases as the sill mat span increases but decreases as the curvature of the sill mats increases. The overlying uncemented backfill has effect to decrease the stability of sill mats. The minimum required cohesion of sill mats increases with the thickness of the uncemented backfill above the sill mats. These results remain valid as long as the stopes are wide enough, and the prevailing failure mode of sill mats is tension by flexure. More work is thus necessary to establish a criterion for the application of this new concept to increase the stability of cemented backfill sill mats against the failure of tension by flexure. [ABSTRACT FROM AUTHOR]

This paper presents an integrated approach for field instrument and numerical modeling to investigate the stability principle and control method of entries that are driven adjacent to a retreating longwall top coal caving panel. The test site is located in the city of Yuncheng, Shanxi Province, China. The instrument results indicated that the deformation and failure process of entries driven adjacent to the retreating longwall panel (EDRLP) could be divided into three stages: a slow initial increase stage (more than 125 m ahead of the mining panel), a rapid and continuous increase stage (125 m ahead of the panel to −100 m behind the panel), and an accelerating increase stage (−100 m to −200 m behind the panel). The stress changes in two ribs obtained with numerical modeling confirmed the described evolution of the mining disturbance. Meanwhile, numerical modeling results also suggested that a roof pre-splitting treatment could significantly decrease the peak stress in the coal pillar and panel rib, thus decreasing coal/rock mass failures and entry convergences. Furthermore, based on the rock formation condition of the test site and the ground response of entries, the detailed parameters of the roof pre-splitting treatment were determined and tentatively applied in the field. The field application suggested significantly improved gateroad performance with the proposed roof pre-splitting scheme. This study's results can help to increase understanding of the ground response of the EDRLP and correlations with roof strata movement. In addition, the proposed control strategy and design principle of the roof pre-splitting treatment can potentially be applied to other projects. [ABSTRACT FROM AUTHOR]

This paper presents the results of a numerical study of the stability of mining roadways in double-roadway excavation with a 6.0-m coal pillar. This excavation method is used to alleviate the imbalance between mining and tunneling and improve the recovery rate of coal resources. The roadways on both sides of the 6.0-m coal pillar were excavated before the mining activities, resulting in a complex stress environment of the coal pillar. A meticulously validated numerical model was developed to investigate the dynamic evolution characteristics of the 6.0-m coal pillar vertical stress and plastic state. The results indicated that the coal pillar entered the residual state and that the solid coal side of the next panel became the main bearing body under the abutment load caused by the mining activity of the 8311 panel. The abutment load transfer process from the 8311 gob to the 8312 panel exhibited obvious non-instantaneity. During the mining of the 8312 panel, an L-shaped high-stress area was formed at the upper end of the working face, owing to the superposition of the abutment load of two adjacent gobs. When the coal pillar transitioned from an elastic state to a plastic state, large deformation and failure occurred. To ensure roadway safety during the service period, the combined control technologies of (1) roof cutting using dense drilling and (2) support strengthening were proposed. Field observations indicated that the stability of the roadway next to the 6.0-m coal pillar was maintained and that the severity of other ground control problems was reduced. [ABSTRACT FROM AUTHOR]

In longwall mining method, the stability of the maingate and tailgate gateroads is of great importance because these excavations are used for essential services such as ventilation and transportation. The aim of this paper is to develop a numerical model to investigate the stability of a maingate gateroad named P8-maingate during caving and extraction of P7 and P8 coal panels of Tabas Parvadeh underground mine. Stability analysis is carried out by critical shear strain criterion, zone plasticity conditions, support system efficiency and strength-stress ratio conditions using FLAC3D, a software based on finite difference numerical methods. In the critical shear strain criterion method, the stability analysis is conducted by comparing the shear strain increments of the element in the model and Sakurai critical shear strain. In the second stability analysis method based on zone plasticity, the zones indicating shear plastic failures are the potential instability-causing regions. In the support system efficiency method, the applied load and pressure are compared to the yielding strength of the support systems to check their stability. In the Strength-Stress Ratio (SSR) method, a safety factor is determined for the gates, and the strength versus stress plots of the gates are assessed to control the stability condition. The results of numerical modeling indicate the stability of the P8-maingate using these methods. The modeling results are then compared to the instrumentation data to investigate the consistency where a good agreement was observed. [ABSTRACT FROM AUTHOR]

Field studies have found that, generally, a roof fall takes place after certain amount of roof strata sagging. Recent applications of auto warning tell-tale (AWTT) instrument as an early warning system at different depillaring panels found it to be simple, economical and efficient during the coal pillar extraction (called depillaring). AWTT is able to watch the set value of roof sagging continuously in time. However, the application of AWTT experienced two practical issues: (i) fixing roof sagging warning value before the fall of overlying strata and (ii) deciding horizon above which its anchorage point of AWTT needs to be installed. Different field experiences noticed that an anchorage horizon at 10 m depth from the ceiling of a gallery works satisfactorily for goaf edge. But fixation of the roof sagging warning value, before the fall of overlying strata, is found to be complex. Generally, the warning value for a given site conditions is determined by extensive field monitoring and correlation of the roof sagging value at the goaf edge with corresponding roof falls. Field studies found that modulus of elasticity and thicknesses of immediate roof are important parameters in deciding the warning value of the roof sagging. These studies also indicated that the variation in size of the left-out ribs and goaf span influence the warning value. These observed facts about the warning value for an AWTT are studied in laboratory using a numerical modelling approach in FLAC3D. The results obtained through developed simulated models are found to be in close agreement with those of the field studies. Attempts are also made to quantify the influence of above-mentioned parameters over the warning value through collated results of different studied models. The obtained results of field and laboratory investigations are presented and discussed in this paper for strata control in a depillaring panel. [ABSTRACT FROM AUTHOR]