Deepak Adhikary

A study of the mechanism of flexural toppling failure of rock slopes

SummaryThe mechanism of flexural toppling failure of jointed rock slopes has been investigated through a series of centrifuge experiments conducted on models manufactured from two types of materials (brittle: a sand-gypsum mixture; and ductile: fibre-cement sheeting). The basal failure plane observed in the centrifuge models, has been found to emanate from the toe of the slope, and orient at an angle of 12 to 20° upward from the normal to the discontinuities. A theoretical model based on a limiting equilibrium approach (Aydan and Kawamoto, 1992) has been adopted to analyse the centrifuge test data. After calibration, the model was found to accurately predict the failure load for all the tests reported in this study. Using this model, a set of charts has been prepared to assist with the analysis of slopes susceptible to flexural toppling.

Physical Modelling of Stress-dependent Permeability in Fractured Rocks

This paper presents the results of laboratory experiments conducted to study the impact of stress on fracture deformation and permeability of fractured rocks. The physical models (laboratory specimens) consisted of steel cubes simulating a rock mass containing three sets of orthogonal fractures. The laboratory specimens were subjected to two or three cycles of hydrostatic loading/unloading followed by the measurement of displacement and permeability. The results show a considerable difference in both deformation and permeability trends between the first loading and the subsequent loading/unloading cycles. However, the micrographs of the contact surfaces taken before and after the tests show that the standard deviation of asperity heights of measured surfaces are affected very little by the loadings. This implies that both deformation and permeability are rather controlled by the highest surface asperities which cannot be picked up by the conventional roughness characterization technique. We found that the dependence of flow rate on mechanical aperture follows a power law with the exponent n smaller or larger than three depending upon the loading stage. Initially, when the maximum height of the asperities is high, the exponent is slightly smaller than 3. The first loading, however, flattens these asperities. After that, the third loading and unloading yielded the exponent of around 4. Due to the roughness of contact surfaces, the flow route is no longer straight but tortuous resulting in flow length increase.

Modelling the deformation of underground excavations in layered rock masses

Abstract Two small-scale models simulating an opening in a flat-lying layered rock mass were manufactured in the laboratory using ilmenite sand and gypsum mixtures and tested on a loading frame until failure. It was observed that the failure of the opening was initiated by tensile cracking at the mid-section of the immediate roof layer. The loading frame test results have been back-analysed using a Cosserat Continuum Finite Element code. In the finite element formulation, the layers have been assumed to be elastic with equal thickness and equal mechanical properties; whereas the inter-layer interfaces (joints) have been assumed to be elastic perfectly-plastic. It is observed that the present Cosserat continuum model accurately predicts the experimental displacements everywhere, except for the roof region where the prediction is accurate up to a certain load level beyond which the experimental displacements increase more rapidly than those predicted by the numerical model. It is shown that the load, at which the measured displacements start to deviate noticeably from those obtained from the numerical prediction corresponds to the critical load that may initiate cracks in the roof layer.

Effect of strata properties and panel widths on chock performance

The selection of optimum chock (support) capacity is very crucial for a successful longwall mining. The selection of chock capacity depends on the site-specific geotechnical parameters, constraints and longwall panel geometry, which are generally not known in detail in priority. Hence, based on the field and laboratory data, various possible combinations should be analyzed to cater for the unforeseeable mining conditions. This paper discusses the use of numerical model for selecting an appropriate chock capacity based on the site-specific geological and geotechnical information and longwall panel geometry. The fracture mechanisms of immediate and main roofs are also discussed for various panel widths and support capacities. For the models considered, the chock convergence is predicted to increase by about 33% due to the increase in face width from 100 to 260 m. Similarly, the massive roof strata are found to yield higher chock convergence compared to bedded strata.

Numerical Study of Mine Site Specific Multiseam Mining and Its Impact on Surface Subsidence and Chain Pillar Stress

This paper investigates various multiseam mining related parameters using mine site specific data and numerical simulations. Two important mining effects—subsidence and stress—are analysed for different possible mining layouts. A geological mine dataset has been used to generate a numerical model. The predicted surface subsidence magnitude and surface profile have been compared under different scenarios to assess potential options in multiseam mining strategies. The effects that seam separation distances, mining offset, panel layout and panel orientation each have on surface subsidence and chain pillar stress magnitude have been investigated. The numerical simulation shows that ascending or descending mining directions have little impact on controlling the surface subsidence in multiseam mining and predicted an almost identical maximum stress development at the chain pillars. Numerical simulations infer that the orientation of the top panels control the subsidence profile.

Assessment of Chock Capacity and Strata Caving for a Longwall Mine

A mine scale numerical analysis of modern day longwall using a 3D Cosserat continuum method has been presented. The effect of mine specific geological conditions on viability of introducing a modern day longwall is comprehensively investigated and analysed in this paper. The various longwall parameters like chock (face support) convergence and strata caving mechanism are evaluated. The varying thickness of the sandstone present in the roof can be seen to have a strong impact on the magnitude and pattern of chock convergence. The paper also discusses the performance of chocks with different capacities under identical conditions. The effect of overlaying sandstone properties and width of the longwall panels have also been investigated. The analyses carried out in this study is expected to provide valuable process guidance during the mine design in relation to selecting the optimal mine geometry and support capacity so that the potential mining hazards could be minimized.

Deficiencies in 2D Simulation: A Comparative Study of 2D Versus 3D Simulation of Multi-seam Longwall Mining

The reliable prediction and management of mining-induced surface subsidence is one of the environmentally challenging issues for the coal mining industry. Because coal mining companies operate under strict environmental accountability, the absence of robust and reliable analysis tools may significantly affect the industry’s ability to gain approval and licenses when significant surface subsidence issues are involved. This issue becomes even more critical in multi-seam mining conditions, where high-stress concentration and large amounts of surface subsidence are expected to generate during multi-seam mining, hence could affect the feasibility and safety of all seams being mined. To obtain mining approval, it is, therefore, imperative to understand the geomechanical effect of mining in one seam on the mining of the underlying/overlying seams, and to accurately predict the magnitude and profile of surface subsidence. Various computer programs using empirical or numerical approaches have been developed to estimate the stresses at pillars and coal seams during multi-seam mining (Bigby et al. 2007; Ellenberger et al. 2003; Mark et al. 2007; SCT 2010; Sears and Heasley 2013). However, empirical-based models have severe limitations, which often make them inapplicable for assessing the feasibility of multi-seam mining at green field sites. Instead, numerical simulations are widely employed for this purpose. Due to the complexity of the problems and the computational times, researchers and engineers generally resort to twodimensional (2D) simulations. The present study assesses the performance of 2D and 3D numerical simulations and presents comparisons of subsidence profiles and stresses in pillars obtained during multi-seam mining. We modeled two different seams, each with four mining panels, using an in-house, 3D, finite element code called COSFLOW (Adhikary et al. 1996; Adhikary and Guo 2002). A unique feature of COSFLOW is the incorporation of Cosserat continuum theory in its formulation (Cosserat and Cosserat 1909). In the Cosserat model, interlayer interfaces (i.e., joints, bedding planes) are considered to be smeared across the mass. In other words, the effects of the interfaces are incorporated implicitly in the choice of stress–strain model formulation. The Cosserat model incorporates the bending rigidity of individual layers in its formulation, unlike other conventional implicit models. COSFLOW produced very accurate results when simulating surface subsidence due to longwall mining at Appin Colliery in New South Wales in Australia (Guo et al. 2004).

Slope Failure in a Foliated Rock Mass with Non-Uniform Joint Spacing: a Comparison Between Numerical and Centrifuge Model Results

Flexural toppling is a mode of slope failure involving the overturning of interacting rock columns formed by a single set of discontinuities which strike more or less parallel to the slope and dip into the rock mass. In the analysis, the rock columns may be visualised as an array of cantilever beams, which bends under its own weight and transfers load to the underlying rock. When the induced tensile stress exceeds the tensile strength of the rock layer in the toe region, layer fracturing starts in this region and may lead to a progressive slope failure. Earlier investigations of the flexural toppling failure of foliated rock slopes (e.g. Aydan and Kawamoto 1992; Adhikary et al. 1996, 1997) have considered only a uniform joint spacing. However, in natural geological conditions, joint spacing is highly non-uniform. This paper investigated the flexural toppling of foliated rock slopes with non-uniform joint spacing for the case of low joint friction angle. The results of two centrifuge tests conducted on small-scale foliated slope models manufactured from silica glass sheets have been compared with the results obtained from the finite element simulations.

Effect of loading rate on sand pile failure: 2D DEM Simulation

Discrete element method (DEM) is a widely used simulation tool to model physical behaviour of granular materials. In this study 2D DEM simulation has been used to simulate the failure of a sand pile loaded at the crest. The model has been calibrated and validated using experimental force-displacement behaviour, angle of repose and particle velocity profile. The effects of numerical loading rates on simulation results have been investigated. The calibrated DEM model showed that the selection of loading rate is crucial in simulating particle assembly behaviour. In the quasi-static state a small change in loading rate does not change the force-displacement behaviour of the model. However, the system becomes unstable and force-displacement behaviour of the granular assembly diverges from the quasi-static state when the loading rate is higher than the quasi-static loading rate.

Numerical modeling of porous flow in fractured rock and its applications in geothermal energy extraction

Understanding the characteristics of hydraulic fracture, porous flow and heat transfer in fractured rock is critical for geothermal power generation applications, and numerical simulation can provide a powerful approach for systematically and thoroughly investigating these problems. In this paper, we present a fully coupled solid-fluid code using discrete element method (DEM) and lattice Boltzmann method (LBM). The DEM with bonded particles is used to model the deformation and fracture in solid, while the LBM is used to model the fluid flow. The two methods are two-way coupled, i.e., the solid part provides a moving boundary condition and transfers momentum to fluid, while the fluid exerts a dragging force to the solid. Two widely used open source codes, the ESyS_Particle and the OpenLB, are integrated into one code and paralleled with Message Passing Interface (MPI) library. Some preliminary 2D simulations, including particles moving in a fluid and hydraulic fracturing induced by injection of fluid into a borehole, are carried out to validate the integrated code. The preliminary results indicate that the new code is capable of reproducing the basic features of hydraulic fracture and thus offers a promising tool for multiscale simulation of porous flow and heat transfer in fractured rock.