12th ISRM Congress

ABSTRACT: Surrounding of a rock tunnel contains various structural discontinuities which requires utilizing numerical tools with ability to solve both continuous and discontinuous domains in a rock mass. This paper presents a three-dimensional numerical manifold method (3-D NMM) in analyzing complete failure process of key blocks in a rock tunnel. Based on the developed 3-D NMM, a rock block falling from the side and roof of a horseshoe-shaped tunnel is studied. 12 discrete blocks are created, from which the most critical key blocks are found. The falling process of the key blocks detaching from the tunnel is simulated. The corresponding critical friction angle is also calculated.
The robustness of the developed method is verified through the rock tunnel stability analysis, which demonstrates the potential ability to analyze the interactive contact among complex blocks in the 3-D numerical manifold platform.

ABSTRACT: A finite element analysis of uniaxial tensile fracture was performed for specimen models of monomineral polycrystalline rock under the assumption that fracturing occurs only at the grain boundaries. The constitutive law of the grain boundaries was derived by applying the associated flow rule in the plastic theory and a tension-shear-softening curve to an extended Coulomb criterion. Unloading of intergranular cracks occurs before the peak strength when the intergranular cracks do not compose the final failure plane that is completely formed near the inflection point of the axial stress-axial strain curve after the peak. During the completion of the final failure plane, the conversion of failure mode from tension to shear occurs at intergranular cracks that have a large angle of the normal direction relative to the loading axis, for which the normal stress is in compression in both the initial elastic stage and the final failure stage.

ABSTRACT: The safety assessment of concrete dam foundations entails the examination of the potential failure mechanisms, typically defined by natural rock discontinuities or the concrete-rock interface. Numerical models which represent the rock mass as a discontinuous medium, in particular discrete element models, are particularly adequate for the analysis of these failure scenarios, given their ability to represent the geologic structure of the rock mass, as well as the concrete structure. The application of discrete element deformable block models to the safety assessment of arch and gravity dam foundations is discussed with reference to specific examples. The issues involved in model generation, such as the representation of the rock discontinuities or the application of joint water pressures, are examined, as well as the procedures for safety factor evaluation.

ABSTRACT: Rock mass is a highly discontinuous medium where secondary structures such as joints are of common existence. For laterally loaded rock socketed piles, joint characteristics such as joint set, spacing and dip angle may each play an important part in determining the p-y behaviour and the corresponding load-deflection response of the pile-rock system. In this study, extensive numerical modeling was undertaken for piles socketed into jointed mudstone using a three-dimensional distinct element computer package, 3DEC. The numerical model was initially calibrated against laboratory model pile tests. The calibrated model was subsequently extended to specifically study the effect of different joint dip angles (30°, 45° and 60°) on the p-y and the pile head load-deflection behaviour of a 1-joint set model. In this study, it was found that joint dip angle has little impact on the p-y and the pile head load-deflection behaviour for a 1-joint set model. The linear response of the pile remained similar for the dip angles simulated up to a pile deflection of 10% of pile diameter. Beyond this, a steeper dip angle demonstrated higher load carrying capacity.

ABSTRACT: The linear elastic theory has been used in the parametric analysis of borehole stresses in the hollow cylinder setting. The analysis has been conducted in terms of two major parameters that introduce different geometries and loading conditions and have significant influence on the critical pressures. The Mohr Coulomb, the Drucker-Prager and the Modified Lade criterion have been used and the safe mud weight window has been defined in each case. Different outer diameters and hole sizes have been considered and their impact on stresses and failure investigated, both for dry and saturated rock. It has been shown that pore pressure plays an important role in borehole stability and that the Mohr-Coulomb criterion is apparently conservative. The Drucker-Prager criterion is non-conservative and over predicts the s₂ strengthening effect. The Modified Lade criterion is a moderate one, between the extremes of the other two criteria.

ABSTRACT: The hard rocks like granite, basalt, quartzite etc. show the sudden failure due to loading and unloading. The phenomena of rock bursting during underground excavation for such hard rocks under high rock cover is quite common. The rocks in general show strain softening behavior under loading and unloading. The strain softening behavior of rocks is captured using strain controlled tests in the laboratory. The rate of strain softening (residual strain/peak strain) is generally more in softer rocks than that in harder rocks.
The paper presents the post peak response of granite under strain controlled loading for confining pressures of 0 to 30MPa in the laboratory. The strength and deformation parameters are determined from laboratory test results. The peak and residual strength parameters are determined using Mohr Coulomb criterion. The variation of cohesion and friction angle with plastic strain is also determined. The rock shows the vertical splitting under confining pressures of 0 and 10MPa and shear failure under other high confining pressures. The rate of softening is less under low confining pressures and high under high confining pressures.
The paper also presents the application of softening behavior of granite to the analysis of large underground caverns (Power house and transformer yard) under plane strain conditions using FLAC. The power house cavern is 22m wide and 44m high and transformer hall cavern is 16m wide and 22m high. The analyses of large underground caverns have been carried out using conventional Mohr-Coulomb model and Mohr-Coulomb model with strain softening effect using FLAC. The support system has been installed in various stages in the form of shotcrete and effect of softening on cavern behavior is studied. The effect of support system on the softening behavior is also studied in detail.
It is seen from the analyses that there is significant effect of the softening on caverns behavior.

ABSTRACT: Hydraulic fracturing is a common technique used in the oil and gas industry for stimulation of tight formations in order to increase productivity. Production from unconventional reservoirs including tight gas sandstones is growing rapidly. When a hydraulic fracture approaches an interface, which could be a second formation or a small pocket of shale accumulation or sand lens, it may get arrested or cross the interface. This depends on the state of in-situ stresses, rock mass properties and the interface characteristics.
This paper presents the results of numerical simulations using PFC2D corresponding to two lab experiments. The modeling carried out to investigate the effect of different parameters on the interaction mechanism. Sample I includes a sandstone block in the middle of a 15 cm mortar cube. In sample II sandstone block is located in the two sides of the mortar block. The macro properties of the samples (UCS, friction coefficient etc) used for this study were estimated from the model micro properties (bond strength, friction) which are the input to PFC models. This was done by performing several simulations including bi-axial, direct and Brazilian tensile tests from which the rock properties were determined through the plot of Mohr Circles corresponding to different stress levels. Similarly, micro and macro hydraulical properties were estimated using permeability simulation tests. A hydraulic fracture was initiated in the centre of the sample and model response was monitored. The results are presented here and conclusions are made.

ABSTRACT: Water plays an important roles in acid mine drainage, both as reactant as well as contaminant carrier. On nondurable slaking rock material such as coal measure rock,water – aswater content in pore space – will control the oxygen diffusion and slaking, thus oxidation rate of sulfide mineral contained in rock. Present study is conducted in order to investigate acid mine drainage release behavior of sulfide mineral containing rock under diverse water content condition affected by different water pouring interval. Five columns with diameter 50mm and 150mm in height were used for leach test. Each column was experienced different pouring interval and flushed in each cycle (6 days) for as much as 9 cycles by using de-ionized water. Leachate as a result of flushing was then measured for pH and electric conductivity as well as sulfate and metal content. Water balance and leachate characteristic due to various water pouring interval were discussed in this paper.

ABSTRACT: In this paper, the authors discuss circumferential stress distribution in a concrete lining and displacement of rock around shaftwall induced by shaft excavation at Horonobe Underground Research Laboratory. Field measurements of the lining stress and rock displacement were conducted between 218m and 220m in depths during excavation of a 6.5m diameter access shaft and the results were analyzed using a three-dimensional numerical model. As a result, it was found that the maximum and minimum stresses occurred in the direction of horizontal minimum and maximum initial stresses, respectively. It was also clarified that difference in the stress between the maximum and minimum values was beyond 10MPa. In addition, it was found that compressive axial strain occurred around a shaft wall after casting concrete lining since rock behavior in the vicinity of shaft wall was controlled by a concrete lining.

ABSTRACT:A new technology for storing LNG in the underground cavern has been developed by combining the concept of groundwater drainage and an ice-ring with insulation system for LNG carriers and above-ground storage tank. Formation of the ice-ring, which is one of the core technologies forming LNG storage system, can be identified by a complex mechanism undergoing between the thermal characteristics of the rock mass and hydro-geological characteristics of groundwater. The key to the technology of LNG storage system have been well demonstrated through the design, construction and operation of the Pilot Plant existing at Daejeon, Korea. The results obtained from the study with regards to the formation of the ice-ring, showed that the freezing temperature of the water flowing through rock joints is affected by the latent heat during freezing, aperture of joints and the flow rate of groundwater. In the present study, the freezing temperature and the penetration length of groundwater was investigated by a thermo-hydraulic coupled analysis and CFD analysis of the groundwater flow in the joints during formation of the ice-ring. The factors affecting the freezing temperature of groundwater during the ice-ring formation are the rate of rise in groundwater level and the joint aperture. In addition, the numerical modeling studies demonstrated that the two factors were affecting each other prominently. The freezing temperature of groundwater in both narrow and wide aperture of a joint can be applied with a value higher than -3°C, which was assumed as freezing temperature in the past. However, these numerical analyses were conducted in a limited range with several assumed conditions. Therefore, further analysis with several other conditions would be necessary to clarify the freezing temperature of groundwater in the rock joints.