Abstract: Despite bad geological condition due to a poor and complex geological formation, many mountain tunnels have been constructed in Japan. Generally prior to the tunnel construction, a rock mass is classified based on the result of pre-investigation. Then supports and excavation method corresponding to the rock class are decided. However, there are many cases where an additional reinforcement of supports and a change of excavation method are necessary because of unexpected rock condition. In the worst case, it is necessary to re-excavate. A large amount of extra cost is needed as a result. In order to avoid this situation, it is very important to evaluate accurately the information about geological condition and tunnel behavior which is obtained in the tunnel face. If the detailed geological information is obtained before next excavation of tunnel face, the evaluation of geological condition may become more accurate. In this paper, geological classification results which have been evaluated in 16 tunnels (total length 31km and 661 sections) of Hokkaido Japan were examined in detail, and the effect of the advancing boring from tunnel face was examined. As a result, the effectiveness of advancing boring for the rational tunnel construction was shown.
Abstract: We model crack propagation and damage induced by deviatoric stress around the crack tip. A new damage model is proposed to describe the damaged zone near fractures, in order to predict the mesoscale geomechanical behavior of rock during hydraulic fracturing. The process of new damage development follows a thermodynamic framework. An associated flow rule is utilized for irreversible strain rate while a non-associated flow rule is applied for damage evolution. Uniaxial tension and triaxial compression tests are simulated at the Gauss point of one element in MATLAB with the new damage model. The results illustrate the influence of anisotropic damage on stiffness degradation and residual strain development. The implementation of this new damage model in the commercial FEM software ABAQUS is undergoing. A preliminary Brazilian tension test is computed for the elastic domain using ABAQUS’ UMAT subroutine. The result agrees well with the analytic solution. The new damage model for rock matches the theoretical expectations, and shows that the proposed model can predict anisotropic damage.
Abstract: A geomechanical modeling study was conducted to investigate stability of major faults during past gas production and future underground gas storage operations in a depleted gas field in the Netherlands. The field experienced induced seismicity during gas production, which was most likely caused by the reactivation of an internal Central fault separating the two major reservoir blocks. A 3D field scale geomechanical finite element model of the gas field was developed with realistic representation of the structural geology and juxtaposition of various lithologies across the Central fault. The model was calibrated to match the subsidence data and the approximate location of the critically stressed, reactivated part of the fault in agreement with the seismological localization of the hypocenters of the past major seismic events. The model predicted a maximum shear slip of up to 2 cm associated with gas production. Additional, but a smaller, fault slip of up to 0.5 cm could be expected during the subsequent phase of cushion gas injection. During annual cycles of gas injection and production, the Central fault is not critically stressed and the predicted stress changes lie in the elastic region. Although the fault slip is unlikely, continuous monitoring of induced seismicity is essential.
Abstract: Here we describe both a sequential, hierarchical multi-scale approach for designing scale-dependent constitutive models at different levels of refinement in the problem as well as an open-source, massively parallel software platform (GEOS) used for implementing the approach. General cross-scale coupling is achieved through a variety of terms, which are tailored to the specific physical mechanisms involved. The focus in this paper is on the specific state of fracture and damage propagation under fluid forcing, including seismic source generation, directionality and criteria for nucleation and growth, especially in the presence of multi-scale discontinuities. To address computational complexity issues, we discuss strategies for handling under-resolution at the crack tip and the design of phenomenological models based on finer scale considerations in two dimensions. We also discuss progress on approaching the three dimensional case as well as preliminary results of simulations to predict the evolution of microseismicity resultant from progressive damage under changing subsurface stress and hydrological conditions.
Abstract: Artificial ground freezing continues to prove as an effective approach to successful underground excavations in weak rock mass 11conditions. Numerous mining and civil projects use artificial freezing worldwide; however uncertainties remain with respect to understanding and predicting behavior of frozen rock mass.It is well established that frozen rock has increased strengths relative to unfrozen states. Empirical data from recent case studies also shows that rock mass ratings of weak rock are increased by up to 38%. This paper discusses ongoing frozen rock testing at the University of British Columbia and development of empirical approaches to ground control in frozen ground. Emphasis is on observed increases in rock mass ratings. Challenges in representative numerical models are discussed, including heterogeneous material properties, distinct deformation rates, and varying temperature effects.
Abstract: Secondary pillar mining at the Doe Run series of underground lead mines near Viburnum Missouri often requires predictive modeling of pillar conditions to determine factors of safety. Pillar strengths used in modeling are based on empirical case studies as well as confinement theory. In some cases, pillars are identified with adverse geological structures, which reduce their strength. Such conditions can include highly persistent jointing, and bands of solution breccia or heavy mineralization. Current practice for such structures involves reducing pillar strengths for modeling by 7 or 11 percent, at the discretion of the engineer. UDEC modeling with Voronoi tessellation to simulate intact rock fractures was undertaken to examine the relationship between pillar height and strength reduction due to persistent jointing. The results of these numerical experiments are intended to provide guidelines for strength reductions in current pillar stability modeling. Current modeling results suggest a linearly increasing strength reduction of up to 19% for pillars with w/h of 0.67 and up to 67% for pillar with w/h of 0.33. Modeled strengths were normalized to the compressive strength of the pillar host rock and compared to empirical strength curves with encouraging results.
Abstract: Several "Thermal/Structural Interactions" full-scale in-situ experiments were fielded at the Waste Isolation Pilot Plant in the mid 1980’s. Data from two of these experiments, the Mining Development Test (Room D) and the Overtest for Simulated Defense High-Level Waste (Room B), have been used previously to help validate the legacy constitutive models and computer codes used to assess the performance of the disposal facility prior to its licensing and operation. Since then, approximately 30 years of software and hardware advances have yielded efficient software frameworks and enabling tools/infrastructure to produce a new generation of high-fidelity simulation tools. One such current state-of-the-art modeling capability is the computer code suite, SIERRA Mechanics. The capability to model waste repositories is a relatively recent addition to SIERRA Mechanics. Consequently, data from the same two WIPP rooms D & B are used in an effort, described herein, aimed at validating the code suite to this class of problems. WIPP Rooms D & B are also being proposed for an international benchmarking exercise between US and German researchers. A review of the salient features for these two rooms that need to be captured in such an exercise will also be described and elaborated.
Park, D. (Korea Institute of Geoscience and Mineral Resources) | Ryu, D.W. (Korea Institute of Geoscience and Mineral Resources) | Choi, B.H. (Korea Institute of Geoscience and Mineral Resources) | Han, K.C. (Korea Institute of Geoscience and Mineral Resources)
Abstract: In the present study, using a computational fluid dynamics code, we numerically investigated the effect of the aspect ratio of rock caverns for underground thermal energy storage (UTES) on thermal stratification during the standby mode of operation without energy flows due to charging and discharging processes. The numerical investigation was based on the Lyckebo storage cavern in Sweden. This cavern has a unique toroidal shape and is the first large-scale rock cavern for UTES. Heat transfer simulations were carried out for different aspect ratios ranging from 1.0 to 4.0, and the heating of the surrounding rock due to longterm TES was taken into account. In these simulations, the variation of the thermal stratification with respect to time was quantitatively examined based on indices for the degree of thermal stratification. The results of the numerical simulations showed that the thermal stratification in rock caverns varied depending on the aspect ratio and the duration of the standby mode. It was also demonstrated that better thermal stratification can be obtained by increasing the aspect ratio, the effect of which was significant when this ratio was below 3.5.
Abstract: This study focuses on micro-mechanical understanding of horizontal proppant flow and transport in a narrow hydraulic fracture. This research is motivated by problems encountered in practice related to proppant placement problems, such as proppant pack formation in shear fractures, and clogging and bridging. For this study a numerical Discrete Element Method coupled with Continuum Fluid Dynamics and new user-defined contact model are used. Particles contact model accounts for lubrication of a thin fluid layer between two approaching particles and dissipate their kinetic energy. Consequently, particles that are driven towards each other by the shear flow stay nearby each other and form clusters. Higher particle concentration and interaction with walls in narrow fracture enhances proppant agglomeration. As a result, stable and unstable packs significantly decrease an overall efficiency of proppant placement, causing abruption of fluid flow and proppant transport. The results of the study give a new understanding of pressure, fluid viscosity, proppant concentration and size, and fracture width on the difference between horizontal proppant and fluid velocities in the fracture and clogging of narrow channels with proppant.
Abstract: The overall strength of a rockmass, as determined by traditional rockmass characterization methods, typically involves an assessment of the intact rock strength and rockmass structure. This paper investigates a variety of approaches to account for the effect of intrablock structure (veins) on rockmass strength. Triaxial and direct shear test data were generated numerically for a theoretical andesitic tuff using FEM. The triaxial results were applied to a deep underground mine drift design case. Direct shear results were compared to an open pit slope case. The approach previously developed by the authors to account for intrablock structure using GSI was compared to a new approach that degrades the intact strength parameters of the material based on UCS and triaxial tests. The GSI and intact parameter approaches were found to be comparable, with only slight differences near zero confining stress. The numerical simulation of direct shear tests showed the importance of veins in the way failure initiates and propagates under that particular loading condition.