Abstract: In this study, the outcomes of an experimental program on the supporting effect of backfilling on the response and stability of abandoned mines, quarries and karstic caves and its verification in actual backfilling applications are presented. The backfilling materials were granular and cohesive, and rock samples were lignite, mudstone, and sandstone of Mitake, Nagakute lignite, Oya tuff and Ryukyu limestone. During experiments, electrical resistivity, acoustic emission responses in addition to conventional load and displacement responses were measured. An acrylic cell with an internal diameter of 100 mm was prepared to provide lateral confinement for back-filled samples and its lateral confinement pressure was monitored in relation to the overall mechanical response during the pre and post-failure of pillars. Several examples of applications for the verification of the backfilling effect are described and the implications in actual practices are discussed.
Chuanying, Wang (Chinese Academy of Science) | Zengqiang, Han (Chinese Academy of Science) | Sheng, Hu (Chinese Academy of Science) | Hengyin, Zhu (No.313 Geological Party of Anhui Bureau of Geology and Mineral Exploration) | Bohua, Wang (No.313 Geological Party of Anhui Bureau of Geology and Mineral Exploration)
Abstract: Structure characteristics of rock and orebody in deep borehole are obtained by borehole camera technology. Based on theory of metallogeny and geostatistics, distribution rules of different kinds of discontinuity are summarized and relevance analysis is carried out between discontinuities’ dominant orientation and lodes’ extension direction. Comparative analysis is carried out between the result of lodes’ connectivity and the actual depth of orebody. The result indicates: (1) In the high-resolution digital borehole image, discontinuities’ orientation and other parameters of rock and orebody in deep borehole can be showed visually. (2) There are some differences between discontinuities’ dominant orientation of different statistical zone in the borehole: the distribution of discontinuities in the rock is discrete with no obvious regular pattern, and the distribution of discontinuities in the orebody is more centralized with apparent dominant orientation. (3) There are more orebody in the boreholes whose orientations are forecasted by the data of ZK61 than in the boreholes whose orientation are not. Forecast results are consistent with a higher degree of actual lodes distribution, suggesting method of analyzing lodes’ extension direction based on digital borehole camera technology is feasible.'ép.
Abstract: Multiple laboratory scale hydraulic fracture stimulation treatments were performed on Colorado Rose Red Granite and medium strength concrete grout. The samples were approximately 30×30×30 cm3 and boundary conditions consisted of a single unconfined test and a true-triaxially confined and heated test in order to simulate Enhanced Geothermal Systems (EGS) reservoir conditions. High viscosity Valvoline® DuraBlend® SAE 80W90 oil and low viscosity brine solution were used as fracturing fluid with granite and analog rock, respectively. Hydraulic fractures were obtained utilizing open hole fracturing intervals. Acoustic Emissions (AE) were monitored throughout testing from six observation points, using wideband piezoelectric transducers manufactured by Physical Acoustics Corporation (PAC), in order to characterize the stimulated reservoir and perform simplified moment tensor analysis to obtain source mechanisms. The simplified moment tensor analysis only utilizes first arrival characteristics to determine crack type classifications from a unified decomposition of eigenvalues. The AE event source mechanism locations were plotted in order to determine if spatial relationships existed. Tensile, shear and mixed mode fractures were observed throughout testing. Differing factors were investigated as potential causes of the tensile and shear distribution including viscosity of fracturing fluid, brittleness of source material, homogeneity of source material, presence of natural fractures and stress conditions.
Abstract: The capability of the numerical DDA method to perform site response analysis is tested. We compare the resonance frequency obtained for a multi-drum column when modeling it with DDA and testing it in the field. When the numerical control parameters are properly selected we obtain a good agreement between DDA and the geophysical site response survey in the field. We find that the choice of the contact spring stiffness, or the numerical penalty, is directly related to the obtained resonance frequency mode in DDA. The best agreement with the physical test is obtained with a relatively soft contact spring stiffness of k = (1/20) Eo*Lo, where Eo is the Young’s modulus of the blocks and Lo is the average block diameter. This optimal k value falls within the range of acceptable k values obtained in preliminary calibration tests performed independently of the field data. We find that the obtained resonance frequency is independent of the time step interval selected. Furthermore, DDA returns only a single resonance mode whereas the geophysical test results indicate two modes. This discrepancy is explained by the fixed base used in the DDA model which inhibits soil structure interactions.
Wang, Yuan (College of Civil and Transportation Engineering and Hohai University and Lawrence Berkeley National Laboratory (LBNL) ) | Hu, Mengsu (College of Civil and Transportation Engineering and Hohai University and Lawrence Berkeley National Laboratory (LBNL)) | Rutqvist, Jonny (Lawrence Berkeley National Laboratory (LBNL))
Abstract: In this paper we present developments and applications of a new code for confined-unconfined seepage analysis based on the Numerical Manifold Method (NMM). We approach the problem using an energy-work-based seepage model which provides a clear definition and physical meaning of the seepage energy terms when assembling the governing equations. A unique feature of our approach is that it enables the application of the NMM to non-homogenous seepage analysis, based on our pipe model analogous to the penalty spring commonly applied in mechanical analysis. We verified the proposed model and the NMM code by comparison of our simulation results to analytical solutions for confined seepage and to available numerical models for a number of unconfined fluid flow examples, including a case with non-homogeneous material domain. We show that NMM, based on a two-cover-mesh system, can in the case of unconfined seepage achieve high accuracy and convergence speed with rather coarse meshes and without the need for remeshing as the phreatic surface changes.
Abstract: It is generally understood that discontinuity intensity can control a number of rock mass characteristics, such as rock mass strength and rock mass modulus, and influence rock mass response and behavior. Three-dimensional modeling can provide useful insights into the spatial variability of rock properties, however, there still are a number of aspects that hinders its full and comprehensive inclusion into rock mechanics applications. In contrast to traditional spatial modeling, where parameters are typified by scalar values (e.g. grade, density, etc.), one-dimensional discontinuity linear intensity parameters are vector variables and are unique to the borehole's orientation. The validity of spatial modeling of one-dimensional discontinuity linear intensity is therefore dependent on both the sampling direction and the degree of rock mass anisotropy. This paper describes some of the difficulties and considerations in the spatial modeling of discontinuity intensity from one-dimensional data. The paper also presents a three-dimensional test method to spatially quantify the degree of rock mass anisotropy. The method also allows practitioners to identify regions of the rock mass that have been isotropically sampled by drilling and where the spatial modeling of discontinuity intensity may be confidently applied.
Abstract: Some significant hazards may occur due to large landslide-dams formed by earthquake induced landslides. It presents serious threats to both life and property from possible upstream flooding as the impounded lake water level rises, possible dam failure and downstream flooding as rapid release of impounded water.
In order to prevent those secondary disasters, we made an assumption that the landslide dam is only formed when a large amount of landslide deposits directly rush into a river with moderate or high-velocities, and then presented a practical prediction method to extract the dangerous slopes as sources of earthquake induced landslide-dam formation. The prediction procedure consists of the following steps: (1) Identification of all the slopes using spatial statistics; (2) The extraction of potential slopes according to our assumption using three spatial relation filters; (3) Stability analysis to determine landslide prone slopes and volume estimation of potential landslide deposits using Limit Equilibrium Analysis. (4) Production of susceptibility map by ranking the exact slopes.
This method has been used to extract slopes those have potential to collapse and form landslide-dams in the catchment of Tongkou river based on the data from 2008 Wenchuan earthquake. The results show that the proposed method is effective and efficient.
Abstract: Faults distributed in surrounding rock mass always play a decisive role for the stability of the tunnel surrounding rock mass. In this paper, a series of numerical experiments is carried out to investigate the fault influence with different thickness on stability of surrounding rock mass and sprayed concrete lining at the construction phase. To simulate these different thicknesses faults without remeshing, a practical method based on the equivalent theory on deformation and strength is established. This method is composed of two parts. The first one is the constraint joint element (COJO element), which is an element used to simulate the fault discontinuity behaviors; the second part is an affecting zone to simulate the fault deformation characteristics, which is a mesh zone with a constant width. The discrepancy on the deformation, stress and plastic region in surrounding rock mass is pointed out via analyzing these results under the researching cases with different fault thickness, as well as the inner force of spayed concrete lining. Some disciplinary and quantificational regularity about the influence of different thickness faults are summarized. This regularity can perform as guidance to the site selection and layout, the design of the supporting system and the construction of underground openings encounter the weak fault.
Abstract: Realistic hydromechanical simulation of carbon dioxide storage into deep saline aquifers is computationally intensive and thereby time consuming. Large field heterogeneous models of stress-sensitive reservoirs with complex flow and geomechanical behavior are sometimes required to be modeled, which is very demanding. Therefore, most recent studies on simulation of CO2 sequestration in saline aquifers have been focused either on short-term migration or near field processes or large scale models with highly idealized geometries. This paper presents a coupled geomechanics-streamline simulation technique for rapid hydromechanical simulation of large heterogeneous reservoirs with elastic geomechanical constitutive relations. Streamline trajectories represent a three-dimensional velocity field during injection of CO2 in porous medium, and therefore are helpful for model order reduction and inclusion of geomechanics in sub domains where streamlines density is relatively high. To assess the robustness and speed of the technique, a large reservoir-cap rock system with large number of grid-blocks was made. Accuracy and speed of streamline-based method were compared to finite volume based flow geomechanical simulations for the same model with the same geometry. Porosity and its relationship to absolute permeability were the primary geomechanically influenced variables studied in the simulations. The effective stress principle was applied to characterize the stress state and governing geomechanical differential equations were implemented based on mass and momentum conservation laws on a C++ platform with the Box Method (subdomain collocation method) as the discretization technique. The streamline tool used was 3DSL which is developed on a FORTRAN platform. Simulation results showed that streamlines can be helpful for fluid flow simulation or hydromechanical coupling particularly during the injection process when the fluid flow mechanism is advection-dominated. The method is demonstrated to increase model efficiency and reduce computational cost, particularly for heterogeneous reservoirs. The inclusion of geomechanics in streamline simulation thus represents a key step in the direction of quantifying uncertainty in CO2 storage process for large scale, heterogeneous systems.
Abstract: The National Institute for Occupational Safety and Health has recently implemented the strength reduction method to evaluate coal mine entry stability in FLAC3D. The method can evaluate the performance of a support system and calculate a stability factor for the entry. For this paper, two studies were analyzed to quantify the impact that cable bolts have on the stability of the entry. The first study investigated the impact of replacing fully grouted bolts with cable bolts for a variety of mining conditions. The strength reduction method was useful for quantifying the additional stability provided by the cable bolts for the mining conditions evaluated. The results were validated with case histories from the Analysis of Roof Bolt Systems (ARBS) database. The second study investigated the benefit of using an angled cable bolt with a steel strap. It was found that with a strap, the cable bolts are better utilized to provide stability. The study also found that the capacity of the cable bolt was not sufficient to provide adequate roof support when used with thicker roof straps. The strength reduction method was useful in quantifying the effectiveness of the cable bolts and determining the weakest link in the support system.