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Some rock engineering designs are based on the Barton shear strength criterion for rock joints. One of the more relevant input parameters of this criterion is the basic friction angle. Nevertheless, no method is suggested to determine this value. Initially, the basic friction angle can be obtained by means of simple tilt tests, but some deeper analyses show that these tests should be carried out for particular block geometries and that results should be carefully interpreted. After observing a decrease of friction angle in artificial joints necessarily associated to time and/or wear, the authors decided to set up an experimental program aimed to differentiate between any kind of time effect and the regular degradation of the slipping surfaces due to surface wear. The results of the study showed that time does not affect test results. However, previous shear or sliding movements largely affect basic friction angle results. A tentative approach is finally proposed to estimate the basic friction angle of rock joints based on this observation.
The basic friction angle (øb) is an essential value in estimating the shear strength of discontinuities according to Barton (1973, 1976). This strength quantifies the stability of engineered or natural rock slopes and underground excavations against various kinds of failures (planar, wedge or toppling) and it is used to calculate suitable safety factor values for engineering designs.
The authors of this paper have accumulated experience in tilt testing on fresh cut rock surfaces during the past years with more than 20,000 tilt tests performed on different types of rocks and specimens of varied geometry (Alejano et al. 2012, González et al. 2014). They found out that the nature of the basic friction angle is more complex than expected, since it can largely depend on test conditions and circumstances of the joint surfaces.
As a result of this testing, it was first observed that the geometry of samples may affect results. In this way samples with ratios l/h over four are preferred, since they fulfill stress distribution conditions (i.e. they ensure compressive normal stresses all along the contact of the sample and the sliding plane). l refers to the length of the sample in the sliding direction and h refers to the height of the sample in the direction normal to sliding. Tests using small specimens are not recommended, as problems may arise related to the curvature of the cut surfaces (Alejano et al. 2012).
Bock, Barbara (Ruhr-University Bochum) | Alber, Michael (Ruhr-University Bochum) | Rogall, Michael (Geological Survey and Mining Authority of Rhineland-Palatinate) | Wehinger, Ansgar (Geological Survey and Mining Authority of Rhineland-Palatinate) | Scherschel, Jürgen (Baugrundinstitut Franke-Meißner und Partner GmbH) | Sachtleben, Volker (Baugrundinstitut Franke-Meißner und Partner GmbH)
As a relict of former subsurface basalt mining activity within the municipal area of the city of Mendig (Germany) an area of about 200,000 m² of disused pillar supported mining openings still exists at about 15-20 m below the current ground surface. After the shutdown of mining, numerous pillars experienced brittle deformation and several surface collapses occurred. To prevent any further harmful damages, a risk assessment with modeling of relevant parts of the mining openings is carried out currently. The geomechanical input data for modeling are based on the results of numerous laboratory tests. The test results indicate that pillar failure is strongly influenced by a combination of particular pillar properties like geomechanical properties as well as geometric pillar properties.
In the course of the 19th and 20th century extensive subsurface mining activities for the exploitation of the well-known "Mendig basalt" have left behind exhausted stope-and-pillar mining plants below the city of Mendig, Germany. During the mining activities, the remaining pillars had to ensure the support of the overburden and to provide a stable and safe surface environment.
For some of the basalt pillars below the meanwhile intensely developed urban areas of Mendig, the uniaxial compression strength (UCS) has been exceeded later on and consequently the corresponding pillars experienced brittle deformation after the shutdown of the mining plants.
In order to assess the currently given rock mass strength of the remaining pillars, the mechanical properties of the pillars (e.g. uniaxial rock and rock mass strength) as well as the geometrical properties of the pillars (e.g. cross sectional area, height, shape, axial orientation and slenderness) have to be considered.
The compilation and interpretation of a corresponding data base is the topic of a current research project. The project includes desk studies and site investigation drillings for the exploration of further suspected subsurface mining areas, 3D laser scanning and geotechnical mapping of known and explored mining areas, as well as core sampling, the execution of laboratory tests, and the numerical modeling for the stability assessment of known mining areas.
It is a widely accepted opinion in the tunnelling community that the primary stress state influences the cutting process, however basically no rigorous analysis regarding these effects has been conducted. In order to better understand these effects, the work has been separated in the following steps:
• A 3D numerical analysis of the secondary stress state in the rock mass while the advance approaches a fault zone, thus causing a stress increase in the face area of the competent pillar;
• 3D numerical analysis of the cutting process on heterogeneous numerical models with brittle softening behavior, with various stress states as determined from the overall 3D analysis.
The results allow definite qualitative statements about the influence of the stress state
As the TBM performance prediction is used to make reliable cost and price estimates as well as to characterize the ground conditions, special attention has been given to this currently insufficiently explored issue. Therefore, the influence of the primary stress state has to be investigated, as it is a widely accepted opinion that it has an influence on the cutting process.
In order to examine the influence of the primary stress state, the research is based on four different fields of activity:
• Numerical modelling of a TBM advance towards a fault and examination of the stresses induced at the face;
• TBM data analysis from real projects (currently pending);
• Numerical simulation of the cutting process with a highly sophisticated numerical model and examination of the influence of the stress boundary conditions on the cutting process;
• Laboratory testing of the cutting process with and without confinement stresses, in order to verify the findings of the numerical analysis.
This paper concerns only the numerical simulation of the TBM advance and estimation of the secondary stresses, and their evaluation regarding cutability.
Rockfalls are very common along the Himalayan roadways because of the highly jointed nature of rock mass. Discontinuities in the rock mass render them extremely anisotropic, reduces the strength and create avenues for different failure mechanisms like planar, wedge and toppling in destabilizing the slopes. The formation of overhangs due to excavation, coupled with the high density of joints make them a highly susceptible zone for the initiation of rockfall activity. The present study area is a part of seismically active Himalayas (Luhri, Himachal Pradesh), where the occurrence of mild to major tremors are quite rampant. Thus, the study concentrates on the possibility of initiation of rockfall activities due to seismic activities using distinct element modelling (DEM) approach. The results shows contrasting difference in the magnitudes between displacement and velocity in static as well as in dynamic case, causing rockfall to initiate in the latter case.
Rockfall is a major concern along transportation corridors in hilly areas, usually prevalent in the jointed rock slopes (Ferlisi et al. 2012 and Budetta 2004). It is a two-stage phenomenon where initial stage is the detachment of blocks while the second stage relates to the motion of the falling body, post failure. The main triggering factors for the detachment of blocks are erosion and weathering along the structural discontinuities, rainfall, earthquakes and others (Dorren 2003, Singh et al. 2010, Asteriou et al. 2012). Studies have shown a good correlation between the landslides density and the areas of strongest ground motion, while their frequency declines on moving away from the epicenter (Meunier et al. 2007). Sepulveda et al. (2005) studied the rock slope failure due to topographic amplification of strong ground motion in case of Pacoima Canyon, California and observed that the most frequent failure was planar and wedge type derived from rockfalls in many cases. The dominant mode of failure can be established from the relationship between the orientation of structural discontinuities and surface topography at any particular site, but the analysis of mechanism behind the initiation of rockfall due to seismic shaking requires the application of rigorous numerical models that have the capability to model large strain of a discontinuous media. Seismically induced rockfall occurs as a combined effect of gravity and seismic acceleration producing short-lived stress, which exceeds the cohesive and frictional strength of the earth materials (Newmark 1965). As a result, slope failure can take place due to slight disturbance in slopes that may have been stable under static loading time. Earthquakes produce two types of ground accelerations. Out of the vertical and horizontal accelerations, the later one is known to cause greater impact on slopes (Romeo 2000). Past studies have shown that even an earthquake of magnitude 4.0 can trigger a rockfall activity (Keefer 1984). The threshold displacement responsible for causing landslides ranges from 2-5 cm (Wilson & Keefer 1985).
In the paper, two modeling strategies for the computation of the effective permeability of heterogeneous, fractured rocks utilizing homogenization over a representative elementary volume (REV) are proposed and compared. One method is based on a new continuum micromechanics model. Here, the REV represents distributed fractures idealized as penny shaped inclusions in a porous matrix. The effective properties computed by this model are anisotropic and depend on the intrinsic properties of the porous matrix and the topology and density of the fractures. We propose a novel Cascade Continuum Micromechanics model (CCM), which is able to predict a fracture percolation threshold for a particular fracture density as a function of the topology of the fractures. The second modeling strategy is based on an Extended/Generalized Finite Element model (XFEM-GFEM) recently developed for numerical simulations of hydraulic fracturing in deep geothermal reservoirs. The predictions for the effective permeability from both models are compared for a REV containing distributed fractures with different aspect ratios and crack densities.
The effective transport property of heterogeneous rocks with diffusively distributed fractures at various scales (from microcracks in the sub-mm to macroscopic fractures in the m range) is strongly influenced by their distribution, orientation and their interactions with the porous matrix material. Reliable estimates for effective transport properties of fractured rocks are relevant in various projects in subsurface engineering, such as the construction of tunneling or caverns, the exploitation of oil and gas and geothermal energy reservoirs or the installation of underground storage systems (see, e.g. Tiab & Donaldson (2012), Economides & Nolte (2000) and references therein). In this paper, we propose two methods to determine the effective permeability of heterogeneous rocks. The first strategy is based on continuum micromechanics. Based on a representative elementary volume element (REV), representing distributed fractures idealized as penny shaped inclusions in a porous matrix, the effective anisotropic permeability is predicted. To investigate the percolation probability of the fracture network, we use a novel Cascade Continuum Micromechanics model (CCM) (Timothy & Meschke 2013). The model predicts a fracture percolation threshold for a particular fracture density as a function of the topology of the fractures. This provides initial estimates for the connectivity characteristics of the fracture system and allows to characterize the interrelations between diffusivity, permeability and the anisotropic stiffness. The second strategy uses computational homogenization based on a discrete representation of distributed cracks using a novel Extended/Generalized Finite Element model (Meschke & Leonhart 2015) recently proposed for numerical simulations of hydraulic fracturing in deep geothermal reservoirs. Both modeling strategies are investigated and compared by means of different configurations of fractured rocks.
The main purpose of this study was to carry out the physical and mechanical characterization of dolomitic limestone from "Camadas de Coimbra" Formation with different degrees of weathering. The study begins with a characterization of the geotechnical properties of the intact stones used in this research. Seven tests (sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability) were carried out. The characterization permitted to understand and evaluate the variation of the mechanical and physical properties throughout the weathering processes. The porosity tends to growth according to the rock weathering degree. Its increase corresponds to a decrease of the ultrasonic velocity and rock density. Considering the totality of the specimens tested, moderate correlation coefficient was found between the longitudinal waves propagation and the point load strength values (r = 0.62). For the longitudinal and transversal waves propagation the correlation coefficient was equal to 0.75.
The main purpose of this work is to contribute to the geotechnical knowledge of several weathering degrees of dolomitic limestone belonging to the "Camadas de Coimbra" Formation. The characterisation of the physical and mechanical properties was carried out.
The study area is located in the city of Coimbra (Central Portugal). Log samples were collected in a prospection campaign performed at Polo I of the University of Coimbra. The rock characterisation was carried out through seven laboratory tests: sound velocity, porosity, density, uniaxial compressive strength, point load strength, Schmidt rebound hardness and slake-durability tests.
Correlations were established based on the test results, in order to define and predict the geotechnical behaviour of dolomitic limestone with different degrees of weathering.
Lithification is a chemical process that involves the precipitation of minerals from the pore fluid, thus affecting the pore structure and the mechanics of inter-granular contacts. In parallel with such process, grain crushing and cement fracture are basic drivers of long-term compaction. In the present study, a micromechanics-based constitutive model for granular rocks is proposed to account for the coupling between these chemical and mechanical processes. The goal is to take into account both chemical strengthening via particle cementation and mechanical degradation associated with grain crushing and bond damage. The main purpose of the model is to provide a tool that, based on simple information about the evolving microstructure of a granular rock, can be able to quantify the evolution of the strength of sedimentary rocks, as well as the alterations of their elastic domain.
Mechanical and transport properties of granular rocks depend on diagenetic processes, i.e. on the long-term phenomena that shape the pore structure of a geomaterial. In particular, lithification (i.e., the gradual formation of cement bonds of a sedimentary rock) is a natural process of fluid-infiltrated porous media exposed to physico-chemical interactions. Such micro-scale processes generate measurable couplings at the macroscopic scale, which are reflected by changes in strength, stiffness, and porosity. Capturing the underlying causes of such couplings is pivotal to understand naturally occurring phenomena, as well as to forecast the long-term implications of human activities.
Continuum approaches for reactive porous media (Coussy 2004, Ulm & Coussy 1996) are useful tools to describe chemo-mechanical couplings. Nevertheless, their state variables are often defined without considering the specific microstructural implications of reactive processes at the pore scale. To overcome this problem, here we postulate that a major source of microstructural alteration is the evolving geometry of the microstructural units (e.g., the cement bonds). To achieve these objectives, a modified form of the Breakage Mechanics theory (Einav 2007, Tengattini et al. 2014) is enhanced by incorporating reactive processes. The effect of chemo-mechanical processes will be incorporated by enhancing the energy potentials with non-mechanical contributions, i.e. by following a strategy similar to recent extensions of the Breakage Mechanics framework for multi-phasic, chemically-active granular media (Buscarnera & Einav 2012, Buscarnera 2012).
Modelling of extra large flat jack (ELFJ) testing at the Bakhtiyari dam site, Iran is the subject of this study. A complete 3D model of dam abutment, exploration gallery, and test set up was constructed. The aim was to reproduce the in-situ test results and determine the extra large jack coefficients manufactured by INTERFELS Corporation. In typical practice the large flat jack (LFJ) coefficients proposed by LNEC is used to calculate the final modulus data. These coefficients are also used to calculate the modulus data from ELFJ, which leads to significant errors. In this study a precise 3D numerical model of typical flat jack test condition was constructed using the Flac-3D code. Accordingly, the flat jack tests conducted at Bakhtiyari dam site, Iran, was simulated numerically. A very good agreement was achieved between numerical and field test results. Moreover, the obtained numerical results provided the coefficients of extra large flat jack. These, coefficients were employed for calculating the in-situ deformation modulus of Bakhtiyari dam rock mass.
Knowledge of rock mass in-situ parameters is essential in the design of large rock engineering projects. Rock mass deformation modulus is one of the most important design parameters. Typically, in most rock engineering projects the rock mass deformation is determined by combining the laboratory and field mapping data of rock mass condition. The combination of lab and field data is carried out through rock mass characterization methods (e.g. RMR, GSI, Q, etc.) aiming at determining rock mass design parameters. Depending on the quality and quantity of data, this approach can be erroneous in determining rock mass design parameters. On the other hand, direct in-situ testing is another method of measuring rock mass parameters. However, in-situ testing of rock is associated with many difficulties and is not feasible for all projects. Flat jack testing is a common method of direct measurement of rock mass modulus. This test is typically carried out for important rock engineering projects such as dams and large caverns. The objective of this study is to demonstrate the mechanisms involved in extra large flat jack testing numerically and to determine the jack coefficients. The study results were used in the flat jack testing program conducted at Bakhtiyari dam site project, Iran.
The anisotropic strength properties of rocks, which are of great importance for the stability of engineering structures, can be determined by doing various laboratory works. In this study, the anisotropic strength properties of sedimentary rocks such as marl and mudstone were examined by making use of the indirect tensile strength test. Two different methods were applied in the test phase. Firstly, the disc shaped specimens prepared by using NX diameter cylindrical samples taken from blocks parallel to the bedding plane were placed in the testing apparatus at different angles from horizontal to perform tests. In the second phase, the NX diameter cylindrical samples were prepared at perpendicular (00), parallel (900) and 450 angles to the bedding plane and tests were performed at these orientation angles. Accordingly, in the tests performed by placing samples in the testing apparatus at different angles, the highest value was gathered at Ψ =0°, and the lowest at Ψ=900 anisotropy angle. In the tests of second phase, the highest value was gathered at φ=900, the lowest at φ=450 orientation angle.
Approximately 95% of sphere is consists of magmatic and metamorphic rocks but important part of earth’s surface, 75% of it, is consists of sedimentary rocks. Important engineering projects like mine, tunnel, subway, underground shelters, slopes etc. are designed due to geological and geomechanical properties of rocks. Thereby, it‘s very important to define strength and deformation behaviour pattern of these type of rocks, for safety and economics of structures which will build in them.
Rock anisotropy is one of the most important factors that effects strength and deformation behaviour of rocks. Most of the geological formations in nature exhibit anisotropic properties. Anisotropy of rocks come into existence because of structural facts like foliation, cleavage, schistosity, joint, micro and macro fractures, sequence of minerals and orientation of particles which constitutes rocks, etc. Suitable design models should be developed for determining effect of structural or inherent anisotropy on constructions which will build in geological formations.
Rocks show different strength and deformation characteristics due to direction. Sedimentary and metamorphic rocks are more anisotropic than igneous rocks. (Ramamurthy 1993). Metamorphic rocks such as slate, shale and gneiss show anisotropic behaviour (Goshtasbi et al. 2006). According to the study "Modelling of inherent anisotropy in sedimentary rocks" by Pietruszczak et al. (2002), sedimentary rocks such as shale, siltstone, claystone, etc. show strong anisotropy depending on orientation.
Frictional strength of discontinuities (joints and faults) is the key rock mass property for analyzing mining induced seismicity or rock slope stability. This paper reports about laboratory tests on discontinuities from coal bearing strata in the Ruhr area. Shear tests have been conducted on discontinuities in shale and sandstones. These were partly weathered and sometimes coated by coal. Friction angles range from 100 for coal-coated joints in sandstone to 400 clean joints in sandstone. The discontinuities were scanned with a 3D laser scanner for estimating the surface roughness. JRC was computed from scan profiles and compared to manual measurements. Implications for fault stability around coal mines are delineated.
It has been shown (Alber 2013) that the frictional strength of discontinuities (joints and faults) is the key rock mass property for analyzing mining induced seismicity. Up to now, however, frictional properties were only back-calculated from past events, i.e. by combining fault plane solutions with numerical modeling. Back-calculations from seismic events in the Saar coal mining district yielded friction angles as low as 90 (Alber & Fritschen 2011). For the in-situ stresses in the Ruhr mining district it was concluded that friction angles should be higher than 200. As coal mining ceases in Germany mine working will be (partly) flooded and faults may be re-activated, leading to induced- seismicity caused by increasing fluid-pressure on discontinuities.
A first hint for such a case may be the 2 earthquakes of magnitudes ML 2.7 and 2.0, respectively, at the abandoned coal mine Ensdorf (Saar) on Sep. 15 and Oct. 10, 2014. It is not clear how the pore pressures at depth developed after mine-closure in 2008. Yet, the ongoing stress-release in form of seismicity makes a strong case for proper characterization of discontinuities.
Estimating the strength of discontinuities is mainly done through laboratory shear tests and/or index tests. In-situ shear tests are rarely executed so that large scale discontinuity properties stem from back-calculations. This paper summarizes results from laboratory shear tests on weathered and coal-coated rock joint in coal measure rocks from the Ruhr area and elaborates on the use of laser-scans for evaluating roughness.