H. Konietzky

Brazilian Tests on Transversely Isotropic Rocks: Laboratory Testing and Numerical Simulations

Abstract The dominant anisotropy (foliation and bedding) of geological materials, especially of foliated metamorphic rocks such as slates, gneisses, phyllites or schists, and sedimentary rocks with bedding planes, leads to complex fracture mechanical behavior. A series of Brazilian tests on Mosel slate were conducted considering different foliation-loading angles. Fracture patterns and strength of samples were analyzed. In addition, the deformation process and failure behavior of the foliated rock samples during the Brazilian tests were simulated using the discrete element method. The influence of anisotropic strength parameters of weak planes was studied numerically. A diagram of failure mode distribution marked with typical failure fracture patterns for Brazilian tests of transverse isotropic rocks was developed, which results in better understanding of failure modes of Brazilian tests on foliated rocks and allows a more reliable interpretation of strength parameters. It reveals, how the microparameters influence the bearing capacity and failure modes of Brazilian tests for anisotropic rocks.

Aggregate base residual stresses affecting geogrid reinforced flexible pavement response

This paper presents findings of an analytical study aimed at investigating the effects of unbound aggregate base residual stresses on the resilient response behaviour of geogrid reinforced flexible pavements. A finite element (FE) based mechanistic response model recently developed at the University of Illinois was used to predict critical pavement responses of unreinforced and geogrid reinforced flexible pavements. The nonlinear analyses performed in the base and subgrade also considered different horizontal compressive residual stress distributions introduced as locked-in initial stresses in the base course due to pavement construction and subsequent repeated traffic loading. The primary focus was to study effects of increased confinement and stiffening around geogrid on improved granular layer moduli and reduced critical subgrade vertical strains/stresses by assigning initial residual stresses around the geogrid tensile reinforcement in the FE pavement continuum analysis. An increase in horizontal confinement resulted in significant increases in the moduli of the base and subgrade layers in the vicinity of the geogrid reinforcement. The degree of structural benefit provided by geogrid reinforcement could be successfully quantified in the response analysis to show the commonly observed technical benefit of geogrids in the field.

A New Large Dynamic Rockmechanical Direct Shear Box Device

Stability and deformation analysis for geotechnical projects like tunnels, underground openings and rock slopes or reservoir engineering problems need reliable data about the behaviour of discontinuities (joints, fractures, bedding planes etc.). Especially, the tremendous progress in numerical simulation techniques require detailed data about deformation, strength and damage characteristics of geomaterials. Shear box tests are widely used in geotechnical engineering to obtain soil mechanical data. Shear box devices for rockmechanical testing are also common, but, by far, not in the same capacity. This is mainly due to the fact that rockmechanical testing equipment, in general, needs much higher forces, higher resolution in deformation measurements and larger specimens. This makes the equipment much more expensive, bigger in size and more complicated in handling. Today, commercial shear box devices for rockmechanical testing are restricted to maximum forces of between 200 and 500 kN and restricted to pure quasi-static mechanical testing (e.g. MTS-816.01, GCTS-RDS-300 or TerraTek-DS-4250). Also, most of the latest published in-house developments have reported maximum forces of about 500 kN, pure mechanical loading and no dynamics [e.g. Gomez (2008); Geertsma (2002); Balthasar et al. (2006); Kim et al. (2006); Jiang et al. (2004); Seidel and Haberfield (2002); Wong et al. (2007)]. Superimposed dynamic testing and/or hydro-mechanical coupled testing was reported by Buzzi et al. (2008) and Barla et al. (2007, 2010), but on significantly smaller samples and devices of quite different types. To investigate higher stress environments at up to about 5,000 m in depth or larger samples at lower stress levels, higher forces are necessary. Based on these circumstances, the idea of a new shear box device with significantly higher forces (up to 1,000 kN) and superimposed dynamic loading (up to 40 Hz) was born. Briefly, this article describes the components, set-up and technical data of this in-house development completed by some selected first test results.

New Methodology to Characterize Shear Behavior of Joints by Combination of Direct Shear Box Testing and Numerical Simulations

A new procedure is presented, which combines big shear box tests on rocks and corresponding numerical simulations with explicit consideration of joint roughness to get deeper insight into the shear behavior of rock joints. The procedure consists of three parts: (1) constant normal load- or CNS-shear box tests with registration of shear- and normal-components of stress and displacements and deduction of basis rock mechanical parameters; (2) high resolution 3D-scanning of joint surface to deduce joint topography; and (3) set-up, run and evaluation of 3-dimensional numerical model with explicit duplication of joint roughness as back-analysis of shear box tests. The numerical back-analysis provides deeper insight into the joint behavior at the micro-scale. Several parameters can be deduced, like micro-slope angle distribution, aperture size distribution, local normal stress distribution and detailed analysis of dilation in relation to shear direction. The potential of the new procedure is illustrated exemplary by shear box tests on slate.

Numerical Simulation of Heterogeneous Rock Using Discrete Element Model Based on Digital Image Processing

Rock is heterogeneous material because it is composed of different minerals, pores, cracks, joints and layering. Independent of the complexity of the behavior being integrated into a model, one should never forget that the most important characteristic of rock material is its heterogeneous nature. Heterogeneities affect significantly the failure of rock (Yuan and Harrison 2005). Experimental results obtained by many studies revealed that rock strength is considerably affected by the existence of different minerals. The nature of minerals suggests that the presence of strong elements such as quartz increases the rock strength and, on the other hand, the abundance of weak minerals such as mica decreases the rock strength. The study of Tuğrul and Zarif (1999) on granitic rocks also showed that the feldspars play an important role in strength reduction. The presence of mineral cleavage and micro-fissures in feldspars within the intact rock sample lowers both the tensile and compression strength. Many researchers consider the heterogeneities by using statistical methods based on the statistical analysis of microstructures. Liu et al. (2004) have introduced the Weibull distribution to characterize the rock heterogeneities. However, the statistical method cannot represent the failure mechanism appropriately due to its inaccuracy to model the actual microstructures at the microscopic level. From a microscopic view, the mechanical behavior of rock is governed by the formation, growth and eventual interaction and coalescence of micro-cracks. These micromechanisms can be considered as a function of rock material texture, which is related in a complex way to the deformability and strength of the mineral grains and cement, grain size, grain shape, grain packing and degree of cementation. (Potyondy and Cundall 2004). Li et al. (2002) investigated progressive cracking of granite plates under uniaxial compression. They found that cracking of granite appears locally and begins with the opening of quartz grain boundaries, which are located in the compressive direction. Different minerals produce distinct behavior during failure. Not only the heterogeneities in mineralogical composition but also the grain shape, grain packing and the contacts between grains have great influence on crack initiation. The mechanism of crack initiation and rock failure process is therefore controlled by mineral composition (grain size, shape) and texture (grain packing and contacts). Therefore, the prediction of these damage processes in rock material at the microscale due to any loading is difficult to characterize within the framework of existing continuum theories. Yue et al. (2003) developed several digital image processing (DIP) techniques to capture actual microstructures of geo-materials. This technique provides a good basis to set up numerical models considering the microstructure in detail taking into account the mineral grain structure. & Xin Tan xintan@hnu.edu.cn

Automated mapping of rock slope geometry, kinematics and stability with RSS-GIS

A GIS-implemented, deterministic approach for the automated spatial evaluation of geometrical and kinematical properties of rock slope terrains is presented. Based on spatially distributed directional information on planar geological fabrics and DEM-derived topographic attribute data, the internal geometry of rock slopes can be characterized on a grid cell basis. For such computations, different approaches for the analysis and regionalization of available structural directional information applicable in specific tectonic settings are demonstrated and implemented in a GIS environment. Simple kinematical testing procedures based on feasibility criteria can be conducted on a pixel basis to determine which failure mechanisms are likely to occur at particular terrain locations. In combination with hydraulic and strength data on geological discontinuities, scenario-based rock slope stability evaluations can be performed. For conceptual investigations on rock slope failure processes, a GIS-based specification tool for a 2-D distinct element code (UDEC) was designed to operate with the GIS-encoded spatially distributed rock slope data. The concepts of the proposed methodology for rock slope hazard assessments are demonstrated at three different test sites in Germany.

Experimental and Numerical Study on Evolution of Biot’s Coefficient During Failure Process for Brittle Rocks

List of symbols r Stress r0 Effective stress e Strain ev Volumetrical strain dij Kroenecker delta symbol p Fluid pressure / Porosity f Increment of water content a Biot’s coefficient B Skempton coefficient E Elastic modulus m Poisson’s ratio K Bulk modulus Ks Bulk modulus of solid phase Mij Stiffness matrix H and R Poroelastic moduli V Volume Vp Volume of pore space kn Normal stiffness of contact ks Shear stiffness of contact

NUMERICAL MODELLING OF IN SITU STRESS CONDITIONS AS AN AID IN ROUTE SELECTION FOR RAIL TUNNELS IN COMPLEX GEOLOGICAL FORMATIONS IN SOUTH GERMANY

Abstract The paper describes three-dimensional numerical modelling studies of in situ stress distributions in complex geological conditions. The modelling was intended to augment and generalise extensive hydraulic fracturing stress measurements carried out to assist in selecting the optimum alignment of an approximately 14 km long tunnel, part of a proposed new rail link between Stuttgart and Augsburg, Germany. The numerical model includes specific representation of seven different geological layers and six geological faults with throws of up to 30 m. Results indicate complex and variable three-dimensional in situ stress conditions along the tunnel routes. This is confirmed by the field measurements. Stress conditions are characterised by strong inhomogeneity and anisotropy with a maximum to minimum principal stress ratio of up to 4:1. The numerical model indicates a large change in orientation of the quasi-horizontal maximum principal stress direction along the tunnel route. This is also observed in the measurement results. Based on the stress profiles from the model, the tunnel routes can be subdivided into four and five sections in each of which the stress conditions are approximately uniform. An initial assessment has been made of the necessary support measures and problems that may be anticipated during tunnel construction by determining a factor of safety for a circular tunnel of a certain diameter in each of the sections defined above.

Voronoi-Based DEM Simulation Approach for Sandstone Considering Grain Structure and Pore Size

Abstract This paper presents a new procedure to create numerical models considering grain shape and size as well as pore size in an explicit and stochastic equivalent manner. Four shape factors are introduced to reproduce shape and size of grains and pores. Thin sections are used to analyze grain shape and pore size of rock specimen. First, a particle-based numerical model is set up by best fitted clumps from a shape library according to thin sections. Finally, an equivalent Voronoi-based discrete element model is set up based on the superimposed particle model. Uniaxial compression and tensile tests are simulated for validation. Both tests indicate that grain boundaries and pores provide preferred paths of weakness for crack propagation, but they also reveal significant differences in terms of intra- and inter-granular fracturing.

Time to Failure Prediction Scheme for Rocks

The lifetime of solids under loads has been studied by many researchers (e.g., Mishnaevsky 1996; Ignatovich 1996; Sun and Hu 1997; Aubertin et al. 2000; Miura et al. 2003; Kemeny 2003, 2005; Guedes 2006; Le et al. 2009; Damjanac and Fairhurst 2010). In this study, the lifetime of rocks under load is governed by microstructural defects such as microcracks. The natural rock contains microcracks which propagate and coalesce under loads. The macroscopic fractures formed by these microcracks could finally lead to the failure of the rock. This process has been simulated by Konietzky et al. (2009), based on cellular automata. As a further development of this research work, this study improved the crack propagation scheme by considering initial crack orientations and wing cracks. The developed scheme is especially valid for cohesive granular material, like rocks, which consists of mineral grains of different shape, size, and mechanical properties, like strength and stiffness. Like seismoacoustic monitoring documents (e.g., Yang et al. 2012; Fortin et al. 2011; Mayr et al. 2011; Yoon et al. 2012), the damage and final failure of rocks are not characterized by the growth of one single crack, but, instead, by the growth and coalescence of many microcracks, which grow at different speeds at the same time. The proposed simulation scheme is an attempt to reproduce this process up to the formation of a final macroscopic failure (macroscopic tensile crack or shear band). As a limitation of the current numerical model, the evaluated lifetime value is influenced by the mesh size.