Thomas Frühwirt

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.

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.

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

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.

Direct Shear Behavior of Planar Joints Under Cyclic Normal Load Conditions: Effect of Different Cyclic Normal Force Amplitudes

The strength of joints under complicated stress conditions has to be analyzed to evaluate the stability of tunnels, foundations and underground excavations. In particular, static loads superimposed by dynamic excitations produced by earthquakes or vibrations induce complex stress pattern. Stein (1999) already proved that an earthquake alters the shear and normal stresses on surrounding faults. Understanding the dynamic behavior of rockmasses and especially joints asweak elements inside them is essential for the design of engineering projects, e.g., for surface and underground excavations, slopes, dam foundations or geothermal reservoirs (e.g., Barton and Choubey 1977; Crawford and Curran 1981; Kana et al. 1996; Lee et al. 2001; Jafari et al. 2003; Bagde and Pertros 2005; Petros and Bagdewas 2005; Belem et al. 2007; Ferrero et al. 2010; Guo et al. 2011; Liu et al. 2011, 2012a; Konietzky et al. 2012; Cabalar et al. 2013; Tao et al. 2013; Thevenet et al. 2013; Lee et al. 2014; Mirzaghorbanali et al. 2014; Nguyen et al. 2014; Zhou et al. 2015, 2017; Du et al. 2016; Zhu et al. 2016; Li et al. 2016, 2017; Dang 2017), but also for geological hazard evaluation (e.g., Hoek and Brown 1980; Hoek and Bray 1981; Babanouri et al. 2011; Liu et al. 2012b, 2013; Liu and Dang 2014; Lin et al. 2015). However, due to the present limitations in laboratory testing, shear tests under dynamic normal loading (DNL) conditions are still very rare. On the other side, earthquakes and rock bursts are very complex dynamic events, where direct shearing under DNL conditions is a common phenomenon. For the first time, Dang et al. (2016a) investigated the direct shear behavior of a plane joint under DNL conditions for different dynamic normal force frequencies and static normal load levels. They found that there is a significant time shift between peak normal stress and peak shear stress, which decreases with increasing normal load, while the relative time shift between peak normal stress and peak friction coefficient is nearly constant. Dynamic normal force frequency has little effect on relative time shift. They also proved that dynamic shear strength cannot be estimated just by simple superposition of static and dynamic loading but needs the consideration of time shift between normal and shear stress. Normal force amplitude is also an important & Wengang Dang dbzkd@126.com