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Many laboratory tests of samples where the rock fractures are based on meso-scale (cm) characterization of effective ‘intact’ strength parameters neglecting the microstructure effects. Understanding fracturing processes at micro-scale (mm) will require models with microstructure data. However, data is lacking on micro-scale. Stylolites are natural rock-rock interlocked interfaces which form by a localized dissolution process and their interface contains minerals and material different from that in the surrounding host rock. Microstructures such as stylolites influence the tensile stress behaviour in a rock formation. We are interested in stylolites because they can act as drains or as barriers to flow. Therefore, we introduce a study where we investigate on micro-scale failure mechanisms of limestone samples with stylolites in diverse orientations. In preceding study (Pluymakers et al., pers. comm.) a series of Brazilian Disc Tests was performed on eleven samples all including stylolites of microscale carbonate samples from the ”Treuchtlinger Marmor” formation from the Molasse Basin (Munich, Germany). All experiments were filmed using a DSLR camera. In this study we aim to develop the use of the Particle Image Velocimetry method to analyze such type of movies. We use the developed Particle Image Velocimetry method to analyze three of the movies of the preceding study, which contained samples where the stylolite is at different angles to the horizontal axis of the sample, so to s3. Two samples has an angle of 90° between the stylolite and s3 and one with an angle of 40°. In this open-source software the pixel displacement is analyzed in frames (i.e. consecutive images, ’before’ and ’after’ image) and it calculates the velocity distribution within the framepairs, but it is also used to derive, display and export multiple parameters of the flow pattern. In this study we, Applied Earth Sciences

Many laboratory tests of samples where the rock fractures are based on meso-scale (cm) characterization of effective ‘intact’ strength parameters neglecting the microstructure effects. Understanding fracturing processes at micro-scale (mm) will require models with microstructure data. However, data is lacking on micro-scale. Stylolites are natural rock-rock interlocked interfaces which form by a localized dissolution process and their interface contains minerals and material different from that in the surrounding host rock. Microstructures such as stylolites influence the tensile stress behaviour in a rock formation. We are interested in stylolites because they can act as drains or as barriers to flow. Therefore, we introduce a study where we investigate on micro-scale failure mechanisms of limestone samples with stylolites in diverse orientations. In preceding study (Pluymakers et al., pers. comm.) a series of Brazilian Disc Tests was performed on eleven samples all including stylolites of microscale carbonate samples from the ”Treuchtlinger Marmor” formation from the Molasse Basin (Munich, Germany). All experiments were filmed using a DSLR camera. In this study we aim to develop the use of the Particle Image Velocimetry method to analyze such type of movies. We use the developed Particle Image Velocimetry method to analyze three of the movies of the preceding study, which contained samples where the stylolite is at different angles to the horizontal axis of the sample, so to s3. Two samples has an angle of 90° between the stylolite and s3 and one with an angle of 40°. In this open-source software the pixel displacement is analyzed in frames (i.e. consecutive images, ’before’ and ’after’ image) and it calculates the velocity distribution within the framepairs, but it is also used to derive, display and export multiple parameters of the flow pattern. In this study we, Applied Earth Sciences

A spring element model that takes into account the dynamic effects associated with fibre failure in composite materials is presented. The model is implemented in a parallel environment to allow a better performance in the prediction of the complex mechanisms associated with longitudinal tensile failure. The model is used to identify the changes in the stress fields around a broken fibre representing fibre failure as a dynamic phenomenon. In light of these changes in the stress fields, the cluster formation and failure development is analysed and the results are compared with the static spring element model. It is observed that the stress redistribution around a broken fibre is strongly dependent on the dynamic effects and it varies with the material in study, specially if the matrix is considered linear elastic. The changes in local stress redistribution are seen to affect the materials tensile behaviour and cluster formation, being this changes higher for a material with an elastic matrix., Peer Reviewed, Postprint (author's final draft)

To assess the effects of geological activities, models that take into account the short and long-term hydro-mechanical coupling including fracture failure are required. A new method to solve shear and tensile failure of fractures is proposed. It focuses in fragile rocks where it is possible to assume an elastic matrix and to impose irreversible displacements in fractures when fracture failure occurs. The formulation is based on an analytical solution combined with numerical methods solved using an iterative algorithm.

In recent years, some geological applications like the CO2 storage or the hydraulic fracturing have attracted the attention not only of the scientific community but also of the general public. One of the reasons of the controversy generated by these topics is the ignorance about some processes that can take place during the realization of this kind of operations. In fact, several questions are still unresolved: how long will the fractures propagate?, Will the contaminated fluid or the released gas leak to aquifers or to the atmosphere? Will the injection process induce seismicity? What magnitude? Hence, the aim of this work (developed as a part of the EU - FracRisk project) is to answer some of these questions identifying the parameters which are key in the hydraulic fracturing operation. In order to do that, a new thermo-hydro-mechanical Finite Element code has been developed in the Kratos framework using C++. As part of this code, a different method to solve the shear and the tensile failure has been proposed. In this method, fractures are represented as a zero thickness element, this means that both sides of the element lie together in the initial configuration (it seems a 1D element in a 2D domain, and a 2D element in a 3D domain) and separate as the adjacent matrix elements deform (Figure 1). Since the hydraulic fracturing operations take place in hard, fragile rocks, an elastic matrix can be assumed and irreversible displacements in fractures when rock failure occurs can be imposed. The formulation used to simulate shear and tensile failure is based on the analytical solution proposed by Okada (1992) and it is part of an iterative process. Thus, the fracture failure is calculated in a straightforward manner, avoiding numerical issues and capturing the domino effect that may occur when the failure process is triggered in a zone. In conclusion, the goal of this work is to analyze the main uncertainties related with the thermo-hydro-mechanical behavior of fractur, Peer Reviewed, Postprint (published version)

Rockburst is an unstable and violent rock failure and it is a hazardous problem in deep underground mines and civil tunnels; it imposes a great danger to safety of workers and investment. Many factors that influence rockburst damage have been identified. In many rockburst case histories, the presence of geological structures such as faults, shear zones, joints, and dykes has been observed near excavation boundaries but their role in rockburst occurrence is still not fully understood. A good understanding of the role of geological structures on rockburst damage is important to anticipate and control rockbursts and constitutes the focus of this thesis. In this research an explicit finite element tool (Abaqus-Explicit) is employed to study unstable rock failure and rockburst processes. First, uniaxial compression tests were simulated to confirm the suitability of the adopted numerical tool for simulating unstable rock failures. Two indicators namely Loading System Reaction Intensity (LSRI) and the maximum unit kinetic energy (KEmax) were proposed to distinguish between stable and unstable failures in laboratory testing conditions. Unstable rock failures under polyaxial unloading conditions were also simulated. The influences of loading system stiffness, specimen‘s height to width ratio, and intermediate principal stress on rock failure were investigated. Next, material heterogeneity (in terms of strength and deformability) was introduced into the models using Python scripting to enhance the efficiency of Abaqus for modeling geomaterials. Numerical simulation results showed that heterogeneous models resulted in more realistic failure modes than homogeneous models. The effect of material heterogeneity on rock failure intensity in unconfined and confined compression tests was investigated. It was observed that when two materials have the same peak strength, the heterogeneous model had more released energy than the homogeneous model due to differences in the failure mode.

Rockburst is an unstable and violent rock failure and it is a hazardous problem in deep underground mines and civil tunnels; it imposes a great danger to safety of workers and investment. Many factors that influence rockburst damage have been identified. In many rockburst case histories, the presence of geological structures such as faults, shear zones, joints, and dykes has been observed near excavation boundaries but their role in rockburst occurrence is still not fully understood. A good understanding of the role of geological structures on rockburst damage is important to anticipate and control rockbursts and constitutes the focus of this thesis. In this research an explicit finite element tool (Abaqus-Explicit) is employed to study unstable rock failure and rockburst processes. First, uniaxial compression tests were simulated to confirm the suitability of the adopted numerical tool for simulating unstable rock failures. Two indicators namely Loading System Reaction Intensity (LSRI) and the maximum unit kinetic energy (KEmax) were proposed to distinguish between stable and unstable failures in laboratory testing conditions. Unstable rock failures under polyaxial unloading conditions were also simulated. The influences of loading system stiffness, specimen‘s height to width ratio, and intermediate principal stress on rock failure were investigated. Next, material heterogeneity (in terms of strength and deformability) was introduced into the models using Python scripting to enhance the efficiency of Abaqus for modeling geomaterials. Numerical simulation results showed that heterogeneous models resulted in more realistic failure modes than homogeneous models. The effect of material heterogeneity on rock failure intensity in unconfined and confined compression tests was investigated. It was observed that when two materials have the same peak strength, the heterogeneous model had more released energy than the homogeneous model due to differences in the failure mode.

A cohesive fracture model considering tension, compression and shear material behaviour is implemented into the hybrid continuum–discrete element method, i.e. Universal Distinct Element Code (UDEC), to simulate possible multiple fracture in soft rocks. The fracture model considers both elastic and inelastic (decomposed to fracture and plastic) displacements. The norm of the effective inelastic displacement is used to control the fracture behaviour. Three numerical examples, including Mode-I, Mode-II and Mixed-mode tests, are conducted to verify the model and its implementation in UDEC. The model is subsequently applied to simulate uniaxial compression and Brazilian disc tests on soft rocks and the results are compared with experimental results. The results indicate that the cohesive fracture model is capable of realistically simulating the combined tensile, shear and mixed-mode failure behaviour applicable to geomaterials.

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impacts has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. This phenomenon has been modeled by means of continuous damage mechanics in the last decades. To minimize pathological mesh sensitivity, a nonlocal formulation is generally considered. Nevertheless, these models fail to properly represent damage initiation and growth around discontinuities, such as notches, damage areas and free boundaries. These inconsistencies are the consequence of using a fixed interaction domain (characteristic length) in the nonlocal formulation. In spite of limited experimental knowledge about the definition of the characteristic length parameter, there is now consensus that this quantity is not constant. In this contribution, an enhanced nonlocal model, where the interaction domain of any gauss point contracts or expands according to the stress-state of its neighbors, is used. This formulation was coupled to the well-known Mazars damage model and implemented within the framework of LS-DYNA using a fully explicit computation scheme. Two sets of numerical studies are presented in this paper. One shows the applicability and limitations of the implemented explicit algorithm to compute nonlocal quantities. The other shows that with this stress-enhanced regularization model it is possible to represent damage initiation and growth more realistically and correct the inconsistencies emerging from the traditional nonlocal formulations., Structural Engineering, Civil Engineering and Geosciences

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impacts has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. This phenomenon has been modeled by means of continuous damage mechanics in the last decades. To minimize pathological mesh sensitivity, a nonlocal formulation is generally considered. Nevertheless, these models fail to properly represent damage initiation and growth around discontinuities, such as notches, damage areas and free boundaries. These inconsistencies are the consequence of using a fixed interaction domain (characteristic length) in the nonlocal formulation. In spite of limited experimental knowledge about the definition of the characteristic length parameter, there is now consensus that this quantity is not constant. In this contribution, an enhanced nonlocal model, where the interaction domain of any gauss point contracts or expands according to the stress-state of its neighbors, is used. This formulation was coupled to the well-known Mazars damage model and implemented within the framework of LS-DYNA using a fully explicit computation scheme. Two sets of numerical studies are presented in this paper. One shows the applicability and limitations of the implemented explicit algorithm to compute nonlocal quantities. The other shows that with this stress-enhanced regularization model it is possible to represent damage initiation and growth more realistically and correct the inconsistencies emerging from the traditional nonlocal formulations., Structural Engineering, Civil Engineering and Geosciences

A 1-m-thick pillar was subject to coupled excavation- and thermal-induced stresses to induce brittle rock mass yielding. The yielding strength of the heterogeneous and fractured rock mass consisting of Aspo diorite was evaluated at eighteen discrete locations using data from the displacement, acoustic emission, and thermal monitoring systems. The average rock mass yielding strength was determined to be 0.59 of the uniaxial compressive strength. The onset of dilation in uniaxial laboratory tests, determined from strain gauge data, was found to occur at approximately 0.45 of the uniaxial compressive strength. It was shown that that the onset of acoustic emission events in situ also occurred when the tangential stress exceeded 0.43 of the uniaxial compressive strength. For sites with absence of in situ data it is recommended that this lower-bound value determined from laboratory data may be used for assessing the in situ rock mass yielding strength. Visual observation and displacement monitoring showed that extent of rock mass yielding is sensitive to small changes in the tangential stress magnitudes. It was determined using three-dimensional modelling that changes in the tangential stress magnitude of approximately 1 MPa was sufficient to cause yielding of the pillar to propagate in what appeared to be intact rock. Observations suggest that without this small stress change yielding of the rock mass would not occur. In other words, there appeared to be a well defined boundary, and if the stresses reached this boundary yielding was observed. However, if stresses were only slightly below this boundary yielding or time-dependant processes were not observed over the monitoring period used in the experiment., QC 20100525

The traditional source model for long-period seismic waves from nuclear explosions consists of a monopole releasing tectonic strain. Tectonic release has been studied since the 1960's, and numerous studies have shown that linear superposition of monopole + double-couple sources can explain many observations of Rayleigh and Love waves. Free surface interactions and the dynamics of shock-wave rebound are responsible for modes of tensile failure which can also lead to permanent deformations affecting long-period excitation. Indeed, the vast majority of nuclear explosions worldwide were conducted under containment conditions that facilitated shock-induced, deep-seated tensile failure. A new source model, which is a superposition of monopole + tectonic release + shock-induced tensile failure, is proposed, the latter source represented by a compensated-linear-vector dipole (CLVD) with vertical axis of symmetry. This CLVD source does not excite Love waves. I draw upon the Toksoz-Kehrer (1972) model for tectonic release where F is an index measuring long-period source strength of the release relative to monopole moment MI. A new index K, analogous to F, is introduced, providing a relative measure of MCLVD, the source strength of tensile failure. MCLVD vanishes for K = 1, and is > 0 in the case of extensional deformation along the vertical axis, e.g., K > 1. Rayleigh waves from the CLVD destructively interfere with waves from the monopole, and polarity reversals occur on all azimuths for K > ~3 in Poisson media. Most Nevada Test Site (NTS) observations support ~1 < K < 3, and as such the new model predicts lower Ms compared to the traditional model involving just tectonic release. This effect of tensile failure on Ms improves mb - Ms discrimination and suggests that anomalously large Ms compared to mb for the North Korean test of 9 October 2006 is due to the absence of tensile failure on this explosion., Presented at the Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (30th) held in Portsmouth, VA on 23-25 Sep 2008. Published in the Proceedings of the Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (30th), p649-657, 2008. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). The original document contains color images.

QC 20100622

The Los Alamos National Laboratory's Ground-Based Nuclear Explosion Monitoring Research and Engineering (GNEMRE) Program has a long-standing magnitude research project in support of regional yield estimation and source discrimination. This project has developed magnitude-yield scaling relationships based on regional P, S phases and coda waves to improve our capabilities to monitor nuclear explosions over broad areas and at low yields. Due to the great variability of regional seismograms, the methods developed for broad-area monitoring must be adaptable to different phases and frequency bands. These requirements, along with an understanding of the transportability of scaling relationships, pose a significant challenge from both practical and theoretical standpoints since any broad-area method needs a sound physical basis to be ultimately successful. Coda wave efforts have recently focused on practical aspects such as schema design for the exchange of calibration parameters and details of signal processing coding to match measurements between institutions, as well as advanced aspects of extending near-regional coda techniques to broad area and far-regional distances. We have developed the MagYield 0.10 schema, which ties to National Nuclear Security Administration (NNSA) Knowledge Base (KB) core schema for the exchange of envelope recipes, coda calibration parameters, and yield regression parameters. Exacting tests of signal processing methods between institutions uncovered issues with a widely used deconvolution code and code-dependent effects of non-ideal bandwidth/Nyquist ratios. Extension of the coda method to broad area far-regional distances showed the importance of 2-D path effects, as well as 2-D phase type (P vs. Sn vs. Lg coda) and 2-D transfer function effects., Published in the Proceedings of the Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (29th), p635-643, Sep 2007. Presented at the Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies (29th) held in Denver, CO on 25-27 Sep 2007. Sponsored in part by AFRL. The original document contains color images.

Fracture mechanics-based multi-level computational modeling and simulation techniques were developed to predict failure strengths of composite scarf joints under tension or compression. Global, local, and element level models were used in the study to calculate the energy release rates at the scarf joints. The study showed that explicit modeling of the resin layer at the scarf joint, where cracks initiate, was important for accurate prediction of the joint failure strengths. In addition, the consideration of the joint interface slope in the fracture model was important especially for compressive joint failure strengths. In terms of the mixed failure criteria for crack propagation, the interactive biquadratic criterion was found to be useful for reliable prediction of joint failure strengths. The predicted strengths were in good agreement with experimental data, which were obtained for two different kinds of polymer composites: e-glass/epoxy and carbon/epoxy., The original document contains color images.

The geological disposal of nuclear waste, in underground openings and the long-term performance of these openings demand a detailed understanding of fundamental rock mechanics. A full scale field experiment: Äspö Pillar Stability Experiment was conducted at a depth of 450 m in sparsely fractured granitic rock to examine the rock mass response between two deposition holes. An oval shaped tunnel was excavated parallel to the σ3 direction to provide access to the experiment and also provide elevated stress magnitudes in the floor. In the tunnel floor two 1.75-m diameter 6-m deep boreholes were excavated so that a 1-m thick pillar was created between them. In one of the holes a confinement pressure of 700 kPa was applied and in the other displacement transducers were installed. The pillar volume was monitored by an Acoustic Emission System. Spatially distributed thermocouples were used to monitor the temperature development as the pillar was heated by electrical heaters. The excavation-induced stress together with the thermal-induced stress was sufficient to cause the wall of the open borehole to yield. The temperature-induced stress was increased slowly to enable detailed studies of the rock mass yielding process. Once the rock mass loading response was observed, the rock mass was unloaded using a de-stress slotting technique. This thesis focuses on the in-situ study of the rock mass response to coupled mechanical thermal loading and thermal-mechanical unloading. The experiment, its design, monitoring and observations are thoroughly described. An estimate of the yielding strength of the rock mass is presented and compared with laboratory test and results from other rock mass conditions reported elsewhere in the open literature. General conclusions about the effect of the confining pressure and the observations from the unloading of the pillar are also presented. Important findings are that the yielding strength of the rock mass has been successfully determined, low con, QC 20100622

The geological disposal of nuclear waste, in underground openings and the long-term performance of these openings demand a detailed understanding of fundamental rock mechanics. A full scale field experiment: Äspö Pillar Stability Experiment was conducted at a depth of 450 m in sparsely fractured granitic rock to examine the rock mass response between two deposition holes. An oval shaped tunnel was excavated parallel to the σ3 direction to provide access to the experiment and also provide elevated stress magnitudes in the floor. In the tunnel floor two 1.75-m diameter 6-m deep boreholes were excavated so that a 1-m thick pillar was created between them. In one of the holes a confinement pressure of 700 kPa was applied and in the other displacement transducers were installed. The pillar volume was monitored by an Acoustic Emission System. Spatially distributed thermocouples were used to monitor the temperature development as the pillar was heated by electrical heaters. The excavation-induced stress together with the thermal-induced stress was sufficient to cause the wall of the open borehole to yield. The temperature-induced stress was increased slowly to enable detailed studies of the rock mass yielding process. Once the rock mass loading response was observed, the rock mass was unloaded using a de-stress slotting technique. This thesis focuses on the in-situ study of the rock mass response to coupled mechanical thermal loading and thermal-mechanical unloading. The experiment, its design, monitoring and observations are thoroughly described. An estimate of the yielding strength of the rock mass is presented and compared with laboratory test and results from other rock mass conditions reported elsewhere in the open literature. General conclusions about the effect of the confining pressure and the observations from the unloading of the pillar are also presented. Important findings are that the yielding strength of the rock mass has been successfully determined, low con, QC 20100622

QC 20100622