Pre-existing Planes Of Weakness Research Articles

Petrographical, petrophysical, and geomechanical characterization of the Pliocene sandstone reservoirs of the Scarab Gas Field, deepwater Nile Delta, Egypt – Inferences on reservoir development

This study presents a comprehensive reservoir characterization of the channel sandstones of the Pliocene El Wastani Formation from the deepwater Scarab field, in Egypt. Routine core analysis, wireline logs, petrographic thin sections, X-ray diffraction, and drilling data were integrated to characterize the petrographical, petrophysical and geomechanical properties of the studied gas reservoirs to infer the implications for reservoir development. The petrographic analysis indicates fine to medium-grained glauconitic subfeldspathic wacke with dominantly primary intergranular porosity and minor secondary porosity contributed by feldspar dissolution. The reservoir facies exhibit pore-filling detrital clay (dominantly illite, smectite mixed-layer clay) along with minor carbonate and silica cementation. These reservoirs are megaporous and consist of an isotropic pore system, with 20–35 % porosity, 2400–3400 mD horizontal permeability, and 1300–2900 mD vertical permeability. Wireline log-based petrophysical assessments indicate excellent reservoir qualities with low shale volume (9–23%) and high hydrocarbon saturation of about 65–85%. Based on the geomechanical analysis, rock-mechanical properties, pore pressure, vertical stress, and minimum horizontal stress magnitudes were interpreted. The production and depletion-related risks were analyzed by utilizing geomechanical modeling which provided a quantitative assessment of drawdown and depletion limit to ensure sand-free hydrocarbon production without the risk of shear slippage on pre-existing weak planes. Considering the average unconfined compressive strength of 14 MPa, the Pliocene reservoir will require a 9.2 MPa depletion or 16.9 MPa drawdown to reach the onset of sanding, while the reservoir can be depleted to a pore pressure level of 4 MPa before it induces shear slippage on the optimally oriented fractures or fault planes. Inferences are drawn on reservoir quality, and reservoir development strategies are discussed accordingly.

Estimation of elastic properties and failure strength of layered rocks with a multi-scale damage approach

Layered rocks (LR) are composed of intact rock matrix (IRM) and various oriented rough weakness planes (e.g., fractures, bedding planes, and foliations). New microcracks can also be created and propagate in IRM during subsequent loading history. Effects of induced microcracks and pre-existing weakness planes on macroscopic mechanical behavior of LR are usually investigated separately. The novelty of the present work is to develop an original multi-scale damage approach to estimate the macroscopic anisotropic elastic properties and failure strength of LR. Two material scales are taken into consideration. At the macroscopic laboratory scale, a set of parallel rough weakness planes are embedded in the IRM, while at the microscopic scale, the IRM contains an elastic solid phase and randomly distributed microcracks. By means of a two-step homogenization technique, the effective elastic properties of LR is first determined. A multi-scale friction-damage model (MFDM) is further developed, by accounting for isotropic growth of closed microcracks and anisotropic evolution of rough weakness planes at two different scales. Subsequently, with the concept of critical damage, an analytical macroscopic strength criterion incorporating failure mechanisms of IRM and weakness planes is derived as an inherent result of the MFDM. Compared with experimental data, it shows that the macroscopic anisotropic elastic properties of LR are well estimated. However, the derived macroscopic strength criterion only provides a qualitative prediction of the anisotropic failure strength of LR for different orientations of weakness planes in conventional triaxial compression tests. By taking advantage of the fabric tensor approach, the strength criterion is further improved to take into account interaction between weakness planes and microcracks. The improved strength criterion is able to quantitatively capture the failure property of LR.

The primary influence of shear band evolution on the slope bearing capacity

Slope bearing capacity is one of the most important characteristics in slope engineering and is strongly influenced by weak planes, loading conditions, and slope geometry. By presenting the evolution of slip surfaces, this paper explored how the slope bearing capacity is affected by widely observed influencing factors. The initiation and propagation of slip surfaces are presented in laboratory model tests of slope using the transparent soil technique. Shear band evolution under various weak planes, loading conditions, and slope geometries were experimentally presented, and slope bearing capacities were analyzed with the process of shear band evolution. This paper verified that slip surface morphologies have a strong relation with the slope bearing capacity. The same slip surface morphology can have different evolutionary processes. In this case, it is the shear band evolution that determines the slope bearing capacity, not the morphology of the slip surface. The influencing factors such as pre-existing weak planes, loading conditions, and slope geometry strongly affect the slope bearing capacity as these factors govern the process of shear band evolution inside the slope.

Discrete fracture network model analysis of the effects of fluid transport on the morphology of a cluster of activated fractures.

Convective transport in low-permeability rocks can be enhanced by the injection of a pressurized fluid to activate preexisting weak planes (fractures). These fractures are initially closed, but fluid-pressure-induced slippage creates void space that allows for fluid flow. The Coulomb-Mohr criterion yields a critical pressure required to open each of the fractures. Due to the intrinsic porosity of the rock, the injected fluid can flow from the fractures' surfaces to the rock matrix through a process referred to as leakoff. Following this activation mechanism, the connectivity of the cluster of activated fractures is strongly dependent on the ratio F_{N} of the standard deviation of the critical pressures to the viscous pressure drop over a fracture's length. Recently, we proposed a continuum model to predict the effects of fluid transport on the morphology of the cluster of activated fractures formed by this process over a specific intermediate range of values of F_{N} [Alhashim and Koch, J. Fluid Mech. 847, 286 (2018)JFLSA70022-112010.1017/jfm.2018.313]. In this paper, the activation process of a discrete well-connected network of preexisting fractures embedded in a highly heterogeneous rock is modeled to analyze the effects of fluid transport on the resulting cluster's morphology for a wider range of F_{N} and show how the idealized averaged equation solution arises in a discrete system. We derive a length scale ξ_{ch}, which is a function of F_{N} above which the viscous pressure drop is important. This length scale, along with the radius of the cluster R and the average separation between the preexisting fractures ξ_{0}, can be used to define distinct growth regimes where different models can be used to describe the growth dynamics and predict the connectivity of the active network. When ξ_{0}∼ξ_{ch}≪R, the cluster is well connected and a linear pressure diffusion equation can be used to describe the cluster's growth. When ξ_{ch}≫R≫ξ_{0}, a fractal network is formed by an invasion percolation process. In an intermediate regime ξ_{0}≪ξ_{ch}≪R, percolation theory relates the porosity and permeability of the network to the local fluid pressure. For this regime, we validate the predictions of the continuum theory we recently developed to describe the cluster growth on length scales larger than ξ_{ch}.

An appraisal of crustal deformation of the East Coprates Planum, Mars

Terrestrial planets in the Solar System have been impacted by meteorites from the very beginning of their existence. Mars is a planet that preserves features of tectonic deformation as well as impact-related deformation. East Coprates Planum (ECP) is an area situated on the equatorial region located just on the south of the dichotomy boundary of Mars, which has been affected by numerous tectonic and impact events from the past. A detailed structural map of ECP was prepared focusing on the wrinkle ridges (WRs) and a few Polygonal Impact Craters (PICs) in the region. The study discusses the relationship between these two geomorphic features. The WRs (established geomorphic signatures of thrust faulting) within this area were studied and various morphometric data generated from them were used to estimate the inclination of subsurface faults and shortening across the ridges. To find out the control of pre-existing weak planes on the formation of PIC rims, the parallelism between them was studied. This study was further used as a proxy to check if there is any buried weak plane in the region and their orientation. Three sets of WRs [(i) N–S, (ii) E-W, and (iii) NNE-SSW] and a set of dykes (ENE-WSW), a solitary dyke of different orientation, and a solitary graben are present. Shortening estimates across the WRs are similar to the previous estimates from both Mars and Moon. This study reveals that along with the N–S and E-W trending WRs, a set of unexposed ​~ ​NNW-SSE trending weak planes controlled the rim geometry of the PICs in this area.

Numerical simulation of complex fracture geometry caused by hydrodynamics in shale with pre-existing weak planes

Three shale models with pre-existing weak planes (WPs) are established in this study based on three real shale outcrops with different weak planes characteristics. A global embedded 3D cohesive zone model (CZM) is used to simulate the fracture geometry of the hydraulic fractures with three structurally anisotropic shale models. The (Khristianovic-Geertsma-de Klerk) KGD model and Blanton's criterion are used to verify the reliability of the proposed model. Several simulation examples are established to study the effects of the stratification distribution, weak plane density, horizontal stress difference (HSD), injection rate, and fluid viscosity on the fracture geometry in the stratification plane and matrix. In addition, the effects of these variables on the stimulated reservoir volume (SRV) are analyzed. Structurally anisotropic shale has a symmetrical fan-shaped fracture network, a central symmetrically inclined fracture network, and an irregular fracture network along the weak plane direction. There are two types of hydraulic fracture (HF) networks: a fracture network composed of WP fractures and another fracture network composed of WP fractures and matrix fractures. A decrease in the weak-plane density, an increase in the approach angle, an increase in the HSD, an increase in the fracturing fluid viscosity, and an increase in the injection rate all contribute to the formation of MFs and a complex fracture network interlacing WP fractures and matrix fractures. In contrast, it is helpful to form WP fractures and fracture network that is only composed of the WP fracture. When WP fractures coexists with matrix fractures, the opening of the WP fracture is generally 2–4 times larger than that of the matrix fracture, which can easily form the main fracture. With an increase in the approach angle, the width of the SRV gradually increases whereas the length of the SRV decreases first and then increases. This study further supports our understanding of the geometry of complex fractures generated by a hydraulic drive in shale and has important reference significance for well completion design and hydraulic fracturing design in shale development.

An Approach for Stability Analysis of Gas/Fluid-Filled Cylindrical Hole in Laminated Materials

An explicit analytical workflow for cylindrical hole stability analyses in general laminated materials that possess transversely isotropic (TI) anisotropy is presented. In this approach, the calculation of the distribution of the stresses around a cylindrical hole and the failure evaluation at the hole wall consider the effects of both material elasticity anisotropy and strength anisotropy caused by material laminated structures. Material strength anisotropy is assumed to be caused by the sliding of preexisting weakness planes oriented parallel to the isotropic plane of the material. The effect of anisotropy on strength is modeled by combining a shear failure criterion for the intact matrix and a weak plane failure criterion for the planes of weakness. We derive critical pressure solutions for the stability of the intact matrix around a hole filled with gas or fluid based on the Mohr–Coulomb failure criterion and Drucker–Prager failure criterion; either one of them can be combined with the weak plane failure criterion to give the solution for hole wall shear failure pressure. The solution for hole wall fracture initiation pressure is derived based on the tensile failure criterion. This approach can be applied to holes of arbitrary orientation in general laminated materials.

Numerical Investigation of a Hydrosplitting Fracture and Weak Plane Interaction Using Discrete Element Modeling

Water inrush caused by hydrosplitting is an extremely common disaster in the engineering of underground tunnels. In this study, the propagation of fluid-driven fractures based on an improved discrete element fluid-solid coupling method was modeled. First, the interactions between hydrosplitting fractures (HFs) and preexisting weak planes (WPs) with different angles were simulated considering water pressure in the initial fracture. Second, the influence of the in situ stress ratio and the property of WPs were analyzed, and corresponding critical pressure values of different interactions were calculated. Lastly, the maximum principal stress and maximum shear stress variation inside the pieces were reproduced. The following conclusions can be drawn: (1) Five different types of interaction modes between HFs and natural WPs were obtained, prone to crossing the WPs under inclination of 90°. (2) The initiation pressure value decreased with an increased in situ stress ratio, and the confining stress status had an effect on the internal principal stress. (3) During HFs stretching in WPs with a high elastic modulus, the value of the maximum principal stress was low and rose slowly, and the maximum shear stress value was smaller. Through comprehensive analysis, the diversity of the principal stress curves is fundamentally determined by the interaction mode between HFs and WPs, which are influenced by the variants mentioned in the paper. The analysis provides a better guideline for understanding the failure mechanism of water gushing out of deep buried tunnel construction and cracking seepage of high head tunnels.

Investigation of Hydraulic Fracturing Behavior in Heterogeneous Laminated Rock Using a Micromechanics-Based Numerical Approach

Understanding hydraulic fracturing mechanisms in heterogeneous laminated rocks is important for designing and optimizing well production, as well as for predicting shale gas production. In this study, a micromechanics-based numerical approach was used to understand the physical processes and underlying mechanisms of fracking for different strata orientations, in-situ stresses, rock strengths, and injection parameters. The numerical experiments revealed a very strong influence of the pre-existing weakness planes on fracking. Geological models for rock without weakness planes and laminated rock behave very differently. Most simulated fractures in the rock without weakness planes were caused by tensile failure of the rock matrix. In an intact rock model, although a radial damage zone was generated around the injection hole, most of the small cracks were isolated, resulting in poor connectivity of the fracture network. For rock models with pre-existing weakness planes, tension and shear failure of these structural planes formed an oval-shaped network. The network was symmetrically developed around the injection well because the strength of the pre-existing weakness planes is generally lower than the rock matrix. The research shows that the angular relations between the orientation of the structural planes and the maximum horizontal stress, as well as the in-situ stress ratios, have significant effects on the morphology and extent of the networks. The strength of the pre-existing weakness planes, their spacing, and the injection rate can dramatically influence the effectiveness of hydraulic fracturing treatments.

A new methodological approach to assess the stability of discontinuous rocky cliffs using in-situ surveys supported by UAV-based techniques and 3-D finite element model: a case study

This paper aims to present a new methodological approach for the stability assessment of coastal cliffs constituted of discontinuous rock masses. The method entails in situ specific geostructural and geomechanical surveys, three-dimensional UAV-based Photogrammetric structural models, laboratory geotechnical tests and, two- and three-dimensional finite element analysis (FEM). The application of the method to a case study is then presented and discussed; it regards a 600 m long sea rocky cliff located at Sant'Andrea (Melendugno, South of Apulia, SE Italy) and faced to the southern Adriatic Sea. Here the cliff is made up of an about 15 m-thick sequence of laminate calcisiltites alternate with bioturbated calcarenites belonging to the Uggiano la Chiesa Fm. (Middle-Upper Pliocene).The structural discontinuities detected with photogrammetry techniques were compared and validated with those derived from conventional in situ survey methods. Later on, the paper discusses assumptions and results of two- and three-dimensional finite element models developed to investigate the potential failure mechanisms of the sea cliff accounting for pre-existing weak planes or discontinuities with unfavourable orientation.The failure mechanisms obtained by both FEM analysis agree well with those typically observed in the study area.

A super-critical stress model for polymodal faulting of rocks

A polymodal faulting phenomenon is further explored here that comprises penecontemporaneous fractures showing finite dispersion of strikes and dips with poles confined in one annular or two arc regions in stereographic projection. The fracture pattern represents three-dimensional strain of rocks with characteristics distinct from previously known bi-, four- or poly-modal faulting, and thus is unable to be interpreted in terms of the corresponding theories such as conjugate faulting, slip on preexisting weak planes and tensile crack interaction, respectively. Here, a super-critical stress model is advanced to decode stress significance of the specific fracture pattern with a particular emphasis on model prediction for orientations of failure surfaces. The model is an extension of the slip model of Reches (1983) that prescribes formation of four sets of faults in orthorhombic symmetry conforming to the Coulomb-Mohr criterion of τ=C+μσ (equivalent to C=F=τ-μσ where C is cohesion of rocks and F is function of shear stress τ, normal stress σ and friction coefficient μ). The mechanic boundary condition means a super-critical stress state (Fmax>C) because stress resolution allows only two sets of planes to coevally have maximum value of F and thus shear fail equally according to Andersonian (1905, 1951) conjugate faulting derivation that corresponds to a critical stress state, i.e., Fmax is just equal to C. This super-critical stress state further means that there are some planes on which F>C certainly on verge of faulting at least possessing a not even lower tendency than those planes on which C=F, and thus polymodal faulting results. The planes with F≥C tend to be areal distribution with arc or annular geometry in stereographic projection of poles. Given the Coulomb-Mohr friction and failure criteria are not fundamentally different, the super-critical model may also apply to initial failure of intact rocks. The new model represents a major reorganization in thinking about shear failure of rocks and fracturing patterns. Several natural examples are provided with fracture patterns fit to the super-critical model proposed here.

The Influence of Pre-Existing Deformation and Alteration Textures on Rock Strength, Failure Modes and Shear Strength Parameters

This study uses the uniaxial compressive strength (UCS), the indirect tensile strength (ITS) and the point load tests (PLT) to determine the strength and deformation behavior of previously deformed and altered tonalite and anorthosite. In general, veined samples show higher strength because the vein material has both cohesive and adhesive properties while fractures have no cohesion, only frictional resistance. This implies that each rock category has to be treated independently and absolute strength predictions are inaccurate. Thus, the conversion factor k is a sample specific parameter and does not have a universal value. The ratio of UCS/ITS appears to be related to the rock strength and can be used to classify rocks based on their strength. The shear strength parameters, the friction angle and the cohesion, cannot be calculated for rocks with pre-existing planes of weakness. Reactivation is favoured only for planes oriented less than 20° to the maximum stress. For planes oriented between 20° and 50° to the maximum stress, failure occurs by a combination of reactivation and newly formed fractures, while for orientations above 50°, new shear fractures are favoured. This suggest that the Byerlee’s law of reactivation operates exclusively for planes oriented ≤10° to the maximum stress.

Earthquake induced soft sediment deformation structures in the Paleoproterozoic Vempalle Formation (Cuddapah basin, India)

Soft sediment deformation structures (SSDS) are preserved in a homogenous dolomudstone lithology of crinkly laminated and heterolithic facies of the Paleoproterozoic Vempalle Formation in the Cuddapah Basin. This basin was formed on the eroded basement rocks of the Eastern Dharwar Craton, India. Deformation structures preserved in this succession include combination of breccia and folds, intrastratal faults, cracks, and dikes belonging mostly to a brittle deformation regime associated with ductile imprints. Based on the observed SSDS, their lateral homogeneity and traceability, draping by undeformed strata, proximity of faults as well as apparent lack of storm signatures and gravity induced mass movement, these fine-grained deformed beds can be tentatively ascribed to a large to intermediate depth earthquake with Richter magnitude scale 4 and above, generated during reactivation of basement faults owing to plume related mantle activity. The occurrence of SSDS in the Vempalle Formation emphasizes the role of downwarping along pre-existing planes of weakness in the Archean basement in the evolution of the Cuddapah Basin, analogous to subsidence in present-day continental margin.

Dykes as physical buffers to metamorphic overprinting: an example from the Archaean–Palaeoproterozoic Lewisian Gneiss Complex of NW Scotland

The early history of polymetamorphic basement gneiss complexes is often difficult to decipher due to overprinting by later deformation and metamorphic events. In this paper we integrate field, petrographic and mineral chemistry data from an Archaean tonalitic gneiss xenolith, hosted within a Palaeoproterozoic mafic dyke in the Lewisian Gneiss Complex of NW Scotland to show how xenoliths in dykes may preserve signatures of early tectonothermal events. The Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses of the Lewisian Gneiss Complex are cut by a suite of Palaeoproterozoic ( c . 2400 Ma) mafic dykes, the Scourie Dyke Swarm, and both are deformed by later shear zones developed during the upper greenschist- to lower amphibolite-facies Laxfordian event (1740 – 1670 Ma). Detailed field mapping, petrographic analysis and mineral chemistry reveal that a xenolith of TTG gneiss entrained within a Scourie dyke has been protected from amphibolite-facies recrystallization in a Laxfordian shear zone. Whereas the surrounding TTG gneiss displays pervasive amphibolite-facies retrogression, the xenolith retains a pre-Scourie dyke, clinopyroxene-bearing metamorphic assemblage and gneissic layering. We suggest that retrogressive reaction softening and pre-existing planes of weakness, such as the c . 2490 Ma Inverian fabric and gneiss–dyke contacts, localized strain around but not within the xenolith. Such strain localization could generate preferential flow pathways for fluids, principally along the shear zone, bypassing the xenolith and protecting it from amphibolite-facies retrogression. In basement gneiss complexes where early metamorphic assemblages and fabrics have been fully overprinted by tectonothermal events, our results suggest that country rock xenoliths in mafic dykes could preserve windows into the early evolution of these complex polymetamorphic areas. Supplementary material: Electron microprobe analyses and analytical spot locations are available at: https://doi.org/10.6084/m9.figshare.c.3809545

Rock strength measurements on Archaean basement granitoids recovered from scientific drilling in the active Koyna seismogenic zone, western India

Reservoir triggered earthquakes have been occurring in the Koyna area, western India for the past five decades. Triaxial tests carried out on 181 core samples of Archaean granitoids underlying the Deccan Traps provide valuable constraints on rock strength properties in the Koyna seismogenic zone for the first time. The data include measurements on granite gneiss, granite, migmatitic gneiss and mylonitised granite gneiss obtained from boreholes KBH-3, KBH-4A, KBH-5 and KBH-7 located in the western and eastern margins of the seismic zone. Salient results are as follows. (i) Increase of rock strength with increasing confining pressure allow determination of the linearized failure envelopes from which the cohesive strength and angle of internal friction are calculated. (ii) Variable differential stresses at different depths are the manifestations of deformation partitioning in close association of fault zone(s) or localized fracture zones. (iii) Fractures controlled by naturally developed weak planes such as cleavage and fabric directly affect the rock strength properties, but the majority of failure planes developed during triaxial tests is not consistent with the orientations of pre-existing weak planes. The failure planes may, therefore, represent other planes of weakness induced by ongoing seismic activity. (iv) Stress-strain curves confirm that axial deformation is controlled by the varying intensity of pre-existing shear in the granitoids, viz., mylonite, granite gneiss and migmatitic gneiss. (v) Frequent occurrences of low magnitude earthquakes may be attributed to low and variable rock strength of the granitoids, which, in turn, is modified by successive seismic events.

Comprehensive evaluation of the stability of the left-bank slope at the Baihetan hydropower station in southwest China

The stability of the left-bank slope is a crucial geological engineering problem at the Baihetan hydropower station, China. Due to continuous excavations on the rock slope, different regions of the surrounding rock mass undergo varying degrees of unloading deformation. It is important to assess the stability of the rock slope from a macroscopic viewpoint by investigating its deformation characteristics and mechanisms. Therefore, in this work, microseismic (MS) monitoring was first employed to detect the progressive rock mass damage in the rock slope subjected to excavation, including the initiation, propagation, coalescence, and interaction of rock microfractures. Numerical modeling was subsequently performed to understand the deformation and failure mechanism of the rock slope. Moreover, traditional surveying approaches (i.e., multiple-point extensometers and inclinometers) and field observations were also used to analyze the deformation and failure characteristics of the rock slope. The MS monitoring results showed that spatiotemporal regularities in the evolution of seismic source locations were indicators of deformation failure and potential sliding surfaces. MS event clustering can be used to delineate activated pre-existing geological structures (i.e., LS331 and LS337). The simulation results show that the deformation and failure characteristics of the rock slope are mainly controlled by pre-existing weak structural planes (i.e., the intraformational faulted zones LS3319, LS331, and LS337 and fault F17). These results agree well with the results of geological data and conventional monitoring data. Our study reveals that an integrated approach combining MS monitoring, numerical modeling, traditional surveying, and field observations leads to a better understanding of the behavior of the rock slope under the influence of excavation as well as greater control of the working faces, ensuring safety under complex geological and excavation conditions.

Numerical investigation of the effect of preexisting discontinuities on hydraulic stimulation

The present work aims to investigate the effect of the orientation and spacing of preexisting planes of weakness (discontinuities) on the process of hydraulic stimulation. Structured assemblies of spheropolyhedra were created and a constant water inflow was applied at their center. The hydraulic pressure and the resulting fracture pattern were monitored during each simulation. Both showed a strong dependence on the geometry of the preexisting discontinuities and the orientation of the stress field. The dependence of the maximum hydraulic pressure in each simulation on the geometry of the preexisting discontinuities was found to be compatible with continuum considerations.

Discrete Element Modeling of Stick-Slip Instability and Induced Microseismicity

Using Particle Flow Code, a discrete element model is presented in this paper that allows direct modeling of stick-slip behavior in pre-existing weak planes such as joints, beddings, and faults. The model is used to simulate a biaxial sliding experiment from literature on a saw-cut specimen of Sierra granite with a single fault. The fault is represented by the smooth-joint contact model. Also, an algorithm is developed to record the stick-slip induced microseismic events along the fault. Once the results compared well with laboratory data, a parametric study was conducted to investigate the evolution of the model’s behavior due to varying factors such as resolution of the model, particle elasticity, fault coefficient of friction, fault stiffness, and normal stress. The results show a decrease in shear strength of the fault in the models with smaller particles, smaller coefficient of friction of the fault, harder fault surroundings, softer faults, and smaller normal stress on the fault. Also, a higher rate of displacement was observed for conditions resulting in smaller shear strength. An increase in b-values was observed by increasing the resolution or decreasing the normal stress on the fault, while b-values were not sensitive to changes in elasticity of the fault or its surrounding region. A larger number of recorded events were observed for the models with finer particles, smaller coefficient of friction of the fault, harder fault surroundings, harder fault, and smaller normal stress on the fault. The results suggest that it is possible for the two ends of a fault to be still while there are patches along the fault undergoing stick-slips. Such local stick-slips seem to provide a softer surrounding for their neighbor patches facilitating their subsequent stick-slips.

Hollow Cylinder Simulation Experiments of Galleries in Boom Clay Formation

In the context of nuclear waste disposal in clay formations, laboratory experiments were performed to study at reduced scale the excavation damaged zone (EDZ) induced by the construction of galleries in the Boom clay formation. For this purpose, thick-walled hollow cylindrical samples were subjected (after recovery of in situ stress conditions) to a decrease in the inner confining pressure aiming at mimicking a gallery excavation. X-ray computed tomography (XRCT) scans of the specimens were carried out through the testing cell before and after the mechanical unloading and allowed to quantify the displacements undergone by the clay as a result of the mechanical unloading. The deformation of the hollow cylinders and the inferred extent of the damaged zone around the central hole are found to depend on the orientation of the specimen with respect to the bedding planes and show a great similarity with in situ observations around galleries and boreholes at Mol URL in the Boom clay formation. In the experiments performed on samples cored parallel to the bedding, the damaged zone is not symmetrical with respect to the hole axis and extends more in the direction parallel to the bedding. It is the same for the radial convergence of the hole walls which is larger in the direction parallel to bedding than in the perpendicular one. In contrast, a test on a sample cored perpendicularly to the bedding did not show any ovalisation of the central hole after the mechanical unloading. These observations confirm the significance of the pre-existing planes of weakness (bedding planes) in Boom clay and the need for a correct consideration of the related mechanical anisotropy.

Use of Reverse Extrusion Method to Determine Undrained Shear Strength

The unconfined compression test (UCT) is one of the most common tools to measure the undrained shear strength of fine-grained soils. Soft to very soft soils often pose problems such that the determination of failure load is unclear using this test. Similarly, fissured clays would endeavor failure along pre-existing planes of weakness. The reverse extrusion test (RET), a mechanical metallurgy tool, has been used recently in some soil mechanics applications. The scope of this investigation is to evaluate the potential of the RET as a substitute for the UCT. Sixty remolded soils and 75 natural soils were subjected to the RET and UCT. Remolded soils were tested on mixtures prepared at the plastic limit. It was found that the ratio of extrusion pressure at failure to the undrained shear strength is relatively consistent and concentrates around 14. The same ratio was found to be 15 for natural soils. The conclusion reached, upon a series of tests on both the remolded and natural soil samples, is that the RET better represents the undrained shear strength of soil than does the UCT, that the practical difficulties with UCT is eliminated by the RET, and the results may be improved by taking into account soil plasticity.