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Ultra-high-performance fibre-reinforced concrete (UHPFRC) specimens with and without longitudinal reinforcement were experimentally tested under direct tensile loading. The variables considered were the volume fraction of fibres (1.0% and 2.0%), the type of steel fibres (straight and hooked end) and the longitudinal steel reinforcement ratio (none and 1.2%). All of the specimens were tested using a servo-controlled fatigue testing machine in displacement control mode. The changes in displacement were monitored using linear variable displacement transducers and the digital image correlation technique. The strain profiles at different loading stages were used to identify the crack evolution process. The average localised strain was found to be 0.2–0.36%, with corresponding crack widths of 0.3–0.6 mm. A uniaxial tensile stress–strain model was developed based on the test results and data from the literature. Longitudinally steel-reinforced specimens showed both stiffening and strengthening effects. Tension-stiffened specimens with 1.0% fibres failed at a higher strain due to the formation of multiple macrocracks. In the specimens with 2.0% fibres, the rebar fractured in a brittle manner due to crack localisation. It is concluded that a higher longitudinal reinforcement ratio is needed to utilise UHPFRC effectively under tension-dominant loads. [ABSTRACT FROM AUTHOR]

The deformation patterns and failure mechanisms of pressurised cavities at shallow depth are relevant to many geotechnical applications, including tunnelling and horizontal directional drilling. In this paper, an experimental study of a reduced-scale pressurised cavity under geostatic stress is presented, in order to measure the effect of cavity length, vertical stress and soil density on soil deformation and failure. X-ray computed tomography is used to acquire images of the system at key stages of the cavity inflation process. A closed-shaped failure region developed around the cavities, beyond which shear planes of elliptic paraboloid shape formed, extending from the bottom of the cavities all the way to the free surface. The plane-strain assumption did not hold beyond the central portion of the longest cavity tested (L = 6D). The volumetric strain and porosity changes inside the shear bands showed significant dilation in dense specimens, but contraction in loose specimens. The average orientation and the thickness of the shear bands were in agreement with those found in the literature for passive arching mechanisms (anchoring). The orientation of the principal strains around the cavity follows catenary shapes, similar to those displayed in active trapdoor mechanisms. [ABSTRACT FROM AUTHOR]

A sensitivity study is presented to evaluate the influence of different parameters on the simulation of an underground excavation in the Callovo-Oxfordian (COx) argillaceous formation performed in the Meuse/Haute-Marne underground research laboratory. An elasto-viscoplastic constitutive law representing the characteristic behaviour of indurated mudrocks and stiff clays has been employed. It incorporates anisotropy, strain-softening, creep deformations and dependence of permeability on damage. In addition, a non-local formulation, able to simulate localised deformations objectively, has been incorporated in the analyses. The following features affecting the excavation have been studied: initial stress, strength and stiffness anisotropy, strength parameters, hydraulic and hydromechanical parameters, and scale effects. A simulation reported in a companion paper provides the base case for benchmarking. The results are compared in terms of extent and configuration of the excavation fractured zone, vertical and horizontal tunnel convergences, and the development and evolution of pore pressures in the rock. From the comparisons, an enhanced understanding of the hydromechanical mechanisms associated with underground excavations in COx claystone, and other similar argillaceous materials, has been achieved. [ABSTRACT FROM AUTHOR]

The scope of the paper encompasses planar and compound sliding motions, which may exhibit creeping behaviour during a certain period but may evolve to a very rapid motion. Thermo-mechanical interactions, at the scale of the sliding surface, are accepted as a critical aspect to explain these motion phases and their relationship. The sliding kinetics and global equilibrium are described at a large scale and the evolving shearing strength at the sliding surface derives from the local analysis of the shearing band and its vicinity. Pore pressures, temperatures and related variables are calculated by resolving a set of balance equations. The paper describes the transition from creeping motions to a rapid event. Results are found in terms of dimensionless numbers. Calculation of the slide evolution requires special numerical techniques described in the paper. Band permeability is found to be the dominant property controlling the triggering of fast motions. The creeping stage and the eventual slide blow-up are intimately linked. This relationship is explored in the paper. The models presented can be readily used to back-analyse relevant case histories or, in principle, even to carry out predictive modelling, provided an adequate calibration is available for the material parameters. [ABSTRACT FROM AUTHOR]

Failure of rock close to a free surface is reproduced in laboratory tests on two types of sandstone. Digital image correlation is used to study the in-plane displacement fields prior to and during the successive formation of surface-parallel fractures. The axial and lateral displacements suggest that the displacement gradient is uniform in both directions at early loading stages; as the axial load increases, the free-face effect starts to influence the displacement contours, especially the lateral displacement field. Localised damage forms a spall before reaching peak stress. Using a simple model, a rock brittleness index is proposed as the ratio of uniaxial compressive strength and surface instability peak stress. [ABSTRACT FROM AUTHOR]

The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over other numerical models available in the literature are that it can simulate the whole process of progressive failure leading to spread. The finite-element results are consistent with previous conceptual models proposed from field observations. The parametric study shows that, depending upon geometry and soil properties, toe erosion could cause three types of shear band formation: () only a horizontal shear band without any global failure; () global failure of only one block of soil; () global failure of multiple blocks of soil in the form of horsts and grabens. [ABSTRACT FROM AUTHOR]

This paper presents a multiscale investigation on the interplay among inherent anisotropy, fabric evolution and strain localisation in granular soils, based on a hierarchical multiscale framework with rigorous coupling of the finite-element method (FEM) and discrete-element method (DEM). DEM assemblies with elongated particles are generated to simulate inherent anisotropy and are embedded to the Gauss points of the FEM mesh to derive the required constitutive relation. Specimens prepared with different bedding plane angles are subjected to biaxial shear under either smooth or rough loading platens. Key factors and physical mechanisms contributing towards the occurrence and development of strain localisation are examined. The competing evolutions of two sources of anisotropy, one related to particle orientations and the other related to contact normals, are found to underpin the development of the shear band. A single band pattern is observed under smooth boundary conditions, and its orientation relative to the bedding plane depends critically on the relative dominance between the two anisotropies. Under rough boundary conditions, the non-coaxial material response and the boundary constraint jointly lead to cross-shaped double shear bands. The multiscale simulations indicate that the DEM assemblies inside the shear band(s) undergo extensive shearing, fabric evolution and particle rotation, and may reach the critical state, while those located outside the shear band(s) experience mild loading followed by unloading. The particle-orientation-based fabric anisotropy needs significantly larger shear and dilation for mobilisation than the contact-normal based one. The asynchrony in evolution of the two fabric anisotropies can cause non-coaxial responses for initially coaxial packings, which directly triggers strain localisation. [ABSTRACT FROM AUTHOR]

The paper describes a three-phase single-point material point method formulation of coupled flow (water and air) for hydro-mechanical analysis of geotechnical problems involving unsaturated soils. The governing balance and dynamic momentum equations are discretised and adapted to material point method characteristics: an Eulerian computational mesh and a Lagrangian analysis of material points. General mathematical expressions for the terms of the set of governing equations are given. A suction-dependent elastoplastic Mohr-Coulomb model, expressed in terms of net stress and suction variables is implemented. The instability of a slope subjected to rain infiltration, inspired from a real case, is solved and discussed. The model shows the development of the initial failure surface in a region of deviatoric strain localisation, the evolution of stress and suction states in some characteristic locations, the progressive large strain deformation of the slope and the dynamics of the motion characterised by the history of displacement, velocity and acceleration of the unstable mass. [ABSTRACT FROM AUTHOR]

The article discusses the stability of a long granular slope within the framework of perfect elastoplasticity. It introduces a modified strategy for assessing the slope safety factor and the concept of the angle of natural repose has been bordered on through considering its dependency on the friction angle and dilatancy parameter. Thus, the problem of slope stability has been tackled along with the mechanical condition which shows the basic role of contractive materials.

The material point method, which combines features of finite-element and particle discretisation methods, has been extended to solve coupled flow-deformation problems in granular media. The method is applied to simulate the construction and failure of the Aznalcóllar dam. The brittle foundation clay is characterised by means of a strain-softening Mohr-Coulomb elasto-plastic model. The rupture process of the foundation is associated with a progressive failure mechanism. The model predicts the development of a localised shearing band that initiates at the toe of the dam at some intermediate stage of construction, and propagates downstream and upstream. The shape and position of the failure surface reproduce actual field observations. [ABSTRACT FROM AUTHOR]

The article presents a study which explores a photographic and photoelastic study of a granular material. It states that a glass ballotini reveals stress conditions information when viewed using polarised light. The study uses procedures that indicated some possibilities for experimental soil mechanics. Results show that the use of graphics and standard image processing software offers comparison of features of the incremental strain and stress fields.

Particle crushing, shear banding, interface abrasion and migration of crushing products all have the potential to influence the behaviour of displacement piles in sands. This paper considers these particulate processes, reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests. The findings offer new insights into the mechanics of displacement piles in sands. [ABSTRACT FROM AUTHOR]

This paper presents a study of the micromechanics of granular materials as affected by the direct shear test scale using the discrete element method (DEM). Parametric studies were conducted to investigate the effects of specimen length and height scales (in relation to the particle size) on the bulk material shear strength and shear banding behaviour in the direct shear test. A mesh-free strain calculation method previously developed by the authors was used to capture and visualise the evolution of strain localisation inside the direct shear box. Simulation results show that the maximum shear strength measured at the model boundaries increases with decreasing specimen length scale and increasing specimen height scale. Micromechanics-based analysis indicates that the local and global aspects of fabric change and failure are the major mechanisms responsible for the specimen scale effect. Global failure along the primary shear band prevails when the specimen length scale and length to height aspect ratio are small, while progressive failure becomes more likely when the specimen length scale and aspect ratio become larger. Further quantifications of the specimen scale effects on the macroscopic behaviour of granular materials rely on the fundamental understanding of quantitative relationships between material fabric, anisotropy and bulk strength. [ABSTRACT FROM AUTHOR]

Precise knowledge of the post-peak constitutive response occurring within shear bands in sands is of keen interest in geomechanics, particularly for accurate modelling of progressive failure phenomena. There is mounting evidence that the displacement field within shear bands in sands is non-uniform and distinguished by distinct meso-scale features: namely, particle force chains. Experimental validation of such features will help elucidate the precise nature of the deformation field within shear bands. This paper presents experimental evidence of the kinematic signatures of force chain activity within shear bands in sands. The meso-scale kinematics are quantified from digital-image-correlation-based, grain-scale displacement analyses performed on digital images of specimens undergoing plane strain compression. As in previous work, the data reveal distinct, systematic patterns in the kinematics along the length of the shear band, which serve as indirect evidence of force chain build-up and collapse. Herein, local volume changes are shown to integrate into this pattern. Temporal changes in these patterns with the progress of deformation are also tracked. It is argued that the changes observed in the kinematics from softening to critical state provide a physical, meso-scale explanation for the progress of global stress-strain response through the post-peak regime. [ABSTRACT FROM AUTHOR]

The paper presents a quantitative assessment of particle orientation, contacts and spatial variation of void ratio in a sand specimen loaded under plane-strain conditions. The specimen has a prismatic shape, and measures 57 mm wide × 121 mm long × 180 mm high. It was prepared using a dry natural silica sand known as F-75 Ottawa sand at a dense packing, and was sheared until failure. The specimen failed along a well-defined single shear band. It was then stabilised by slowly percolating a low-viscosity epoxy. Cylindrical subsamples 8 mm in diameter and 50 mm high were cored from the stabilised specimen at regions of interest for further investigation. A synchrotron X-ray microtomography technique was used to acquire high-resolution three-dimensional images of the cores. The scans have a voxel size of 15·06 m, which is high enough to visualise individual sand particles. Post-scanning analyses included mapping the spatial distribution of void ratio and particle contacts inside and outside the shear band. A good correlation was found between void ratio and particle contacts. Statistical distributions of particle orientation within and outside the shear band were calculated: they show that most particles inside the shear band orient in the - plane towards the -axis, which is perpendicular to the shear band with angle in the range 70-90°. Particles outside the shear band have no preferred orientation in the - plane, and form chains with preferred angles of 30-60° from the direction of the applied major principal stress. [ABSTRACT FROM AUTHOR]

A detailed study of spatial variation of void ratio and shear band thickness measurements is presented in this paper. A prismatic sand specimen that initially measured 57·4 mm wide × 120·5 mm long × 182·1 mm high was sheared under plane-strain (biaxial) loading conditions. The specimen was prepared at a relative density of 79% using F-75 Ottawa sand. X-ray computed tomography was used to scan the specimen before shearing and after the onset of the shear band. The specimen failed through a single shear band with an inclination angle of 65·6° measured from the direction of the minor principal stress. Computer algorithms were developed to calibrate CT images and quantify void ratio (e) variation within the specimen. CT data analysis revealed cross-sectional spatial variation in void ratio where density is higher in regions close to the edges of the specimen due to membrane effects and confining pressure with no significant variation in void ratio in the axial direction (top to bottom) before shearing. The shear band was easily identified from the CT images, and analysis of void ratio showed a noticeable jump in void ratio profile across the shear band. A detailed statistical summary of the thickness of the shear band and variation of void ratio across the shear band is presented and discussed. [ABSTRACT FROM AUTHOR]

Laboratory tests on samples of sand typically show instabilities in response related to shear band nucleation and evolution. Influences of constitutive characteristics (for instance, plastically driven elastic anisotropy), sensitivity to changes in constitutive parameters, and effects of testing conditions (for instance, related to stress conditions or to membrane penetration or stretching) on different types of material instability occurring in the post-localisation behaviour of sand specimens are investigated. Comparison between model prediction and available experiments shows that certain constitutive and testing features, which do not much influence the global response of a sample, may nevertheless strongly affect the critical and post-critical behaviours. The results obtained show that the proposed approach is general enough to permit the investigation of effects of experimental features that have not previously been studied. [ABSTRACT FROM AUTHOR]

The results of an analysis concerning a landslide of great dimensions, which occurred at Senise (Southern Italy) on 26 July 1986, are presented. The landslide was of a translational type, and the failure developed within a thin clayey silt layer interbedded by a slightly cemented sand formation. Both these soils experienced a pronounced strain-softening behaviour during laboratory tests. From the analyses performed using the traditional limit equilibrium approach, it has been found that the average operational resistance along the sliding surface was less than that of the peak and greater than the residual resistance deduced from the laboratory tests. As a consequence, it has been postulated that a progressive failure had occurred. Such a failure process can be appropriately studied using methods that are capable of simulating the formation and development of shear zones, where strain is localised. From a computational point of view this presents many difficulties, because the numerical procedures currently used are very often affected by a lack of convergence, and the solution may depend strongly on the mesh adopted. In order to overcome these numerical drawbacks, in the present study use is made of both an elasto-viscoplastic constitutive model and a non- local elasto-viscoplastic model implemented in a finite element code. The Mohr-Coulomb plastic law is adopted, and the strain-softening behaviour of the soil is simulated by reducing the strength parameters with the accumulated deviatoric plastic strains. The results of the analysis show that a progressive failure occurred owing to deep excavations carried out at the toe of the slope. In addition, the soil mass involved in the sliding and the location of the failure surface deduced from the numerical simulation are found to be very similar to those observed. [ABSTRACT FROM AUTHOR]

The article presents insights into 1D localization theory and micromechanical constitutive laws. Continuum constitutive models enjoy widespread use in geotechnical engineering, particularly in applications that are beyond the scope of current discrete element simulations. The micromechanical model, along with the previously mentioned shear band analysis, has been shown to predict many features of shear banding, including: the inclination angle and onset, thickness evolution, compaction and dilation, and their dependence on the confinement pressure and void ratio.