Solid behavior of anisotropic rigid frictionless bead assemblies

We investigate the structure and mechanical behavior of assemblies of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. Three different loading paths are explored: triaxial compression, triaxial extension and simple shear. Generalizing recent results [1], we show that the material, despite rather strong finite sample size effects, is able to sustain a finite deviator stress in the macroscopic limit, along all three paths, without dilatancy. The shape of the yield surface is adequately described by a Lade-Duncan (rather than Mohr-Coulomb) criterion. While scalar state variables keep the same values as in isotropic systems, fabric and force anisotropies are each characterized by one parameter and are in one-to-one correspondence with principal stress ratio along all three loading paths.The anisotropy of the pair correlation function extends to a distance between bead surfaces on the order of 10% of the diameter. The tensor of elastic moduli is shown to possess a nearly singular, uniaxial structure related to stress anisotropy. Possible stress-strain relations in monotonic loading paths are also discussed

Criteria for rapid sliding II.: thermo-hydro-mechanical and scale effects in Vaiont case

Thermally induced excess pore pressures have been included into a two-wedge evolutive model of Vaiont landslide. The problem requires the solution of a system of four coupled balance equations for the shear bands and the surrounding rock as well as the joint equation of motion of the entire slide. The model predicts the high velocities observed and is consistent with other data (slide geometry, residual strength, and conditions on the sliding surface). The interpretation of a sensitivity analysis suggests that there exists a threshold permeability band, in the range 10- 8 to 10- 10 m/s, which separates potentially fast motions from slow motions. This conclusion is maintained if the scale of the landslide is reduced.Peer ReviewedPostprint (author's final draft

Seasonally Frozen Soil Effects on the Seismic Performance of Highway Bridges

INE/AUTC 12.0

Numerical modeling of compacted fills under landing mats subjected to aircraft loads

Rutting failures are prominent in expedient airfields constructed with AM2 landing mats over soft existing subgrades. There are many issues that must be addressed when approaching this multiaceted problem. The load transfer mechanism occurring at interlocking mat joints and the mat-soil interface bonding condition affect near surface subgrade response. The repeated loading coupled with lateral aircraft wander causes significant principal stress rotation in the subgrade. This kneading action then causes variations in the excess pore-water pressure and a subsequent softening of the soil. The purpose of this study is to investigate the critical factors that lead to subgrade rutting failures in landing mats constructed over soft subgrades. A three dimensional finite element (3D FE) model of a landing mat system over soft subgrade is implemented under both static and pseudo-dynamic loading conditions with aircraft wander. To capture the complex stress histories induced by the simulated moving gear loads over the unique structural features of the AM2 mat system, an elastoplastic kinematic hardening constitutive model, the Multi-Mechanical Model, is developed, calibrated and used to represent the subgrade response. Under both static and pseudo-dynamic loading, the FE model results match very well with the stress and deformation results from full-scale instrumented testing of the AM2 mat over 6 CBR subgrade. Results show that incorporating the load transfer mechanism occurring at the mat joints and varying the mat-soil interface condition affect the near surface subgrade deformation and stress responses that contribute to rutting failures. Furthermore, rotation of the principal stress axes and changes in excess pore-water pressures occur in the subgrade because of the moving tire load. These phenomena contribute to extension of the field of deformation influence around the trafficked area in the subgrade and upheaval at the edges of the test section. Findings of this study show that although layered elastic analysis procedures are the basis of current airfield design methodologies, critical design features and the corresponding deformation responses can be better modeled using the FE approach. Furthermore, the proposed 3D modeling approach implementing aircraft wander can provide a reliable platform for accurately simulating the subgrade response under pseudo-dynamic loading conditions

Experimental Investigation of Plastic Deformations Before Granular Avalanche

We present an experimental study of the deformation inside a granular material that is progressively tilted. We investigate the deformation before the avalanche with a spatially resolved Diffusive Wave Spectroscopy setup. At the beginning of the inclination process, we first observe localized and isolated events in the bulk, with a density which decreases with the depth. As the angle of inclination increases, series of micro-failures occur periodically in the bulk, and finally a granular avalanche takes place. The micro-failures are observed only when the tilt angles are larger than a threshold angle much smaller than the granular avalanche angle. We have characterized the density of reorganizations and the localization of micro-failures. We have also explored the effect of the nature of the grains, the relative humidity conditions and the packing fraction of the sample. We discuss those observations in the framework of the plasticity of granular matter. Micro-failures may then be viewed as the result of the accumulation of numerous plastic events

A numerical study of the suitability of rigid inclusion ground reinforcement beneath caisson quay walls

The objective of this study was to determine whether rigid inclusions are suitable for reinforcement of the foundation of a caisson quay wall functioning as a container terminal. Apart from their brittle behaviour under lateral loading, rigid inclusions are well suited to the large uniform loads and stringent post-construction deflection tolerances associated with container terminal structures. Their inherent strength and stiffness means they have certain advantages over other stiffening columns commonly used for ground reinforcement in port expansion projects. Their mechanical properties allow construction to unrestricted heights at any construction rate and, in theory, RIs can be applied to all soil types. Additionally the locations of many ports coincide with rivers, deltas and estuaries which are associated with poor soil conditions often requiring ground improvement. Their suitability is of practical significance to port planners and engineers who are faced with the challenge of providing satisfactory foundation performance that is cost effective. The addition of RI ground reinforcement for this structural application would allow for greater flexibility in meeting these challenges. The literature review for this study was broad in its scope with emphasis placed on describing the mechanics of the problem, analysis methods and suitable installation methods for execution in the marine environment. One of the key outcomes of the literature review was identifying the problem of lateral loading due to "free-field" lateral ground movements. In light of this, suitable strategies for limiting and accommodating lateral loading of the RIs were proposed. A numerical study of the proposed ground improvement scheme was undertaken using the 3D finite element method. The key model outputs were caisson deflections and RI forces, moments and stresses, for the various simulated construction phases up to operational conditions. The model results were assessed in terms of the key foundation performance criteria which were related to STS crane rail tolerances and limiting tensile stresses in the RIs. This study found that for a firm clay subsoil condition the proposed RI ground reinforcement scheme met the foundation performance criteria for this structural application provided (i) strategies to limit lateral loading were implemented and (ii) the RIs were reinforced over the length where they were not fully compressed. While this study provided insights into the behaviour of RIs for this structural application, ultimately suitability is a function of range of factors, in addition to the limited technical performance criteria derived for this study

Investigation of Lateral Stress Relief on theStability of PHI = 0 DEG Slopes Using Laboratory, Fracture Mechanics, and Finite Element Method Approaches

Total stress analyses of purely cohesive cut slopes utilize the undrained shear strength for slope stability analyses. These slopes can have an in-situ lateral earth pressure that is greater than the vertical pressure. Excavations into these materials results in expansion of the slope face due to release of confining pressure. When strains exceed that which can be internally absorbed through elastic deformation, failure planes or cracks may develop at the toe of the slope. However, conventional limit equilibrium methods of slope stability analysis do not account for the in-situ stress conditions or the development of shear zones or cracks that occur from lateral stress relief. Progressive failure of the slope may occur if internal lateral stresses are large enough to cause stress concentrations in front of the advancing toe cracks. Finite element methods using substitution methods reveal two distinct shear cracks at the toe of slope consisting of a horizontal and an inclined failure plane while a tension zone develops in the backslope region. The formation and extension of the shear cracks are strongly dependent on ko and they can extend to approximately 1/4 of the slope height due to initial lateral stress relief. Classical limit equilibrium solutions regarding the critical slope height have been revised to account for lateral stress relief. Analyses indicate good agreement with published case histories and they reveal how the shear zones propagate to create progressive slope failure in stiff clay slopes under total stress analyses

Smooth particle hydrodynamics study of surface defect machining for diamond turning of silicon

Acknowledgments The authors would like to thank EPSRC (EP/K018345/1) and Royal Society-NSFC International Exchange Scheme for providing financial support to this research.Peer reviewedPublisher PD