Pre-failure instability of sand under dilatancy rate controlled conditions

Experimental results are presented in this paper to show that a runaway type of pre-failure instability can occur for sand under dilatancy rate controlled conditions when an appropriate strain increment ratio, dεv/dε1, is imposed. This type of instability is similar to the runaway type of instability observed for very loose sand under undrained conditions. Whether a soil element will undergo pre-failure instability depends on the difference between the strain increment ratio of the soil obtained from drained test, under a specified effective confining pressure, (dεv/dε1)s, and the strain increment ratio imposed during the test, (dεv/dε1)i, rather than the absolute magnitude of (dεv/dε1)i. Based on the experimental data obtained in this study it was found that an instability line can be determined from a series of strain path tests conducted at different effective confining pressures but with the same dεv/dε1 by joining the peak points of the effective stress paths to the origin in the q–p′ stress space. This line is similar to the instability line obtained from undrained tests on loose sand. The instability tests under dilatancy rate controlled conditions indicate that the stress ratio at the onset of instability obtained in the instability tests coincide with the peak stress ratio line. This suggests that the peak stress line can be used to predict the onset of instability under dilatancy rate controlled conditions in the same way as the use of instability line to predict the onset of instability under undrained conditions

A study of principle stress rotation on granular soils using DEM simulation of hollow cylinder test

This study presents a numerical modelling of HCT (Hollow Cylinder Test) using DEM (Discrete Element Method) by developing of TRUBAL code on granular soils. Due to high expenditures of HCT, a verified numerical modelling of this test was developed. In addition, this numerical model can be used to address the gap in understanding the relationship between the particle-scale interactions and the overall material response. In the introduced numerical model (HCTBALL), plane and cylindrical walls were defined to apply the boundary forces and stresses. Also, an efficient method was presented to apply the torque. The displacements of inner and outer walls were interdependent while applying the torsion to control the intermediate principal stress parameter (b). To verify the model, the results of experimental hollow cylinder tests on Firoozkooh sand under monotonic loading and drained condition were employed. Therefore, the simulations were compared qualitatively with experiments to verify the presented model. It is observed that the deviatoric stress at failure, increased with a decreasing rate by increasing the confining pressure. By increasing the rotation angle of principal stress direction, the difference between deviatoric stresses in specific confining pressures was decreased

A laboratory study of anisotropic geomaterials incorporating recent micromechanical understanding

This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stress–strain–strength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stress–force–fabric relationship. It was observed that intermediate principal stress parameter b(b = (σ2 − σ3)/(σ1 − σ3)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane

Static, cyclic and post liquefaction simple shear response of sands

An experimental study of static, cyclic and post cyclic undrained simple shear response of reconstituted water pluviated Fraser River sand is presented and compared to its triaxial behaviour from an earlier study. Static and cyclic behaviour was assessed over a range of void ratios that included the loosest deposition state using different confining stress levels. The effect of reconstitution technique on undrained behaviour was also investigated. It is shown that the method of specimen reconstitution has a profound influence on its undrained response. If laboratory results are to be meaningful in field application, the specimen reconstitution technique must duplicate the deposition process of the deposit to be modelled. The static undrained response in simple shear is contractive only for the loosest of the accessible void ratios, regardless of the level of confining stress. For a given initial void ratio and confining stress level, simple shear response is much less contractive than triaxial extension response. The criteria for contractive deformation during cyclic loading based on triaxial studies are shown to be also valid under cyclic simple shear. The influence of confining stress level on liquefaction resistance is shown to increase with relative density in a manner similar to that found under triaxial conditions. The cyclic resistance at the loosest state is essentially independent of the confining stress level. At denser states, however, the reduction factor K in simple shear is not as high as under the triaxial conditions. The reduction factor C used to adjust cyclic triaxial liquefaction resistance to an equivalent field simple shear condition is shown to be larger than currently adopted in design, and its value is dependent on both density as well as confining stress level. The strain level over which sand deforms essentially at zero stiffness in post liquefaction loading is dependent on relative density and maximum strain experienced during cyclic loading. Post liquefaction response of cyclically liquefied sand is shown to be essentially similar to that of the sand liquefied by static load-unload cycle. The undrained simple shear and triaxial behaviour of the silty Syncrude sand is shown to be, in general, similar to that of Fraser River sand.Applied Science, Faculty ofCivil Engineering, Department ofGraduat

Fabric, initial state and stress path effects on liquefaction susceptibility of sands

An experimental study aimed at improving the understanding of the mechanics of liquefaction is presented. Two aspects of the undrained behaviour; (i) the influence of initial state, characterized by void ratio, principal stresses and their directions, and fabric under a given undrained stress path, and (ii) the effect of undrained stress path at a given initial state, were studied in a systematic manner. Triaxial, simple shear and hollow cylinder torsional shear devices were used in the experimentation to enforce a range of stress paths that simulate the loading insitu. The effect of membrane penetration was duly addressed in order to confidently measure the truly undrained behaviour of sands. The influence of soil fabric was determined using triaxial and simple shear tests on specimens reconstituted by different techniques. In addition to specimens reconstituted in the laboratory, undisturbed, in-situ frozen sands from four different sites were tested to assess the relevance of the behaviour of reconstituted specimens to the sand in-situ. It is shown that in-situ sands and those water pluviated in the laboratory are inherently anisotropic, and their undrained behaviour is direction dependent. At a given initial state, they often strain soften to an essential steady state in triaxial extension, but invariably strain harden in compression. Moist tamped sands, on the other hand, strain soften both in compression and extension. The domain of states in void ratio effective stress space accessible to alluvial in-situ sands is shown to be similar to those that ensue on pluviation. The behaviour of water pluviated sands is similar to that of alluvial in-situ sands, at a given initial stress state and undrained stress path, both under static and cyclic loading conditions - an indication that they both possess very similar fabric. This opens up the possibility of using water pluviated sand specimens to characterize the behaviour of in-situ sands, an attractive economical alternative. It is shown that the initial effective stress state plays a dominant role on the subsequent undrained behaviour at a given void ratio. Hollow cylinder torsional shear tests on water pluviated fabric were used to assess the influence of the initial axisyrnmetric and non-axisymmetric stress states on undrained shear. The behaviour due to an increase in deviator stress alone, and that due to simultaneous changes in deviator stress and the direction of principal stresses is investigated. A larger inclination of major principal stress to vertical results in a softer behaviour at all levels of static shear. An increase in static shear at a given inclination of the major principal stress promotes more strain softening response. Even a small undrained perturbation may trigger flow failure in a sand that is otherwise stable if drained, in the event the initial stress state is highly anisotropic together with larger inclination of major principal stress to the vertical. It is demonstrated that flow failure may be triggered by a mere rotation of principal stress directions only. The sense of principal stress rotation with respect to the initial inclination of the major principal stress also plays a prominent role on the undrained behaviour, and hence on strain development. It is shown that the strain induced anisotropy evolves at an accelerated rate due to rotation of major principal stress direction, compared to an increase in effective stress ratio alone at fixed direction of principal stresses. It is shown that both steady state and quasi steady state can be treated within the same framework. The friction angle mobilised at these states is independent of fabric, void ratio, effective confining and static shear stress levels, direction of principal stresses, and undrained stress path. This angle appears to be a unique material property. Contrary to the commonly held notion, the minimum undrained strength is dependent on the stress path, initial confining and static shear stress levels, fabric and the directions of principal stresses, in addition to void ratio. This implies that the steady/quasi-steady state is unique in the effective stress space, but not so in the void ratio-stress space. However, the minimum undrained strength normalised by the major principal stress appears to be dependent only on void ratio and the direction of major principal stress (i.e loading mode). The friction angle mobilised at the trigger of strain softening is also dependent on the initial stress state and the loading mode.Applied Science, Faculty ofCivil Engineering, Department ofGraduat

A comparison of different laboratory techniques to simulate stress and moisture history of hard rock mine tailings

In thickened tailings technology, tailings are deposited at solids concentrations sufficient to prevent segregation and allow for formation of gently sloped stacks. Post-deposition, thickened tailings are known to gain strength through a combination of hindered settling, desiccation, and consolidation. Recently, it is understood that the shear strength and geotechnical stability of the stack is dependent on the degree of desiccation or drying time for a given layer, as well as consolidation history during subsequent deposition. This paper presents some preliminary investigations into how best to reproduce this stress history for element testing. Three laboratory methods for simulating stress history of thickened tailings layers are introduced for preparing samples for testing in an NGI type simple shear apparatus. In the first method, tailings are reconstituted in the simple shear mold, desiccated to different degrees, and re-wetted before shearing. The second method consists of simulating the thickened tailings deposition in a column, followed by sample extraction using a shearing thin-wall sampler. In the third method, a flume is employed to simulate the movement of thickened tailings layers from the deposition point down the beach. The third method is intended to assess whether the movement of tailings down the beach constitutes an important part of stress history, and influences the evolution of the fabric. This paper recommends the most appropriate method for preparing samples to assess the geotechnical behaviour of thickened tailings. It is concluded that desiccation to the shrinkage limit could significantly increase the monotonic shear strength of hard rock mine tailings; however, with continuing desiccation beyond the shrinkage limit, additional monotonic strength gain is minimal. [All papers were considered for technical and language appropriateness by the organizing committee.]Non UBCUnreviewedOthe

Numerical performance assessment of buried corrugated metal culvert subject to service load conditions

Two- and three-dimensional continuum finite element methods were used to assess the mechanical response of buried metal culverts subjected to service load conditions. The analysis explored the influence of the culvert profile, cover depth, section aspect ratio, and service load magnitude on culvert deformation, thrust, and section moment response. The numerical predictions were calibrated using third-party experimental data. For shallow burial conditions, the culvert section profile was found to be an influential parameter on the predicted culvert response where the soil–culvert interaction mechanisms and load transfer processes are governed by nonlinear behaviour. Models that incorporate the corrugated culvert profile provided better estimates of the culvert thrust than the simplified computational models that are typically used in analysis. Results from this study demonstrate that existing simplified methods cannot capture the local variation (i.e., amplitude, waveform) of the membrane strain and section thrust particularly between the crest and trough of corrugation.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Generalized Fragility Relationships with Local Site Conditions for Probabilistic Performance-based Seismic Risk Assessment of Bridge Inventories

The current practice of detailed seismic risk assessment cannot be easily applied to all the bridges in a large transportation networks due to limited resources. This paper presents a new approach for seismic risk assessment of large bridge inventories in a city or national bridge network based on the framework of probabilistic performance based seismic risk assessment. To account for the influences of local site effects, a procedure to generate site-specific hazard curves that includes seismic hazard microzonation information has been developed for seismic risk assessment of bridge inventories. Simulated ground motions compatible with the site specific seismic hazard are used as input excitations in nonlinear time history analysis of representative bridges for calibration. A normalizing procedure to obtain generalized fragility relationships in terms of structural characteristic parameters of bridge span and size and longitudinal and transverse reinforcement ratios is presented. The seismic risk of bridges in a large inventory can then be easily evaluated using the normalized fragility relationships without the requirement of carrying out detailed nonlinear time history analysis

Buried corrugated steel culvert failure mechanisms due to environmental deteriorations

Environmental factors and ageing influence deterioration mechanisms in buried metal culverts that may impair mechanical performance over the service life. Corrosion occurs, primarily at the culvert haunches, due to changes in the water level and repeated exposure to air circulation. This action provides a pathway for water to flow into the backfill that may cause erosion and the development of soil voids adjacent to the culvert. In this study, the behavior of corrugated metal culverts, with a circular cross section subject to overburden and surface loads, is simulated using calibrated finite element modelling procedures. A three-dimensional model of a deteriorated culvert-soil system was developed for different cover depths. The relationship between the culvert wall section loss, due to corrosion, on the load carrying capacity is investigated. Erosion voids are created around the buried culverts with distinct sizes and shapes. For the parameters examined, the soil erosion had a greater influence on pipe/soil interaction mechanisms, which ultimately influenced culvert performance and serviceability, than the effect of corrosion. The erosion void size and location were influential factors on the culvert performance that could result in local bucking and a decrease in the service life