Physical modelling of debris flow in a small-scale flume has been carried out to investigate the internal stress-transfer mechanisms within unsteady, saturated, and segregating granular free-surface flows. Measurements of the internal velocity fields within model flows were obtained via planar laser-induced fluorescence and particle image velocimetry. Normalized velocity profiles taken at a section over the flow duration were found to essentially collapse onto a single curve, the shape of which was dependent on the particle-size distribution. While all flows exhibited internal basal slip and shear, for tests on well-graded materials that are most representative of debris flows, the shear rate was found to reduce towards the surface to near-zero, exhibiting near plug-flow. Dimensional analysis shows that particles of different size within these flows experienced different dominant stress-transfer mechanisms -frictional, collisional or viscous. Rapid grain-size segregation therefore is both due to and results in different modes of stress transfer within a single flow. This means that in a segregating and hence, stratified system, different flow regimes will act concurrently at microscale and mesoscale. Results highlight the complexity of debris flows, so that it may be undesirable to ascribe a single microscale constitutive behaviour throughout, and further calls into question the concept of flow regimes for debris flows based on bulk measurements.Key words: debris flow, dimensionless number, flow regime, plane laser-induced fluorescence, flume model tests.Résumé : La modélisation physique de l'écoulement des débris dans un canal à petite échelle a été réalisée pour étudier les mécanismes de transfert de contraintes internes au sein des flux à surface libre granulaires instables, saturés et séparés. Les mesures des champs de vitesse internes au sein des flux modèles ont été obtenues par vélocimétrie à fluorescence induite laser plane a par Image de particule. Les profils de vitesse normalisés pris à une section sur la durée d'écoulement ont été trouvés à se replier essentiellement sur une seule courbe, dont la forme était dépendante à la distribution de la taille des particules. Alors que tous les flux ont exposé un glissement basal interne et de cisaillement, pour les essais sur des matériaux bien classés qui sont les plus représentatifs des flux de débris, le taux de cisaillement a été trouvé à se réduire vers la surface près de zéro, présentant presque un écoulement piston. L'analyse dimensionnelle montre que des particules de taille différente au sein de ces flux ont connu différents mécanismes de transfert de contrainte dominante -de frottement, collisionnel, ou visqueux. La ségrégation rapide à taille de grain est donc à la fois en raison de résultats et dans différents modes de transfert de contrainte dans un seul flux. Cela signifie que dans un système de ségrégation et donc, stratifié, les différents régimes d'écoulement agissent simultané-ment à l'échelle micro et méso. Les résultats mettent en...

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The motion of debris flows, gravity-driven fast moving mixtures of rock, soil and water can be interpreted using the theories developed to describe the shearing motion of highly concentrated granular fluid flows. Frictional, collisional and viscous stress transfer between particles and fluid characterizes the mechanics of debris flows. To quantify the influence of collisional stress transfer, kinetic models have been proposed. Collisions among particles result in random fluctuations in their velocity that can be represented by their granular temperature, T.

It is becoming increasingly necessary to construct on land that was previously considered inappropriate for construction, such as soft clay. The properties of soft clay make it highly compressible and low in shear strength, meaning that bearing capacity failure and excessive settlement are of concern.Piled embankments are a ground improvement technique that can provide a solution for this problem. Piled embankments have the ability to transfer the greater part of the embankment load and any surcharge to more competent material at greater depth due to the 'arching' concept. Consequently, the soft foundation soil has little direct impact on the performance of the embankment. The concept of 'arching' of granular soil over an area where there is partial loss of support from underlying strata has long been recognised in the study of soil mechanics (e. g. Terzaghi, 1943). However, a number of competing theories exist to quantify this behaviour in piled embankments.In addition, the use of geotextile reinforcement in piled embankments placed above the pile caps in principle provides a number of technical as well as economical benefits. As the embankment fill is placed, tension is created in the reinforcement and it is the vertical component of this tension that transfers the embankment load onto the piles and reduces the load carried by the soft clay hence transferring the load of the embankment on to the piles.Differential settlement can be a problem for piled embankments of low height.Significant differential settlement can cause undesirable effects on any structures constructed on the embankment. 'Arching' limits the amount of differential settlement in embankments and the use of geotextile geogrid can also potentially have additional benefits.This thesis presents a series of centrifuge tests examining the performance of unreinforced and reinforced piled embankments constructed over soft subsoil in terms of stress acting on the subsoil, and differential movement at the surface of the embankment. A large range of embankment heights are considered, and the results for stress on the subsoil are compared with existing predictive methods, allowing generic conclusions to be drawn regarding the predictions of various methods.The effect of a 'working platform' below pile cap level and thus directly loading the subsoil is also considered, and used to examine the concept of a 'Ground Reaction Curve' (Iglesia et al, 1999) for arching in the embankment. In principle this can be used to consider compatibility of displacements at the base of the embankment, and thus improve design simultaneously considering the effect of arching in the embankment and underlying support from the subsoil and layers of reinforcement acting in tension.

Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Threedimensional pore topology is extracted from a series of 2D images and imported into computational fluid dynamics (CFD) simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV-and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterize the variance of flow 1 velocity with the three-dimensional pore topology. Finally, the fluid-particle interactions obtained from CFD results show a larger variance in the angular particle packings.