A novel technique for the objective assessment of particle shape is presented. The technique uses Fourier shape descriptors and image analysis of scanning electron microscope photographs of sand grains to provide an accurate quantification of particle morphology and texture. Three lower order Fourier descriptors, denoted ‘signature descriptors’, provide measures of elongation, triangularity and squareness, while an additional descriptor, denoted ‘asymmetry’, provides a measure of particle irregularity. Together, these four descriptors quantify the overall shape of soil particles (defined as ‘morphology’). A summary of higher-order descriptors provides textural information that is related to local roughness features (defined as ‘texture’). The results of studies on three silica sands (two standard, laboratory-use and one natural, unprocessed) and one carbonate sand are presented. Breakage of particles by crushing is shown to affect the morphological signature differently depending on the type of sand, though it does not significantly alter texture. The study highlights the value of microscopy, combined with image analysis, in revealing sand grain shape and texture, and shows that simple statistical tools may be used to translate the information provided by relatively few grains to that of a larger body of soil.

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The presence of a pore fluid is recognized to significantly increase the mobility of saturated over dry granular flows. However, the mechanisms through which pore fluid increases mobility may not be captured in experimental flows of small volume typical of laboratory conditions. Here we present the results of dry and initially fluid saturated or “wet” experimental flows of near‐monodisperse coarse‐grained ceramic particles in a large laboratory flume for five source volumes of 0.2–1.0 m3. Measurements include flow height, velocity profile, pore pressure, and evolving solid volume fraction, as well as the final deposit shape. The dry experiments constrain the frictional properties of the common granular material and comparison with wet flows permits an independent evaluation of the interstitial fluid effects. These results demonstrate that flow dilation and strong variation in the velocity profile are directly linked to a greatly increased mobility for wet granular flows compared to dry, and a significant influence of scale as controlled by source volume on flow behavior. Excess pore pressure need not be present for these effects to occur.

On 4 September 2010, a magnitude Mw 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicentre of the earthquake was located in the Darfield area about 40 km west of the city of Christchurch. Extensive damage was inflicted to lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers and wetlands throughout Christchurch and Kaiapoi. Unreinforced masonry buildings also suffered extensive damage throughout the region. Despite the severe damage to infrastructure and residential houses, fortunately, no deaths occurred and only two injuries were reported in this earthquake. From an engineering viewpoint, one may argue that the most significant aspects of the 2010 Darfield Earthquake were geotechnical in nature, with liquefaction and lateral spreading being the principal culprits for the inflicted damage. Following the earthquake, an intensive geotechnical reconnaissance was conducted to capture evidence and perishable data from this event. The surveys were performed on foot, by car and from a helicopter over a period of six days. A broad-brush field reconnaissance was conducted in the first two days, followed by pin-point investigations at specific locations including detailed site inspections and field testing using: Dynamic Cone Penetration Test (DCPT), Swedish Weight Sounding (SWS), and Spectral Analysis of Surface Waves (SASW). This paper summarizes the observations and preliminary findings from this early reconnaissance work.

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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...

Physical modelling of debris flows has been carried out in the geotechnical drum centrifuge at ETH Zürich. A new apparatus to model debris flows in the centrifuge is described. The apparatus permits the final reach of a typical debris flow to be modelled within the centrifuge, with unconsolidated material flowing down a slope to deposit as a fan around the drum. Experiments are described for both fixed base conditions and erodible bases. Tests to examine the verification (modelling) of models show that debris flow behaviour is governed mainly by friction and consolidation processes, although some resolution is required between flow behaviour downslope and flow arrest during runout. The results are compared with bulk parameters determined for field-scale debris flows. It is found that some important flow mechanisms, such as contact-dominated behaviour and high pore pressures, are developed that are closer to those developed at field-scale than tests conducted at 1g. Velocity profiles for erodible beds are compared with a semi-empirical expression derived for experimental debris flows at 1g. Normalized velocity profiles are found to be in agreement; however, absolute velocities differ from those predicted. Scaling, the limitations of the apparatus, and potential for future work are discussed.

A rigid walled 'transparent soil' permeameter has been developed to visually study the mechanisms occurring during seepage induced internal erosion in susceptible granular media under upward flow. The experiments use borosilicate glass particles in place of soil, and an optically matched oil mixed with fluorescent dye in place of water. The technique known as Plane Laser Induced Fluorescence (PLIF) enables a two-dimensional plane of particles and fluid to be viewed inside the permeameter, away from the walls. Results of tests have provided close agreement with those of other researchers on soil of comparable particle size grading. Unstable materials showed migration of fine grains under mean hydraulic gradients as low as i = 0.25, while stable materials eventually failed by heave at hydraulic gradients close to unity. Internally unstable soils where the loads were predominantly supported by the coarser fraction exhibited suffusion (fines migration without disruption of the load bearing system); those supported by both coarse and fine particles exhibited suffosion (i.e. volume change during fines migration). Quantitative image analysis conducted on one unstable sample showed areas of open void space migrating though the sample at low hydraulic gradients near critical, as defined by Skempton and Brogan (1994). This occurred before the externally measured local hydraulic gradients began to significantly diverge from the mean. The testing technique developed shows that optically matched glass and oil behave mechanically similarly to soil and water, and that the PLIF technique coupled with image analysis can provide additional insight to the mechanisms of internal erosion. Main Text Click here to download Main Text Hunter & Bowman post review-cleaner.docx 'f effective stress of finer particles specific gravity unit weight of permeant ' average effective stress 'z average effective stress across a section at depth, z w unit weight of water