DEM simulations of thermally activated creep in soils

Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each intergranular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range 20.8 to 21; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared quantitatively with the test data of soils from the literature. The effects of activation energy are considered in relation to the different magnitudes of creep encountered in sands and clays.published_or_final_versio

Dem Simulations Of Soil Creep Due To Particle Crushing

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Bearing capacity and settlement of circular shallow foundations using a nonlinear constitutive relationship

The design of shallow foundations is dominated by issues of settlement rather than bearing capacity per se. The ability to predict the settlement of foundations at a given factor of safety is hence of key importance in design. In this paper, the energy method for a linear-elastic, perfectly plastic method utilizing the von Mises’ yield criterion with associated flow developed and reported by McMahon et al. in 2013 is extended to consider the nonlinear behaviour of soil. The energy method is used to investigate the load–settlement behaviour of shallow foundations by utilizing an ellipsoidal cavity-expansion mechanism and deformation fields within the boundaries of the classical Hill and Prandtl mechanisms. An elastic mechanism obtained from an analysis in ABAQUS was also investigated using this energy method. The upper-bound approach demonstrates that the cavity-expansion mechanism produces a better solution at small values of settlement, whereas at greater settlements the Prandtl mechanism is shown to produce a more optimal upper-bound solution. The first author would like to thank the financial support he received from the Cambridge Australia Trust (Poynton Scholarship) and the Principals of UK Universities (Overseas Research Students Awards Scheme) throughout his studies in Cambridge.This version is the author accepted manuscript. It can also be viewed on the NRC Research Press website at: http://www.nrcresearchpress.com/doi/pdf/10.1139/cgj-2013-027

Understanding ground deformation mechanisms for multi-propped excavation in soft clay

Deep excavation works are carried out to construct underground infrastructures such as deep basements, subways, and service tunnels. The execution of these deep excavation works requires the use of retaining walls and bracing systems. Inadequate support systems have always been of major concern, as excessive ground movement induced during excavation could cause damage to neighboring structures, resulting in delays, disputes, and cost overruns. To gain a better understanding of the mechanisms involved in soil excavations, centrifuge model tests of deep excavations in slightly over-consolidated soft clay have been carried out using a newly developed testing system, in which the construction sequence of a multi-propped wall for deep excavations can be simulated in flight. Deformation mechanisms are observed using Particle Image Velocimetry. Settlements of the ground surface and changes in pore water pressure are monitored during the excavation. The effects of prop stiffness, wall rigidity, and excavation geometry on the characteristics of ground deformation and soil-structure interaction are demonstrated and discussed. The use of the conservation of energy within the framework of the mobilizable strength design in calculating ground movements is validated and shown to perform satisfactorily.This is the author accepted manuscript. The final published version can be found on the publisher website at: http://www.sciencedirect.com/science/article/pii/S0038080614000286 Copyright © 2014 Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved

Forehead Skin Blood Flow in Normal Neonates during Active and Quiet Sleep, Measured with a Diode Laser Doppler Instrument

Changes in forehead skin blood flow during active and quiet sleep were determined in 16 healthy neonates using a recently developed semi-conductor laser Doppler flow meter without light conducting fibres. Measurements were carried out at a postnatal age varying from 5 hours to 7 days. The two sleep states could be distinguished in 17 recordings. The mean skin blood flow values during active sleep were significantly higher (p<0.01) than those during quiet sleep, the mean increase being 28.1%. The variability of the flow signal, expressed as the coefficient of variation, changed significantly from 23.1% during active sleep to 18.2% during quiet sleep

Run-out of cut-slope landslides: mesh-free simulations

This study uses an incompressible smoothed-particle hydrodynamics (ISPH) model to investigate the run-out and deposit morphology of granular materials flowing down cut slopes. The primary aim is to study the influence of various factors on the run-out and to summarise a quantitative relationship for direct use in landslide hazard management. In the model, the granular materials are modelled as a rigid perfectly plastic material with a Coulomb yield surface. The coupled continuity equation and momentum equation are solved by a semi-implicit algorithm. The model is first validated and its results are carefully compared with various controlled experiments regarding granular flows. The model reproduces the flows and correctly predicts the deposition profiles under various conditions. Then, the computational results are used to study the run-out and mobility of landslides. For granular columns collapsing onto a flat surface, a normalised run-out and a new scaling relationship are proposed, which are supported by numerous measured and numerical results. A similar relationship for the run-out of granular rectangles on steep slopes has also been explored. It is found that the normalised run-out is mainly determined by the slope angle and the normalised drop height. Furthermore, three types of idealised cut-slope landslides are simulated to study the influence of the initial landslide shape on the run-out. It is found that the normalised run-out of these idealised cut-slope landslides is smaller than that of granular rectangles on slopes of the same angles and drop heights. The difference between the run-outs is found to be mainly determined by the proportion of the whole mass that initially lies above a predictable discontinuity plane.The research is supported by the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement no. PIAG-GA-2012-324522 “MPM-DREDGE”

The effects of woodland habitat and biogeography on blue tit Cyanistes caeruleus territory occupancy and productivity along a 220 km transect

© 2018 The Authors The nesting phenology and productivity of hole-nesting woodland passerines, such as tit species (Paridae), has been the subject of many studies and played a central role in advancing our understanding of the causes and consequences of trophic mismatch. However, as most studies have been conducted in mature, oak-rich (Quercus sp.) woodlands, it is unknown whether insights from such studies generalise to other habitats used by woodland generalist species. Here we applied spatial mixed models to data collected over three years (2014–2016) from 238 nestboxes across 40 sites – that vary in woodland habitat and elevation – along a 220 km transect in Scotland. We evaluate the importance of habitat, biogeography and food availability as predictors of mesoscale among-site variation in blue tit Cyanistes caeruleus nestbox occupancy and two components of productivity (clutch size and fledging success). We found that habitat was not a significant predictor of occupancy or clutch size but that occupancy exhibited pronounced biogeographic trends, declining with increasing latitude and elevation. However, fledging success, defined as the proportion of a clutch that fledged, was positively correlated with site level availability of birch, oak and sycamore, and tree diversity. The lack of correspondence between the effects of habitat on fledging success versus occupancy and clutch size may indicate that blue tits do not accurately predict the future quality of their breeding sites when selecting territories and laying clutches. We found little evidence of spatial autocorrelation in occupancy or clutch size, whereas spatial autocorrelation in fledging success extends over multiple sites, albeit non-significantly. Taken together, our findings suggest that the relationship between breeding decisions and breeding outcomes varies among habitats, and we urge caution when extrapolating inferences from one habitat to others