Numerical evaluation of three non-coaxial kinematic models using the distinct element method for elliptical granular materials

This is the accepted version of the following article: [Jiang, M. J., Liu, J. D., and Arroyo, M. (2016) Numerical evaluation of three non-coaxial kinematic models using the distinct element method for elliptical granular materials. Int. J. Numer. Anal. Meth. Geomech., 40: 2468–2488. doi: 10.1002/nag.2540.], which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/nag.2540/fullThis paper presents a numerical evaluation of three non-coaxial kinematic models by performing Distinct Element Method (DEM) simple shear tests on specimens composed of elliptical particles with different aspect ratios of 1.4 and 1.7. The models evaluated are the double-shearing model, the double-sliding free-rotating model and the double slip and rotation rate model (DSR2 model). Two modes of monotonic and cyclic simple shear tests were simulated to evaluate the role played by the inherent anisotropy of the specimens. The main findings are supported by all the DEM simple shear tests, irrespective of particle shape, specimen density or shear mode. The evaluation demonstrates that the assumption in the double-shearing model is inconsistent with the DEM results and that the energy dissipation requirements in the double-sliding free-rotating model appear to be too restrictive to describe the kinematic flow of elliptical particle systems. In contrast, the predictions made by the DSR2 model agree reasonably well with the DEM data, which demonstrates that the DSR2 model can effectively predict the non-coaxial kinematic behavior of elliptical particle systems.Peer ReviewedPostprint (author's final draft

Use of tire derived aggregate in tunnel cut-and-cover

A case-history is reported in which tire derived aggregate (TDA) was successfully applied to reduce the weight of fill upon a cut-and-cover railway tunnel. Subsequent 3D numerical analyses are used to explore the effect of different assumptions about the constitutive model of the TDA material. Alternative dispositions of TDA around the tunnel section are also examined. Reductions of up to 60% in lining bending moment may be achieved. For the case analyzed the elastic description of the TDA has little influence on tunnel lining loads, although is important for fill settlement estimates.Peer ReviewedPostprint (author's final draft

La boîte à outils géotechnique de demain: conception des structures géotechniques selon EN 1997: 202x

This paper shows how three new concepts – ‘Design Cases’ (introduced in prEN 1990), the ‘Geotechnical Design Model’ (prEN 1997-1), and the ‘Ground Model’ (prEN 1997-2) – are combined (in prEN 1997-3) to provide a comprehensive and flexible set of tools for the design of specific geotechnical structures. The paper presents flow charts divided between: a) reliability management, b) ground modelling, c) verification of the design, and d) structure execution, which provide guidelines for navigating prEN 1990 and prEN 1997.Postprint (published version

DEM simulation of soil-tool interaction under extraterrestrial environmental effects

In contrast to terrestrial environment, the harsh lunar environment conditions include lower gravity acceleration, ultra-high vacuum and high (low) temperature in the daytime (night-time). This paper focuses on the effects of those mentioned features on soil cutting tests, a simplified excavation test, to reduce the risk of lunar excavation missions. Soil behavior and blade performance were analyzed under different environmental conditions. The results show that: (1) the cutting resistance and the energy consumption increase linearly with the gravity. The bending moment has a bigger increasing rate in low gravity fields due to a decreasing moment arm; (2) the cutting resistance, energy consumption and bending moment increase significantly because of the raised soil strength on the lunar environment, especially in low gravity fields. Under the lunar environment, the proportions of cutting resistance, bending moment and energy consumption due to the effect of the van der Waals forces are significant. Thus, they should be taken into consideration when planning excavations on the Moon. Therefore, considering that the maximum frictional force between the excavator and the lunar surface is proportional to the gravity acceleration, the same excavator that works efficiently on the Earth may not be able to work properly on the Moon.Peer ReviewedPostprint (author's final draft

Macroelement modeling of SSI effects on offshore wind turbines subject to large number of loading cycles

In this paper, the hypoplastic macroelement formulation proposed by [1] has been modified in order to extend its range of applicability to offshore structures subject to cyclic loads with very high number of cycles, with particular attention to fatigue phenomena and cyclic displacement accumulation. A series of FE analysis has been performed to model the soil–foundation interaction processes of a prototype of offshore wind turbine, for which the geometrical characteristic of the superstructure and foundation, the soil conditions and the predicted environmental (wave and wind) loads were known. The study, carried out in parametric form, has allowed to better understand the role played by the modified cyclic part of the macroelement model in reproducing the shake–down effects as observed in small–scale model tests.Peer ReviewedPostprint (published version

A probabilistic Bayesian methodology for the strain-rate correction of dynamic CPTu data

Dynamic Cone Penetration Tests (CPTu) profile offshore sediments by impact penetration. To exploit their results in full the measured data is converted to obtain a quasi-static equivalent profile. Dynamic CPTu conversion requires calibrated correction models. Calibration is currently done by using paired (i.e., very close) quasi-static and dynamic tests. It is shown here that paired test data, which may be inconvenient to acquire offshore, are not strictly necessary to convert dynamic CPTu data. A new probabilistic methodology is proposed to call upon quasi-static results from a much wider area in the conversion procedure. Those results feed the prior distribution of a converted profile, within a Bayesian updating scheme where strain rate coefficient and correction model error are also described by updated stochastic variables. The updating scheme is solved numerically using the Transitional Markov Chain MonteCarlo sampling algorithm. To avoid undue influence of local profile heterogeneity, the statistic treatment of the quasi-static CPTu data takes place in the frequency domain, using a discrete cosine transform (DCT). The new procedure is applied to a CPTu campaign offshore Nice (France): dynamic tests are converted with equal precision using quasi-static data acquired at distances orders of magnitude larger than what was previously employed.Peer ReviewedPostprint (author's final draft

Undrained strength from CPTu in brittle soils: a numerical perspective

Static liquefaction of soils that have a brittle undrained response (hydraulic fills, mine tailings or sensitive clays) may lead to sudden failures of large consequence. Given the importance of undrained failure, obtaining precise estimates of peak and residual yield strength is important. The CPTu plays a major role in the geotechnical characterization of these geomaterials and so do CPTu-based estimates of undrained strength. Most of the methods available for CPTu-based estimation of undrained strength are empirical, based on correlation with other laboratory or field tests. When such correlations are established difficulties appear due to variable disturbance affecting the reference laboratory samples and parasitic effects, such as unaccounted for partial drainage during penetration or unknown side friction, affecting the cone results. Such difficulties are not present when using numerical simulation. The paper builds upon a series of CPTu simulations using a model able to represent brittle undrained failure. Confounding factors such as partial drainage and cone side friction are systematically varied to examine their effect on the results. The results are then employed to examine the performance of several empirical methods frequently employed to obtain peak and residual strength from CPTu.The authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa Programme for Centres of Excellence in R&D” (CEX2018-000797-S).Postprint (published version

Influence of the particle size distribution on the Critical State Properties of Mine Tailings

Arguably, critical state soil mechanics (CSSM) is now the preeminent methodology for understanding static liquefaction of mine tailings, having been used in the mining industry by the expert panels retained to investigate recent TSF failures. One of the key ingredients of the CSSM framework is the assessment of a critical state line, which separates contractive from dila-tive states. A critical state line is often defined by a linear relationship and two parameters, namely the altitude of the critical state line at 1 kPa (¿) and its slope (¿). In this study, we use the TAILENG mine tailings database to investigate potential relationships between the particle fea-tures and the particle size distribution and the critical state properties. Towards this end, the crit-ical state line is evaluated for a range of mine tailings with broad gradations and compressibility, defining ¿and ¿, with known particle size distributions. This information is subsequently used to investigate potential correlations. Insights from the observations are shared, and potential funda-mental mechanisms in explaining correlations between the critical state properties and particle features are discussed.Postprint (published version

Sphericity measures of sand grains

The sphericity of a grain should measure the similitude of its shape with that of a sphere. Sphericity is a shape descriptor of long-standing interest for sedimentology. Now it has gained also interest to facilitate discrete element modelling of granular materials. True sphericity was initially defined by a surface ratio that requires three-dimensional (3D) grain surface measurement. That kind of measurement has been practically impossible until recently and, as a consequence, a number of alternative 3D measures and 2D proxies were proposed. In this work we present results from a study of grain shape based on x-ray tomography of two different sand specimens, containing more than 110.000 particles altogether. Sphericity measures were systematically obtained for all grains. 2D proxy measures were also obtained in samples of oriented and not-oriented grains. It is shown that the 2D proxy best correlated with true sphericity is perimeter sphericity, whereas the traditional Krumbein-Sloss chart proxy is poorly correlated. 2D measures acquired through minor axis projection are more closely related to 3D measures than those acquired using random projections.Peer ReviewedPostprint (author's final draft

Microscopic calibration of rolling friction to mimic particle shape effects in DEM

It is widely recognised that particle shape influences the mechanical response of granular materials [1-2]. Rolling resistance elasto-plastic contact models are frequently used to approximate particle shape effects in simulations using the Discrete Element Method (DEM) [3-4]. Such contact models require calibration of several micro-parameters, most importantly a rolling resistance coefficient. In this work, the rolling resistance has been calibrated to reproduce the triaxial tests – in terms of mechanical and kinematic responses – of two different sands: Hostun and Caicos sands. The value of rolling resistance is directly linked to true sphericity, a basic measure of grain shape, as originally proposed in Rorato et al. (2018) [5]. When shape measurements are performed [6], this link enables independent evaluation of the rolling resistance coefficient for each particle. It does also allow the characteristic shape variability of natural soils to be easily taken into account