Marcos Arroyo

Homogeneity and Symmetry in DEM Models of Cone Penetration

A three—dimensional numerical model was implemented to PFC3D Code to simulate cone penetration test in Ticino sand in a calibration chamber. The model is calibrated using laboratory test results. The full/half/quarter calibration chamber was used to examine the effect of symmetry on the results. Examination of specimen homogeneity was done by (i) visual observation of the network of the contact forces developing between particles, (ii) examination of the porosity, d50 and Cu distributions inside the specimen by using a representative elementary volume (REV). Some overall results from these simulations are also presented here and compared with the experimental results from a calibration chamber test database.

Sand production simulation coupling DEM with CFD

Sand production in oil wells is often predicted using continuum fluid-coupled models. However, a continuum approach cannot capture important features of the sanding problem, such as erosion and localised failure. This shortcoming of continuum-based analyses can be overcome using the particulate discrete element method (DEM). However, these models, apart from issues of computational cost, have the disadvantage of being difficult to calibrate. One way forward is to calibrate DEM models to capture the response observed in continuum models, where the material parameters can be selected with greater confidence. Adopting this philosophy here, a 3D numerical model based on DEM coupled with Computational Fluid Dynamics was built to simulate sand production around perforations. In the first instance, the basic DEM model (i.e. a dry case) is calibrated against a well-known poro-elastoplastic analytical solution by Risnes et al. (1982). Subsequently, a range of hydrostatic scenarios involving different levels of pore pressure and effective stress are considered. The numerical model shows an asymmetry of the eroded zone that is related to initial microscale inhomogeneity. The stress peak of the analytical solution at the elastic-plastic interface is smoothed because of that asymmetry. The presence of hydrostatic fluid decreases the plastic region and reduces the amount of sand produced. This is not due to changes in effective stress but rather by the particle-scale stabilizing effect of the fluid drag.

Evaluation of self-combustion risk in tire derived aggregate fills

Lightweight tire derived aggregate (TDA) fills are a proven recycling outlet for waste tires, requiring relatively low cost waste processing and being competitively priced against other lightweight fill alternatives. However its value has been marred as several TDA fills have self-combusted during the early applications of this technique. An empirical review of these cases led to prescriptive guidelines from the ASTM aimed at avoiding this problem. This approach has been successful in avoiding further incidents of self-combustion. However, at present there remains no rational method available to quantify self-combustion risk in TDA fills. This means that it is not clear which aspects of the ASTM guidelines are essential and which are accessory. This hinders the practical use of TDA fills despite their inherent advantages as lightweight fill. Here a quantitative approach to self-combustion risk evaluation is developed and illustrated with a parametric analysis of an embankment case. This is later particularized to model a reported field self-combustion case. The approach is based on the available experimental observations and incorporates well-tested methodological (ISO corrosion evaluation) and theoretical tools (finite element analysis of coupled heat and mass flow). The results obtained offer clear insights into the critical aspects of the problem, allowing already some meaningful recommendations for guideline revision.

Steady state of solid-grain interfaces during simulated CPT

Abstract It has recently been shown (Arroyo et al. [1]) that 3D DEM models are able to reproduce with reasonable accuracy the macroscopic response of CPT performed in calibration chambers filled with sand. However, the cost of each simulation is an important factor. Hence, to achieve manageable simulation times the discrete material representing the sand was scaled up to sizes that were more typical of gravel than sand. A side effect of the scaled-up discrete material size employed in the model was an increased fluctuation of the macro-response that can be filtered away to observe a macroscopic steady-state cone resistance. That observation is the starting point of this communication, where a series of simulations in which the size ratio between penetrometer and particles is varied are systematically analyzed. A micromechanical analysis of the penetrometer-particle interaction is performed. These curves reveal that a steady state is arrived also at the particle-cone contact level. The properties of this dynamic interface are independent of the initial density of the granular material.

Sensitivity to damping in sand production DEM-CFD coupled simulations

A three dimensional numerical model based on Discrete Element Method (DEM) and coupled with Computational Fluid Dynamics (CFD) was implemented to simulate sand production. Simulations with no fluid flow conditions and with fluid flow conditions have been performed and the sensitivity of the simulations to numerical damping is studied.

Numerical Analysis of Soil Ploughing Using the Particle Finite Element Method

This contribution illustrates some results from a parametric analysis of the factors affecting soil ploughing. The numerical simulations rely on the Particle Finite Element method, a method known for its capabilities to tackle large deformations and rapid changing boundaries at large strains. A total stress analysis – assuming a quasi-incompressible elastic model along with a Tresca plastic model - is used to simulate a clayey soil behavior. As a first step, a two-dimensional geometry is used and the effect of contact roughness in the horizontal force along the chip formation is assessed.

Discrete Simulation of Cone Penetration in Granular Materials

The simulation of penetration problems into granular materials is a challenging problem as it involves large deformations and displacements as well as strong non-linearities affecting material behaviour, geometry and contact surfaces. In this contribution, the Discrete Element Method (DEM) has been adopted as the modelling formulation. Attention is focused on the simulation of cone penetration, a basic reconnaissance tool in geotechnical engineering, although the approach can be readily extended to other penetration problems. It is shown that DEM analysis results in a very close quantitative representation of the cone resistance obtained in calibration chambers under a wide range of conditions. DEM analyses also provides, using appropriate averaging techniques, relevant information concerning mesoscale continuum variables (stresses and strains) that appear to be in agreement with physical calibration chamber observations. The examination of microstructural variables contributes to a better understanding of the mechanisms underlying the observed effects of a number of experimental and analysis features of the cone penetration test.

DEM Investigation of Particle Crushing Effects on Static and Dynamic Penetration Tests

A 3-dimensional discrete element method model has been developed to simulate static and dynamic rod penetration test in a calibration chamber. The chamber has been filled with a scaled analogue of Fontainebleau sand. Crushing effects on penetration results have been examined. It has been found that particle crushing reduces penetration resistance for both static and dynamic tests. Microscale observation of crushed particles has been conducted. It is shown that dynamic impact causes more crushing events. The crushed particles are distributed within 2–3 radius from the rod for both tests.

Hydraulic conductivity from CPTu on-the-fly: a numerical evaluation

Permeability is important in many geotechnical applications. The current (cone penetration test that gathers piezometer data (CPTu)) practice to obtain permeability values relies on dissipation tes...

Marchetti flat dilatometer tests in a virtual calibration chamber

Calibration chambers are frequently used to verify, adapt, or both verify and adapt empirical relations between different state variables and in situ test results. Virtual calibration chambers (VCC) built with 3D discrete element models may be used to extend and partially substitute costly physical testing series. VCC are used here to explore the mechanics of flat dilatometer penetration and expansion. Results obtained for a simulation of physical tests in Ticino sand are presented. Blade tip resistance during penetration is in good agreement with the experiments. A piston-like design is used for the blade so that larger displacements may be applied than it is possible with a membrane. Initial piston pressures in the expansion curves are very low, strongly affected by the scaled-up grain sizes. Despite that difficulty, expansion curves may be easily interpreted to recover dilatometer moduli ED close to those observed in the physical experiments. Particle-scale examination of the results allows a firmer understanding of the current limitations and future potential of the technique.