Joanna Butlanska

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.

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.

Calculating strain in 3D DEM simulations

Particulate DEM allows us to simulate and evaluate in detail the evolution of localizations in particulate material, whether bonded/cemented or unbonded. DEM simulations generate a wealth of particle scale data including particle displacements, velocities and contact forces. Traditionally in geomechanics we understand material response based upon a continuum mechanics framework that considers stress and strain. There is little debate as to how to calculate stress from DEM simulation results, however there is no consensus on how to calculate strain. Most of the methods proposed in the literature to calculate strain have considered the overall response of an assembly of grains, rather than the local in homogeneities that are associated with shear band evolution. This paper outlines the challenges associated with quantifying strain based upon DEM simulation results and demonstrates that a local wavelet based homogenization approach as proposed by may have advantages over triangulation based linear interpolation.

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.

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.