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Soil arching is significantly influenced by relative density, while its mechanisms have barely been analyzed. A series of DEM numerical simulations of the classical trapdoor test were carried out to investigate the multi-scale mechanisms of arching development and degradation in granular materials with different relative density. For analysis, the granular assembly was divided into three zones according to the particle vertical displacement normalized by the trapdoor displacement δ. The results show that before the maximum arching state (corresponding to the minimum arching ratio), contact forces between particles in a specific zone (where the vertical displacement of particles is larger than 0.1δ but less than 0.9δ) increase rapidly and robust arched force chains with large particle contact forces are generated. The variation in contact forces and force chains becomes more obvious as the sample porosity decreases. As a result, soil arching generated in a denser particle assembly is stronger, and the minimum value of the arching ratio is increased with the sample porosity. After the maximum arching state, the force chains in this zone are degenerated gradually, leading to a decrease in particle contact forces in microscale and an increase in the arching ratio in macroscale. The recovery of the arching ratio after the minimum value is also more significant in simulations with a larger relative density, as the degeneration of contact force chains is more obvious in denser samples. These results indicate the importance of contact force chain stabilities in specific zones for improving soil arching in engineering practice. [ABSTRACT FROM AUTHOR]

This paper presents numerical results from a two-dimensional discrete element method (DEM) simulation study on the influence of lateral boundary condition on shaft resistance of a pile driven into a crushable sand. The study was conducted by comparing simulation results from the pile penetration test and the interface shear test employing parallel-bonded agglomerates for modeling of the particle breakage. The interface shear test was performed under three different types of normal boundary conditions; namely, constant normal load (CNL), constant normal stiffness (CNS), and constant volume (CV) boundary conditions. For the pile penetration test, a series of sampling windows were identified on the initial ground configuration to monitor the stress-strain, volume change, and particle breakage behavior of particle groups located within the sampling windows. A detailed investigation was then undertaken by comparing the behavior of particle groups with that from the interface shear test to find out which type of normal boundary condition best describes the lateral boundary condition in the pile penetration test. It has been found that the behavior of a particle group reaches the peak state below the pile tip and the critical state after it reaches the pile shaft. The influence of normal boundary condition on the stress ratio at the critical state is not obvious. The conventional interface shear test (i.e., CNL) can provide valuable information on the determination of skin friction along the pile shaft. Key words: pile penetration, interface shear test, constant normal stiffness (CNS), crushable sand, particle breakage, discrete element method (DEM) simulation. Cet article presente des resultats numeriques a deux dimensions a partir d'une etude de simulation avec methode aux elements discrets (DEM) sur l'influence des conditions aux limites laterales sur la resistance d'un pieu enfonce dans un sable ecrasable. L'etude a ete faite en comparant les resultats de la simulation de l'essai de penetration du pieu et l'essai de cisaillement de l'interface employant des agglomerats lies longitudinalement pour la modelisation de la rupture des particules. L'essai de cisaillement de l'interface a ete effectue dans trois differents types de conditions aux limites normales, a savoir, les conditions sous charge normale constante (CNL), sous la rigidite normale constante (CNS), et sous les limites de volume constant (CV). Pour l'essai de penetration du pieu, une serie de fenetres d'echantillonnage a ete identifiee sur la configuration initiale du sol pour surveiller la contrainte--deformation, le changement de volume et le comportement de la rupture de particules de groupes de particules situes dans la fenetre d'echantillonnage. Une enquete detaillee est ensuite effectuee en comparant le comportement des groupes de particules avec ceux de l'essai de cisaillement de l'interface pour connaitre le type de condition limite normale qui decrit le mieux la condition a la limite laterale dans l'essai de penetration de pieu. Il est constate que le comportement d'un groupe de particules a atteint l'etat maximal au-dessous de la pointe du pieu et l'etat critique apres avoir atteint l'axe de pieu. L'influence des conditions de limites normales sur le rapport des contraintes a l'etat critique n'est pas evidente. L'essai de cisaillement de l'interface classique (c.-a-d., CNL) peut fournir des informations precieuses sur la determination de la friction le long de l'axe du pieu. [Traduit par la Redaction] Mots-cles: penetration de pieu, essai de cisaillement de l'interface, rigidite normale constante (CNS), sable ecrasable, rupture de particules, simulation de la methode des elements discrets (DEM)., Introduction The mechanical behavior of soil-pile interface has a deciding influence on the overall reaction of driven piles in sand (Ooi and Carter 1987). The unit pile shaft resistance (Tf) [...]

Shear wave method has been increasingly popular in assessing the liquefaction potential of granular soils. Two particle-scale parameters, the inter-particle friction and the shear modulus of grains, play vital roles in correlation between Cyclic Resistance Ratio (CRR) and shear wave velocity corrected by overburden stress (Vs1). Series of drained one-dimensional compression tests were simulated on samples of different inter-particle friction angles assigned during preparation stage. Uniformity coefficients of these Particle Size Distribution (PSD) curves are 2 and 4 whose average particle size d50 are identical. The shearing results, as well as their assigned inter-particle friction angles form calibration curves for real sands. Dissimilar PSD curves result in different calibration outcomes. For Silica sand no.8, these curves give divergent inter-particle friction angles. This study calibrates particle shear modulus for Silica sand no.8 as well. Different PSD curves give divergent values of particle shear modulus. PSDs show impacts on calibrations of both vital parameters, which have converse effects on CRR-Vs1 curves. This study suggests that the CRR-Vs1 correlation should be independent of PSDs.

The estimation of the skin friction of onshore or offshore piles in sand is still a difficult problem for geotechnical engineers. It has been accepted by many researchers that the mechanism of driving piles in the soil has shared some similarities with that of an element shear test under the constant normal stiffness (CNS) condition. This paper describes the behaviour of an element of soil next to a pile during the process of pile penetration into dense fine sand using the 2D-DEM numerical simulation software. A new CNS servo was added to the horizontal boundary while maintaining the vertical stress constant. This should simulate the soil in a similar manner to that of a CNS pile-soil interface shear test, but allowing the vertical stress to remain constant which is more realistic to the field situation. Shear behaviours observed in these simulations were very similar to the results from previous researchers’ lab shearing tests. With the normal stress and shear stress obtained from the virtual models, the friction angle and the shaft friction factor β mentioned in the API-2007 offshore pile design guideline were calculated and compared with the API recommended values.

The Discrete Element Method (DEM) was used to simulate the mechanical behaviour of a reservoir sandstone. Triaxial tests were carried out using 3D-DEM to simulate the stress-strain behaviour of a sandstone with comparisons made between the numerical tests and the laboratory tests. The influence of isotropic unloading was investigated, which was found to have impacts on bond breakages and was successfully captured in the 3D shearing processes. It was found that bond breakages correlated strongly with the stress-strain behaviour of the sandstone affecting the peak strength. It was also found that unloading affected the bond breakages, which then changed the mechanical behaviour of sandstone. The tangent stiffnesses of simulated virgin and cored samples under different confining stresses were compared. From the tangent stiffnesses, gross yield envelopes and the yielding surfaces for unloaded samples and virgin samples were plotted and analysed in detail.