Mesh sensitivity in discrete element simulation of flexible protection structures

The Discrete Element Method (DEM) has been employed in recent years to simulate flexible protection structures undergoing dynamic loading due to its inherent aptitude for dealing with inertial effects and large deformations. The individual structural elements are discretized with an arbitrary number of discrete elements, connected by spring-like remote interactions. In this work, we implement this approach using the parallel bond contact model and compare the numerical results at different discretization intervals with the analytical solutions of classical beam theory. Successively, we use the same model to simulate the punching test of a steel wire mesh and quantify the influence of a different number of elements on the macroscopic response

A coupled damage-plasticity DEM bond contact model for highly porous rocks

In view of the significant stress loss induced by structural collapse when simulating high-porous soft rocks using traditional damage bond models in DEM ( discrete element method) modelling, a novel damage bond contact model is proposed to capture the ductile failure of high-porous cemented soft rocks. To address the unrealistic physical contact distribution resulting from the use of spherical particles in DEM modelling and consider the physical presence of broken bonds, far-field interaction is introduced between grains when two untouched particles reach a specific activation gap, enabling the generation of stable, highly porous open structure samples while using spherical DEM particles. The final results demonstrate that this newly developed model facilitates the transition from the purely elastic rock-like behaviour stage to the transitional ductile failure stage of porous soft rocks, as well as reproduces the softening/hardening response of soft rocks under different confinements

A coupled damage-plasticity DEM bond contact model for highly porous rocks

In view of the significant stress loss induced by structural collapse when simulating high-porous soft rocks using traditional damage bond models in DEM ( discrete element method) modelling, a novel damage bond contact model is proposed to capture the ductile failure of high-porous cemented soft rocks. To address the unrealistic physical contact distribution resulting from the use of spherical particles in DEM modelling and consider the physical presence of broken bonds, far-field interaction is introduced between grains when two untouched particles reach a specific activation gap, enabling the generation of stable, highly porous open structure samples while using spherical DEM particles. The final results demonstrate that this newly developed model facilitates the transition from the purely elastic rock-like behaviour stage to the transitional ductile failure stage of porous soft rocks, as well as reproduces the softening/hardening response of soft rocks under different confinements

G-PFEM Numerical Study of the Downward Trapdoor Problem

The downward trapdoor provides a valuable framework for investigating stress distribution and ground movement during tunnel excavation. Numerous efforts have been made to devise analytical methods based on experimental observations. The area above the trapdoor experiences gravitational flow, and it becomes an active soil pressure condition with the lowering of the trapdoor. Due to the substantial discontinuous movement between the trapdoor and the adjacent stationary support, significantly discontinuous deformations arise at the boundary between the active zone above the trapdoor and the stationary zone. This complexity renders the trapdoor problem challenging by traditional continuum analysis methods. To confront this challenge, the present study employs the Particle Finite Element Method for geotechnical applications (G-PFEM) to simulate the downward trapdoor problem. The effectiveness of this approach is demonstrated by replicating a model experiment included in the literature. The simulation captures the gravitational flow of the surrounding ground as the trapdoor descends, and the numerical results, encompassing stress distributions, ground displacements, and surface settlements, closely correspond to experimental data. Moreover, to underscore the advantages of employing G-PFEM, a larger displacement is applied to the trapdoor. The results indicate significant changes in ground displacement and corresponding earth pressure as the trapdoor displacement increases, ultimately leading to substantial slope failure like large deformation

Backtesting AVaR and VaR with a simulated copula

The aim of this study is to verify whether the average value at risk (AVaR) can be a good alternative to the value at risk (VaR) for estimating portfolio losses, especially regarding tail events. To achieve this aim, we use a copula framework to estimate the dependence between the stock returns of a portfolio composed of 94 components of the S&P100 index to compute the AVaR and VaR and compare the results with respect to the Gaussian exponentially weighted moving average (EWMA). To compute the simulated returns, we employ the algorithm used by Biglova et al. (2014) in portfolio selection problems and then backtest the model with Kupiec’s and Christoffersen’s tests. The results are coherent with the literature; in particular, the VaR computed both via the copula and via the EWMA seems to fail to provide an accurate risk measurement while the AVaR with the copula and EWMA appears to be more reliable

Development of an acquisition system for high deformation barriers using low-cost IMU sensors and Image Analysis

Meso and full-scale impact tests have historically been used to assess the capacity of high-deformation barriers used against natural hazards and to validate numerical models. However, the data acquired from such experiments is typically limited to peak barrier elongation and occasionally force-time-displacement curves acting on specific structural elements. In rare occasions, complex and expensive procedures such as 4D photogrammetry are employed. Herein, a procedure is developed to obtain a barrier deformation data in three dimensions using low-cost MEMS sensors and consumer-grade cameras. The procedure is validated against LIDAR data for both quasi-static and dynamic conditions

Model CPTs in Chalk

Cemented geomaterials exist in many parts of the world. Structure and bonding largely influence their strength, stiffness, permeability and other hydromechanical properties. Despite the CPT being the most widely used geotechnical engineering soil characterization tool, most existing correlations between penetration resistance and soil parameters apply only to uncemented granular deposits. Application of existing correlations to cemented geomaterials such as soft rocks can produce misleading interpretation making CPT application more challenging. In particular, CPT databases correlating tip resistance with yield stress of the intact material show a wide scatter prompting the need for a better understanding of the mechanics of cone penetration in soft bonded materials. In this work, 1g small-scale model CPTs are performed in a soft rock, whilst in-test X-ray techniques help to reveal mechanisms behind the penetration process. Thereafter, experimental results are compared to field scale results and those modelled using the Particle Finite Element method which is geared toward large deformation analyses. The combined interpretation of the experimental and numerical data is then used to discuss some of the unique attributes of CPT behaviour in soft rock