A DEM-FEM coupled numerical model has been employed to analyze the behavior of a platform reinforced by geotextile (GTX) over soft soil under cyclic loading. Upon calibration, the model enabled the quantification of reinforcement mechanisms (soil confinement by GTX frictional effect and increase of loading capacity by GTX tensioned membrane effect). Initially, frictional forces dominated over the tensioned membrane effect, but as deflection accumulated with successive cycles, the latter gained prominence. Moreover, the model provided insights into the load transfer through the GTX, and its escalating involvement with each cycle.

Building roads or platforms on weak soil is a remaining challenge. The reduction of granular material required for the subbase is more and more crucial, for cost reasons but also for a much lower impact of the earthworks on the environment. Geosynthetics have proved that they offer an effective and efficient solution. Design methods using geosynthetics have been developed for subbase stabilisation but may strongly differ depending on the model and the calibration behind. An experimental study, completed by numerical modelling has been carried out to understand better the behaviour of geosynthetic at the base of a granular layer on a weak soil.

Cavity formations by soil dissolution or underground collapses are at the origin of large surface subsidence that constitutes a risk of damage or failure for infrastructures. Soil reinforcement with geosynthetics positioned at shallow depth is an economical and functional solution to reduce the induced surface settlements. Previous research has mainly focused on the load transfer mechanism and the arching effect in cohesionless reinforced backfills when the cavity opens. Experimental and numerical studies dealing with cohesive soils are very rare, although this situation is commonly found in practice. To overcome this lack of knowledge, a numerical study based on Discrete Element Modelling is carried out to better understand the load transfer mechanisms that are mobilized in cohesive embankments prone to underground cavity opening. The results are compared with experimental data obtained on a small-scale laboratory model in terms of vertical and horizontal displacements of both soil and geosynthetics. The numerical results focus on the collapse mechanisms of the cohesive embankment, the load transfer mechanisms, the shape of the vertical load distribution acting on the geosynthetic layer, the strain and traction forces within the geosynthetic sheet.

L’histoire des géosynthétiques a débuté il y a plus de cinquante ans. Elle s’écrit toujours grâce à des ingénieurs, des chercheurs et des passionnés qui voient dans ces produits une réponse concrète aux défis environnementaux de notre société.Les géosynthétiques d’hier à aujourd’hui présente les notions et concepts de base nécessaires à l’emploi, la mise en oeuvre, le dimensionnement et la durabilité de ces produits qui n’ont eu de cesse de s’améliorer au cours du temps pour répondre à des défis techniques et environnementaux toujours en évolution.Cet ouvrage permet, grâce à des retours d’expérience acquis sur plusieurs décennies, d’établir le bienfondé de ces techniques et d’en comprendre leur concept et leur évolution. Il s’adresse aussi bien aux néophytes qu’aux ingénieurs confirmés curieux d’en savoir plus sur un sujet aussi passionnant.

The results of an experimental campaign of reinforcement of thin cohesive soil embankments in the case of cavity collapse are presented. In particular, the aim is to test the effectiveness of a new type of bi-stiffness geosynthetic. A coupled DEM-FEM numerical model is validated based on these results and allows a better understanding of the soil- geosynthetic interaction phenomena mobilized during the collapse. Comparison between the numerical and experimental results obtained with the two types of reinforcement (mono-stiffness and reversed bi-modulus) make it possible to underline the interest of the innovative product developed.

Ce chapitre permet de présenter les différents types de produits manufacturés qui constituent la famille des géosynthétiques, d’en donner les définitions et d’en présenter sommairement les fonctions et domaines d’application principaux. Les caractéristiques essentielles à leur fonction ainsi que leur mode de caractérisation sont déclinées en termes de propriétés mécaniques, hydrauliques ou d’interface.

Six full-scale unpaved test sections were constructed to investigate the benefit obtained from inclusion of geosynthetic layer. Two geotextiles were placed at the interface between the base layer and the subgrade. The results allowed the estimation of reinforcement efficiency. In addition, a numerical model coupled between the discrete element method and the finite element method has been calibrated based on the experimental results.

Geosynthetic-reinforced and pile-supported (GRPS) embankments are becoming more popular as a solution for addressing soil structural instability. The interaction between the geosynthetic-pile-subsoil-embankment elements is crucial to the load transfer mechanism and performance of GRPS embankments. Several analytical models for GRPS embankment design have been proposed, but their performance and applicability still require further validation. This research presents a three-dimensional numerical investigation of the load transfer mechanism of GRPS embankments using the finite difference approach, considering the combined interaction between the soil embankment, geosynthetics, pile, and subsoil. The importance of these crucial aspects in the GRPS embankment design technique is highlighted, as well as their influence and sensitivity. The following elements, in descending order, influence the load and settlement efficacies of the GRPS embankments: soft soil stiffness, embankment height, geosynthetic stiffness, and embankment soil density, according to this research. Furthermore, the use of geosynthetics reduces differential settlements and mitigates soil yielding above the pile heads. The numerical findings are then compared to four well-known design standards, with the subsurface stiffness, geosynthetic stiffness, embankment height, and fill soil density all being varied simultaneously to measure their performance. The findings of the comparison revealed that these techniques differ greatly in their ability to forecast load efficacy and differential settlement. Depending on the geometric properties of the embankment and material properties, all of the selected design methods produce over-predictions or under-predictions.

This paper presents an experimental thorough study on geocell reinforced loose sand (Dr = 25%), focusing on the improvement of the shear strength in undrained conditions. The experimental program includes a series of undrained compressive tests (CU) performed on loose sand samples with and without reinforcement. The experimental tests are performed for various geocell height (Hg = 15, 25 and 35 mm) and at different level of effective confining pressure (sigma'(c) = 50, 100 and 200 kPa). The obtained results show a significant effect of geocells height and initial confining pressure on the shear strength of the reinforced sand. The apparent cohesion of the reinforced soil increases linearly with the axial strain while the friction angle is rather constant and slightly dependent on the sample deformation.

Afin d’evaluer la pertinence des techniques de renforcement de sol granulaire par geotextile, les mecanismes de transfert de charge a l’œuvre lors de la formation d’une cavite sont etudies. Les travaux sont issus de simulations numeriques basees sur un couplage entre des methodes aux elements finis et aux elements discrets. Le sol (assemblage granulaire sans cohesion) est modelise par elements discrets, et le renforcement (nappe geosynthetique) par elements finis. Les resultats mettent en evidence l’influence des caracteristiques geometriques de l’essai ainsi que du mode de d’ouverture de la cavite sur la formation, l’evolution et l’intensite du phenomene.

Trajectory analysis is often needed to handle rockfall hazards. It is crucial to understand and to refine ground and rock interactions during a rockfall, which can be related to the elements involved. They include the topology, ground and rock nature, but also the volume and shape of the falling blocks. This work focuses on replicating field observations using a 3D discrete elements model (DEM) in order to further analyze the possible rockfall deposition areas, which may not be accessible due to the limited number of experimental data available. The numerical model implements blocks of more realistic shapes that were reconstructed from in situ blocks obtained by photogrammetry. The dissipation of kinetic energy at the collision point is suitably managed. In the experimental campaign, dozens of boulders (rock block) releases were conducted on two slope profiles of a quarry located in Authume (France). Block passing heights, velocities and runout distances were assessed at specific ground points. We analyzed lateral spreads, propagation distances, and energy balances computed for 3 different block geometries. These numerical results were confronted with experimental observations. Although time-consuming compared to lumped mass and rigid body dynamics models where the impact duration is zero, the DEM used in this work is versatile thanks to an explicit consideration of geometrical effects throughout the life of multiple contacts. It allows to simulate quite accurately a multitude of configurations. Among the properties of the terrain and the blocks, geometric features are shown to be crucial. For the sake of efficiency, a simplified shape coefficient based on block's elongation is proposed.

This study deals with an innovative type of protection structure for gravity-driven natural hazards such as landslides (slope failures, rockfalls, etc.) consisting of a vertical wall made up of interconnected concrete blocks. This type of articulated structure presents many advantages including reduced footprint, versatility and easy maintenance. The response of such a structure under impact is investigated considering projectiles with kinetic energies of 520 and 1020 kJ, based on real-scale impact experiments and numerical simulations. The finite difference model is described in detail as well as the experiments. The model was developed focusing on the global structural impact response while keeping the computation time reasonable. The model parameter calibration is based on data in the literature and complemented with specific measurements. The experimental data allows us to describe the impact response of the structure and identify the main mechanisms controlling this response (sliding, tilting, and fracturing). The simulation results revealed that the model is efficient in mimicking this response, in terms of deformation amplitude and evolution with time. Finally, the numerical model made it possible to highlight complex mechanisms that were not possible to experimentally determine such as the different energy dissipation modes within the wall.

The occurrence of a sinkhole in an area may compromise the safety of the existing structures and infrastructures. Therefore preventive solutions are necessary. Recently, a coupled DEM-FEM numerical model has been developed to better account for the failure mode of reinforced soil layers during cavity openings and the interaction between the collapsed soil and the geosynthetic sheet. An experimental campaign on a small scale trapdoor model gave the possibility to validate the numerical model. In that case, a usual linear elasto - perfectly plastic Mohr Coulomb constitutive law and its failure criterion have been chosen to represent the cohesive soil. A good agreement with the experimental results has been observed. To be able to reproduce the behavior of various cohesive materials, an advanced constitutive law has been tested in place of the usual model based on the Mohr Coulomb criterion.

A discrete element study of a reinforced granular embankment by rigid inclusions, submitted to cyclic loadings is presented. The discrete element method (based on molecular dynamics method) is used to understand the load transfer mechanisms into the granular layer (just above the inclusions) during cyclic loadings. The microscale study showed that the soil above the rigid inclusions retrieve the larger forces, illustrating on the role of the granular layer as a load transfer layer, while the settlement of both the granular layer and loading slab increase with loading. The efficiency of load transfer to piles and ability (capacity of the granular material to postpone the overloads to the piles) decreases with cycling, but keeping high values at the end of cycles. The transfer of forces in the granular layer is achieved by two mechanisms, interacting together (inverted pyramid above rigid inclusions and arching), confirming results found in the literature.

The sophistication of geotechnical structures (new technologies and innovative materials in civil engineering) implies the use of increasingly sophisticated numerical models for their design. The soil–structure interaction problem addressed in this chapter is studied using the discrete element method (DEM). The discrete methods apply to quasistatic problems and see their main interest in problems that involve large deformations or collapsed areas, as well as to cyclic or dynamic applications, easily modeled given the specific mathematical formulation used in DEM. After presenting briefly the DEM and the theoretical concepts used to take account of the soil–inclusion interaction, the chapter presents different illustrations of this method and, in particular, the applications to geotechnical structures in interaction with rigid piles (quasistatic and cyclic loadings) or with flexible and deformable reinforcements (geosynthetic sheets).

Subsidence can result from the collapse of underground cavities. The impact of such subsidence on existing structures and infrastructures is generally dramatic. Geosynthetic reinforcement (GSY) is an attractive mitigation solution that can be used to reduce this impact. This paper focuses on the mitigation solutions over existing cavities mainly on the GSY mitigation method. A large-scale physical model (1-g) is used to study the subsidence mechanisms and to estimate the efficacy of GSY for both cohesive and granular overlying soils. The results show that the presence of GSY reduces the ground movement due to the cavity progress toward surface, even under significant overload (traffic, localised foundation, etc.). The deformation of the GSY and the scenario for ground surface movement (subsidence or sinkhole) depend on both the soil type and overload intensity. The experimental results are compared to the analytical solutions proposed to design the GSY for cohesive and granular soils. In particular, the influence of the vertical stress distribution acting on the GSY is investigated. Different geometries of stress distribution are proposed for granular soils as a function of the loading mode (self-weight or localised overload). For cohesive soils, the action of the collapsed soil on the GSY sheet is found to be well estimated by considering the effect of a simplified system composed of two well localised punctual forces. The analytical and experimental results obtained are rather similar, proving the relevance of the analytical models in predicting the behaviour of reinforced soil layers taking into consideration the real stress distribution deduced from the experimental results.

Un modèle physique réduit à 1 × g de 6 m3a été développé pour étudier les conséquences de mouvements de terrains, les mécanismes d’interactions entre les mouvements du sol et les ouvrages et les moyens de mitigation. Les mouvements de terrains sont reproduits à l’aide de vérins. Deux sols analogues (granulaire et cohésif) sont utilisés au-dessus de cavités ainsi reproduites. Cet article présente deux applications montrant la contribution de la modélisation physique. La première application concerne l’évaluation des dommages induits dans une structure en maçonnerie à l’aide de la corrélation d’images. La deuxième application concerne le dimensionnement d’un géosynthétique de renforcement au-dessus de cavités souterraines. Les deux applications montrent la capacité de la modélisation physique à combler le manque de données nécessaires à la validation des approches analytiques et empiriques.

Three-dimensional numerical analyses using the discrete element method are conducted to investigate several fundamental aspects related to soil-structure interaction and mobilization mechanisms in the geosynthetic-reinforced and pile-supported embankments. The contributions of the soil arching, tensioned membrane effect, friction interaction, subsoil support, and punching failure are investigated. The results indicated that the inclusion of the geosynthetic enhances the stress transfer from the subsoil to piles due to the tensioned membrane action, and the stress distribution is more uniform as compared to piled embankment without geosynthetic. However, the tension distribution in geosynthetic is not uniform and the maximum tension occurs near the pile edge. Numerical results also proved that the subsoil provides substantial support and reduces the reinforcement tension while shear stresses are mobilized along the upper and lower sides of soil-geosynthetic interfaces. These mechanisms should be considered in theoretical models to produce a more realistic approach. Finally, ten available design methods are reviewed and compared to the numerical results to assess the performance of analytical models. The results showed that the design method of Pham, CUR 226 design guideline, and EBGEO design standard agree well with the numerical results and are generally better than the results of all other methods.

This paper focuses on experimental and numerical studies aiming to better characterize the rebound kinematics between a natural soil and rock boulders using single‐block collision models. The variability of the dissipative parameters needed to reproduce the experimental results was studied considering two different approaches: stochastic transfer matrix and discrete element model (DEM). An experimental process taking into account all possible variables related to the collision was setup for small‐scale tests involving cylindrical and triangular boulders that introduces natural variability during impact. The objective of the experimental tests is to identify the relationship between incident and reflected velocities (before and after impact, respectively), in order to improve the predictability of the two numerical proposed approaches. The experimental results help to quantify the amount of energy losses during impacts and thus expands the knowledge to reproduce more realistic rockfall events. This work is divided in three main axes: (i) statistical analysis of experimental trajectories based on experimental tests; (ii) stochastic approach predicting in a simple way from initial velocities the kinematic of blocks after rebounds; and (iii) calibration of a discrete element model including probabilistic analysis based on Monte Carlo approach. This way, the two collision approaches presented in this work are able to predict rebounds in terms of averages, standard deviations, and distributions.