Mechanical performance and sustainability assessment of reinforced soil walls

Soil reinforced retaining wall structures are materiallymore efficientthan competing construction solutions such as gravity and cantilever walls. Nevertheless, the behaviour and interactions between the com ponent materials are com plex and not fully understood. Current design methods are typically limited to simple cases with respect to material properties, geometry, and boundary conditions. Advanced numerical models using finite element and/or finite difference methods offer the possibilityto extend the understanding ofthese systems and to predictwall performance under operational conditions. In this Thesis, numerical models were developed and shown to give satisfactory predictions ofwall behavior when compared with results of instrumented physical structures. The verified models were useful for sensitivity analyses using a range ofwall geometries and boundaryconditions, material parameters and different constitutive models. As examples ofthe obtained results, the compressibility ofthe precast panel bearing pads significantly modified the axial vertical facing load but has no significant effect on the tension developed in the soil reinforcement layers. Also, the stiffness ofthe foundation soil has greater effect on the tension developed in soil steel reinforcing elements than for polymeric reinforcement layers.lt has been possible to perform sensitivity analysis using parameters that define soil-structure interactions. Such interactions have been analyzed using different commercial software programs and bydefining them with elements from the continuum media using 2D and 3D models. Laboratory reinforcement pullout tests using steelladder and polymeric strips were performed as part of the Thesis. Those parameters that have the greatest influence on soil-reinforcement interaction are identified, quantified, and compared to default-design wlues anda range ofvalues used to calibrate numerical models. From the results of2D and 3D numerical models suitable correlations have been obtained to allow 2D models to be used in plane strain reinforced soil walls with discontinuous soil reinforcement elements in the running walllength ofthese structures. With a proper sustainability assessment it has been possible to make quantitative comparisons between reinforced soil wall structures and other alternativés performing the same function (such as gravity and cantilever walls) construcfed to different heights. Using a modelbased on the multi-attributeutilitytheoryand wlue analysis decision-making, the best solutions with least negative impact were identified in an example set of alternative earth retaining wall options from a sustainable perspective. The results include possible scenarios based on the relative importance ofthe three pillars ofsustainability(i.e., environmental, economic, and sociallfunctional) as judged bydifferentstakeholders. Reinforced soil walls turned outto be the best choice in most cases analyzed, based on a quantitative end score. The models and analysis methodologies developed as part ofthis Thesis work have improved understanding ofthe behavior ofthese structures, and offered possibilities to improve and optimize designs in the future.Els murs de contenció amb sòl reforçat són estructures materialment més eficients que altres solucions constructives alternatives, com ara els murs de gravetat o en voladís. No obstant això, el seu comportament i les interaccions entre els materials que componen aquestes estructures són complexos i no entesos completament. Els mètodes de disseny actuals solen estar limitats a casos senzills respecte a les propietats dels materials, la geometria i les condicions de contorn. Models numèrics avançats utilitzant elements finits i/o diferències finites ofereixen la possibilitat d'ampliar la comprensió d’aquests sistemes estructurals i de predir el comportament de l'estructura en condicions de servei. En aquesta Tesi s'han desenvolupat models numèrics que han demostrat donar prediccions satisfactòries del comportament d’aquest tipus de murs quan es comparen amb resultats obtinguts d'estructures físiques instrumentades. Aquests models verificats han estat útils per a poder fer anàlisis de sensibilitat segons diferents geometries del parament i condicions de contorn, paràmetres dels materials i diferents models constitutius. Com a exemple dels resultats obtinguts, s’ha determinat que la capacitat de compressió de les peces de recolzament dels panells prefabricats modifica de manera significativa la càrrega desenvolupada vertical axial en el parament, però no té un efecte significatiu en la tensió desenvolupada a les capes de reforç del sòl. O també, que la rigidesa del sòl de fonamentació té un efecte més gran sobre la tensió desenvolupada en elements de reforç metàl·lics que en polimèrics. Ha estat possible dur a terme anàlisis de sensibilitat utilitzant els paràmetres que defineixen les interaccions sòl-estructura. Aquestes interaccions han estat analitzades utilitzant diferents programes comercials numèrics i definint-les amb elements del medi continu tant en models 2D com en 3D. Com a part de la Tesi, s'han de dut a terme assaigs de laboratori d'extracció de reforços tipus malla metàl·lica i banda polimèrica. Els paràmetres que tenen una influència principal en la interacció sòl-reforç han sigut identificats, quantificats i comparats tant amb els valors per defecte de disseny com amb valors reportats a la literatura utilitzats per a calibrar models analítics, permetent el calibratge dels models numèrics generats. Dels resultats dels models 2D i 3D s’han obtingut correlacions que permeten concloure que els models 2D en deformació plana són adequats per a representar el funcionament de les estructures de sòl reforçat amb elements de reforç discontinus a la direcció del parament. Mitjançant una avaluació adequada de la sostenibilitat ha estat possible fer comparacions quantitatives entre estructures de sòl reforçat i altres alternatives constructives que compleixen la mateixa funció (com els murs de gravetat o en voladís) construïdes a diferents altures. Mitjançant un model basat en la teoria de la utilitat multiatribut i d’anàlisi de valor per a la presa de decisions, es van identificar els processos més representatius i de major impacte des d’un punt de vista sostenible. Els resultats obtinguts inclouen un ajust basat en possibles escenaris de presa de decisió per la importància relativa dels tres pilars de la sostenibilitat (ambiental, econòmic, i social/funcional). L'alternativa de sòl reforçat va resultar ser la millor, obtenint una puntuació més alta en gran part dels escenaris de presa de decisió considerats. En base a una puntuació quantitativa final, els murs de sòl reforçat van resultar ser la millor opció en la majoria dels casos analitzats. Els models i metodologies d'anàlisi desenvolupades com a part de aquest treball de Tesi han millorat la comprensió del comportament d’aquestes estructures, i ofereixen possibilitats per a millorar i optimitzar els seus dissenys en el futurLos muros de contención con suelo reforzado son estructuras materialmente más eficientes que otras soluciones constructivas alternativas, tales como los muros de gravedad o en voladizo. Sin embargo, su comportamiento y las interacciones entre los materiales que componen estas estructuras son complejos y no completamente comprendidos. Los métodos de diseño actuales suelen estar limitados a casos sencillos con respecto a las propiedades de los materiales, la geometría y las condiciones de contorno. Modelos numéricos avanzados utilizando elementos finitos y/o diferencias finitas ofrecen la posibilidad de ampliar la comprensión de estos sistemas y de predecir el comportamiento de la estructura en condiciones de servicio. En esta Tesis se han desarrollado modelos numéricos que han demostrado dar predicciones satisfactorias del comportamiento de este tipo de muros cuando se comparan con resultados obtenidos de estructuras físicas instrumentadas. Estos modelos verificados han sido útiles para análisis de sensibilidad según diferentes geometrías del paramento y condiciones de contorno, parámetros de los materiales y diferentes modelos constitutivos. Como ejemplo de los resultados obtenidos, la capacidad de compresión de las piezas de apoyo de los paneles prefabricados modifica de manera significativa la carga vertical axial desarrollada en el paramento, pero no tiene un efecto significativo en la tensión desarrollada en las capas de refuerzo del suelo. O también, que la rigidez del suelo de cimentación tiene un mayor efecto sobre la tensión desarrollada en elementos de refuerzo metálicos que en poliméricos. Ha sido posible llevar a cabo análisis de sensibilidad utilizando los parámetros que definen las interacciones suelo-estructura. Tales interacciones han sido analizadas utilizando diferentes programas numéricos comerciales y definiéndolas con elementos del medio continuo tanto en modelos 2D como 3D. Como parte de la Tesis, se han llevado a cabo ensayos de laboratorio de extracción de refuerzos tipo malla metálica y banda polimérica. Los parámetros que tienen una mayor influencia en la interacción suelo-refuerzo han sido identificados, cuantificados y comparados tanto con los valores por defecto de diseño como con valores reportados en la literatura usados para calibrar modelos analíticos, permitiendo la calibración numérica de los modelos generados. De los resultados de los modelos 2D y 3D se han obtenido correlaciones que permiten concluir que los modelos 2D en deformación plana son adecuados para representar el funcionamiento de las estructuras de suelo reforzado con elementos de refuerzo discontinuos en la dirección del paramento. Con una evaluación adecuada sostenibilidad ha sido posible hacer comparaciones cuantitativas entre estructuras de suelo reforzado y otras alternativas constructivas que cumplen la misma función (tales como los muros de gravedad o en voladizo) construidas a diferentes alturas. Mediante un modelo basado en la teoría de la utilidad multiatributo y análisis de valor para la toma de decisiones, se identificaron los procesos más representativos y de mayor impacto desde un punto de vista sostenible. Los resultados obtenidos incluyen un ajuste basado en posibles escenarios de toma de decisión por la importancia relativa de los tres pilares de la sostenibilidad (ambiental, económico, y social/funcional). La alternativa de suelo reforzado resultó ser la mejor, obteniendo una mayor puntuación en gran parte de los escenarios de toma de decisión considerados. En base a una puntuación final cuantitativa, los muros de suelo reforzado resultaron ser la mejor opción en la mayoría de los casos analizados. Los modelos y metodologías de análisis desarrolladas como parte de este trabajo de Tesis han mejorado la comprensión del comportamiento de estas estructuras, y ofrecen posibilidades para mejorar y optimizar sus diseños en el futuroPostprint (published version

A thermo-hygro-mechanical model for concrete shrinkage: preliminary study

The concrete shrinkage is a common effect of the concrete material behavior during concrete strengthening. The shrinkage is usually a function of the particular concrete material features, its quality, curing process, structural element size, and thermal-hydraulic boundary conditions during the working stress life-time conditions. Typically, grid-geometry steel bars with reinforcement function (rebar) are placed close to the concrete contour surface (coating spacing) to avoid the undesired shrinkage cracks with consequent material section loss. In addition to the solution through classical steel-grid rebar, there are other alternatives considering new materials and methodologies during concrete casting which are still under development (as the use of master fibers is) and may be also appropriate to avoid concrete shrinkage cracking with reducing material and installation costs.The concrete shrinkage is a common effect of the concrete material behavior during concrete strengthening. The shrinkage is usually a function of the particular concrete material features, its quality, curing process, structural element size, and thermal-hydraulic boundary conditions during the working stress life-time conditions. Typically, grid-geometry steel bars with reinforcement function (rebar) are placed close to the concrete contour surface (coating spacing) to avoid the undesired shrinkage cracks with consequent material section loss. In addition to the solution through classical steel-grid rebar, there are other alternatives considering new materials and methodologies during concrete casting which are still under development (as the use of master fibers is) and may be also appropriate to avoid concrete shrinkage cracking with reducing material and installation costs.Preprin

Vertical facing panel-joint gap analysis for steel-seinforced soil walls

This paper reports the results of a numerical parametric study focused on the prediction of vertical load distribution and vertical gap compression between precast concrete facing panel units in steel-reinforced soil walls ranging in height from 6 to 24 m. The vertical compression was accommodated by polymeric bearing pads placed at the horizontal joints between panels during construction. This paper demonstrates how gap compression and magnitude of vertical load transmitted between horizontal joints are influenced by joint location along the height of the wall, joint compressibility, and backfill and foundation soil stiffness. The summary plots in this study can be used to estimate the number and type (stiffness) of the bearing pads to ensure a target minimum gap thickness at the end of construction, to demonstrate the relative influence of wall height and different material component properties on vertical load levels and gap compression, or as a benchmark to test numerical models used for project-specific design. The paper also demonstrates that although the load factor (ratio of vertical load at a horizontal joint to weight of panels above the joint) and joint compression are relatively insensitive to foundation stiffness, the total settlement at the top of the wall facing is very sensitive to foundation stiffness. This paper examines the quantitative consequences of using a simple linear compressive stress–strain model for the bearing pads versus amultilinear model that is better able to capture the response of bearing pads taken to greater compression. The study demonstrates that qualitative trends in vertical load factor are preserved when a more advanced stress-dependent stiffness soil hardening model is used for the backfill soil as compared with the simpler linear elastic Mohr–Coulomb model. Although there were differences in vertical loads and gap compressionwith the use of both soilmodels for the backfill, the differenceswere small and not of practical concern.Peer ReviewedPostprint (author's final draft

Numerical study of the influence of the interaction distance, the polymeric strips pre-tensioning, and the soil–polymeric interaction on the performance of back-to-back reinforced soil walls

This study describes the results of a series of 2D finite element method (FEM) numerical models of 6 m high back-to-back reinforced soil walls using the geotechnical software PLAXIS. These structures are used to support embankments, especially for bridge abutment approaches. The quantitative influence of problem geometry, strip pre-tensioning, strip type, and surcharging on horizontal displacements, development of soil shear and plastic zones, lateral earth pressure, and reinforcement loads is presented. The numerical results demonstrate how this type of reinforced soil walls perform jointly at a certain distance of interaction between the two opposite walls. The walls of the two opposing sides clearly interact with each other when they are close enough and with an overlapping reinforcement layout. Pre-tensioning load can contribute to achieving vertical wall-facing alignment at the end of construction. Using perforated/holed strips, the tensile loads at the end of construction were reduced by about 30% due to the improved polymeric–soil interface strength and stiffness.Peer ReviewedPostprint (published version

3D simulations of gas injection on callovo-oxfordian claystone assuming spatial heterogeneity and anisotropy

A series of gas injection tests on Callovo-Oxfordian (COx) claystone from the Bure underground research laboratory (URL) in France were carried out at the British Geological Survey (BGS). The tests were performed using a triaxial apparatus specifically designed to capture small volumetric strains induced by the injected gas flow and consequent material dilatancy. The long-duration experiments were monitored throughout. Measurements also included pressure, stresses (axial and radial stresses prescribed for each test stage), rate of gas inflow, gas outflow volume as well as pore-pressures observed at various points of the sample. A coupled hydro-gas-mechanical 3D numerical model has been developed to simulate the tests. Initial permeability is assumed heterogeneous throughout the specimen and embedded fractures are incorporated in the formulation. Gas pressure-induced deformations during the test lead to variations of permeability due to changes in matrix porosity and, especially, fracture aperture as well as fracture orientation due to material anisotropy. A programme of sensitivity analyses involving the variation of different aspects and parameters of the model contributes to a better understanding of the phenomena and highlights its complexity. The model is able to reproduce the observed behaviour of the tests.DECOVALEX is an international research project comprising participants from industry, government and academia, focusing on development of understanding, models and codes in complex coupled problems in sub-surface geological and engineering applications; DECOVALEX-2019 is the current phase of the project. The authors appreciate and thank Andra for funding this work, as well as all DECOVALEX-2019 Funding Organizations Andra, BGR/UFZ, CNSC, US DOE, ENSI, JAEA, IRSN, KAERI, NWMO, RWM, SÚRAO, SSM and Taipower for their financial and technical support of the work described in this report. The authors wish to acknowledge Elena Tamayo-Mas, Jon Harrington, Jean Talandier, Gilles Armand and Alex Bond for their constructive comments and discussions during the modelling work carried out in the context of DECOVALEX-2019, as well as the reviewers of the manuscript during the submission process of this study, which allowed to substantially improve the paper in a clear way. The authors wish also to acknowledge the support of the International Centre for Numerical Methods in Engineering (CIMNE) and the funding received from the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa Programme for Centres of Excellence in R&D” (CEX2018-000797-S). The statements made in the report are, however, solely those of the authors and do not necessarily reflect those of the Funding Organizations.Peer ReviewedPostprint (published version

Three dimensional thermo-hydraulic modelling for KBS-3H alternative

The KBS-3H disposal alternative is composed by horizontally placed supercontainers comprising the canisters with the spent nuclear fuel surrounded in both drift axis and radial directions by compacted bentonite blocks (buffer) enclosed in a perforated shell. The different internal gaps between the supercontainer components and the one between the buffer blocks and the host rock have direct effects on the buffer behaviour. This paper presents a Thermo-Hydraulic (TH) Three-Dimensional (3D) numerical model developed to analyse a particular geometry assuming three different gap state conditions and providing results of the temperature, liquid pressure, and evolution of the degree of saturation. The material parameters, constitutive models, and assumptions made were carefully selected with regards to laboratory measurements reported in directly-related bibliography. The modelling settles the importance of understanding the groundwater flow through the rock mass and from fractures in the rock in order to achieve reliable predictions regarding buffer saturation, since it is known that the saturation times could range from few years to one thousand years depending on the hydrogeological conditions in the rock. The obtained results lead to full saturation times of 50 to 100 years. In addition to the rock hydraulic conductivity and fracture transmissivity, the saturation process was directly affected by the material properties of the buffer and gap presence between the buffer blocks and the host rock. Finally, in connection with thermal evolution, the thermal conductivity of repository components and the behaviour of air gaps in the buffer were key variables.Peer ReviewedPostprint (author's final draft

A bridge foundation analysis

This document presents the analysis with CODE_BRIGHT finite element program of a geotechnical case. The problem analyzed is related to a mechanical analysis of soil structure interaction considering different alternatives for the foundation of a bridge in El Prat de Llobregat (Highway A-2)i. The comparison of displacements shows that an alternative solution using shallow foundations can be considered in addition to the originally proposed, composed by sheet walls.Peer Reviewe

Sustainability of geosynthetics-based landslide stabilization solutions

This paper considers the sustainability of geosynthetics-based solutions to mitigate landslide risks. The different types of geosynthetics are briefly described, along with their functions and applications relevant to landslides, emphasizing reinforcement. The paper identifies the sustainability factors to consider when applying geosyn-thetics for these purposes. The paper then presents an overview based on existing literature to illustrate how geosynthetics typically outperform traditional methods across a range of sustainability criteria across the entire life cycle. The paper shows lastly how the value integrated model for sustainable evaluations (MIVES) tool can be applied to evaluate and compare alternative methods for remediation of landslides and recommends further studies using this tool.Peer ReviewedPostprint (published version

Short-term thermo-mechanical numerical modelling of reinforced soil walls with polyester strap reinforcements

Polyester (PET) materials have become more common as reinforcement solution in reinforced soil walls (RSW). It has been shown that strength and stiffness of geosynthetics products, including PET, is load-, time-, and temperature-dependent. Consequently, the mechanical response of these materials is influenced by in-soil conditions. The present study describes viscoelastic and visco-plastic constitutive formulations used to model PET strap reinforcement layers in thermo-mechanical finite element models. The models are demonstrated using an idealized 15-meter high RSW with concrete facing panels, including loading due to a road at the top of the structure. Reinforcement model parameters were calibrated using laboratory measured data. Analyses include temperature boundary conditions representing a Mediterranean climate for a 1-year period following end of construction. Calculated stress and strain values were in accordance with values found in the literature. The results of this study are a precursor for the long-term modelling of RSWs under operational conditions subjected to changing atmospheric boundary conditions.The authors wish to thank Aaron Kim from GECO Industrial (Korea, Rep.) for providing polymeric strap data from manufacturing quality control records. The authors wish to acknowledge the support of the Department of Civil and Environmental Engineering (DECA) of the Universitat Politécnica de Catalunya�����BarcelonaTech (UPC) and the International Centre for Numerical Methods in Engineering (CIMNE) and the funding received from the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa Programme for Centres of Excellence in R&D” (CEX2018-000797-S-20-4).Peer ReviewedPostprint (published version

Modeling soil-facing interface interaction with continuum element methodology

Soil-facing mechanical interactions play an important role in the behaviour of earth retaining walls. Generally, numerical analysis of earth retaining structures requires the use of interface elements between dissimilar component materials to model soil-structure interactions and to capture the transfer of normal and shear stresses through these discontinuities. In finite element method software programs, soil-structure interactions can be modelled using “zero-thickness” interface elements between the soil and structural components. These elements use a strength/stiffness reduction factor that is applied to the soil adjacent to the interface. However, in some numerical codes where the zero-thickness elements (or other similar special interface elements) are not available, the use of continuum elements to model soil-structure interactions is the only option. The continuum element approach allows more control of the interface features (i.e., material strength and stiffness properties) as well as the element sizes and shapes at the interfaces. This paper proposes parameter values for zero-thickness elements that will give the same numerical outcomes as those using continuum elements in finite element and finite difference commercial software. The numerical results show good agreement for the computed loads transferred from soil to structure using both methods (i.e., zero-thickness elements and continuum elements at interfaces). Both different interface modelling approaches can give very similar results using equivalent interface property values, and demonstrates the influence of choice of numerical mesh size on the numerical outcomes when continuum elements are used at the interfaces.Peer ReviewedPostprint (published version