An interdisciplinary approach towards improved understanding of soil deformation during compaction

International audienceSoil compaction not only reduces available pore volume in which fluids are stored, but it alters the arrangement of soil constituents and pore geometry, thereby adversely impacting fluid transport and a range of soil ecological functions. Quantitative understanding of stress transmission and deformation processes in arable soils remains limited. Yet such knowledge is essential for better predictions of effects of soil management practices such as agricultural field traffic on soil functioning. Concepts and theory used in agricultural soil mechanics (soil compaction and soil tillage) are often adopted from conventional soil mechanics (e.g. foundation engineering). However, in contrast with standard geotechnical applications, undesired stresses applied by agricultural tyres/tracks are highly dynamic and last for very short times. Moreover, arable soils are typically unsaturated and contain important secondary structures (e.g. aggregates), factors important for affecting their soil mechanical behaviour. Mechanical processes in porous media are not only of concern in soil mechanics, but also in other fields including geophysics and granular material science. Despite similarity of basic mechanical processes, theoretical frameworks often differ and reflect disciplinary focus. We review concepts from different but complementary fields concerned with porous media mechanics and highlight opportunities for synergistic advances in understanding deformation and compaction of arable soils. We highlight the important role of technological advances in non-destructive measurement methods at pore (X-ray tomography) and soil profile (seismic) scales that not only offer new insights into soil architecture and enable visualization of soil deformation, but are becoming instrumental in the development and validation of new soil compaction models. The integration of concepts underlying dynamic processes that modify soil pore spaces and bulk properties will improve the understanding of how soil management affect vital soil mechanical, hydraulic and ecological functions supporting plant growth

Engineered metabarrier as shield from seismic surface waves

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation. However, for seismic excitation, where energy is mostly carried by surface waves, energy reflection and redirection might lead to harming surrounding regions. Here, we propose a seismic metabarrier able to convert seismic Rayleigh waves into shear bulk waves that propagate away from the soil surface. The metabarrier is realized by burying sub-wavelength resonant structures under the soil surface. Each resonant structure consists of a cylindrical mass suspended by elastomeric springs within a concrete case and can be tuned to the resonance frequency of interest. The design allows controlling seismic waves with wavelengths from 10-to-100 m with meter-sized resonant structures. We develop an analytical model based on effective medium theory able to capture the mode conversion mechanism. The model is used to guide the design of metabarriers for varying soil conditions and validated using finite-element simulations. We investigate the shielding performance of a metabarrier in a scaled experimental model and demonstrate that surface ground motion can be reduced up to 50% in frequency regions below 10 Hz, relevant for the protection of buildings and civil infrastructures

Towards specific T–H relationships: FRIBAS database for better characterization of RC and URM buildings

FRIBAS database is an open access database (https://doi.org/10.5281/zenodo.6505442) composed of the characteristics of 312 buildings (71 masonry, 237 reinforced concrete and 4 mixed types). It collects and harmonizes data from different surveys performed on buildings in the Basilicata and Friuli Venezia Giulia regions (Southern and Northeastern Italy, respectively). Each building is defined by 37 parameters related to the building and foundation soil characteristics. The building and soil fundamental periods were experimentally estimated based on ambient noise measurements. FRIBAS gave us the opportunity to study the influence of the main characteristics of buildings and the soil-building interaction effect to their structural response. In this study, we have used the FRIBAS dataset to investigate how the building period varies as a function of construction materials and soil types. Our results motivate the need of going beyond a ‘one-fits-all’ numerical period–height (T–H) relationship for generic building typologies provided by seismic codes, towards specific T–H relationships that account for both soil and building typologies

Calibrated 3D Computational Modeling of Soil-Structure Systems and Liquefaction Scenarios

Three-dimensional (3D) computational simulation is increasingly allowing for insights into the mechanics of seismic soil-structure system response. Calibration is being facilitated by field, full-scale, and centrifuge model laboratory data. Computational algorithms and scenario-specific graphical user-interfaces are gradually permitting the routine adoption of such geometrically realistic simulation environments. This paper presents an overview of salient recent 3D soil-foundation-structure earthquake response simulations. Developments related to graphical user-interfaces (OpenSeesPL, http://cyclic.ucsd.edu/openseespl) are summarized, demonstrating the current and evolving capabilities towards performance-based earthquake engineering (PBEE). From an OpenSeesPL-generated lateral push-over analysis of a large pile-group, it is shown that corner piles may shoulder a significantly higher level of load (axial, shear, and bending). Evolution of large tensile forces in these piles may warrant careful consideration. Modeling of liquefaction response mechanisms are also discussed, highlighting the role of cyclic mobility and influence of permeability in dictating the level of associated ground shear deformations, and related countermeasure performance

Towards specific T–H relationships: FRIBAS database for better characterization of RC and URM buildings

FRIBAS database is an open access database composed of the characteristics of 312 buildings (71 masonry, 237 reinforced concrete and 4 mixed types). It collects and harmonizes data from different surveys performed on buildings in the Basilicata and Friuli Venezia Giulia regions (Southern and Northeastern Italy, respectively). Each building is defined by 37 parameters related to the building and foundation soil characteristics. The building and soil fundamental periods were experimentally estimated based on ambient noise measurements. FRIBAS gave us the opportunity to study the influence of the main characteristics of buildings and the soil-building interaction effect to their structural response. In this study, we have used the FRIBAS dataset to investigate how the building period varies as a function of construction materials and soil types. Our results motivate the need of going beyond a 'one-fits-all' numerical period-height (T-H) relationship for generic building typologies provided by seismic codes, towards specific T-H relationships that account for both soil and building typologies

Analysis of simplified time of liquefaction triggering methods by laboratory tests, physical modelling and numerical analysis

The damage resulting from earthquakes can result from the combination of seismic excitation and/or due to a build-up of excess pore pressure in the soil (liquefaction). These two effects are related since the reduction of soil stiffness due to a decrease in effective stress, modifies the seismic response of the soil deposit. Therefore, the expected level and type of damage is dependent on the amount of seismic energy reaching the ground surface before liquefaction. The development and validation of simplified liquefaction assessment methods to provide reasonable estimates of the build-up of excess pore pressure is essential for improving estimates of the level of seismic demand (ground shaking and permanent ground deformation) that may be experienced by a building. This paper presents two methods, one based on equivalent cyclic stress loading, and another based on the cumulative strain energy, which are used to predict the evolution of the pore pressure build up throughout time. The centrifuge tests performed in ISMGEO (Italy) during the LIQUEFACT project (www.liquefact.eu) were used as a benchmark to evaluate the predictive performance of the methods. Additionally, a series of one dimensional soil column effective stress and total stress analyses and single soil element simulations were run. Available laboratory tests were used to calibrate the parameters of the simplified methods, as well as input parameters for the numerical simulations. The results showed that both simplified methods had considerable bias. A direct comparison of the effective stress analyses, a set of effective stress analyses with limited drainage, and the centrifuge results, highlighted that the centrifuge experiments exhibited significant pore water flow that was not captured in the simplified models. Comparisons between the irregular loading in the one dimensional analyses compared to the uniform loading in the element tests highlighted shortfalls in the conversion from irregular to equivalent uniform loading. Comparisons between stress demands from total stress, effective stress and the simplified methods illustrated the limitations of relying on the total stress acceleration to estimate demands on a soil in a liquefying deposit

Assessment of the effects of soil variability in the modeling of liquefiable soils

La licuación del suelo puede tener consecuencias catastróficas en términos de daños estructurales y pérdida de vidas humanas. Una de las principales incertidumbres al momento de realizar modelos para predecir la respuesta del suelo ante el fenómeno de licuación es la variabilidad espacial de las propiedades del suelo causada por su naturaleza heterogénea. Esta investigación tiene como objetivo evaluar los efectos de la variabilidad espacial de un depósito de suelos licuables. Para desarrollar este análisis, se implementó un modelo determinístico de elementos finitos. El suelo se modeló utilizando un modelo constitutivo de múltiples superficies de fluencia, el cual fue calibrado empleando ensayos triaxiales cíclicos. Posteriormente, se desarrolló una evaluación estocástica del problema empleando el Método de Elementos Finitos Aleatorios (RFEM), en el que se modeló la densidad relativa como un campo aleatorio gaussiano correlacionado espacialmente, considerando la variabilidad esperada en condiciones experimentales y en las condiciones in situ del suelo. Al final, se determinaron los efectos de la variabilidad espacial del suelo comparando los resultados de las simulaciones determinísticas y estocásticas con resultados experimentales de centrífuga.MaestríaMagister en Ingeniería Civi

Effect of Liquefaction Induced Lateral Spreading on Seismic Performance of Pile Foundations

Seismically active areas are vulnerable to liquefaction, and the influence of liquefaction on pile foundations is very severe. Study of pile-supported buildings in liquefiable soils requires consideration of soil-pile interaction and evaluation of the interaction resulting from movement of soil surrounding the pile. This paper presents the results of three-dimensional finite difference analyses conducted to understand the effect of liquefiable soils on the seismic performance of piles and pile groups embedded in stratified soil deposits using the numerical tool FLAC3D. A comparative study has been conducted on the performance of pile foundations on level ground and sloping ground. The soil model consists of a non-liquefiable, slightly cemented sand layer at the top and bottom and a liquefiable Nevada sand layer in between. This stratified ground is subjected to 1940 El Centro, 2001 Bhuj (India) earthquake ground motions, and harmonic motion of 0.3g acceleration. Parametric studies have been carried out by changing the ground slope from 0° to 10° to understand the effects of sloping ground on pile group response. The results indicate that the maximum bending moments occur at boundaries between liquefiable and non-liquefiable layers, and that the bending moment increases with an increase in slope angle. The presence of a pile cap prevents horizontal ground displacements at ground level. Further, it is also observed that the displacements of pile groups under sloping ground are in excess of those on level ground due to lateral spreading. Doi: 10.28991/CEJ-SP2021-07-05 Full Text: PD

Finite Element Modeling of Seismic Wave Propagation in Earthen Embankments

The detection of internal seepage zones in embankments (dams and levees) by geophysical seismic techniques such as seismic refraction is limited by a number of factors. Some factors are associated with inversion and smoothing problems during processing, while others are associated with the natural characteristics of embankments and seepage anomalies. In this research, changes in the seismic response associated with: embankment soil compositions and moisture, characteristics of the seepage zone, presence of water in the reservoir, and shape of embankment was studied via 2D and 3D finite element (FE) embankment models. Artificial reflections from external boundaries and numerical dispersion were first examined in the frame work of COMSOL. A combination of an absorbing layer and dashpot elements produced minimal reflections. The numerical dispersion study suggested a mesh composed of 5 quartic (4th order) elements per wavelength and a time step of 1/4 of 1/20 of the minimum period to be optimal. COMSOL models were verified by comparing to the analytic solutions for a transient point source in an unbounded media. The agreement of arrival times from a point source and a line source were also ascertained for an elastic half space model. The seismic response of dry and wet seepage zones in an embankment were evaluated for 2D longitudinal and transverse models. The zones considered in this study do not cause substantial deviations on the first arrival times but behave as scatters and their signatures were, predominantly, wavelet distortion. Wet (high impedance) zone produces a higher amplitude wavelet that is delayed in time, whereas a dry (low impedance) zone produces an earlier arriving lower amplitude, first arriving wavelet. Processing algorithms such as tomography that can incorporate such finite frequency effects may improve the detection of internal seepage in earthen embankments. The results from preliminary 3D models suggest that the water in the reservoir and the embankment\u27s 3D shape have no effect on the first arrival times of seismic waves