Sidi Mohammed Elachachi

Soil Spatial Variability and Structural Reliability of Buried Networks Subjected to Earthquakes

Dysfunctions and failures of buried pipe networks like sewer networks are studied from the point of view of the heterogeneity of geotechnical conditions in the longitudinal direction and of the applied action (seismic action). Combined soil defects (differential settlements along the pipe, landslides, voids surrounding the pipe, etc.) and peak ground acceleration (PGA) induce stresses (which leads to an ultimate limit state ULS) and displacements (which constitute a violation of a serviceability limit state SLS). It is remarkable to note that the influence of the variability of the soil is not reflected in current European standards. A model has been developed which includes a description of the soil spatial variability, within the frame of geostatistics, where the correlation length of soil properties is the main parameter and a mechanical description of the soil–structure interaction of a set of buried pipes with flexible connections resting on the soil by a two parameter model (Pasternak model). Reliability analysis is performed on the sewer by using a Response Surface Model (RSM), with the reliability index calculated for two limit states: Serviceability limit state, corresponding to a too large “counterslope” in a given pipe, which can prevent the normal flow of fluids, and Ultimate limit state, corresponding to a too large bending moment, thus bending stress, which can cause cracks in the pipes. The response in time domain of a buried pipe subjected to natural ground motion records and by taking into account a longitudinal variability of the properties of the soil is modeled. Several conclusions are drawn: Soil heterogeneity induces effects (differential settlements, bending moments, stresses and possible cracking) that cannot be predicted if homogeneity is assumed and the magnitude of the induced stresses depends mainly on four factors: the soil-structure length ratio, which combines the soil fluctuation scale and a structural characteristic length (buried pipe length), a magnitude of the soil variability (i.e. its coefficient of variation), a soil-structure stiffness ratio, and a structure-connection stiffness ratio (relative flexibility).

Soil-structure interaction effects on RC structures within a performance-based earthquake engineering framework

Performance-based earthquake engineering (PBEE) has emerged as a powerful method of analysis and design philosophy in earthquake engineering. Structures are generally assumed to be fixed at their bases in the process of analysis and design under dynamic loading. Response of structures under earthquakes is strongly influenced by the soil-structure system. In soil-structure interaction (SSI) problems, the ability to predict the coupled behaviour of the soil and the structure is essential and requires combined soil and structure models. In the context of PBEE, this paper combines structural behaviour and seismic response analysis of SSI systems. Related to SSI analysis, several issues are studied, such as relative importance of soil parameters, and relative foundation/soil stiffness ratio, in regards to a specified aspect of the system response (e.g. response parameters). A simplified approach is proposed to consider SSI effects on the nonlinear seismic response of a reinforced concrete structure using the nonlinear replacement oscillator method. This oscillator is characterised by an effective ductility along with the known effective period and damping of the system for the elastic condition. The N2 method is used to determine the nonlinear response and extended to include SSI in the design. It is confirmed that the response of the structure depends not only on its dynamic characteristics and on the seismic excitation characteristics but also on the external environment surrounding the base of the structure, i.e. the interaction between the structure, the foundation and the soil. The proposed approach is validated and compared with time history analysis and two other recognised methods.

Evaluation of the seismic vulnerability of quasi symmetrical reinforced concrete structures with shear walls

ABSTRACT The vulnerability assessment of structures is a key element for earthquake prevention and mitigation strategies. In this study, we focus at one structures type, namely a quasi symmetrical reinforced concrete structure with structural walls. The structure is discretized into a set of macro-elements, each macro-element being representative of a structures floor. The estimated performance level achieved during a seismic action is obtained from capacity curves (based on static pushover analysis). The performance analysis of the structure is a probabilistic analysis since both the response spectrum (action) and the materials properties (resistance) are random. A parametric study was conducted to determine the vulnerability cumulative distribution functions and to identify the parameters that control them.

Modélisation de la fiabilité structurale des réseaux enterrés dans un environnement hétérogène

ABSTRACT The reliability of buried pipes from the point of view of their longitudinal behavior was studied, in order to analyze the consequences of the longitudinal heterogeneity of the geotechnic conditions on the mechanical behavior of the elements of the network. The spatial variability of the soil is modelled within the framework of the random fields theory, local rigidity being defined by a random variable with its average, its coefficient of variation and its spatial organization, by means of a correlation length. The reliability of the system is analyzed in terms of forces in the pipes (mainly bending moments, which are compared with the ultimate moments of the cross-sections) and in terms of displacements (misalignment, compared with the threshold values, prejudicial with a good hydraulic operation of the system).

Global sensitivity analysis of soil structure interaction system using N2-SSI method

This paper applies the Sobol’s variance-based global sensitivity analysis to determine the contribution of input parameters to the structural lateral displacement considering damping and taking into account soil structure interaction (SSI) process. This method includes the main effect, the total effect and provides specific parametric interaction for each input. The considered input parameters are soil Poisson’s ratio, soil density, shear wave velocity, soil damping and structural damping. The N2-SSI simplified procedure is used to estimate the lateral displacement and damping of SSI model of RC structure. This simplified nonlinear method allows the assessment of the seismic performance considering the SSI. The results of the sensitivity analysis indicate that soil damping and structural damping greatly affect on the overall damping and can vary according to soil type. Whereas, the maximum lateral displacement is most sensitive to shear wave velocity and soil damping for very soft and soft soils, and it is sensitive to shear wave velocity and structural damping for hard and rock soils. This study reveals the importance of considering more than one output in the sensitivity study for the same input parameters. A same parameter can be considered as deterministic if it has a small influence on the output and it can be considered as probabilistic if it has significant influence on another output.

Influence of spatial variability of soil friction angle on sheet pile walls’ structural behaviour

ABSTRACT The uncertainty in terms of soil characterisation is studied to assess its effect on the structural behaviour of extended structures as sheet pile walls. A finite element model is used. This integrates a numerical model of the soil–structure interaction together with a stochastic model that allows characterising the soil variability. The model serves in propagating the variability and the system parameter uncertainties. Discussion is mainly focused on two points: (1) testing the sensitivity of the structural behaviour of a sheet pile wall to different geotechnical parameters and (2) assessing the influence of spatial variability of soil properties on the structural behaviour by identifying the most sensitive geotechnical parameter and the most significant correlation length values. The findings showed that in assessing the sheet pile wall’s structural behaviour, there are spatial variability parameters that cannot be considered negligible. In this study, soil friction angle is found to be an important parameter.

Response of buried pipes taking into account seismic and soil spatial variabilities

Heavy damage to pipelines has occurred in many strong earthquakes. Because pipes extend over long distances parallel to the ground, their supports undergo differential motions during an earthquake. Furthermore the soil spatial variability and the soil-pipe interaction contribute to the appearance of additional stresses and deformations. Herein, a frequency domain spectral analysis for obtaining the response of pipes to correlated or partially correlated non stationary random ground motion and spatially random soil is presented. The space-time variability of the ground motions and of the soil spatial variability are modeled following a stochastic description through random process (fields) and spectral analysis. The key results are that soil heterogeneity induces significant effects (differential settlements, bending moments, counter-slopes) that cannot be predicted if soil homogeneity is assumed. The spatial variation of the ground motion could increase locally the structural response depending on the soil-structure interaction and the pipe-joint stiffness ratio in segmented pipes.

Effects of spatial soil heterogeneities on structural behavior of a steel sheet pile

This article aims to introduce spatial variability effect on the behavior of extended structures as the sheet piles. The importance of these aspects can provide elements of response to experts in charge of the standards of design for these structures. Two main problems are identified, the first one is about the most appropriate way to model the variability of soil and its interaction with the sheet pile. The second one is to take into account the uncertainties and their propagation through a model. A 2-D mechanical model of the sheet pile has served to propagate the soil variability and the system-parameter uncertainties and allowed to study the response of the different structural elements of the structure. The spatial soil heterogeneities show clearly its effect on the behavior of the structure. This analysis has opened up new possibilities to advance in researches on longitudinal aspects for these structures.

Analyse multi-échelle de la résistance du câble porteur d'un pont suspendu: Description de la corrosion et de ses effets par une approche probabiliste

ABSTRACT Suspension cables of bridges structures suffer from the continuous aggression of the environment. Its effects appear through the corrosion phenomena inducing of strong modifications of the geometrical and mechanical characteristics of the components, which lead to a notable reduction of the resistant capacity of the cable according to time, being able to bring sometimes to its partial rupture. An analysis to evaluate the factors affecting the long-term performance of cable was carried out to develop a strength capacity model, to consider the lifespan and to thus evaluate the risk of failure.