Mourad Karray

Significance and evaluation of Poisson's ratio in Rayleigh wave testing

Important progress has been made in the past 20 years in the use of surface-wave testing in soil characteriza- tion. However, the effect of Poissons ratio on the construction of the theoretical dispersion relationships, associated with the inversion process, has not received enough attention and remains poorly documented. Five ideal profiles with different degrees of variation of shear-wave velocity with depth and three published case records are considered in this paper to study the effect of Poissons ratio on Rayleigh wave phase velocities. The effect of the variation of Poissons ratio on the evaluation of shear-wave velocity profiles (Vs) is also examined. Poissons ratio is generally assumed in surface-wave test- ing, and therefore the paper also examines the possibility of evaluating its value using a multi-mode inversion process. The results of surface-wave testing obtained at two experimental sites are then used to illustrate the potential of surface- wave testing to evaluate the Poissons ratio profile in addition to the Vs profile. The impact of Poissons ratio in Rayleigh wave testing is shown to be significantly more important than previously demonstrated. The error resulting from Poissons ratio does not depend solely on the magnitude of the inaccuracy. A multi-mode inversion process is shown to be a useful tool to determine the Poissons ratio profile, leading to a more accurate soil characterization.

IDENTIFICATION AND ISOLATION OF MULTIPLE MODES IN RAYLEIGH WAVES TESTING METHODS

The spectral-analyses-of-surface-waves (SASW), developed in the early eighties, has constituted an important step in the use of surface waves for definition of shear wave velocity profiles without intrusion. The SASW testing procedure was designed to minimize the contribution of higher Rayleigh modes and thus assumes that the average dispersion curve is representative of the fundamental Rayleigh mode. A number of numerical studies have however demonstrated that this basic assumption is valid only when the shear wave velocity varies regularly with depth. This study demonstrates, based in numerical simulations and an experimental case, that the multi-Rayleigh mode problem can occur in various situations even if the shear wave velocity increases regularly with depth. Techniques are proposed in order to identify and separate the energy of higher modes. The applicability of those techniques is demonstrated with simulated and experimental cases.

Investigation of the Concrete Lining after the Mont Blanc Tunnel Fire

After the Mont Blanc tunnel fire, investigations for assessing the damage to the concrete lining were performed with the objective of determining the need for repairs. It was an opportunity to test new methods. Two innovative methods were tested: colour measurements inside the lining, and the Modal Analysis of Seismic Waves (MASW) method, normally used for soil investigation. These two methods are presented in this paper. Some results from other more classical methods are given, showing results in good accordance. From the identification of burnt concrete to the measurement of concrete thickness, the MASW method is under development for other applications in the world of tunnels like impervious membrane position.

Detection des cavites sous les pavages par l'analyse modal des ondes de Rayleigh (MASW)

Le probleme de cavites sous les pavages au voisinage des ponceaux est relie a l’erosion du sol a travers des ponceaux defectueux. Ceci se traduit par le developpement d’une cheminee jusqu’a provoquer un affaissement jusqu’a la surface, creant ainsi un cratere dans le pavage routier. L’apparence de cratere dans le pavage constitue un danger pour les usagers, en particulier dans le cas des routes et autoroutes achalandees (circulation a frequence elevee). L’utilisation des ondes de surface pour la caracterisation des sols de fondation sous un pavage est problematique compte tenu de la difficulte d’exciter le sol a travers un pavage rigide. La methode « modal analysis of surface waves (MASW) » permet de placer les capteurs a la surface du pavage et d’exciter le sol a l’exterieur de celui-ci. Cette configuration a ete testee au moyen de simulations numeriques qui montrent qu’il est possible de caracteriser le sol en dessous d’une structure de pavage relativement rigide. Des essais « MASW » ont egalement ete r...

Numerical analysis of the effect of wall roughness in deep excavations in sand

Abstract Construction work in many instances requires ground excavation with either vertical or near-vertical face. In such situation, the excavation should be supported by temporary or permanent retaining braced system. The aim of the retaining system is to limit the ground settlement, which affects the lateral pressure acting on the retaining system. The lateral earth pressure on deep excavations used in design is determined from apparent pressure diagrams. These diagrams were established on field measurements from retaining systems that keep pace with neither current techniques nor work specifications nor wall types. The parametric studies were accomplished with the aid of numerical modeling that is based on the finite element method. The selected constitutive model for this research is the Mohr-Coulomb model. This parametric study was performed to explore the effect of retaining wall roughness on the distribution of lateral pressure. The results of numerical modeling generally conform to the classical design approaches using the apparent pressure diagrams that were developed by Peck (1969) and Tschebotarioff (1973). Finally, some practical recommendations are offered for geotechnical design of supported deep excavations.

Prediction of axial capacity of piles driven in non-cohesive soils based on neural networks approach

AbstractThis paper presents an application of two advanced approaches, Artificial Neural Networks (ANN) and Principal Component Analysis (PCA) in predicting the axial pile capacity. The combination of these two approaches allowed the development of an ANN model that provides more accurate axial capacity predictions. The model makes use of Back-Propagation Multi-Layer Perceptron (BPMLP) with Bayesian Regularization (BR), and it is established through the incorporation of approximately 415 data sets obtained from data published in the literature for a wide range of un-cemented soils and pile configurations. The compiled database includes, respectively 247 and 168 loading tests on large- and low-displacement driven piles. The contributions of the soil above and below pile toe to the pile base resistance are pre-evaluated using separate finite element (FE) analyses. The assessment of the predictive performance of the new method against a number of traditional SPT-based approaches indicates that the developed ...

Ultimate Lateral Resistance of Piles in Cohesive Soil

Abstract The ultimate lateral resistance of piles in cohesive soil is studied using the well-known finite difference code, FLAC2D. The Modified Cam Clay (MCC) constitutive relation is adopted in the analyses to model the cohesive soil behavior, whereas the structural pile model with three degree of freedoms, available in FLAC2D library, is adopted to model the piles. The reliability of Bromss method, still used in the current design practice of piles under lateral loads, is verified. Comparisons between the ultimate lateral resistances of piles and those deduced from the graphs proposed by Broms (1964) are presented in graphs. Different factors thought to affect the lateral resistance of piles in cohesive soil, not adequately considered in Bromss method, such as clay stiffness, pile length, pile diameter and axial load are parametrically studied. A special concern is devoted to elucidate the effects of over-consolidation ratio (OCR) on the ultimate lateral resistance of piles in cohesive soil.

Numerical investigation of the lateral response of battered pile foundations

Three-dimensional finite difference parametric analyses have been carried out to investigate the influence of vertical loads on the lateral performance of battered piles in both sandy and clayey soils. Different batter angles and soil stiffness were considered to examine the salient features of this complex soil–structure interaction problem. Numerical results show that the lateral response of battered piles embedded in sands is influenced by the value and the direction of the pile inclination, the presence of vertical loads as well as the soil state of density. When the lateral load acts in the opposite direction of the pile inclination (negative batter angle), the lateral response of the piles increases substantially with the batter angle as well as the sand packing. The response of piles at positive batter angles, however, does not appear to vary considerably with the batter angle or the sand density. The effect of the vertical loads on lateral response of battered piles in sands is found to be very pronounced. On the other hand, the lateral response of piles embedded in a clayey soil at negative batter angles increases greatly with the inclination angle and does not vary with its undrained shear strength. The lateral response of the piles with positive inclination angles is independent of the batter angle and the soil stiffness. Moreover, the presence of a vertical load prior to the application of a lateral load to a battered pile does not alter its lateral capacity.

Load sharing ratio of pile-raft system in loose sand: an experimental investigation

This technical note presents the results of a series of tests carried out on instrumented model pile-raft foundation to investigate the load sharing ratios between the foundation’s components where a loose sand layer is encountered near the ground surface. Four different pile-raft models with two different pile numbers and lengths were considered. In each model, the settlement of the raft and the load of each pile were monitored during the load application process. The experiments showed the effectiveness of pile length on reducing the overall settlement of the pile-raft system. The results showed also that the proportion of load transferred to any pile in a closely spaced pile group increases with its distance from the centre of the group. Unlike some previous studies that classified soil profiles containing loose sands near the surface as unfavourable situations for piled rafts, the experimental results herein represent an evidence of the efficiency of piled rafts even in soil profiles with loose sands immediately beneath the raft. Indeed, ignoring the contribution of the raft would lead to unnecessary over-conservatism in the design of piled rafts.

Displacement functions for layered and inhomogeneous soils

Abstract The risk associated with structures subjected to dynamic loading has created the need for rigorous dynamic analysis that takes into account soil-structure interaction. A key step of this analysis consists of estimating the dynamic response of the foundations by calculating their impedance functions (i.e. dynamic stiffness K and damping C). The purpose of this work is to evaluate first the displacement functions of strip footings on the surface of typical homogeneous soils in which the shear velocity, Vs, increases as a function of the effective overburden stress only. Calculations are performed over a sufficient range of oscillating frequency ratios a0. Displacement functions are also evaluated for different combinations of two-layer soil systems. The results are presented in order to recommend the thickness of soil needed to capture soil-structure interaction. It is concluded that the displacement functions of a two-layer system can be related to the displacement functions of homogeneous half-space (according to the ratio of S-wave velocities between these layers).