Associate Professor George Kouretzis

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. Shakedown analysis is an attractive method for determining the capacity of geostructures to sustain repeated loads involving a large number of cycles, e.g. rolling and sliding train wheel loads. Its main advantage is that it comes at a significantly reduced computational cost, compared to standard time-domain analyses. An essential component of shakedown analysis is the derivation of closed-form solutions to compute stresses due to the external repeated loads, a task that is not always feasible for complex problems as the ballasted rail track discussed herein. To tackle this, we present in this paper the use of finite element tools to obtain a quasi-lower-bound shakedown load numerically. The proposed method is based on the computation of the three-dimensional elastic stress field numerically, and the estimation of the shakedown load iteratively via an optimisation subroutine implemented in ABAQUS. Following a short presentation of this concept, we compare the elastic stress fields from models featuring varying degree of complexity, with the aim of identifying an optimal discretisation of the problem. This approach can be used for optimising the design of ballasted track structure, and this concept is briefly presented via a parametric study.

© 2019 John Wiley & Sons, Ltd. This paper presents a new analytical model for calculating the steady-state impedance of pile groups subjected to vertical dynamic loads. The derived solution allows considering effects of radially but also vertically propagating soil waves on the soil attenuation function, pile interaction factor, and pile group impedance. The proposed model provides accurate estimates of the soil stress field and of the response of the pile group in the low as well as in the high-frequency range, unlike earlier solutions based on the plane-strain model to describe the soil surrounding the piles, which ignores the vertical soil stress gradient. The latter assumption results in underestimating pile group impedance and overestimating radiation damping for frequencies lower than the cutoff frequencies of the system, which are explicitly captured with the proposed solution.

© 2019, © 2019 Taylor & Francis Group, LLC. This study investigates the influence of vertical components (VCs) of seismic excitations on the response of short and tall (slender) superstructures, and compares them with the system response under only horizontal components (HCs). The first part of the study examines the capability of the bounding multi-surface plasticity model to identify dynamic soil properties and soil¿pile¿superstructure system response to the HCs of a strong ground motion using existing results from dynamic centrifuge tests. The influence of VCs on short and tall superstructures is then investigated, and compared with system response under only HCs. A procedure is proposed to select strong vertical ground motion to complete a nonlinear time-history analysis with a three-dimensional finite element model. Step-by-step modelling techniques are described with a direct-method framework to simulate complex soil¿pile¿superstructure systems. This study emphasises the role of VCs in the general response of the system in regard to selected damage parameters for pile foundations. The results indicate that VCs may have increasing effects on axial loads and maximum pile-cap displacements for short superstructures. In addition, contrary to the vertical vibration period of the superstructure, the variation of the superstructure¿s horizontal vibration period with the peak ground acceleration ratio (V/H) may be significant.

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. This paper presents an analytical methodology that provides the dynamic response of an elastic pile embedded in viscoelastic non-uniform soil overlying a rigid base, when subjected to a harmonic vertical load. The non-uniform soil comprises two parts, featuring independent properties: the substratum soil, below the pile toe, is represented by a viscoelastic layer of finite thickness resting on rigid bedrock, while the soil surrounding the pile shaft is modelled as a series of infinitesimally thin independent viscoelastic layers. Following validation of the methodology, where we show that the presented solution degenerates to published solutions for simpler pile embedment conditions, we use results of a parametric investigation to discuss the mechanisms governing the dynamic response of the non-uniform soil¿pile system. The presented solution allows modelling realistically the contribution of end-bearing resistance to vertical vibrations of piles in cases of practical interest when non-linearity effects can be effectively ignored.

© 2015 Elsevier Ltd. This paper presents an analytical method to compute the dynamic response of a thin-walled pipe pile due to a vertical transient point load acting on its head. Inspired from challenges faced during the interpretation of low-strain integrity tests on pipe piles, the proposed method moves beyond the widely used one-dimensional wave theory to consider the asymmetric nature of the problem, and stress wave propagation along both the vertical and circumferential directions. Coupling of pipe pile-viscoelastic soil vibration is considered via modeling the outer and inner soil as a series of infinitesimally thin layers in perfect contact with the pile, and their low-strain properties are directly introduced in the solution. The methodology is validated against numerical results, before discussing the mechanisms governing the dynamic response of the pipe pile-soil system to the impact load, with emphasis on the vertical velocity measured at a hypothetical receiver placed on the pile head. Additional results from a parametric analysis are used to provide insights on the accurate estimation of the arrival time of the receiving wave, and the optimal location of the receiver.

© 2015 American Physical Society. We investigate numerically the mechanisms governing horizontal dragging of a rigid cylinder buried inside granular matter, with particular emphasis on enumerating drag and lift forces that resist cylinder movement. The recently proposed particle finite element method is employed, which combines the robustness of classical continuum mechanics formulations in terms of representing complex aspects of the material constitutive behavior, with the effectiveness of discrete element methods in simulating ultralarge deformation problems. The investigation focuses on the effect of embedment depth, cylinder roughness, granular matter macromechanical properties, and of the magnitude of the cylinder's horizontal displacement on the amplitude of the resisting forces, which are discussed in light of published experimental data. Interpretation of the results provides insight on how the material flow around the cylinder affects the developing resistance, and a mechanism is proposed to describe the development of a steady-state drag force at large horizontal movements of the cylinder.

This paper presents a study on the amplification of horizontal soil stresses during flat dilatometer test (DMT) blade penetration based on three-dimensional total and effective stress numerical analyses, while considering stress-flow coupling and large deformations. The focus here is on saturated clays, and the effect of soil stress history on the horizontal stress index is discussed in detail. The obtained results appear to be in good agreement with published and new field data, leading to the proposal of two new expressions for estimating the overconsolidation ratio and the earth pressure coefficient at rest directly from flat dilatometer tests in estuarine clays.

© 2015 Elsevier Ltd. We present an analytical study on the vertical vibration of an elastic pile embedded in poroelastic soil. The poroelastic soil is divided into a homogeneous half-space underlying the pile base and a series of infinitesimally thin independent layers along its shaft. The dynamic interaction problem is solved by extending a method originally proposed for an embedded rigid foundation. The validity of the derived solution is verified via comparison with existing solutions. Arithmetical examples are used to demonstrate the sensitivity of the vertical pile impedance to the relative rigidity of the two soil parts.

A new perspective on the numerical simulation of cone penetration in sand is presented, based on an enhanced critical state model implemented in an explicit-integration finite element code. Its main advantage, compared to similar studies employing simpler soil models, is that sand compressibility can be described with a single set of model parameters, irrespective of the stress level and the sand relative density. Calibration is based on back-analysis of published centrifuge experiments, while results of the methodology are also compared against independent tests. Additional analyses are performed to investigate sand state effects on cone penetration resistance, in comparison with empirical expressions from the literature. © 2013 Elsevier Ltd.

Numerical simulation of cone penetration in sand is performed by means of a computationally efficient critical state model implemented in an explicit-integration finite element code. Its main advantage, compared to other published studies employing simpler soil models such as the Drucker-Prager, is that sand compressibility can be described with a single set of model parameters, irrespective of the stress level and the sand relative density. Calibration of the constitutive model is based on back-analysis of published centrifuge tests results, and consequently the predictions of the numerical methodology are compared against independent tests. Additional analyses are performed for proposing a new simplified formula to correlate the cone penetration resistance with the in situ sand relative density. © (2014) Trans Tech Publications, Switzerland.

A large-deformation numerical methodology is applied to simulate the interaction effects for a pipeline installed in a trench backfilled with loosely deposited dry sand, focusing on shallow buried pipelines subjected to lateral displacements relative to the surrounding soil. Based on the backfill-pipeline deformation mode under shallow embedment conditions, described in previous experimental studies, analyses are performed while considering only the critical state shear strength parameters of the backfill. The numerical methodology is validated against experimental full-scale test measurements from the literature, for pipelines buried in uniform dry loose and medium sand. Parametric analyses are performed to generate approximate formulas and charts for calculating (i) the maximum force on the pipeline and (ii) the minimum trench dimensions to eliminate interaction with the surrounding natural ground. Application of the proposed approach in the prediction of independent full-scale test results for a pipeline embedded in a shallow trench demonstrates its effectiveness, and underlines the effect of trench dimensioning on the response of the pipeline. © 2013 Elsevier Ltd.

Diverse vertical embedment response is observed for partially embedded pipelines when experimentally tested under similar initial and boundary conditions. Although vertical resistance of pipelines is presented through simple analytical solutions, a number of factors contribute to complications in implementing these theories into practice. The objectives of this research is to provide a more detailed investigation on the vertical embedment for the partially-embedded pipelines (PEPs) using a coupled large deformation finite element (CLDFE) analysis with contact. A modified Cam Clay (MCC) model represents the elastoplastic response of the soil. The model of pipeline embedment investigates the effect of drainage condition on heave forming with respect to rate of penetration. Besides, effect of frictional contact on the heave development and wedging effect is investigated and design-related considerations are proposed. It is shown that depending on the rate of pipeline penetration and soil consolidation rate, the pipeline penetration response can be categorised as undrained, partially drained or fully drained. © (2014) Trans Tech Publications, Switzerland.