Associate Professor George Kouretzis

This paper presents a mathematical model for the analysis of axially-loaded end-bearing piles embedded in a two-layer elastic soil deposit. The governing equations of the soil layers surrounding the pile are cast by means of a Tajimi-type continuum formulation, and this allows deriving a closed-form expression that provides the frictional stresses that develop at the soil-pile interface. The pile is treated as two virtual one-dimensional elastic rods, and pile displacements are obtained by solving a system of linear equations that results from considering the boundary and continuity conditions. This results in improved efficiency compared to existing Tajimi-based solutions for piles in layered soil, at no expense of accuracy. The model is used to study the effect of the relative stiffness of the soil layers on the serviceability behavior of axially-loaded piles, and drawn conclusions of practical importance.

A mathematical formulation for the seismic analysis of floating piles embedded in a saturated soil layer to vertically propagating S waves is presented in this paper. Pivotal to the formulation is the establishment of a closed-form expression for the lateral soil reaction to pile displacement, and results from expressions for the free-field and scattered soil displacements obtained from Biot's governing equations. Subsequently the pile is analysed as Euler-Bernoulli beam and the soil column between the pile tip and the bedrock is analysed as one-dimensional shear beam. This results in expressions for the kinematic response factors, that provide the frequency-dependent pile head displacement and rotation as function of the free-field soil movement. Validation of the developed model is followed by a discussion on the sensitivity of pile response to the two-phase nature of soil, focused on informing researchers and practitioners on the uncertainties introduced in conventional seismic analysis, where the foundation soil is modelled as single-phase medium.

This paper presents the derivation of closed-form expressions for the interpretation of piezocone tests in clay on the basis of an undrained cavity expansion solution. The cavity expansion solution employs the modified Unified Hardening model to describe the undrained behaviour of normally- and over-consolidated clays upon shearing. The derived expressions correlate the measured tip resistance and pore pressure with the undrained shear strength and the over-consolidation ratio, using a cone factor that, unlike empirical formulas, is calculated directly from the constitutive model parameters and the cone geometry. The cavity expansion solution is validated via comparison of its results against published studies, and accordingly the accuracy of the proposed expressions is benchmarked against field data. We show that, despite the simplifications introduced in the derivation, the calibrated expressions are capable of reproducing the field data with reasonable accuracy.

This paper presents a numerical solution that describes the response to forced vertical vibrations of rigid strip footings founded on the surface of a poroelastic soil layer of finite thickness. To mathematically describe this mixed boundary value problem, we relax the boundary conditions and cast it as a pair of dual integral equations, which are transformed to a system of linear equations by means of Jacobi orthogonal polynomials. The system is solved numerically to provide the frequency-dependent footing compliance, and the associated nearby soil displacement. Results obtained with the model at hand underline the important contribution of standing waves to the response of the soil-foundation system, and draw the limits of existing solutions that consider the foundation soil as infinite half-space, where standing waves cannot form.

This paper presents an analytical solution that provides the response to vertical dynamic loads of pile groups with arbitrary numbers of end-bearing piles. The solution allows accounting for pile¿pile interaction effects on pile group response in a robust manner, as both wave diffraction and scattering are explicitly considered. The effects of soil layering are introduced by using the plane strain model to mathematically describe wave propagation in the subsoil. In addition to accounting for realistic soil conditions, the adopted plane strain formulation eliminates the need to calibrate the Winkler spring and dashpot parameters to obtain the impedance of pile groups. The presentation of the solution is accompanied by an arithmetic study focused on exploring the effects of scattered waves on the response of pile groups in layered soil.

This paper presents an experimental study on quantifying the effects of soil suction on the resistance offered by compacted unsaturated backfills to uplift of buried steel pipes and identifying the mechanisms that contribute to increased resistance compared to similar pipes buried in dry sand. This is achieved by means of 1-g physical model experiments, with the pipe buried in sandy loam¿Kaolin soil beds of varying water content (suction), compacted to the same dry unit weight. The main experiments are supplemented by benchmarking experiments performed in dry sand of similar grain size distribution, as well as in compacted soil beds inundated with water to achieve conditions close to full saturation. The experiments are supported by a detailed characterisation study of compacted sandy loam¿Kaolin mixtures and mini-CPT tests performed to evaluate the uniformity of the soil beds. Measurements of the reaction developing on the pipe as function of its uplift displacement are co-evaluated together with images of the failure mechanisms obtained using particle image velocimetry and continuous measurements of soil matrix suction. We conclude with a simplified method to predict the peak reaction to pipe uplift that allows considering the contribution of suction and the tensile¿shear failure mechanism observed during the experiments.

This paper presents a systematic study on the effect of radial soil deformations on the dynamic response of floating piles subjected to axial loads. The study is facilitated by a novel mathematical model, that allows rigorously considering both vertical and radial soil displacements generated by axial pile vibration, under an extended Tajimi-type framework. Following the presentation of the mathematical model and the solution, we show that the model produces similar results compared to rigorous numerical boundary integral methods. Then we compare results of the proposed model against an earlier model that is formulated on the basis of confined elastodynamics i.e. on ignoring radial soil deformations. We demonstrate that the confined model overestimates stiffness and underestimates damping of the pile impedance in the low frequency range, regardless of the problem conditions, and we explain the mechanisms that lead to these discrepancies. Although the proposed model is based on the elastodynamic theory, findings of this study are not limited to small-strain problems, as the identified mechanisms are solely associated with the effect of radial soil deformations, and not with the constitutive behavior of soil.

This paper presents an analytical solution for the analysis of axially-loaded compressible floating piles embedded in a homogeneous elastic soil layer of finite thickness. The mathematical model is based on a Tajimi-type elastic continuum formulation, that allows deriving an accurate closed-form expression for the frictional stresses developing at the soil-pile interface. The pile and its underlying soil column are considered as one-dimensional axially deforming rods, and solution of their governing equations leads to closed-form expressions for pile and soil displacements and stresses. The proposed model provides results of similar accuracy to boundary integral solutions for the special case of floating piles in elastic half-space, and to numerical analyses for the general problem of floating piles embedded in a layer of finite thickness. As such, the model is used to prepare charts for the settlement analysis of floating piles in practice, based on a parametric analysis that unveils the sensitivity of the axial response of floating piles to the different problem parameters.

This paper presents a mathematical model for calculating stresses due to moving deep-sea mining vehicles transferring dynamic loads to the seabed via twin or multiple caterpillar tracks. Expressions for the distribution of stresses in three dimensions are obtained for the special case of twin caterpillar tracks by using the triple Fourier transform technique, while assuming viscoelastic seabed behavior. Accordingly we establish stress attenuation functions, that allow robustly treating more complex track layouts, at no expense of accuracy. The proposed closed-form expressions can be used together with elasticity or plasticity theory methods to predict the performance of mining vehicles in different seabed conditions. We use a numerical case study to demonstrate that ignoring the dynamic nature of transferred loads may result in erroneous stress predictions, and thus vehicle performance estimates.

This paper presents an analytical study on the kinematic response of single cylindrical end-bearing piles embedded in homogeneous saturated soil, due to the propagation of seismic P-waves from the bedrock. The governing equations of soil are based on Biot¿s theory, while the pile is modelled as one-dimensional rod bonded to its surrounding soil. Solution of the pile and soil governing equations results in closed-form expressions for pile displacement and for the frictional force at the soil¿pile interface. These expressions are employed to study the difference in the kinematic response of piles embedded in saturated two-phase soil, compared to piles in single-phase soil, and investigate the sensitivity of the axial kinematic response of piles to the main problem parameters. We show that ignoring the two-phase nature of saturated soil may result in significantly underestimating the amplitude of strong motion transferred to the base of pile-supported structures.

This paper presents an air pluviation system, developed to facilitate 1g physical model tests in granular soils. The deposition process is fully automated and requires minimal input from the operator, thereby significantly reducing the time required to deposit large volumes of granular material, improving the uniformity of the prepared specimens and the reliability of test results. The components comprising the pluviation system have been calibrated to produce loose-to-very dense sand beds, of relative density that ranges between Dr = 7% and > 100% of the maximum density achieved with the procedures described in the pertinent standards. The testing chamber where sand is deposited is instrumented with an array of pressure sensors, and the rig is equipped with a miniature cone penetration testing (mini-CPT) device. Measurements from the earth pressure sensors and cone tip resistance profiles are used to evaluate how friction at the sand-chamber interfaces affects the distribution of geostatic stresses inside the chamber, the uniformity of sand beds and boundary effects during deposition and during mini-CPT testing. The air pluviation system allows preparing layered sand profiles by adjusting the deposition parameters on the fly, and this feature is demonstrated through the analysis of mini-CPT tests performed in layered sand beds.

This paper presents an analytical method to determine the response of rigid strip footings to harmonic horizontal loads, while considering the foundation soil as poroelastic, two-phase medium. The derived solution is the first of its kind that allows considering in a robust manner the coupled effects of soil deformation and flow on the dynamic response of strip footings to horizontal loads. The strategy for treating this complex problem analytically comprises transforming the mixed boundary conditions to a pair of dual integral equations that provide the contact stresses at the soil-footing interface, which are solved by means of Jacobi orthogonal polynomials. Presentation of the solution is followed by a discussion on how soil flow parameters affect the dynamic response of strip footings to horizontal loads, and the importance of considering the foundation soil as two-phase medium.

In this paper we revisit the problem of modelling analytically the kinematic interaction between a single pile and its surrounding soil under the action of seismic shear waves, by means of a Tajimi-type continuum elastodynamic model in three dimensions. The model provides the steady-state kinematic response of a cylindrical end-bearing pile embedded in a homogeneous viscoelastic soil layer, subjected to vertically propagating harmonic S-waves. Results of the model are first validated against the results of numerical simulations, and the results of an existing, approximate solution. Accordingly, we employ the model in a parametric study, where we investigate the sensitivity of the seismic response of piles to certain key problem parameters, including pile slenderness, soil-pile relative stiffness, excitation frequency and fixity conditions at the pile head. The solution yields closed-form expressions for pile deformations and for the soil resistance developing on the pile, that do not require introducing fitting coefficients.

This paper presents a mathematical model that provides the response to lateral dynamic loads of single floating piles embedded in a homogeneous viscoelastic soil layer. The formulation results in closed-form expressions of the pile's swaying, rocking and cross swaying-rocking compliance components, that do not require solving complex integral equations or iterative calculations. The model exploits Tajimi's elastodynamic theory to compute the dynamic soil resistance to pile movements. One of its main features is allowing for the effect of the thickness of the soil layer on the response of the soil-pile system, by properly accounting for resonance effects associated with the formation of standing waves. The model is validated by comparing its results for the special case of a floating pile embedded in elastic half space against published analytical and numerical results. The paper concludes with a parametric study, focused on exploring under which conditions standing waves may result in ¿softer¿ response of the soil-pile system, a phenomenon that may be important for the serviceability analysis of foundation systems sensitive to resonance effects.

This paper presents an analytical solution developed to describe the dynamic interaction between a single end-bearing pile subjected to a dynamic axial load on its head, and its surrounding poroelastic soil. The main feature of the solution is that both the two-phase soil and the pile are treated as three-dimensional continua. This allows capturing the propagation of compressional and shear waves in three dimensions, and their effect on pile response. The soil surrounding the pile is treated as porous medium and its response is described with Biot's poroelastic theory, while the pile is treated as linear elastic solid, considering explicitly both vertical and radial pile displacements. The solution yields expressions for the displacement and velocity admittance at the pile head in the frequency domain, and allows computing pile head velocity time histories. These are applied to study the effects of wave propagation in three dimensions on pile response in the frequency- and in the time-domain, and identify the conditions under which these effects may be important for the analysis of piles in practice.

This paper presents an analytical solution that provides the dynamic response of laterally-loaded pile groups with an arbitrary number of piles installed in layered soil. The solution allows modelling robustly pile-to-pile interaction effects via a new stress-based interaction factor, that considers the fact that soil displacements along the periphery of large-diameter piles of a group subjected to lateral loads will not be in-phase. Layered soil conditions are introduced in the solution by modelling wave propagation in the soil surrounding the piles with the popular plane-strain model and considering soil as viscoelastic material. These idealisations greatly simplify the formulation, at no expense of accuracy when considering loading of relatively stiff piles under serviceability conditions. The solution provides the Winkler modulus for each pile in the group as function of problem parameters that have direct phenomenological meaning, and allows direct calculation of the horizontal impedance of the entire pile group. The presentation concludes with results of a parametric analysis, to demonstrate the effect of considering layered soil conditions and the cross-sectional dimensions of the piles on the response of the group.

This paper presents a series of physical modelling tests performed to measure the resistance developing during lateral dragging of a rigid pipe buried in loose to very dense dry sand. The experiments were performed in a small-scale prototype developed to model sand-pipe interaction during relative ground movement episodes while accurately controlling the density and uniformity of sand around the pipe. Digital imaging and particle image velocimetry equipment are integrated with the rig, so as to track the evolution of the failure surface developing in sand with increasing pipe displacements. Auxiliary components of the rig allow investigation of the effects of pipe kinematic constraints and embedment method on the results obtained. Accordingly, the measurements obtained with the developed prototype are compared against results from similar studies, with the intention of shedding some light on the scatter observed in published data, and on the provisions from different pipe stress analysis guidelines. It is shown that current simplified methods may underestimate the lateral reaction developing on pipes in very dense sand beds, and analysis models built around these methods may under-predict pipe strains. To alleviate this, a modified expression is proposed for estimating the peak reaction of lateral elastoplastic soil springs, and an upper bound of this reaction is provided for design purposes.

An isotach elastoplastic constitutive model devised by Yang et al. (2016) and referred to as the Hunter Clay (HC) model attempts to capture a number of key behaviours of soft soils within a critical state framework, namely destructuration, fabric anisotropy and rate dependency, the latter often manifesting in creep settlement. Finite element implementation of the HC model is a useful means by which its application to practical problems can be facilitated. However, there are a number of significant challenges associated with the translation of isotach elastoplastic models into a finite element setting. In this paper, a detailed discussion of these challenges is undertaken and a new finite element implementation of the HC model is subsequently developed. This includes sophisticated numerical integration algorithms which employ automatic time substepping for solution of the governing finite element equations. The ability of the implemented HC model to predict the mechanical behaviour of soft soils under 1D compression is investigated via simulation of laboratory tests carried out on Ballina clay by Pineda et al. (2016) and Parkinson (2018).

This paper presents results of scaled physical model tests performed to measure the reaction developing on a rigid pipe buried in dry sand when the pipe is subjected to vertical downwards movement relative to its surrounding soil. The aim of this experimental study is to evaluate the efficacy of methods used to determine the properties of vertical bearing springs, an integral part of beam-on-nonlinear Winkler spring models used for the analysis of buried pipelines subjected to permanent ground displacements. We show that bearing capacity formulas used in practice to estimate the ultimate reaction developing on buried pipes may provide reasonably accurate estimates, provided that they are used together with sand friction angle values that account for the fact that granular materials do not obey an associative flow rule, and with bearing capacity factors compatible with the mode of sand failure observed in the tests. We also provide evidence suggesting that laying pipes in loose sand backfillsdoesnothaveabeneficial effect on the reaction developing on the pipe, compared to medium dense sand, and we recommend against using loose sand material properties for the estimation of the properties of vertical bearing springs.

Permanent ground movements due to seismic fault rupture, ground subsidence or liquefaction-induced lateral spreading may result to failure of buried pipelines crossing the affected areas, due to the development of excessive bending and axial pipe strains. Verification of pipe integrity against such actions and design of mitigation measures is based on analyses with nonlinear beam-on-non-linear Winkler foundation models, where the soil reaction developing on a pipe as result of differential ground movements is modelled by means of elastoplastic springs. This note presents a new set of guidelines and expressions for determining the parameters of Winkler springs applicable to the analysis of steel pipes backfilled with sand and subjected to differential movement with a lateral, vertical-upwards and/or vertical-downwards component. The guidelines, collectively referred to as UoN recommendations, are accordingly employed in the parametric stress analysis of a buried steel pipe crossing an oblique seismic fault. Results of this parametric analysis demonstrate that adoption of the UoN recommendations in the analysis workflow results in considerably lower pipe axial and bending strains, compared to analyses based on current conservative methods, and thus more realistic and economical design solutions.

¿his paper presents an analytical method for calculating the steady-state impedance factors of pile groups of arbitrary configuration subjected to harmonic vertical loads. The derived solution allows considering the effect of the actual pile geometry on the contribution of pile-soil-pile interaction to the response of the group, via the introduction of a new dynamic interaction factor, defined on the basis of soil resistance instead of pile displacements. The solution is first validated against a published solution for single piles that accounts for the effect of pile geometry on the generated ground vibrations. Accordingly, we show that the derived soil attenuation factor agrees well with existing solutions for pile groups in the high frequency range, but considerable differences are observed in both the stiffness and damping components of the computed impedance when the relative spacing between piles decreases. Numerical results obtained for typical problem parameters suggest that ignoring pile geometry effects while estimating the contribution of pile-soil-pile interaction in the response may lead to inaccurate results, even for relative large pile group spacings.

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.

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.

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.