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Thermo-active structures are underground facilities that enable the exchange of thermal energy between the ground and the overlying buildings, thus providing renewable means of space heating and cooling. Although this technology is becoming increasingly popular, the behaviour of geotechnical structures under additional thermal loading is still not fully understood. This paper focuses on the use of underground tunnels as thermo-active structures and explains their behaviour through a series of finite-element analyses based on an existing case study of isothermal tunnels in London Clay. The bespoke finite-element code adopted – the Imperial College Finite Element Program (ICFEP) – is capable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. The complex coupled interactions between the tunnel and the surrounding soil are explored by comparing results from selected types of coupled and uncoupled simulations. It is demonstrated that: (a) the thermally induced deformation of the tunnel and the ground are more critical design aspects than the thermally induced forces in the tunnel lining, and (b) the modelling approach in terms of the type of analysis, as well as the assumed permeability of the tunnel lining, has a significant effect on the computed tunnel response and, hence, must be chosen carefully.

This paper describes the results obtained from a field testing campaign on laterally loaded monopiles conducted at Cowden, UK, where the soil consists principally of a heavily overconsolidated glacial till. These tests formed part of the PISA project on the development of improved design methods for monopile foundations for offshore wind turbines. Results obtained for monotonic loading tests on piles of three different diameters (0·273 m, 0·762 m and 2·0 m) are presented. The piles had length-to-diameter ratios (L/D) of between 3 and 10. The tests included the application of monotonic loading incorporating periods of constant load to investigate creep effects, and investigations on the influence of loading rate. Data are presented on measured bending moments and inclinations induced in the piles. Inferred data on lateral displacements of the embedded section of the piles are determined using an optimised structural model. These field data support the development of a new one-dimensional modelling approach for the design of monopile foundations for offshore wind turbines. They also form a unique database of field measurements in an overconsolidated clay, from lateral loading of piles at a vertical distance above the ground surface.

The PISA Joint Industry Research Project was concerned with the development of improved design methods for monopile foundations in offshore wind applications. PISA involved large-scale pile tests in overconsolidated glacial till at Cowden, north-east England, and in dense, normally consolidated marine sand at Dunkirk, northern France. This paper describes the experimental set-up for pile testing, with unique features of load-application mechanisms and built-in fibre optic strain gauges. New procedures are described for the interpretation of pile loading data, and specifically for providing precise interpretation of pile displacements.

This paper describes a one-dimensional (1D) computational model for the analysis and design of laterally loaded monopile foundations for offshore wind turbine applications. The model represents the monopile as an embedded beam and specially formulated functions, referred to as soil reaction curves, are employed to represent the various components of soil reaction that are assumed to act on the pile. This design model was an outcome of a recently completed joint industry research project – known as PISA – on the development of new procedures for the design of monopile foundations for offshore wind applications. The overall framework of the model, and an application to a stiff glacial clay till soil, is described in a companion paper by Byrne and co-workers; the current paper describes an alternative formulation that has been developed for soil reaction curves that are applicable to monopiles installed at offshore homogeneous sand sites, for drained loading. The 1D model is calibrated using data from a set of three-dimensional finite-element analyses, conducted over a calibration space comprising pile geometries, loading configurations and soil relative densities that span typical design values. The performance of the model is demonstrated by the analysis of example design cases. The current form of the model is applicable to homogeneous soil and monotonic loading, although extensions to soil layering and cyclic loading are possible.

The PISA design model is a procedure for the analysis of monopile foundations for offshore wind turbine applications. This design model has been previously calibrated for homogeneous soils; this paper extends the modelling approach to the analysis of monopiles installed at sites where the soil profile is layered. The paper describes a computational study on monopiles embedded in layered soil configurations comprising selected combinations of soft and stiff clay and sand at a range of relative densities. The study comprises (a) analyses of monopile behaviour using detailed three-dimensional (3D) finite-element analysis, and (b) calculations employing the PISA design model. Results from the 3D analyses are used to explore the various influences that soil layering has on the performance of the monopile. The fidelity of the PISA design model is assessed by comparisons with data obtained from equivalent 3D finite-element analyses, demonstrating a good agreement in most cases. This comparative study demonstrates that the PISA design model can be applied successfully to layered soil configurations, except in certain cases involving combinations of very soft clay and very dense sand.

Owing to the repeatedly observed strong vertical ground motions and compressional damage of engineering structures in recent earthquakes, the multi-directional site response analysis is increasingly critical for the seismic design of important structures, such as nuclear power plants and high earth dams. However, the site response to the vertical component of the ground motion has not been the subject of detailed investigation in the literature. Therefore, in this paper, the vertical site response due to vertical ground motion is investigated by employing both analytical and numerical methods. First, a one-dimensional frequency domain analytical solution, which can be employed for vertical site response analysis in practice, is studied and compared against time domain finite-element (FE) analyses for the two extreme soil state conditions (i.e. undrained and drained conditions). The vertical site response is further investigated with hydro-mechanically (HM) coupled FE analysis, considering solid–fluid interaction. The parametric studies undertaken show that the predicted vertical site response is strongly affected by the parameters characterising the hydraulic phase, namely, soil permeability and soil state conditions, both in terms of frequency content and amplification. The subsequent corresponding quantitative investigation, of the frequency content and amplification function of the vertical site response, shows that depending on the soil permeability the response is dominated by the two types of compressional waves (fast and slow waves). Notably, the parametric studies identify a range of permeability that significantly affects dynamic soil properties in terms of P-wave velocities, damping ratios and vertical site response, and this range is relevant for geotechnical earthquake engineering applications. It is therefore recommended that coupled consolidation analysis is necessary to simulate this effect accurately at such permeability-dependent intermediate transient states between fully undrained and drained conditions. Finally, this work suggests a simple modification of standard total-stress site response analysis to account for vertical ground motion and solid–pore fluid interaction. In order to simulate the attenuation of the response due to solid–pore fluid interaction effects, it is suggested to employ additional HM viscous damping in total-stress analysis, further to the one used to account for hysteretic material damping. This additional viscous damping can be quantified based on the empirical curves proposed in the paper.

Thermo-active foundations utilise heat energy stored in the ground to provide a reliable and effective means of space heating and cooling. Previous studies have shown that the effects of temperature changes on their response are highly dependent on their interaction with the surrounding ground. Consequently, it is necessary to consider this interaction and include both the thermal and mechanical behaviour of the ground in design. This paper addresses this issue by performing state-of-the-art finite-element analyses using the Imperial College Finite Element Program, which is capable of simulating the fully coupled thermo-hydro-mechanical behaviour of porous materials. First, the Lambeth College pile test is analysed to demonstrate the capability of the adopted modelling approach to capture the observed response under thermo-mechanical loading. Subsequently, a detailed study is carried out, demonstrating the impact of capturing the fully coupled thermo-hydro-mechanical response of the ground, the use of appropriate boundary conditions and the uncertainty surrounding thermal ground properties. It is demonstrated that the modelling approach has a large impact on the computed results, and therefore potentially on the design of thermo-active piles. Conversely, the effects of thermal conductivity and permeability of the soil are shown not to influence the pile behaviour significantly.

The necessity of accounting for small-strain behaviour of soils in numerical analyses of the serviceability limit states of geotechnical structures is well established both nationally and internationally. This has led to further development of appropriate soil constitutive models, as well as to further advances in accurate laboratory measurements of small-strain stiffness. The current paper considers recent laboratory research into the behaviour of London Clay, performed at Imperial College in conjunction with the major ground works for Heathrow Terminal 5 in London, UK. This research has shown the small-strain response of London Clay to be different to that assumed previously and in particular to that determined from good-quality commercial experiments. Both sets of small-strain stiffness data are applied in this paper in finite-element analyses of a deep excavation and a tunnel construction in London Clay, with the objective of investigating their effect on predicted ground movements.

A discrepancy has been identified in existing overstress theory based elastic-viscoplastic models regarding the definition of the Φ function, which determines the magnitude of the viscoplastic strain increment. Based on existing experimental data, some authors define the function Φ assuming that any given loading surface is a locus of constant volumetric viscoplastic strain rate. Others, instead, assume that a loading surface is a locus of constant Φ. This paper examines the implications of using either of the above approaches. It is shown that the two approaches yield very similar predictions for stress states far from failure (for stress levels up to approximately 0·6), however, critical-state conditions are reproduced only when the current loading surface is assumed to be a locus of constant Φ.

The results of a finite-element parametric study, performed to investigate the nature of soil arching in the geosynthetic-reinforced fill above a void, are presented. The fill response to the formation of a void is compared with the arching theories of Terzaghi and Hewlett and Randolph. It was found that Terzaghi's approach is capable of describing all aspects of behaviour observed in numerical analyses, whereas Hewlett and Randolph's formulation may only be applied to a limited number of cases. It was also found that the accuracy with which Terzaghi's approach predicts the vertical stress at the base of the fill layer depends mostly on the value of the stress ratio K in the shear zone propagating from the edges of the void. The values suggested in the literature are shown to be too low, resulting in a highly overestimated vertical stress at the base of the fill. A suitable value of K is suggested from the numerical results, which is independent of the shape and size of the void, and of the fill and geosynthetic properties.

A review of the first 60 years of Géotechnique publications shows clearly how the subject of soil mechanics has evolved. In terms of constitutive and numerical modelling of soil, early forms of numerical analysis involved hand calculations of ultimate states applying classical methods of analysis: limit equilibrium, limit analysis or stress field solutions. Consequently, the soil was considered to behave as a rigid plastic material, and to follow one of the two basic failure laws of classical soil mechanics, namely the Tresca or Mohr–Coulomb failure criteria. For assessing the deformation of structures, soil was normally considered to be linear elastic. The foundations of modern numerical analysis and constitutive modelling were laid in the early to mid 1960s, with the development of the finite element method and the postulation of the critical state framework of soil behaviour respectively. Clearly, the continuous advancement of computer power has been essential in applying new developments to modern geo-technical analysis. This paper reviews some of the main milestones in the evolution of geotechnical analysis in the past 60 years, commenting, where appropriate, on what problems still lie ahead.

The pre-failure behaviour of overconsolidated soils is well known to be non-linear and inelastic. Over the last two decades experimental work has shown that pre-failure behaviour is also dependent on the previous stress history of the soil. However, some research has also suggested that, if enough time is allowed for creep to take place, then there is no effect of recent stress history; moreover, what affects soil stiffness is the direction of the stress path relative to the gross yield surface. This paper investigates the effect of two scenarios on the surface settlement trough associated with tunnelling. The first assumes that the previous stress history of the soil has an effect on soil stiffness, whereas the second assumes that creep/ageing periods have erased any memory of the previous stress history, and it is only the stress path direction that influences soil stiffness. A three-surface kinematic hardening model, which can simulate both scenarios, is employed in finite element analyses of tunnel construction. The predictions of the kinematic hardening model for the simulation of each scenario are compared with predictions using a non-linear elastic model and with field measurements.

The paper investigates the effect of the deviatoric shape of the yield and plastic potential surfaces on the failure height of an embankment founded on a soft clay deposit. Different combinations of the shapes of the surfaces in the deviatoric plane are explored in a critical state model that predicts isotropic strength, and which is used in order to model the clay foundation. The shapes considered start from the widely used circular shape and move on to the Mohr–Coulomb hexagon and advanced shapes, such as the Lade shape. For all combinations the same undrained strength profile in triaxial compression is adopted for the clay foundation. Finite element analyses investigate the failure height of the embankment assuming plane-strain conditions. Moreover, two ways of modelling the fill material are adopted in the analyses, and the effect of each of these on the failure height of the embankment is explored.

This paper investigates a number of issues related to the modelling of a retaining structure used to support an excavation in 3D finite element analyses. In particular, the effects of wall stiffness in different coordinate directions and the rotational fixity in the corner of the excavation are examined. Both square and rectangular excavations are analysed and compared with the equivalent axisymmetric and plane strain analyses, normally used as approximations for modelling purposes. The chosen geometry, construction sequence and soil conditions are based on a proposed deep excavation at Moorgate in London (next to the Moor House development), which will form part of an underground station for the Crossrail project. The objective of the study is to provide a detailed assessment of wall and ground movements and structural forces in the wall in the light of different modelling assumptions. The study has wider application to a variety of projects that include the development of infrastructure, the construction of deep basement car parks and buried structures, and the effect that these have on the surrounding areas.

Finite element analyses of geotechnical problems that involve partially saturated soils are commonly carried out using conventional fully saturated soil models. Such models, however, cannot successfully reproduce partially saturated soil behaviour. This paper investigates the influence of partial soil saturation on the behaviour of piles. A total stress constitutive model for partially saturated soils has been developed and implemented into the Imperial College finite element Program (ICFEP). This model, which is presented here, was used to perform a series of single pile analyses. The results of these analyses show the effect of partial saturation and in particular of fluctuations of the groundwater table on the behaviour of single piles. The effects of a rising groundwater table on pile behaviour are also investigated in the case of a pile foundation of a high-rise building in London. The paper indicates where prediction of pile capacity and displacement under different load and groundwater table fluctuations may be very different when using fully or partially saturated constitutive models.

The need to build embankments on soft clay deposits is widespread, including flood defences, rail and motorway embankments. The difficulties in constructing embankments on these deposits are well known. The permeability of the ground is usually low, so that construction is generally performed under undrained conditions and, as a consequence of the low undrained shear strength of the clay, these structures are of a limited height, usually only 3–4 m. The classical design of an embankment is often based on a limit equilibrium approach, in which the soil is considered as an isotropic material. However, soft clays are almost always anisotropic, having both stiffness and strength dependent on the orientation of principal stresses. This complicates any design analysis, and as sufficient data are not usually available to define the magnitude of anisotropy, empirical factors are introduced into conventional design procedures. This paper attempts to demonstrate the effect of soil strength anisotropy on the behaviour of soft clay embankments through finite element analysis using an anisotropic soil model. The finite element predictions are compared with data from a full-scale test embankment brought to failure. The dangers of ignoring anisotropy, or treating it empirically, are highlighted.

Three-dimensional analyses of tunnel excavation and lining construction are rarely performed in practice, because it is difficult to reproduce the exact boundary conditions involved in this process, and because such analyses require considerable computing resources if realistic non-linear soil behaviour is to be reproduced. This paper presents a pseudo-three-dimensional analysis of tunnel construction by utilising the Fourier series aided finite element method (FSAFEM). The theory behind this approach, and its validation when applied to a tunnelling problem, are described. The method is then used to predict the behaviour of tunnels forming part of the Seoul Subway system in Korea. The results are compared with field measurements.

It is well known that Ko-consolidated natural sediments have anisotropic strength, yielding and stiffness characteristics, particularly at low overconsolidation ratios. For example, the undrained triaxial compression shear strengths developed after Ko-consolidation far exceed those available in extension. This paper describes recent hollow cylinder apparatus research into other types of stress-induced anisotropy, reporting experiments on dense silt samples that involved rotation of the principal stress axes during consolidation. The results show strikingly different anisotropies from those developed by Ko-consolidated samples. The effective stress paths chosen for the laboratory studies were guided by the conditions expected under multi-stage embankments built on soft ground, but the results have considerable significance in other practical geotechnical problems.

This paper investigates the effect of several parameters on the pull-out capacity of bucket foundations in soft clay. These parameters are load inclination, skirt length, foundation diameter, soil adhesion and soil anisotropy. Initially the soil is assumed to be isotropic soft clay, and is represented using a form of the modified Cam clay model. Some recent laboratory experiments performed in a hollow cylinder apparatus, which were designed to investigate strength anisotropy of a particular silt soil, are then used to represent the behaviour of an anisotropic soft clay. The anisotropic behaviour is simulated using anisotropic soil model MIT-E3. Both models are then used in finite element analyses to predict the behaviour of bucket foundations on pull-out. All analyses were performed using the three-dimensional Fourier series aided finite element method (FSAFEM).

Practical geotechnical problems invariably involve rotations of the principal stress axis direction and changes in the relative magnitude of the intermediate principal stress. To assess stiffness characteristics under such conditions requires a hollow cylinder apparatus (HCA) with independent controls and accurate measurements of the component stresses (σz, σr, σθ and τ2θ) and strains (ɛz, ɛr, ɛθ and γzθ). This paper describes how the anisotropy of a dense quartzitic silt was explored over the non-linear range by conducting tests on 254 mm diameter HCA samples, and on smaller triaxial specimens. It is shown that the stiffness characteristics depend strongly on the stress path direction and major principal stress axis inclination. K0 consolidated samples show a particularly stiff response to torsional shear straining and the silt's compliance matrix is non-symmetrical and non-unique, even at small strains. The cross-coupling terms are different from those expected conventionally. For example, τzθ stress changes can affect ɛz, ɛr and ɛθ strongly and the soil tends to compress vertically when subjected to either torsional shear straining or uniaxial lateral loading. The patterns of anisotropy are altered by overconsolidation. Dans la pratique, les problèmes géotechniques font invariablement intervenir la rotation de l'orientation des axes des contraintes principales et la variation de l'importance relative de la contrainte principale intermédiate. L'évaluation des caractéristiques de rigidité dans de telles conditions exige l'emploi d'un appareil à cylindre creux à commandes indépendantes et la mesure précise des composantes de la contrainte (σz, σr, σθ et τzθ) et de la déformation (ɛz, ɛr, ɛθ et γzθ). L'article décrit l'étude de l'anisotropie d'un limon quartzeux dense dans la gamme non linéaire à l'aide d'essais effectués sur des cylindres creux d'un diamètre de 254mm et sur de plus petites éprouvettes de compression triaxiale. Les résultats montrent que les caractéristiques de rigidité sont fortement influencées par la direction des parcours de contrainte et par l'inclinaison prédominate des axes des contraintes principales. Les éprouvettes de sol consolidé K0 manifestent une réponse particulierèment rigide à la déformation de cisaillement en torsion, et la matrice d'élasticité du limon est asymétrique et non unique, même à de petites contraintes. Les couplages réciproques différent de ceux auxquels on pourrait normalement s'attendre. A titre d'exemple, des changements de la contrainte τzθ peuvent fortement influencer ɛz, ɛr et ɛθ, et le sol tend à se comprimer verticalement quand il est soumis à une déformation de cisaillement en torsion ou à une charge latérale uniaxiale. Les modes d'anisotropie sont modifiés par la préconsolidation.