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An experimental campaign is conducted on a 100g centrifuged monopile model, impact driven, into saturated dense sand, down to three embedding depths, corresponding to three slenderness ratios of 5, 4 and 3. These models are instrumented with optical fibres to measure the bending moment profile along the monopile. A new method is developed to determine the experimental soil reaction curves by considering the distributed moment part from the measured bending moment. This distributed moment is assessed by the one-dimensional finite-element (1D FE) model of the PISA (pile soil analysis) method. The key features of this study are: (a) the monopile behaves from pure rotation to combined rotation–flexure as the slenderness ratio is enhanced from 3 to 5, inducing a less pronounced linear rotation–deflection response at ground level; (b) although the distributed moment assessed in the PISA project is very low, the developed method is useful to explore the experimental local behaviour of laterally loaded monopiles; (c) the 1D FE PISA model captures reasonably well the overall behaviour of laterally loaded centrifuged driven monopiles, even if it is less accurate for the local behaviour. [ABSTRACT FROM AUTHOR]

Underground transportation systems often involve multiple tunnels constructed closely together. Previous studies mainly focus on interaction between circular tunnels; by contrast, interaction mechanisms involving non-circular tunnels are not well understood. In this study, four physical three-dimensional centrifuge tests were performed in dry sand, simulating the response of existing circular and horseshoe-shaped tunnels to a newly excavated tunnel. Two different ratios between pillar depth and tunnel diameter (P/D) of 0.5 and 2.0 were considered. Furthermore, three-dimensional numerical back-analyses considering small-strain stiffness were undertaken. Results reveal that the ground settlement above an existing horseshoe-shaped tunnel is less sensitive to pillar depth than for circular ones. Furthermore, for P/D = 0.5, the existing horseshoe-shaped tunnel experiences both vertical and horizontal compression; more stress reduction occurs vertically than horizontally. A circular tunnel for the same pillar depth becomes compressed vertically but elongated horizontally; stress reduction around the existing circular tunnel is less vertically than horizontally. However, for P/D = 2.0, both types of tunnel become elongated vertically and compressed horizontally because of a larger reduction in vertical stresses than horizontal ones. These results demonstrate that both pillar depth and shape profoundly affect tunnel deformation mechanisms. [ABSTRACT FROM AUTHOR]

A series of centrifuge tests was conducted to explore the deformation mechanisms and earth pressures mobilised in loose and dense sand for a complete set of passive movement modes of a rigid retaining wall: rotation about the top and base and translation. Sand deformations were measured by particle image velocimetry and earth pressures were observed by a Tekscan pressure mapping system. Simplified linkages between wall displacements, sand strains and earth pressures were built. Superposition of such results would allow designers to predict retaining wall behaviour in sand during the construction sequence as well as ultimate collapse in a mobilisable strength design procedure. [ABSTRACT FROM AUTHOR]

This study conducted both three-dimensional physical model tests and numerical back-analyses to examine the performance of a single pile subjected to twin tunnelling beneath the pile toe in dry sand. Moreover, numerical parametric analyses were carried out to assess the impact of working load level and tunnel volume loss on pile behaviour. The findings revealed that the first tunnelling induced a 1.9%dp (pile diameter) settlement in the pile when the pile toe was 0.5 D (tunnel diameter) above the twin tunnels, which had a 1.0% volume loss. The settlement was accompanied by an upward load transfer mechanism. The second tunnelling had a reduced impact and induced only 76% of the settlement caused by the first tunnelling. The study also showed that tunnelling-induced shearing behaviours weakened beneath the pile toe with a decrease in working load, resulting in reduced pile head settlement. Moreover, the computed pile settlement increased by 178% when the tunnel volume loss increased from 0.5% to 2%. The findings of this study offer valuable data for the validation and calibration of numerical models. In addition, the outcome from this study provides useful insights into the prediction of the pile-soil-tunnel interaction when subjected to twin tunnelling. [ABSTRACT FROM AUTHOR]

Thermally induced ratcheting of a thermo-active pile is the accumulation of net and irreversible pile head displacement upon heating–cooling cycles. Although this kind of phenomenon has been observed in vertically loaded piles in sand, it is unknown whether this exists in laterally loaded cases, and also what underlying mechanisms occur under the thermomechanical flexural soil–pile interaction. This study presents a series of centrifuge tests and finite-element simulations of the thermomechanical behaviour of a laterally loaded thermo-active pile in sand. A new model reinforced concrete (RC) was used in the centrifuge tests to mimic the thermal and mechanical properties of a prototype RC pile realistically. Ratcheting was evident in laterally loaded piles and its extent was more significant when the working horizontal load was higher. The ratcheting phenomenon was attributed to the accumulation of soil plastic strain due to the cyclic mechanical loading induced by pile thermal horizontal expansion and contraction, soil dilation upon soil–pile interface shearing and creep. The additional bending moment induced by the thermal action did not induce yielding within the pile. A subsequent numerical sensitivity study suggested that ignoring the softening behaviour of the sand would lead to underestimation of the magnitude of the accumulative thermally induced pile head lateral displacement. [ABSTRACT FROM AUTHOR]

Two model piles with outer diameter D = 50 mm are loaded laterally at 100g in a large-beam geotechnical centrifuge. The normal strains on both the tensile and compressive sides are measured using fibre Bragg gratings. An incremental method is introduced to define the pivot point. The testing and analytical programme enables the effect of the embedding depth and load eccentricity to be quantified. The key findings are as follows. (a) The piles generate asymmetric tensile and compressive strains during bending, and the tension–compression asymmetry becomes more pronounced at the pile toe and for shorter piles. (b) The piles transition from flexure to rotation as the embedding depth is decreased from 9D to 3D, where the uniqueness of the ground-level rotation and deflection (θg–yg) relationship disappears. (c) The reaction and deflection (P–y) relationship flattens with increasing embedding depth but seems independent of the load eccentricity. [ABSTRACT FROM AUTHOR]

A macro-element model for predicting the load–displacement behaviour of a spudcan foundation in clay overlying sand when subjected to combined vertical, horizontal and moment loading is introduced. Observations from detailed drum centrifuge tests that measured the effect of the underlying sand layer on the foundation behaviour are combined with finite-element results and theoretical developments to derive the components of the model. The yield surface defined by the centrifuge test results suggests that as the spudcan nears the underlying sand layer, the absolute horizontal capacity remains relatively constant, while the vertical and moment capacities increase at approximately the same normalised rate. The model is demonstrated to accurately predict foundation behaviour by retrospectively simulating the experimental results. This macro-element model has the advantage that it can be integrated into the structural analyses of jack-up platforms required for site-specific assessments. [ABSTRACT FROM AUTHOR]

Screw piles can be used as foundations for offshore energy applications, thanks to their silent mode of installation and considerable uplift capacity, although a significant upscaling of onshore dimensions is necessary. However, the crowd (vertical) force necessary to install upscaled screw piles was previously shown to be far too great for practical installation. Although guidance recommends that the pile's vertical displacement must be one helix pitch (helix height) per each pile revolution, it is shown in this paper that a lower vertical displacement per revolution can significantly reduce the necessary crowd force during installation or even generate some pull-in. In addition, it was shown that the uplift stiffness and capacity of the pile were enhanced by this installation process, at a shallow (relative) depth in sand. This paper gathers 19 centrifuge tests, with varying screw pile geometries (shaft diameter, base shape), sand relative density and advancement rates. A predictive framework for the pull-in potential of a given pile geometry was proposed to assess its ability to be installed with a reduced crowd force. [ABSTRACT FROM AUTHOR]

This paper reports a series of centrifuge tests conducted on a stiffened suction caisson anchor in a medium dense calcareous sand. The aims were to provide insight into the behaviour of the anchor during installation and under operational monotonic loadings, and to propose corresponding design approaches to be used in practice. The sand was collected from the Pluto field of Australia's North-West Shelf and characterised through consolidated-drained triaxial compression tests and cone penetration tests (CPT). For assessing caisson installation resistance, a conventional shear resistance method and a CPT-based method were calibrated hinging on the measured caisson installation resistance, effective radial stress, cone tip penetration resistance and friction angle of the sand. Under inclined loadings, with reducing loading angle at the padeye (and hence mooring angle at the mudline), the ultimate capacity increased and the rotational failure mechanism changed to a more lateral translational mechanism. For assessing ultimate capacity, an extended analytical model was presented and verified against the centrifuge test data capturing stress and dilatancy with a simple yet robust systematic approach to include particle breakage. Particle breakage under inclined loading was shown to increase with reducing loading angle. The anchor vertical-horizontal failure envelope was approximated by a parabolic expression. • Installation resistance of a suction caisson anchor in calcareous sand. • Capacity of a suction caisson anchor in calcareous sand. • Shear resistance method and CPT-based method for assessing installation resistance. • Extended analytical model for assessing ultimate capacity under operational loadings. • Evolution of failure mechanism, anchor rotation, and sand particle breakage with loading angle. [ABSTRACT FROM AUTHOR]

A series of centrifuge tests was conducted to explore the deformation mechanisms mobilised in loose and dense sand for a complete set of active rigid retaining wall movement modes: rotation about the base and top and translation. The sand deformation was measured by particle image velocimetry and data relating to displacements and strains are reported. A simplified deformation mechanism is proposed for sand behind a rigid wall rotating about the top and this is validated with the centrifuge test results. The paper reveals that the sand deformation caused by wall translation can be well characterised by the superposition of mechanisms with equal but opposite wall rotation about the top and base. Integration of these displacement mechanisms together with a constitutive law into an equilibrium solution for wall bending would allow designers to predict deformations during construction as well as ultimate collapse. [ABSTRACT FROM AUTHOR]

Soil liquefaction induced manhole uplift causes significant disruptions to vital lifelines following major earthquakes. In the past decades, remediation measures have been proposed to tackle this problem for newly constructed manholes. Nonetheless, mitigation measures for existing manholes were rarely reported, which leaves existing manholes facing high uplift risk in future earthquakes. Aiming to address this problem, this study proposes a series of mitigation measures, including (1) opening a drainage hole at the manhole base, (2) installing a smooth permeable pile under the manhole, and (3) installing a permeable pile with spiral wings under the manhole. The experimental results revealed that these three methods can mitigate manholes uplift to different extents. The drainage opening can mitigate the manhole uplift by releasing the pore water pressure on the manhole invert and draining water into the manhole chamber. The smooth permeable pile can further mitigate the manhole uplift with more drained water and greater retained sidewall friction due to the lowered liquefaction degree in the surrounding soil as a result of the extended drainage effect. However, the additional pull-out resistance expected from the spiral wings was of little contribution in view of the slight uplift difference comparing to the smooth permeable pile. The above observations are also validated with theoretical force equilibrium. [ABSTRACT FROM AUTHOR]

Fault deformation can cause severe damage to structures located within the fault zones. A logarithmic spiral model was developed by D. A. Cole and P. V. Lade in 1984 to predict the shapes and locations of fault ruptures in sand caused by dip–slip faulting, based on 1g sandbox tests. However, it shows poor predictions for reverse fault ruptures observed in centrifuge tests. In this study, centrifuge modelling of reverse faulting was performed to explore the reasons for the poor predictions by the logarithmic spiral model. These faulting tests were conducted in free-field conditions with different sand densities and soil thicknesses. Based on results of centrifuge tests from both the present study and previous research studies, a hyperbolic spiral model is proposed to predict approximately the shapes and locations of reverse fault ruptures in sand. The results show that the 45° − ϕmax/2 rather than 45° − ψmax/2 controls the surface direction angle of the outcropping rupture. Validation of the hyperbolic spiral model shows a promising prediction in centrifuge tests both in the present study and previous research studies. [ABSTRACT FROM AUTHOR]

This paper presents results from a systematic investigation of footing response in four different types of sands placed and tested under controlled conditions in a geotechnical centrifuge. A parallel series of cone penetration tests (CPTs) was performed to examine correlations between CPT end resistance (qc) and footing bearing pressure (q). The experimental results show that there is a unique relationship between the normalised bearing stress (q/qc) and the settlement normalised by footing diameter (s/B), irrespective of footing diameter, stress level, sand grading, mineralogy and angularity. These results are shown to compare well with findings from published databases of field-scale footing tests when allowance is made for the significant effects of creep. The study enables simple and robust recommendations to be established for the ultimate capacity (defined at s/B = 0·1) and allowable bearing pressure of a footing on sand. [ABSTRACT FROM AUTHOR]

This paper discusses the instability of an offshore monopile in association with wave-induced liquefaction of sand beds. Centrifuge wave tests in a drum channel were performed with viscous scaling introduced such that the time-scaling laws for fluid wave propagation and consolidation of the soil were matched. The relationships between liquefaction and the start and development of monopile instability in a sand bed under wave loading were investigated. The monopile started significant structural rocking when liquefaction occurred and progressed to one-third of the monopile embedment depth. The residual displacement of the pile increased markedly with the downward progress of the liquefied zone, eventually leading to the collapse of the pile. The characteristics of the onset of liquefaction around the monopile were also investigated. It was found that liquefaction first took place at the sides of the monopile rather than at the front or rear presented to the direction of the travelling waves, which highlights the importance of the rotation of the principal stress axes induced in the sand bed under the passage of progressive waves. The experimental results further demonstrated the effect of embedment of the pile in a dense layer. It was found that embedding the pile in the dense layer with thickness equal to or more than half of the pile embedment depth was effective in preventing the significant inclination and collapse of the pile. The present study also clarified the effects of scour protection around the monopile on wave-induced liquefaction and pile instability. It was found that the scour protection increased the liquefaction resistance depending on the diameter ratios of the scour protection and the pile, however, it could not prevent the collapse of the monopile once liquefaction occurred. These results show the importance of soil stratification on the wave-seabed-structure interaction in light of wave-induced liquefaction. [ABSTRACT FROM AUTHOR]

This paper details analysis of deformation behaviour of silica and carbonate sands under a rectangular foundation subject to uniaxial vertical load based on results from a series of centrifuge model tests. A multiscale particle image velocimetry/digital image correlation (PIV/DIC) technique was used to record and analyse the foundation tests with high resolution and measurement precision. Cone penetrometer and pressuremeter tests provide in situ soil characterisation of the tested sand sample in the centrifuge environment. The soil behaviour is analysed through foundation load–settlement response and the observed soil deformation measurements. Different soil deformation mechanisms and strain behaviours were observed in the different sands tested, and the particle shape effect is considered, with data from scanning electron microscopy, to explain the differences. The results and analyses contribute towards better understanding of different soil behaviours under shallow foundations in different sands. [ABSTRACT FROM AUTHOR]

Centrifuge modelling and finite element analysis are powerful tools of research on the lateral pile/soil interaction. This paper aims at presenting the main results of experimental and numerical analysis of the pile response under monotonic lateral loading in sand. After description of the experimental devices, it focuses on the determination of the load-transfer P-Y curves for rigid and semi-rigid piles embedded in dry dense sand by using the experimental bending moment profiles obtained in centrifuge tests, as well as by a three-dimensional finite element models using ABAQUS Software. The elastic perfectly plastic Mohr-Coulomb constitutive model has been used to describe the soil response, and the surface-to-surface contact method of ABAQUS software has been used to take into account the nonlinear response at soil/pile interface. The analysis methodology has allowed to propose a hyperbolic function as a model to construct P-Y curves for rigid and semi-rigid piles embedded in dry dense sand, this model is governed by two main parameters, which are the initial subgrade reaction modulus, and the lateral soil resistance, the latter has been formulated in terms of Rankine's passive earth pressure coefficient, the sand dry unit weight, and the pile diameter. [ABSTRACT FROM AUTHOR]

This study investigated the change in monotonic and cyclic capacity of a plate anchor across different degrees of consolidation in dense sand. To quantify the effect of consolidation on anchor capacity, a framework is introduced and validated using centrifuge model anchor test data. The centrifuge tests considered a rectangular plate loaded at varying rates in dense sand, under both monotonic and irregular cyclic conditions, at a fixed embedment depth and with a horizontal load inclination (at the seabed). In order to vary from drained to undrained conditions, the sand was saturated using both water and a viscous pore fluid with viscosity approximately 700 times higher than water. The anchor's ultimate monotonic capacity in dense sand increased by up to 173% as the consolidation response evolved from drained to undrained with generation of dilation-induced suction. This increase in capacity across the consolidation regime can be adequately quantified using the proposed framework; however, uncertainty arises in achieving the theoretical undrained capacity. Both drained and undrained irregular cyclic loading resulted in anchor capacity increases of up to 33%, attributed to soil volume changes associated with cyclic densification under drained cyclic loading and excess pore pressure dissipation under undrained cyclic loading. [ABSTRACT FROM AUTHOR]

This paper presents centrifuge tests undertaken to investigate the combined VH capacities of suction caisson in sand under multidirectional loadings following series of low magnitude horizontal cyclic loading. These loading paths are relevant to foundations of array of floating renewables such as wave energy converters or wind turbines. The centrifuge tests mainly provide information about the V–H yield envelopes in different loading directions and the Hx–Hy yield envelopes in the horizontal plane so that the effect of multidirectional loading can be analysed. The evolution of the yield envelope after cyclic loadings is first presented. Yield envelopes for load change angle β of 60°, 90°, 120° from the initial horizontal cyclic loading direction are then presented, and the mechanisms for increased capacity for β > 90° and reduced capacity for β < 90° are discussed. [ABSTRACT FROM AUTHOR]

Sand erosion and scouring caused by waves and marine currents result in gradual increase of local seabed inclination and formation of slopes around hydraulic structures and offshore foundations. During this process, shear stresses in the soil body increase monotonically which may lead to static liquefaction and damage of the adjacent offshore infrastructure. This paper presents the details of a newly developed static liquefaction triggering actuator to be used at an enhanced gravity condition in a geotechnical centrifuge. This actuator simulates the steeping process of submarine sand layers due to scouring and enables the investigation of failure mechanisms in submerged slopes. The details of the centrifuge test set-up designed and constructed to simulate the process of triggering static liquefaction in loose sand layers are presented. Furthermore, the performance of the novel integrated model preparation facility using sand fluidization is explained. The set-up was used to conduct several centrifuge tests at four different slope steepening rates to investigate the slope steepening rate effects. Moreover, the effect of viscosity of the submerging pore fluid on the behaviour of the slopes at the onset of failure is investigated. The Coriolis effect on loose saturated sand samples during increase of g-level is examined as well. Results show that the built-up of pore pressure due to local shear deformations can be detected and considered as one of the triggering mechanisms of this kind of submarine slope instabilities. [ABSTRACT FROM AUTHOR]

This technical note considers experimental data on the long-term response of a suction caisson in sand and sand over clay to lateral cyclic loading. Installation of the caisson under suction in a geotechnical centrifuge provides insight into the contribution of this installation process, as well as the effects that soil drainage and consolidation in the clay layer have on the accumulated caisson rotation and change in stiffness. The tests focused on sand over clay, and considered variations in the cyclic load magnitude and symmetry. One-way cyclic loading in sand over clay is seen to result in higher rotation than two-way loading, which contrasts with findings from previous studies in sand. Excess pore pressure dissipation in the clay layer leads to strength increases that stabilise caisson rotation and increase stiffness. The rate of accumulation in caisson rotation is observed to be the same from centrifuge and single gravity tests, while the initial rotation differs with stress level, drainage regime, loading magnitude, soil profile and installation method. The centrifuge tests are considered collectively with equivalent single gravity tests to form a basis for predicting the long-term response of a monopod suction caisson. [ABSTRACT FROM AUTHOR]

Skirted foundations are an attractive foundation concept in the offshore energy sector, both for wind turbines and oil and gas platforms. Most of the evidence of skirted foundation behaviour under combined vertical, horizontal and moment (VHM) loading in sand has been collected from small-scale model experiments conducted at unit gravity on the laboratory floor. This paper presents results from a series of centrifuge experiments of skirted foundations on loose silica sand at relevant prototype stress levels. The vertical load-penetration curve is shown to be predicted well using established analytical methods. Centrifuge modelling results provide experimental evidence of the complex effects of the interaction of skirt aspect ratio and relative stress level on the VHM yield surface. A conservative and design-oriented solution based on the yield envelope approach describes available foundation capacity within the established framework of strain-hardening plasticity theory. [ABSTRACT FROM AUTHOR]

In this study, centrifuge model tests and numerical analyses were conducted to investigate the deformation mechanisms of an existing tunnel due to normal faulting in sand. With the presence of a tunnel in sand stratum, normal faulting-induced ground surface settlements above the existing tunnel were significantly reduced (i.e., shielding effects). These shielding effects provide by the existing tunnel were decreased with the tunnel burial depth. When a kinematically constrained boundary condition was applied in the tunnel, the induced longitudinal strains at the tunnel crown were much larger than those observed in a tunnel with a free boundary condition. [ABSTRACT FROM AUTHOR]

Highlights • Centrifuge testing results of greenfield tunnelling in sands. • Empirical relationships for the soil volume loss and the width parameters of settlement troughs. • Formulas for predicting settlement troughs by using modified Gaussian curves. • Semi-analytical equations to replicate vertical and horizontal ground displacements. • Proposed formulas account for the cover-to-diameter ratio, relative density, and volume loss. Abstract Empirical formulas and closed-form solutions provide, in many cases, good predictions of tunnelling-induced ground movements which, when combined with their computational efficiency, make them valuable for tunnel-structure interaction analyses. For sandy soils, however, there is a shortage of available methods that can adequately describe the changes in soil deformation patterns that occur as a result of tunnel volume loss, soil relative density, and geometrical parameters. In this paper, two approaches are adopted to describe centrifuge test outcomes for tunnelling in dry silica sand of varying relative density. Firstly, empirical expressions for the prediction of settlement trough shape and magnitude are presented; additionally, a set of equations is given to describe the settlement troughs using modified Gaussian curves. Next, semi-analytical expressions (modifying an elastic analytical solution for incompressible soil and ovalized tunnel) are developed to predict both vertical and horizontal movements within the ground. Results from both methods can capture the main effects that cover-to-diameter ratio, relative density, and volume loss have on surface and subsurface ground movement profiles. The proposed expressions can be used for the calibration/verification of tunnel-structure interaction analysis methods by using outputs from comparable centrifuge tests; once verified, these methods may be more widely applied to other scenarios and used within design or risk-assessment exercises. [ABSTRACT FROM AUTHOR]

The suction bucket foundation is turning to be an increasingly prevalent foundation for offshore wind turbines (OWTs). In this study, a series of centrifuge tests are performed to investigate the vertical bearing capacity of the suction bucket foundation in sandy soil under drained condition. Four bucket models are tested by considering the influence of aspect ratios (APs). The foundations are tested under compressive and tensile vertical loadings. The bucket foundation is in the general shear failure mode, and the ultimate bearing capacity increases with the AP. The tensile bearing capacity is smaller than the compressive capacity. The sharing factor of the base capacity and the skirt friction of the suction bucket foundation is determined with the assistance of the tensile test. The bucket lid provides the majority of the bearing capacity. The internal skirt friction is larger than the external skirt friction due to the soil constraint effect and the increased vertical stress. Several analytical methods are proposed to estimate the bearing capacity, and the results are compared with the test data. The bearing capacity equation is more applicable to the "wide-shallow" bucket foundations. The recommended design method is regarded as the upper bound when calculating the ultimate bearing capacity. [ABSTRACT FROM AUTHOR]

Tunnel construction is vital for the development of urban infrastructure systems throughout the world. An understanding of tunnelling-induced displacements is needed to evaluate the impact of tunnel construction on existing structures. Recent research has provided insight into the complex mechanisms that control tunnelling-induced ground movements in sands; however, the combined influence of relative tunnel depth and soil density has not been described. This paper presents data from a series of 15 plane-strain centrifuge tests in dry sand. The cover-to-diameter ratio, C/D, of the tunnels ranges between 1·3 and 6·3, thereby including relatively shallow and deep tunnels. The sand relative density varies between 30 and 90%, corresponding to loose and dense soils. The effects of C/D, soil density and volume loss on vertical and horizontal soil movements, shear strains and ground reaction curves are discussed. Analysis of surface and subsurface settlement trough characteristics shows that the mechanisms are non-linear and the effects of soil relative density and volume loss on deformation patterns are highly dependent on C/D. The role of soil arching in the definition of the displacement mechanisms and a discussion of the implications of the results to the assessment of damage to existing structures are also provided. [ABSTRACT FROM AUTHOR]

A systematic study into the response of monopiles to lateral cyclic loading in medium dense and dense sand was performed in beam and drum centrifuge tests. The centrifuge tests were carried out at different cyclic load and magnitude ratios, while the cyclic load sequence was also varied. The instrumentation on the piles provides fresh insights into the ongoing development of net stresses, bending moments and deflections as cycling progresses. Parallels between the test results and corresponding cyclic triaxial tests are drawn. The paper combines the results from this study with those from previous experimental investigations to provide empirical design recommendations for monopiles subjected to unidirectional cyclic loading. [ABSTRACT FROM AUTHOR]

Abstract The monopile has been widely used to support offshore and coastal structures. A series of centrifuge tests has been performed to investigate the bearing capacity of large diameter monopiles in sandy soil. Both static tests and cyclic tests have been conducted for open-ended and close-ended model piles, and the effects of influence factors, such as loading rates, embedment depths, and loading histories are considered. The piles are then loaded by a sequence of compressive-tensile loadings to estimate the tension capacity, from which the shaft friction is derived. The cyclic load tests are performed with five varying load intensities, and the accumulated settlement is assessed. The centrifuge tests indicate that the pile bearing capacity tends to increase with the initial penetration depth, and the stress state of soil greatly influences the pile behavior. The tensile shaft friction is smaller compared to the compression test. The capacities of the piles reduce significantly under the axial cyclic load, and the maximum cyclic load intensity should be limited to 75% of the ultimate bearing capacity. The API design method is used to calibrate with the centrifuge test results. The method overestimates the bearing capacity at larger depths, and a conservative reduction factor is required. [ABSTRACT FROM AUTHOR]

The increasing trend of offshore wind turbines in deeper waters has necessitated the usage of alternative foundations such as the bucket foundation (including monopod and tripod). To investigate and compare the lateral monotonic and cyclic behaviour of the monopod and tripod, a series of centrifuge tests were carried out in medium dense sand, which were fabricated with an identical amount of material. The monotonic centrifuge tests were back-analysed using an advanced hypoplastic model to offer further insights into the test results. Under a monotonic lateral loading, the tripod exhibits a 78% higher initial lateral stiffness than the monopod, but it yields at a 71.4% smaller rotation, suggesting a more brittle response. While subjected to lateral cycling, the rotation of the monopod increases with number of cycles but at a decreasing rate, following a power function. Similarly, the dynamic stiffness of the monopod also increases with number of cycles. Differing from the monopod, the tripod exhibited a “self-healing” behaviour. Both the rotation and dynamic stiffness increases during the first few hundreds of cycling but decreases thereafter, leading to little variation. This unique feature in cumulative rotation and dynamic stiffness of the tripod makes it superior to the monopod. [ABSTRACT FROM AUTHOR]

New analytical models have recently been presented for evaluating the peak bearing capacity of circular footings penetrating sand-over-clay and the subsequent capacity once embedded in the underlying clay. However, in lieu of any analytical model, current methods of analysis use an inaccurate linear interpolation for the bearing capacity between the peak and the clay surface. This paper proposes an analytical model for calculating the bearing capacity of circular footings after the peak failure in sand-over-clay soils and before the footing enters the clay. Based on observations from visualisation experiments conducted in a geotechnical centrifuge, and consistent with the silo analysis approach used in predicting the peak capacity, the failure mechanism is a vertical frustum of sand progressively being forced into the underlying clay. Analysis of the capability of the new model to retrospectively simulate a database of 66 geotechnical centrifuge tests is provided. Motivation for improved prediction methods comes from the offshore mobile jack-up industry where concerns remain over the punch-through of large 20 m diameter spudcan footings in sand-over-clay. Accurate prediction of not just the peak bearing capacity but the subsequent load-displacement behaviour is required in order to assess the effectiveness of operational methods used to mitigate punch-through incidents. [ABSTRACT FROM AUTHOR]

There has been a constant trend towards larger mobile jack-ups capable of operating for extended periods in deeper water and in harsher environmental conditions. This is increasing both the size of their spudcan footings and the operational bearing pressures. Though some new analytical methods to predict the load-penetration profile have been proposed, and shown to fit centrifuge experiments well, these methods were calibrated mainly using experimental and numerical data for sand overlying soft clay. This paper reports six centrifuge tests simulating spudcan installation in sand overlying a stiff clay. These are complemented with large-deformation finite element analyses, simulating the continuous penetration of the spudcan in sand over stiff clay. Modified Mohr-Coulomb and Tresca models were used to describe the sand and clay behaviour, accounting for the effects of strain softening on the response of the soil. These results provide confidence that a recently published analytical method to predict the peak capacity can indeed be extended to high bearing resistances and stronger underlying clays. Bearing capacity factors used to predict capacity in the underlying clay have, however, been updated to reflect the new database of results. A new formulae that explicitly accounts for increasing strength with depth profiles is provided. [ABSTRACT FROM AUTHOR]

A key aspect of permanent offshore structures is protection against scour. This is typically in the form of a blanket of coarse gravel or cobbles surrounding the structure. These coarse particles are selected for their high resistance to being displaced by strong currents and thus protect the underlying finer sand particles from scour. However, in the event of an earthquake, the foundation sand may be susceptible to some degree of liquefaction. This research investigates the effects of seismic-induced liquefaction over a scour blanket, and if sinking is inhibited by some combination of the additional effective stress imposed by the gravel together with the interlocking resistance that develops when coarse particles are subjected to relative displacements. In order to evaluate the stability of scour protection blankets, a programme of physical modelling was carried out, involving the assessment of different configurations of stone layers over a liquefiable material, and a monopile-type foundation. Models were subjected to scaled base shaking equivalent to earthquake loading. A mass-balance of particle sinkage showed that a filter layer was critical for maintaining the integrity of the armour stones. Based on displacement and pore water pressure measurements, it was found that the presence of the scour protection blankets improved the response of the liquefiable sand under seismic loading, and even inhibited the occurrence of liquefaction. This implies that a well-designed scour protection blanket can assist in protecting against earthquake effects also. [ABSTRACT FROM AUTHOR]

This paper presents a simplified procedure for predicting the seismic slip of a vegetated slope. This is important for more precise estimation of the hazard associated with seismic landslip of naturally vegetated slopes, and also as a design tool for determining performance improvement when planting is to be used as a protective measure. The analysis procedure consists of two main components. Firstly, Discontinuity Layout Optimisation (DLO) analysis is used to determine the critical seismic slope failure mechanism and estimate the corresponding yield acceleration of a given slope. In DLO analysis, a modified rigid perfectly plastic (Mohr–Coulomb) model is employed to approximate small permanent deformations which may accrue in non-associative materials when subjected to ground motions with relatively low peak ground acceleration. The contribution of the vegetation to enhancing the yield acceleration is obtained via subtraction of the fallow slope yield acceleration. The second stage of the analysis incorporates the vegetation contribution to the slope's yield acceleration from DLO into modified limit equilibrium equations to further account for the geometric hardening of the slope under increasing soil movement. Thereby, the method can predict the permanent settlement at the crest of the slope via a slip-dependent Newmark sliding block approach. This procedure is validated against a series of centrifuge tests to be highly effective for both fallow and vegetated slopes and is subsequently used to provide further insights into the stabilising mechanisms controlling the seismic behaviour of vegetated slopes. [ABSTRACT FROM AUTHOR]

The paper presents results from a centrifuge-scale study of the lateral response of piles in variably cemented sands. These sands were characterised using in situ cone penetration tests (CPTs) as well as laboratory element tests. Experimental results and numerical back-analysis of the centrifuge tests confirm that the lateral ground stiffness and capacity correlate well with the CPT end resistance ( qc). The centrifuge data are combined with measurements made in other field-scale tests obtained in a strongly cemented sand, enabling development of a CPT-based formulation for load transfer curves that is consistent with observations and numerical analyses. The formulation is shown to provide reasonable predictions of lateral load response in a number of cemented sands with qc values in the range 2-50 MPa. [ABSTRACT FROM AUTHOR]

When a jack-up rig is installed in close proximity to an existing piled jacket platform, the massive soil movements induced by spudcan penetration may damage the adjacent piles. In 2012 the authors reported the findings of an extensive centrifuge model study on spudcan-pile interaction in soft clay. As a seabed profile may consist of soft clay over stiff soils and the spudcan-pile interaction phenomenon in such case is expected to be significantly different from that in soft clay, the earlier study is extended to investigate the interaction of a spudcan with adjacent piles in a soft clay overlying sand profile. Besides centrifuge tests on model spudcans, tests on half-spudcans placed behind the transparent window of the model container have been conducted to evaluate the soil movements and failure mechanism for various model configurations and soil conditions. The experimental results and findings are presented in this paper. [ABSTRACT FROM AUTHOR]

A complete analytical method to describe the full load-penetration resistance profile of a mobile jack-up spudcan footing penetrating a sand over clay stratigraphy is described. It is based on both large deformation finite-element analyses and geotechnical centrifuge experiments. The coupled Eulerian-Lagrangian (CEL) approach is used to accommodate the large deformations of a spudcan footing penetrating sand overlying clay. Modified Mohr-Coulomb and Tresca models describe the sand and clay behaviour, with modifications accounting for the effects of strain softening on the response of the soil. The CEL results are shown to match centrifuge tests well, allowing the numerical study to be extended parametrically, and with confidence, to cover the range of layer geometries, sand relative densities and footing shapes that are of practical interest to offshore jack-ups. The results are used to ( a) assess the performance of an existing model to predict the peak resistance in the sand layer (extending its range of application to medium dense to dense sands and to conical footings of angle 0° to 21°), and ( b) develop an expression for the bearing capacity factor when the footing penetrates into the underlying clay. Using the analytical formulas proposed, retrospective simulations of centrifuge tests show that the method provides a reasonable estimate of the peak punch-through load, the behaviour in the underlying clay, as well as the punch-through distance; the latter being a basic reflection of the severity of a potential punch-through failure. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that the use of a discretely-spaced row of piles can be effective in reducing the deformations of slopes in earthquakes. In this paper, an approximate strain-dependant Newmark sliding-block procedure for pile-reinforced slopes has been developed, for use in analysis and design of the piling scheme, and the model is validated against centrifuge test data. The interaction of the pile within the slipping soil was idealised using a non-linear elasto-plastic ( P–y ) model, while the interaction within the underlying stable soil was modelled using an elastic response model in which (degraded) soil stiffness is selected for an appropriate amount of shear strain. This combined soil–pile interaction model was incorporated into the improved Newmark methodology for unreinforced slopes presented by Al-defae et al. [1] , so that the final method additionally incorporates strain-dependent geometric hardening (slope re-grading). When combined with the strain-dependent pile resistance, the method is therefore applicable to analysis of both the mainshock and subsequent aftershocks acting on the deformed slope. It was observed that the single pile resistance is mobilised rapidly at the start of a strong earthquake and that this and the permanent slope deformation are therefore strongly influenced by pile stiffness properties, pile spacing and the depth of the slip surface. The model shows good agreement with the centrifuge test data in terms of the prediction of permanent deformation at the crest of the slope (important in design for selecting an appropriate pile layout/spacing i.e. S / B ) and in terms of the maximum permanent bending moments induced in the piles (important for appropriate structural detailing of the piles), so long as the slip surface depth can be accurately predicted. A method for doing this, based on limit analysis, is also presented and validated. [ABSTRACT FROM AUTHOR]

The operation of energy piles in summer can expel excess heat of the buildings into the ground by the use of a heat pump. Despite having been implemented for decades, the design of energy piles still relies heavily on empiricism, as there is limited understanding about heating effects on pile capacity. A series of centrifuge model tests on aluminum energy piles in medium dense saturated sand is reported in this study to investigate heating effects on the settlement patterns as well as capacities of single piles. In total, four in-flight pile load tests under three different temperatures, namely 22, 37, and 52 °C, and different loading sequences were carried out. Variations of pile capacity were interpreted with the help of a nonlinear elastic analysis. The test results show that after heating at zero applied axial load, toe resistance of the pile was mobilized as a result of constrained downward thermal expansion of the pile. Heating to a higher temperature caused the neutral plane to shift towards the pile toe due to a larger degree of mobilization of end-bearing resistance. It is also found that for a pile under a maintained working load, the pile head heaved initially by 1.4% D ( D, pile diameter) when the temperature increased by 30 °C, but it gradually settled to 0.8% D after 4 months of continuous heating at the constant temperature. The post-pile settlement is believed to be caused by thermal contraction of sand. Subsequent pile load tests show that pile capacities increased by 13% and 30% with incremental temperatures of 15 and 30 °C, respectively. With an increasing temperature, shaft resistance increased but at a reducing rate. At a higher elevated temperature, toe resistance increased more rapidly than shaft resistance due to a larger downward expansion of the pile. For simplicity, earth pressure coefficients with values of 1.1 K0 and 1.3 K0 were found to be suitable for estimating the capacities of aluminum model piles with temperature increments of 15 and 30 °C, respectively. [ABSTRACT FROM AUTHOR]

Water injection is currently used to reduce pile installation resistance in a variety of soil conditions. However, the mechanisms governing load reduction remain unclear, and there are uncertainties over the effect of water injection on the long-term behaviour of the installed pile. This paper offers some insights based on centrifuge model tests, with the aim of clarifying the installation mechanism. A novel approach to scaling was required, and the particle size was deemed to scale with enhanced gravity. A three-stage variable permeability mechanism is proposed and validated against a series of instrumented pile installations. Various pile installations were completed using different flow rates and injection geometries to investigate the differing injection effects. Base resistance was eliminated by forming a fluidized region around the pile toe, extending to two pile radii. However, fluidization could not be maintained beyond a critical depth of installation, and base resistance increased thereafter. [ABSTRACT FROM AUTHOR]

Shallow foundations supporting bridge piers, building frames, shear walls and monuments are often subjected to extreme lateral loading such as wind in offshore environments, or strong seismic shaking. Under such loading conditions, foundations may experience a host of non-linear phenomena: sliding on and uplifting from the supporting soil or even soil failure in the form of development of ultimate bearing capacity mechanisms. This type of response is accompanied by residual settlement and rotation of the supported structural system. Nevertheless, inelastic foundation performance can provide potential benefits to the overall seismic integrity of the structure. Thanks to such non-linearities, energy dissipation at or below the foundation level may eventually limit the seismic demand on structural elements. Several theoretical and experimental studies have provided encouraging evidence to this effect. This paper has a dual objective: first, to study the behaviour of shallow foundations under vertical and lateral monotonic loading and under lateral slow cyclic loading of progressively increasing amplitude; second, to explore the differences in foundation response between reduced-scale 1 and centrifuge 50 model testing. Emphasis is placed on interpreting their discrepancies by unveiling the role of scale effects. The role of soil densification due to multiple loading cycles with uplifting is also highlighted. [ABSTRACT FROM AUTHOR]

Currently monopiles are the most common foundation solution for offshore wind turbines. The design of monopiles relies on empirical data from tests performed on long, slender, small-diameter piles loaded predominantly in shear. In contrast, a monopile is a large-diameter, relatively short pile on which load is applied with a large eccentricity. With centrifuge tests as the basis, this paper investigates the behaviour of a rigid pile loaded with a high eccentricity. A test series was carried out to simulate idealized monotonic load cases for monopiles supporting an offshore wind turbine. Centrifuge tests were performed on model monopiles subjected to stress distributions equal to prototype monopiles with pile diameters ranging from 1-5 m and eccentricities ranging from 8.25-17.75 pile diameters. It was possible to identify a unified response of all of these tests by using dimensional analysis and Rankine's passive earth pressure coefficient as a normalization parameter. The normalized ultimate soil resistance was unaffected by acceleration level and load eccentricity, indicating that the failure mechanism was the same for all tests. Based on the centrifuge tests, a reformulation of soil-pile interaction curves is presented. The normalized initial stiffness of the soil-pile resistance curves was seen to increase linearly with depth in the centrifuge tests. The reformulation differs from current guidelines in terms of the shape of the interaction curve and magnitude of ultimate resistance. [ABSTRACT FROM AUTHOR]

Basement excavation may induce unsymmetrical and highly skewed loadings and (or) stress changes in an existing tunnel, not only in the transverse, but also in the longitudinal direction of the tunnel. Although basement–tunnel interaction has attracted intense academic interest recently, it is often simply treated as a plane strain problem. In this study, however, based on a dimensional analysis of the governing parameters, two three-dimensional centrifuge tests were designed and carried out in dry sand to investigate the effects of a basement excavation on an existing tunnel located in two horizontal offsets in relation to the basement. In addition, a preliminary three-dimensional numerical analysis was conducted to back-analyse the centrifuge test and to investigate the effects of the tunnel cover-to-diameter and unloading ratios on the existing tunnel. For the specific conditions simulated and soil type tested, a maximum heave of about 0.07% of the final depth of the basement excavation (He) was induced in the tunnel that ran parallel to and beneath the basement. On the contrary, a maximum settlement of 0.014% He was induced in the tunnel located at the side of the basement. For the former tunnel, the influence zone by the basement excavation on vertical tunnel displacement along the longitudinal direction was 1.2L (basement length). By inspecting the measured strains in the longitudinal direction of the existing tunnel, it was found that the inflection point, where the shear force is at a maximum, was located at 0.8L away from the basement centre. Due to stress relief from the basement excavation, the tunnel located directly beneath the basement was vertically elongated, but the one that lay at the side of the basement was distorted. A preliminary numerical parametric study found that tunnel heave decreased as the cover-to-diameter ratio increased, but at a reduced rate. [ABSTRACT FROM AUTHOR]

Earthmoving equipment working on a progressively placed cover layer over mine tailings often experiences catastrophic 'rotational' (or 'punch-through') failure, with potential for loss of the equipment and harm to the operator. Although the performance of foundations on a homogeneous sand or clay slope is routinely calculated and investigated, comprehensive investigation of a thin, stronger slope overlying a softer layer is scarce. This paper reports the results of centrifuge model tests undertaken to provide insight into strip foundation behaviour during penetration, with freedom in horizontal displacement and rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). Variables were the set-back of the edge of the foundation from the crest of the slope, the height of the slope relative to the foundation size, and the normalised strength of the lower clay layer. Soil movement was captured continuously by a digital camera, and subsequently quantified through particle image velocimetry (PIV) analysis. The load-penetration responses were separately recorded. The effect of normalised set-back ratio ( = /), slope height ( = /) and clay strength ( / ) on the evolving soil flow mechanisms and the penetration resistance profile is discussed in the context of the likelihood and severity of failure. Rotational failure, with a peak in penetration resistance followed by some reduction, occurred for all cases investigated except for a higher set-back of = 1·5. The severity of failure was greater the closer the proximity of the footing to the slope crest, and the greater the height of the slope, whereas it reduced as the normalised strength of the lower layer increased. Typical critical failure occurred in clear shear planes pushing a (nominally) rigid block of soil, with the shape of a hemisphere followed by a wedge, towards the slope. [ABSTRACT FROM AUTHOR]

Experimental observations on the behaviour of a small-scale model strip footing on dense sand subjected to combined loading under an enhanced gravitational field are presented. These observations are interpreted within the framework of plasticity-based force-resultant models, established earlier under 1 conditions, with the aim of describing changes of behaviour between small-scale 1 and centrifuge models. The comparison reflects the tendency of 1-based observations to overestimate, on the unsafe side, the normalised deviatoric load capacity for low levels of / . On the other hand, the 1-based response is found to be conservative compared to centrifuge-obtained responses for high levels of / . [ABSTRACT FROM AUTHOR]

Microbial-induced calcite precipitation (MICP), a novel bio-mediated ground improvement method, was explored to mitigate liquefaction-prone soils. Geotechnical centrifuge tests were used to evaluate cementation integrity and the response of MICP cemented sands to dynamic loading. The cementation integrity testing reveals a change in behaviour from 'soil like' to 'rock like', with an increase in treatment level. Results from dynamic testing demonstrate a clear increase in resistance to liquefaction of MICP-treated sands compared to untreated loose sand. The MICP sands were treated to varying levels of cementation (light, moderate and heavy cementation levels) and assessed using non-destructive shear wave velocity measurements. The centrifuge models were all subjected to ground motions consisting of sine waves with increasing amplitudes. Accelerations, pore pressures and settlements were measured in the soil during shaking, and the changes in soil behaviour and post-shaking shear wave velocity for soils prepared to different cementation levels are discussed. Increased resistance to liquefaction was demonstrated with a decrease in excess pore pressure ratios in the MICP-treated models, as well as in reduced post-shaking settlements; however, surface accelerations were amplified at heavy levels of cementation. A tradeoff between improving liquefaction resistance and minimising undesirable higher surface accelerations needs to be considered when designing the soil improvement level. [ABSTRACT FROM AUTHOR]

Abstract: Centrifuge experiments were conducted to learn about the factors that affect mobilised resistance during rapid load testing of piles in sand. We studied the influence of pore water pressure during rapid load tests and its effect on the widely used unloading point method to derive static pile capacity. This paper describes the testing programme and the test set-up. We present typical measurement results from a total of 36 rapid and 12 static load tests, as well as the effects of the loading rate and excess pore pressures on pile resistance. The tests confirm that a rapid load test can overestimate static capacity due to pore water pressure when testing piles in medium to fine sands. The results of the pore pressure measurements show a combination of positive and negative excess pore pressure in the zone around the pile base, which can be explained by compression, volumetric behaviour during shearing and pore fluid flow around the pile. [Copyright &y& Elsevier]

Piled foundations are often subjected to cyclic axial loads. This is particularly true for the piles of offshore structures, which are subjected to rocking motions caused by wind or wave actions, and for those of transport structures, which are subjected to traffic loads. As a result of these cyclic loads, excessive differential or absolute settlements may be induced during the piles' service life. In the research presented here, centrifuge modelling of single piles and pile groups was conducted to investigate the influence of cyclic axial loads on the performance of piled foundations. The influence of installation method was investigated and it was found that the cyclic response of a pile whose jacked installation was modelled correctly is much stiffer than that of a bored pile. During displacement-controlled axial load cycling, the pile head stiffness reduces with an increasing number of cycles, but at a decreasing rate; during force-controlled axial load cycling, more permanent settlement is accumulated for a bored pile than for a jacked pile. The performance of individual piles in a pile group subjected to cyclic axial loads is similar to that of a single pile, without any evident group effect. Finally, a numerical analysis of axially loaded piles was validated by centrifuge test results. Cyclic stiffness of soil at the base of pre-jacked piles increases dramatically, while at base of jacked piles it remains almost constant. [ABSTRACT FROM AUTHOR]