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This study evaluated the deformation behaviour, vertical stress, strain, and failure mode of a 14 m-high (H) geogrid-reinforced slope supporting an 8 m-high embankment using centrifuge models. Tests were conducted at an acceleration of up to 40g. Additional tests were conducted to investigate the effects of layer spacing (0.5 to 1.0 m at prototype scale) and slope inclination (45° to 90°) on the performance of the reinforced slopes. The maximum strain (∼0.04%) occurred in the reinforcement layer nearest to the crest of the slope. Potential slip surfaces were identified from peak strains in the reinforcement layers. The vertical stress was redistributed owing to the tensile behaviour of the geogrid, and vertical stress within the soil slope was transferred to the area near the face of the slope (0.26–0.77H from the face). As the layer spacing increased (from 0.5 to 1.0 m), the potential slip surface moved away from the slope face with increasing deformation, which reduced the overall stability of the reinforced slope. The model with a vertical slope generated the largest reinforcement strain (∼0.2%), vertical settlement, and surface bulging compared with the model with a mild slope. [ABSTRACT FROM AUTHOR]

Increasing the resilience of structures to seismically induced soil liquefaction is a major objective for the earthquake engineering community. Increasingly, soil drains are being used to mitigate the liquefaction hazard. By facilitating drainage, excess pore pressures can be limited to acceptable levels, thus reducing settlement and rotation of structures founded on liquefiable soils. However, drain design is currently based on procedures developed for the free-field. Structures change the stress distribution and flow patterns in the soil, altering the occurrence of liquefaction. At present, none of the available engineering procedures considers soil–structure interaction (SSI) in the design of drains. In dense urban environments, further complications arise from interaction between neighbouring structures. To investigate the influence of pre-fabricated vertical drains (PVDs) on structure–soil–structure interaction (SSSI), a series of geotechnical centrifuge tests was conducted to compare the performance of an isolated structure, a pair of similar structures without mitigation and a pair of structures where PVDs were present around just one building. Results show that, without improved drainage, SSSI can reduce permanent rotation of structures. However, where PVDs were placed around one of a pair of structures, the consequences for permanent rotation of and inertial demand on the adjacent unmitigated superstructure were detrimental. [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]

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill. [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]

This paper reports a forensic investigation on the unexpected collapse of a 13-storey high-rise residential building in Shanghai soft clay. Prior to the building collapse, a shallow excavation was carried out on one side of the building while a slope was filled on the opposite side. Despite the observed breakage of all the piles in the field, a preliminary forensic numerical analysis adopting a typical undrained shear strength (cu) profile in Shanghai suggests that the piles are very unlikely to break. The surprising contrast has led to this detailed forensic study on the underlying mechanisms for the building collapse, through a site-specific characterisation of the cu profile, three-dimensional centrifuge modelling and numerical back-analysis. Although the typical cu profile in Shanghai increases almost linearly with depth, the clay in this site consists of two major layers with distinctive strength – that is, a relatively soft clay layer (average cu = 23 kPa, thickness = 13·5 m) underlain by a relatively stiff clay layer (average cu = 58 kPa, thickness = 18·2 m). By reproducing this cu profile in the centrifuge test and numerical analysis, the resulting direction of the building collapse and the depths of pile breakage were broadly consistent with those observed in the field. It was found that the surcharge of the fill slope caused the bearing capacity to be exceeded in the upper soft clay layer. This led to the development of a sliding wedge under the surcharge, which extended horizontally (at a depth of 11 m) across the pile foundations to reach the excavation side, and therefore the breakage of the piles. If a typical strength profile in Shanghai (with cu linearly increasing with depth) were adopted, the failure mechanism could be significantly altered from the aforementioned deep slip failure to a shallow passive failure, causing much smaller bending moment in each pile. [ABSTRACT FROM AUTHOR]

This paper evaluates the performance of currently available analytical procedures to assess building response to tunnelling‐induced ground displacements. The focus is on methods that account for the interaction between the soil and the structure during tunnelling. These methods relate the soil to the building stiffness and are often called Relative Stiffness Methods (RSMs). Results from centrifuge model tests are used to evaluate the ability of these RSMs to predict building deformations. This evaluation benefits from detailed building models including facade openings, intermediate walls and strip footings. The range of RSM predictions was large, and the accuracy of each RSM was quantified. It was found that no RSM accurately predicts flexural building deformations. Recommendations that consider the building‐to‐tunnel position to achieve accurate predictions are indicated. This contribution provides a better understanding of the performance of currently available criteria to assess the risk of urban tunnelling. [ABSTRACT FROM AUTHOR]

The rapid development of modern metropolises has led to a shortage of surface space and, in response, engineers have pursued alternatives below ground level. Shafts are commonly used to provide temporary access to the subsurface for tunnelling and, as permanent works, are utilised for lifts or ventilation purposes. The construction sequence of axisymmetric shafts makes them a dramatically simple solution. In addition, circular shafts are inherently stiffer than other plan geometries. Those are perhaps the reasons why circular shafts are preferred in situations of restricted space or unfavourable ground conditions. However, due to the lack of case histories reporting ground movements induced by shaft construction, no empirical prediction method for subsurface soil displacements exists. The work presented here seeks to provide clearer insights into surface and subsurface soil displacements induced by circular shaft construction by means of an analysis of measurements obtained from centrifuge tests and available field data. Novel empirical equations and procedures are then suggested for practical use. [ABSTRACT FROM AUTHOR]

This paper presents a description of testing procedure and results of the LEAP-GWU-2015, and an investigation of the effect of the radial gravity field in centrifuge modelling. Dynamic responses of two models are compared. One had a planar surface with a 5° slope relative to the base of the container; the other had a curved surface to maintain a constant slope angle with respect to the radial g-field. For the centrifuge tests employed in this study, the slope and base excitation are in the tangential direction to spinning centrifuge. Results show that spikes in acceleration records due to cyclic mobility associated with lateral displacements appeared on both models. However, for the plane surface model, acceleration spikes in the negative (upslope) direction are more prominent and the residual downslope ground deformation was larger. Larger lateral displacement was observed in the plane model, while surface displacements in the curved model are smaller and uniform along the length of the model. For the plane model, the radial gravity acting near the slope top is almost constant and gradually increases towards the slope toe which causes a net increase in effective slope angle. Curving the model ground surface is recognized as leading to uniform lateral displacements along the length of the model surface. • Under the LEAP-GWU-2015, a physical model test under specified conditions is conducted. • Dynamic responses of a saturated sloping ground forming either curved or plane surface are compared. • Larger upslope acceleration and residual deformation was observed in the plane surface model. • In the curved model, displacements were smaller and uniform along the length of the model. • For centrifuge modelling, importance of curving the model ground surface was by the comparison. [ABSTRACT FROM AUTHOR]

The rocking response of embedded foundations in sand is studied, combining centrifuge modelling and numerical analysis. In total, 51 centrifuge model tests are conducted at ETH Zurich, varying the embedment ratio D/B and the factor of safety against vertical loading (bearing capacity), Fs. The experimental results are used to validate three-dimensional finite-element models, which are subsequently employed for a parametric study. The initial two phases of non-linear response are studied, namely quasi-elastic and non-linear response, while the third phase (plastic response) will be examined in a forthcoming publication. Regarding small-strain rocking stiffness, Kr (first phase), an extended formula is proposed, accounting for soil inhomogeneity along the embedded sidewalls. Kr is shown to decrease with Fs, due to initial soil yielding from vertical loading. The formula is further extended to account for the role of Fs, offering improved estimation of small-strain rocking stiffness. With respect to the non-linear response (second phase), it is shown that the degradation curves of rocking stiffness with rotation can become approximately dimensionless, if the rotation θ is normalised to a characteristic rotation, θs. The increase of D/B leads to a reduction of the normalised rotation θ/θs at which the non-linear response is initiated, due to the increasing participation of sidewall friction. [ABSTRACT FROM AUTHOR]

The hybrid foundation of combining a friction wheel with a monopile is an innovative solution for offshore structures subjected to large lateral-moment loading. In this paper, the lateral-moment response of the monopile-friction wheel foundation in saturated sand are investigated via centrifuge tests and three-dimensional finite element method (FEM). A series of tests on the monopile, hybrid foundations with wheels of different diameters and thicknesses, and single wheel foundation were conducted. The results show that the lateral bearing capacity and stiffness improve significantly by adding a wheel to monopile, and the improvement follows the diameter or thickness of the wheel. An extensive experimental research regarding to the influential factors such as the embedment of the wheel and the vertical load is also presented. By means of FEM, the load transfer mechanism, interaction between the foundation and soil, and the bending moment in the pile are illustrated to study how the wheel contributes to the performance of the foundation system. Moreover, the effects of load eccentricity and vertical load are investigated by FE analyses. • Centrifuge tests and finite element (FE) analyses are performed about the hybrid foundation. • Parametric studies are conducted via centrifuge tests. • The load transfer mechanism, interaction between foundations and soils are illustrated by FE analyses. • The effects of load eccentricity and vertical load on lateral bearing performance are investigated. [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]

Diving upon pullout of an installed anchor leads to an increase in embedment depth and, hence, capacity under operational loadings. It is particularly critical for dynamically installed anchors (DIAs) in calcareous sediments where the embedment depth is generally relatively shallow. This paper proposes a novel fish DIA featuring an elliptically shaped shaft, which reduces hydrodynamic drag resistance. Every cross section of the anchor shaft is elliptical, and the widest part is in the middle. The shaft is shaped to be thicker near the head to lower the mass centroid and increase its diving potential. The padeye is fitted perpendicularly to the wider part of the shaft, so that the maximum resistance area can be mobilized during pullout loading. The dynamic installation and monotonic pullout performance of the fish DIA in calcareous silt were assessed through a series of beam centrifuge tests at 133.3 times Earth's gravity, varying the drop height (hence the impact velocity), reconsolidation period after installation and load inclination angle at the mudline. The anchor tip embedment depths of 1.02-1.42 times the anchor length lay in the range for OMNI-Max and torpedo DIAs in calcareous silt. A dimensionless embedment depth-based model, total energy-based models, and a conventional shear resistance model were found to be adequate for the prediction of embedment depths. The tracked anchor trajectory confirmed that the fish DIA dove under inclined mudline loading (θ0 < 45°), as opposed to being pulled out for torpedo and OMNI-Max DIAs. The normalized vertical and horizontal holding capacities of the fish DIA were respectively 4.0 and 5.6 times those of a four-fin torpedo. [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]

Abstract Earthquake induced liquefaction can cause structures with shallow foundations to experience large settlement and rotation, and can cause subsurface structures to uplift. The performance of structures with basements, which intuitively combine these two problems, is not understood. In this paper, data from three dynamic centrifuge tests on structures with wide basements are examined. The ratio of upward to downward vertical forces was varied, and a symmetric and asymmetric superstructure was tested. Digital image correlation was used to capture the soil displacements, providing novel insight into the co-seismic soil-structure interaction. The inclusion of wide basements was shown to reduce the overall settlement of structures by providing an increased uplift force during the liquefied period. For symmetric structures, symmetric soil displacements occurred around the basement during consecutive half-cycles of sinusoidal shaking, resulting in negligible accumulation of rotation. In contrast, significant rotation was accumulated for an asymmetric structure as a result of the P − δ effect due to the eccentric mass. Highlights • Wide basements can reduce the settlement of structures during liquefaction. • P- δ effects cause large rotation of an asymmetric structure with a wide basement. • Basements enable full liquefaction below, which isolates the structure. [ABSTRACT FROM AUTHOR]

Abstract Gravity Wharf Structures are widely used worldwide to form shallow and deep water ports. They are large structures with a height of 25–50 m depending on the water depth. When these structures are to be sited on loose to medium dense sand, earthquake induced liquefaction settlements present a significant risk. This often requires expensive soil replacement or other ground improvement techniques. In this paper, the dynamic behaviour of these large structures that exert very high bearing pressures on the foundation soil was investigated for the first time. The level of settlements they can suffer due to soil liquefaction was investigated using dynamic centrifuge testing. It will be shown that the full liquefaction does not occur below the structure even when the free field soil fully liquefies during strong earthquakes. However there will be some stiffness degradation owing to excess pore pressure generation and consequent structural settlements. The level of these settlements are considered to be acceptable from a Service Limit State (SLS) perspective. In addition to this the hydro-dynamic pressures that act on the Gravity Wharf structure were also investigated. Highlights • High gravity centrifuge testing of Large Gravity Wharf Structures subjected to earthquake loading. • Liquefaction of free-field and partial liquefaction of foundation soil were observed. • Settlement of these large structures following moderate and strong shaking are discussed. • Hydro-dynamic pressures observed experimentally are compared to the conventional Westergaard's approximate solutions. [ABSTRACT FROM AUTHOR]

The parkable piezoprobe is a site investigation tool for measuring the coefficient of consolidation, in situ offshore, at shallow embedment depths. The device applies a similar bearing pressure to subsea infrastructure so it reaches a comparable self-weight penetration when 'parked' at the seabed, representative of an unburied pipeline. Instrumentation on the device allows the dissipation of penetration-induced excess pore pressure to be recorded at various locations on the surface. From these dissipation responses the coefficient of consolidation can be inferred, which is a key parameter in the design of many offshore structures founded in surficial soil, such as pipelines or shallow foundations. The intent is that this device is deployed from a seabed frame, while other activities such as penetrometer testing or sampling take place in parallel. This paper presents robust interpretation methods for the parkable piezoprobe by using a combination of centrifuge experiments and large deformation finite-element analyses. The centrifuge tests demonstrate that the penetration response of the parkable piezoprobe is adequately captured by existing bearing capacity models, allowing the optimum device weight to be identified. A comprehensive interpretation method is then developed for the dissipation stage. This yields accurate estimates of the coefficient of consolidation, even for cases where there is no prior knowledge of the soil parameters or the depth to which the device embeds under its own self-weight. [ABSTRACT FROM AUTHOR]

This paper presents a predictive model for the settlement of shallow-founded structures on liquefiable ground during earthquakes. The model is based on the results of an extensive fully coupled, three-dimensional numerical parametric study of soil–structure systems, validated with centrifuge experiments as well as a database of case history observations. The results of the numerical study provided insight into the relative importance and influence of each input parameter and the functional form of the predictive model for a structure's permanent settlement. The case history database helped validate and refine the predictive model, accounting for complexities of the ground motion and site conditions in the field. Non-linear regression and latent variable analysis were used to develop model coefficients. The uncertainty around model estimates was modelled by a lognormal distribution. An additional logistic model was provided to estimate the probability of insignificant settlement (defined as less than 1 cm). The proposed probabilistic procedure considers variations in site conditions as well as the presence and properties of a building in three dimensions. By including the case history database in its validation and adjustment, the model captures all mechanisms of settlement below the foundation, including volumetric and deviatoric strains as well as ejecta. The total uncertainty around its predictions is rigorously characterised, which is a necessary step before the benefits of performance-based seismic design can be realised in the evaluation and mitigation of the liquefaction hazard. [ABSTRACT FROM AUTHOR]

This paper examines the interaction between the spatial variations in binder concentration (i.e. cement slurry concentration) and in situ water content, in cement-mixed soil, using field and model data as well as statistical analysis and random field simulation. The field data are first analysed to shed light on the spatial variation in the in situ water content, including its scale of fluctuation. A statistical model is then developed which takes into account the variation in binder concentration and in situ water content. This leads to a two-parameter model for the prediction of the mean, variance and probability distribution function of the strength of the cement-treated soil. The scale of fluctuation for the variation in binder concentration arising from imperfect mixing within a cement-mixed column is then examined using centrifuge model data. This indicates that the scale of fluctuation in binder concentration is much shorter in range than that of the in situ water content. The combined effect of these two scales of fluctuation is then studied by simulating the resulting random field using Monte-Carlo simulations. This indicates that the size of the sampling region has a significant effect on the scale of fluctuation that is captured. If the sampling region is of a similar size to the column diameter, the measured scale of fluctuation reflects that of the binder concentration. As the size of the sampling region increases, so does the measured scale of fluctuation. This explains the wide range of scales of fluctuation that have been reported for cement-treated soil. To capture both scales of fluctuation in core sampling, some boreholes should be sunk at close spacings of less than a column diameter, in order to capture short-range variation. [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]

Punch-through failure of mobile jack-up rigs is still a main concern for spudcan foundations. Using centrifuge model tests and numerical modelling, this paper examines the mitigating effect of top-mounted sleeves and a downward skirt on spudcan punch-through in medium dense sand overlying clay. The findings show that the top-mounted sleeves and downward skirt are both useful separately for reducing punch-through distance. The top-mounted sleeve functions by interfering with soil backfilling above the spudcan and maintaining confinement to the sides. This results in an earlier recovery in penetration resistance and reduces the peak-to-trough drop in resistance. The downward skirt appears to mobilise the soil further ahead of the spudcan, which reduces the peak penetration resistance and increases the rate of post-trough recovery in penetration resistance. A combination of a top-mounted sleeve with a downward skirt appears to confer a smaller peak-to-trough drop in resistance and higher rate of post-peak recovery, and thereby a much smaller punch-through distance. The results would suggest that, compared with the downward skirt, the top-mounted square sleeve with a similar area ratio to the existing lattice leg is supposed to be a more effective option in mitigating spudcan punch-through failure. [ABSTRACT FROM AUTHOR]

The permanent ground deformation during the earthquake faulting induces moderate to severe damages to the underground tunnels. The majority of the investigations on the behavior of the tunnels against fault offset were in the rock medium. There is very little available in the literature about the fault-tunnel interaction in alluvial soil. This paper has studied the interaction between a continuous tunnel and reverse faulting within a dense sandy alluvial deposit. An experimental centrifuge test, alongside the numerical modelling, has been utilized for this purpose. It has been shown that the longitudinal strains of an infinite tunnel were much higher than that of a finite length tunnel. The results have also displayed that there are critical cross-sections along the tunnel's length that maximum curvatures, moments, shears, and axial forces occur in them. The numerical parametric study on the variation of the Fault Zone Width (FZW) and reinforcement content (ρs ) have shown that higher ρs values would be more useful in tunnel resistance against faulting. Besides, smaller FZW will make higher deformations, moments, and forces in the concrete lining. The optimum rs value has been obtained as 4%. [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]

Highlights • Seismic response of offshore wind turbine is investigated via centrifuge modelling. • Mechanisms of the structural performance during the earthquake are explained. • Failure modes of soil and soil-structure interactions are demonstrated. • Influences of the wheel specification are illustrated. Abstract Some large capacity offshore wind turbines are constructed in seismically active areas. The occurrence of soil liquefaction during an earthquake can result in severe failures of the offshore wind turbine. The seismic response of the structure and the failure mechanism of the soil-structure interactions are necessary to investigate. In this study, the seismic response of an innovative hybrid monopile foundation is investigated through a series of centrifuge tests. The seismic performance of the combined system of the superstructure, foundation, and soil are demonstrated. Five hybrid foundation models are tested by considering the influence of the foundation thicknesses and diameters, and a monopile foundation is tested for comparison. Centrifuge test results reveal that the hybrid monopile foundation is effective in reducing the lateral displacement during the shaking. In the saturated condition, soil keeps its strength and stiffness beneath and adjacent to the foundation. The hybrid foundation system tends to settle more due to the larger shear stress caused by the soil structure interactions. Influences of the wheel specifications are illustrated. The foundations with larger thicknesses lead to smaller lateral displacements and lower tendencies of liquefaction, but the settlements are intensified. The larger diameter foundation provides a longer drainage path for the excess pore water pressure. With a similar weight, the structure settles less during the earthquake. [ABSTRACT FROM AUTHOR]

Research efforts have been done to determine the limit of validity of Darcy's Law in various fields of engineering where viscous conditions of flow are likely exceeded. In geotechnical and geological engineering, the application of centrifuge modeling for studying flow through porous materials is complex because the seepage velocity scales proportionally to centrifuge gravity, resulting in greater potential for exceeding the limit of viscous flow. This limit is usually estimated based on Forchheimer's Law and the concept of critical Reynolds number, Rcritic, but its interpretation remains ambiguous. This study provides new insights and establishes a connection between different theoretical approaches available in the literature. Centrifuge permeability tests were conducted at different gravitational levels for different materials. Effects of the characteristics of the porous media and centrifuge acceleration on the flow behavior were evaluated. Results show that parameters of Forchheimer's Law remain constant regardless of the centrifuge acceleration. Values of Rcritic were obtained in a range from 0.2 to 11 depending on the characteristics of material. The interpretation of the limit of validity of Darcy's Law was analyzed based on different definitions of Reynolds number and the Forchheimer number, and critical velocities of flow and hydraulic gradients were estimated. [ABSTRACT FROM AUTHOR]

Abstract Monopiles are one of the most commonly used offshore foundation for wind turbines. Their static capacity, p-y curve and cyclic loading behaviour have been studied using 1 g tests and centrifuge tests, but there is little experimental data regarding their natural frequency, especially using centrifuge testing. The design of offshore wind turbine foundations is largely governed by natural frequency as resonance due to cyclic loading can cause damage and even failure. Understanding the dynamic response of the monopile under free vibration is thus critical to design. This paper presents the results of novel monopile (large diameter) and single pile (small diameter) tests in a centrifuge to for the first time directly determine the natural frequency (f n) of the pile-soil system. An experimental methodology was used to define the natural frequency via measured acceleration and force time histories and their fast Fourier transforms (FFT) under a force applied at a controlled frequency. The effects of pile diameter, embedded length, free length of the tower and soil density on f n were investigated in the centrifuge tests. The same models used in the centrifuge test at 50 g were also tested at 1 g in order to assess the relevance of earlier 1 g investigations into system behaviour. The measured natural frequency of wind turbine monopiles in centrifuge models during harmonic loading from a piezo-actuator, confirmed that soil structure interaction at an appropriate stress level must be taken into account to obtain the correct natural frequency. The experimental data was compared to theoretical solutions, giving important insights into the behaviour of these systems. Highlights • Centrifuge model of wind turbine foundation. • Soil structure influence on the natural frequency of a wind turbine structure. • Experimental methodology to determine the natural frequency of a wind turbine structure. • Comparison of physical and theoretical dynamic response of a wind turbine foundation. [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]

Suction caissons have been used for numerous oil and gas installations and are increasingly considered as a foundation solution for offshore wind turbines (OWTs). There can be significant differences between the two offshore energy applications in the load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). This paper investigates the response of suction caissons in dense sand to a range of cyclic vertical loading histories relevant to a jacket-supported OWT, with an emphasis on cyclic tensile loading. The findings are based on a series of experiments performed in a centrifuge, such that soil stresses reflect those in the prototype. The caisson was installed using suction at enhanced gravity, followed by cyclic loading and then caisson extraction. The installation and extraction results are discussed in a companion paper. This paper focuses on the caisson load–displacement response under vertical cyclic loading. The centrifuge experimental results reinforce findings from previous work, add insights into the load transfer mechanisms and provide confidence in their applicability to the prototype, both qualitatively and quantitatively. The results highlight the complexity of the caisson response, particularly under tensile loading, with the influences of average load, cyclic load amplitude and drainage discussed in detail. [ABSTRACT FROM AUTHOR]

Suction caissons are a promising foundation concept for supporting offshore wind turbines. Compared to applications in the oil and gas industry, where most practical experience exists, significant differences arise in terms of load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). In a set of two companion papers, this contribution investigates the response of suction caissons in dense sand through a series of centrifuge experiments. The caisson was installed using suction, followed by sequences of cyclic loading and then extraction, all steps completed continuously in-flight. This first paper discusses installation, limiting capacities and drainage, whereas the second paper focuses on vertical cyclic loading into tension. The work demonstrates that suction caisson installation behaviour is well described by existing calculation methods. Tests performed at different installation rates demonstrate that careful assessment of the pumping rate is needed to ensure successful installation, with low pumping rates resulting in premature refusal. In the centrifuge tests, full skirt penetration was achieved without apparent loosening of the soil plug. The limiting capacity in tension, measured during the testing at both fast and slow uplift rates, was also well described by existing calculation methods. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a description of testing procedure and results of the LEAP-GWU-2015, and an investigation of the effect of the radial gravity field in centrifuge modelling. Dynamic responses of two models are compared. One had a planar surface with a 5° slope relative to the base of the container; the other had a curved surface to maintain a constant slope angle with respect to the radial g-field. For the centrifuge tests employed in this study, the slope and base excitation are in the tangential direction to spinning centrifuge. Results show that spikes in acceleration records due to cyclic mobility associated with lateral displacements appeared on both models. However, for the plane surface model, acceleration spikes in the negative (upslope) direction are more prominent and the residual downslope ground deformation was larger. Larger lateral displacement was observed in the plane model, while surface displacements in the curved model are smaller and uniform along the length of the model. For the plane model, the radial gravity acting near the slope top is almost constant and gradually increases towards the slope toe which causes a net increase in effective slope angle. Through the comparison the importance of curving the model ground surface is recognized as leading to uniform lateral displacements along the length of the model surface. Highlights • Under the LEAP-GWU-2015, a physical model test under specified conditions is conducted. • Dynamic responses of a saturated sloping ground forming either curved or plane surface are compared. • Larger upslope acceleration and residual deformation was observed in the plane surface model. • In the curved model, displacements were smaller and uniform along the length of the model. • For centrifuge modelling, importance of curving the model ground surface was by the comparison. [ABSTRACT FROM AUTHOR]

This paper presents lateral responses of an existing 2 × 2 pile group to advancement of side-by-side twin tunnels (with 1% volume loss) at various depths in dry sand using 3D centrifuge and numerical modelling. The tunnels were located near mid-depth of the pile shaft (test SS), next to (test TT) and below the toe (test BB) of the pile group. The largest and the smallest lateral movement of the pile cap was induced after first tunnelling in test TT and test SS, respectively. The shearing force was induced in piles as a reaction of pile deflection in tests TT and BB. [ABSTRACT FROM AUTHOR]

A series of lateral tests conducted in a centrifuge on displacement piles in kaolin is used to examine the effects of pile shape, pile end-condition and clay overconsolidation on the lateral load transfer (p–y) curves in soft clay. These experimental tests are supported by finite-element analyses, which examine the responses of the circular, square and H pile sections used in the centrifuge tests. The experimental and numerical results reveal an important effect of pile shape on p–y curves, but no discernible effect of the pile end condition. These results also indicate comparable dependencies of net ultimate pressures on normalised depth and undrained shear strength. The findings enable development of a new formulation for p–y curves in soft clay, which incorporates the effects of pile shape and uses the triaxial compression consolidated undrained shear strength as the reference undrained strength. [ABSTRACT FROM AUTHOR]

Earthquake-induced liquefaction is typically viewed as an undrained phenomenon with undrained element tests forming the core of knowledge built around it. However, there is evidence to suggest that partial drainage could be taking place during an earthquake. In this paper two dynamic centrifuge tests are presented, in which drainage was restricted for a part of the soil by enclosing it within a chamber, in order to assess its importance. The hypothesis of undrained behaviour was found to be inappropriate for liquefied sand, even within the timescale of an earthquake. Fluid flow during the seismic motion was inevitable. Its effect on pore pressures and shear stress–shear strain response was controlled by the proximity of the boundaries. [ABSTRACT FROM AUTHOR]

Abstract Tunnelling-induced ground movements are complicated and investigations into them normally require some simplifications. This paper provides a brief literature review which highlights the advantages of adopting simplifications in physical modelling and addresses some of the deficiencies in the assessment of soil deformation due to a simulated tunnel excavation. A set of centrifuge tests modelling a tunnel heading located at different depths in clay was carried out at 125 g. The tunnel was modelled by a semi-circular cavity partly supported by a stiff lining. The unlined tunnel heading was supported by a thin rubber bag supplied with compressed air pressure. Tunnel excavation was simulated by reducing air pressure. The induced ground movements at the subsurface and surface were measured by a 2D image analysis and a new, novel 3D imaging system. The results show that the experiment successfully reproduced key aspects of tunnelling-induced soil deformation in practice. In addition, a new equation to predict horizontal displacements in the longitudinal direction is proposed. [ABSTRACT FROM AUTHOR]

Structures with shallow foundations resting on liquefiable layers can suffer excessive settlement in the event of an earthquake. The state of practice often estimates the settlement of structures using empirical methodologies. Commonly, these are based on case histories or estimations developed for the free field. Their reliability has been contested due to uncertainties regarding the dominant deformation mechanisms in the presence of a structure. Here, six dynamic centrifuge tests are presented, investigating the response of structures with shallow foundations resting on liquefiable layers of different thickness. Particle image velocimetry (GeoPIV) was used to capture the developed deformation mechanisms. A structure resting on a deep liquefiable layer was found to settle primarily due to increased lateral soil displacements taking place beneath a bulb of stiffer soil formed below the foundation. In shallower layers, this bulb reached the base of the layer, transmitting large accelerations to the structure and promoting a rocking response. Settlement in this case was generated due to increased soil displacement from under the edges of the foundation. In no case were methodologies aimed for the free field able to account for the salient settlement-generation mechanisms. [ABSTRACT FROM AUTHOR]

The optimal placement of geogrid reinforcement in clay liners subject to differential settlement was investigated both numerically and with centrifuge modelling. Two unreinforced liners, a liner reinforced at the top-quarter depth, a liner reinforced at the bottom-quarter depth and a double reinforced liner were modelled in the centrifuge. Differential settlement was induced on the model liners by lowering a trapdoor overlain with sand. By considering: 1) the magnitude of differential settlement required to induce micro-cracks in the liners, 2) the strain fields across the liners during differential settlement and 3) the distribution of these strain fields, it was found that dividing the available reinforcement equally between the top-quarter and bottom-quarter of the liner, i.e. double reinforcement, represents the optimal reinforcement strategy. [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]

This paper describes a project-based cone penetration test (CPT) calibration on a carbonate sand fill that was hydraulically placed and subsequently densified by vibrocompaction. The project involved the development of an artificial island constructed offshore from the United Arab Emirates for oil and gas production. Carbonate sands have crushable grains and can be significantly more compressible than silica sands, so it was determined that the semi-empirical CPT-based correlations for silica sands were not applicable and a soil-specific calibration was needed for post-densification characterisation of density, shear strength and compressibility. The CPT calibration investigation was primarily undertaken in a centrifuge, and then checked with supplementary tests in a large calibration chamber. In this paper, analysis of the calibration data follows three different threads for comparison with more typical siliceous sands: ( a) relationships between CPT tip resistance, void ratio and vertical effective stress; ( b) a relationship between CPT tip resistance and state parameter; and ( c) an approach based on cavity expansion theory, which provides predictive capability for the CPT once soil properties have been measured in the laboratory. The test results are compared with silica sands, in terms of void ratio or the state parameter ψ. [ABSTRACT FROM AUTHOR]