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Tailings dams have relatively high failure rates throughout the world and the consequences of these failures often result in significant loss of life and damage to the environment and property. However, the triggers and failure mechanisms are typically hypothesized and not well understood. To investigate potential triggers and the corresponding failure mechanisms, two centrifuge model tests were conducted on loose slopes made of gold tailings using a scaled viscous fluid to induce instability in flight. A numerical back-analysis was also carried out to investigate and verify the associated mechanisms. Two failure mechanisms were observed in the centrifuge tests. In the first test, large seepage forces caused sloughing at the toe. The initially drained instability at the toe induced significant positive excess pore pressures due to the loose state, as well as to the initially higher degree of saturation in the toe region, triggering localized liquefaction at the toe (undrained response). Due to the localized liquefaction, the tailings at the toe could not support the tailings upstream of the toe, triggering a retrogressive flowslide failure. In the second test, a slope failure occurred due to drained instability, i.e., failure occurred once the drained factor of safety approached unity. No liquefaction was evident, due to the initially lower degree of saturation in the toe region, as well as to the slower rate of shearing compared to the first test. As revealed by both physical and numerical simulations, the structural collapse of the soil resulted in the drained instability of the slope, which triggered a slide-to-flow failure. [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]

Seismic design of soil-nailed walls requires demonstrations of tolerable ranges of wall movements, especially when a surcharge load exists near the wall. In this study, the effect of surcharge location on seismically induced wall movements was investigated using four centrifuge tests. The axial tensile forces, developed along the soil nails during the seismic loadings, were also measured during the tests. At 50g centrifugal acceleration, model tests represented a 12 m high prototype wall reinforced with five rows of soil nails. To apply a surcharge stress of 30 kPa at the specified location relative to the wall for each model test, a rigid footing was placed on the soil surface. The model soil-nailed walls were subjected to three successive earthquake motions. Surprisingly, it was found that the model wall with the footing located behind the soil-nailed region experienced the largest seismic movements, even more than when the footing was directly behind the wall. Furthermore, the tests showed that the lower soil nails played a key role in the wall stability during earthquake shaking, acting as a pivot for the pre-collapse cases tested, whereas the upper soil nails needed to be sufficiently extended to contribute properly to the seismic stability of the wall. [ABSTRACT FROM AUTHOR]

Understanding the stability of trees under lateral loads arising from natural hazards (e.g. extreme weather and debris flows) is important, as fallen trees can become a potential threat to life and infrastructure. Two 1 : 20 scale three-dimensional printed analogue root system models, with architectures from field-surveyed root architecture data, were used to simulate the push-over behaviour of trees in silty sand under different conditions in the centrifuge. The peak overturning moments obtained were verified against data from field winching tests. Horizontal roots orientated in the loading direction and the central taproot complex contributed most to the overturning resistance. Increasing soil matric suction due to a lowering of the water table, increasing the loading rate and considering the presence of the fine root fraction all resulted in higher moment capacity and rotational stiffness of the root systems. The overturning behaviour was ductile in fully saturated soil and more brittle in partially saturated cases, with more root breakages in the windward horizontal roots and the taproot complex in the latter. These results suggest that it is important to measure the groundwater conditions when conducting winching tests and demonstrate a connection between soil effective stress, total root breakage area and peak moment resistance. [ABSTRACT FROM AUTHOR]

Geogrid reinforced soil walls (GRSWs) constructed using low-permeable backfills often experience failures when subjected to rainfall. The objective of this paper is to employ centrifuge modelling to investigate the effect of geogrid types on the performance of GRSW models constructed with low-permeable backfill, when subjected to rainfall intensity of 10 mm/h. A 4.5 m radius large beam centrifuge facility was used, and rainfall was simulated using a custom-designed rainfall simulator at 40 gravities. Digital Image Analysis (DIA) was employed to understand the deformation behaviour of GRSWs with low stiffness geogrid layers with and without drainage provision subjected to rainfall. Additionally, the effect of varying stiffness of geogrid reinforcement layers across the height of GRSW was also investigated. The interpretation of DIA helped to quantify displacement vector fields, face movements, surface settlement profiles and geogrid strain distribution with depth. Irrespective of drainage provision, GRSWs reinforced with low stiffness geogrid layers experienced a catastrophic failure at the onset of rainfall. However, GRSW reinforced with geogrid layers of varying stiffness was observed to perform well. This study demonstrates the effective use of DIA of GRSWs subjected to rainfall along with centrifuge-based physical model testing. • This study highlights the performance of GRSWs subjected to rainfall in a geotechnical centrifuge at 40 gravities. • The use of DIA is explored for understanding the deformation behaviour of GRSWs and geogrid strain distribution at the onset of rainfall. • DIA facilitates in analysing the behaviour of GRSWs at the onset of climatic events (like rainfall) in a geotechnical centrifuge. [ABSTRACT FROM AUTHOR]

Plate anchors are an attractive technology for mooring floating facilities; as relative to piles, suction caissons, and drag anchors, they provide a much higher capacity relative to their mass. Plate anchors may experience an extreme loading event that will cause geotechnical failure, although they will still retain a residual capacity. The displacement associated with bringing the anchor to failure will induce excess pore pressures that initially reduce soil strength but will dissipate over time, leading to regains in soil strength and hence anchor capacity. This paper considers the time scales and magnitude of this anchor capacity regain through a series of model scale experiments conducted in a geotechnical centrifuge. The experiments involved vertical loading of pre-embedded horizontally orientated circular anchors in normally consolidated kaolin clay. The results show that anchor capacity regain is a function of consolidation time and the level of resistance maintained on the anchor, with the longest consolidation time and highest maintained resistance leading to a capacity regain of approximately 60%. These capacity increases are described here using a simple hyperbolic function, which provides a basis for estimating the time needed for the residual anchor capacity to regain sufficient capacity following a movement event. [ABSTRACT FROM AUTHOR]

In this study, the seismic response and liquefaction mitigation effect of the soil-cement grid improved ground subjected to large earthquake loadings are studied through dynamic centrifuge tests. The model tests included soil-cement grid improved and unimproved model grounds, both of which have a 15-m-thick liquefiable layer underlain by a 2.5-m-thick coarse sand layer. The pre-cast soil-cement grid adopted in this study enables the dynamic responses of the model closer to the real improved ground. The recorded responses of accelerations, excess pore pressures and the deformation of the enclosed soil in the improved ground were carefully analysed with the comparison of the ground without improvement. It shows that the soil liquefaction and post-shaking settlements were effectively mitigated by the soil-cement grid even under very strong shakings. And the restriction effect of the soil-cement grid on dynamic shear strain of the enclosed soil was the most prominent in the middle height, regardless of the intensities of the shaking events. Such mitigating "waist effect" could mainly be attributed to the dynamic soil-grid interaction during shaking. However, the underlain soil layer may experience larger shear strain due to the increasing inertial force of the overlying ground improved by the soil-cement grid. [ABSTRACT FROM AUTHOR]

In this study, the monotonic and cyclic behavior of an offshore wind turbine with a monopile foundation installed in a sand layer were evaluated in the centrifuge. A simplified offshore wind turbine was modeled, and the lateral load was applied to the tower under displacement control. The monotonic loading test evaluated ultimate lateral load capacity and bending moment profiles under different loading levels. During cyclic loading, variations of moment-rotation responses, cyclic stiffness, and bending moments along the pile were observed. The initial rotational stiffness of the monopile decreased as the loading level increased. In the fatigue limit state (FLS) and service limit state (SLS) loading conditions, no noticeable variation in stiffness was observed with the number of cycles. However, in the ultimate limit state (ULS), the stiffness of the monopile increased during the first few cycles, followed by a decreasing rate of increase, and reached a certain value. The loading rate had a weakening effect on the monopile–soil interaction, which was supported by the bending moments induced in the monopile. [ABSTRACT FROM AUTHOR]

Large soil movements arising from the installation of jack-up spudcan foundations may induce detrimental effects on piles supporting adjacent permanent jacket platforms. Particularly, in a dense sand-over-clay profile, there is a risk of spudcan rapid penetration due to underlying clay bearing capacity failure, known as spudcan punch-through failure, which can cause further damage to piles in the proximity. This study investigates the foundation interaction between the spudcan and adjacent pile in a sand-overlying-clay soil profile by way of centrifuge modelling. Issues investigated include thickness of the upper sand layer with incidence of sand plug beneath the spudcan and spudcan–pile clearance. The experimental findings reveal that spudcan punch-through hazard and the magnitude of induced pile moments amplifies primarily due to the increase in the thickness of the upper sand. In addition, the magnitude of induced pile moment in a sand-over-clay profile is significantly higher than that in a single clay profile because of the post-peak bearing behaviour observed in stratified soil, yielding to a greater risk to pile integrity. Soil pressure profiles on the piles are also back-analysed. Two generalised induced soil pressure profiles are deduced, with the first profile representing the onset of spudcan punch-through and the second profile denoting the situation of post-punch-through failure. [ABSTRACT FROM AUTHOR]

Two centrifuge model tests were conducted, each with three large diameter steel tubular piles installed under similar conditions, i.e., diameter (Φ) = 2 m; thickness (t) = 25 mm; loading height from the rock surface (HL) = 6.5 m, but different rock socketing depths (dr), i.e., 2 m, 3 m, and 4 m, respectively, in prototype scale. Two additional 1 g model tests were conducted using the same model pile and ground. The results indicate that the pile lateral resistance increased with an increase in the rock socketing depth to diameter ratio (dr/Φ) in both 1 g and 50 g models. However, the difference between the two gravitational acceleration levels became visible in the non-linear behaviour as the imposed displacement increased. Specifically, the 1 g models showed larger residual displacement and less stiffness in reloading than the 50 g models, particularly under cyclic loading. Two types of ultimate failure modes were observed, i.e., rock failure and pile structural failure with local buckling just above the rock surface. The latter failure mode was only attained in the pile with a dr/Φ ratio of 2 in a 50 g models among the test conditions adopted in the models, but not in the 1 g model. [ABSTRACT FROM AUTHOR]

The ultimate capacity of a novel type of piled foundation called an 'impression' pile has been investigated using centrifuge modelling techniques. The name derives from the small discrete impressions created in the side walls of a bored cast in situ pile to increase the soil/pile friction such that the impressions form nodules on the shaft of the concreted pile. The technology is suitable for bored piles in overconsolidated clay, such as London Clay. The experiments explored the influence of geometrical parameters such as the vertical spacing of the impressions, their number at each level and their shape. The data show a consistent increase in pile capacity of 40% when the impressions extend over 85% of the pile length. The ultimate capacity of the pile is primarily affected by the length of the pile which is impressed, the number of nodules at a given cross-section and the spacing of the nodules in the vertical direction, as long as this is greater than a threshold value. According to the experimental evidence, a block failure occurs for a spacing lower than this threshold value. Plastic failure mechanisms for the impression pile were established. These were used successfully to calculate the ultimate capacity of the impression piles tested with an error of less than 10%. [ABSTRACT FROM AUTHOR]

This work introduces a scaling analysis of sub-aerial axisymmetric column collapses of glass beads and Newtonian glycerol-water solutions mimicking some of the behaviours of debris flows. The beads were in a size range where their inertia partly decouples their collapse behaviour from the water column. Experiments explored a range of viscous, surface tension and particle inertia effects through systematic variation of particle size and fluid viscosity. Crucially a geotechnical centrifuge was used to access elevated effective gravitational accelerations driving the collapse, allowing field-scale viscous and surface tension effects to be replicated. Temporal pore pressure and run out front position evolution data was extracted using a pressure transducer and high speed imaging, respectively. A least-squares fitted scale analysis demonstrated that all characteristic dimensionless quantities of the acceleration phase could be described as a function of the column-scale Bond number Bo , the Capillary number Ca , the system size r ∗ , and the grain-fluid density ratio ρ ∗ . This analysis demonstrated that collapses as characterised by pore pressure evolution and front positions were controlled by the ratio of Bo / Ca . This indicates that grain-scale surface tension effects are negligible in such inertial column collapses where they may dominate laboratory-scale granular-fluid flow behaviour where geometric similarity between grain and system size is preserved. [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]

Soil erosion and slope instability caused by seepage and rainfall are major problems, especially in mountainous areas. Many researchers focus on a new technologies or materials to stabilise soil slopes. In this study, the novel geosynthetic cementitious composite mat (GCCM) was studied for its ability to reinforce soil slopes. A series of centrifuge tests were performed on the soil slope model under calibrated seepage and rainfall conditions. Medical gypsum plaster sheet, which has an equivalent strength and stiffness to GCCM, was used to reinforce a model soil slope. The results showed that GCCM-reinforcement could reduce slope displacement by contributing its high stiffness and creating an interface frictional force with the slope. In addition, the GCCM could delay the increase in pore-water pressure in the soil slope during rainfall, thus diminishing the hydraulic force acting on the slope, even if the slope surface was not fully covered by GCCMs. Overall, the results indicate that GCCM has good slope reinforcement potential. [ABSTRACT FROM AUTHOR]

Soil bioengineering using vegetation has been considered an environmentally friendly solution to improve slope stability. Although several studies have demonstrated the contribution of vegetation to slope stability, a gap in understanding the mechanisms of grass root–soil interactions under rainfall conditions remains. This study investigates the effects of the roots of vetiver grass (Chrysopogon zizanioides) on the hydromechanical behaviour of an unsaturated soil slope using the centrifuge modelling technique. The changes in pore water pressure and slope deformation were monitored during the test. The monitored data were subsequently back-analysed and interpreted using seepage–stability analyses. In addition, this study focused on evaluating the effect of roots on slope stability, considering safety and pore water pressure during rainfall. Results revealed that the vetiver roots remarkably affected the initial suction of the slope by increasing the soil's air-entry value. The increased suction and the additional cohesion provided by the roots enhanced slope stability under rainfall conditions. • Effects of grass roots on the hydromechanical behaviour of a soil slope were tested. • Root-induced changes in soil water retention affected the suction distribution in the slope. • Long roots that have high surface area and more forks have greater stabilisation effects. • Plants with a large canopy area are favourable for use in bioengineered slopes. [ABSTRACT FROM AUTHOR]

Integral abutment bridges (IABs) are robust structures that avoid the use of bearings and expansion joints and are relatively maintenance-free compared to conventional bridges. The seismic design code of IABs is not fully developed, and the complex soil-abutment interaction is not well understood. Therefore, research was carried out at the Schofield centre, the University of Cambridge, to understand the backfill-abutment interaction under earthquake loading, aimed at developing design guidelines for the industry. Understanding the mechanics by which the foundation soil stiffness and strength govern the abutment deformation and, thus, the earth pressures generated behind the abutment is essential. Two centrifuge tests have been conducted simulating an abutment with the conventional abutment-deck connection (or semi-integral abutment bridge), where moment restraint is released. In this paper, the dynamic response of the abutment founded on dry and liquefiable sandy soil is compared. Different deformation modes have been observed depending on the relative abutmentsoil stiffness. The abutment experienced minimal base displacement in dry sands. Conversely, the abutment witnessed cyclic rotational ratcheting about the deck level in liquefiable soil. The dry soil test helped identify the zones where soil stiffness and strength loss can be critical. In the case of the saturated test, the water table level was up to one-third of the abutment height, fully saturating the foundation soil while the backfill height was dry. The comparative results highlight the vulnerability of semi-integral abutment walls to liquefaction-induced failure, as witnessed in the 2011 Christchurch earthquakes.

Recently, due to more excavation projects near existing tunnels, the research on tunnel deformation control has become very important and urgent. However, the systematic study of the control effectiveness of partition piles, one of the commonly used methods for controlling tunnel deformation, was not mature yet. In response to this situation, a series of centrifuge modelling studies were performed to explore how nearby existing tunnels respond to excavations with partition piles in a dry sand foundation. The tests provided insights into the variations of the horizontal displacement of retaining walls, surface settlement, the horizontal and vertical displacements of the tunnel, and the circular induced strain of the tunnel. The test results showed that the horizontal displacement of retaining walls exhibited a cantilever-type displacement pattern. The most significant surface settlement occurred near the retaining wall and decreased as the distance from the retaining wall increased. The overall deformation of the tunnel roughly showed an ellipsoidal deformation pattern in the horizontal direction. The maximum horizontal displacement was observed at the right shoulder of the tunnel, while the maximum vertical displacement occurred at the right arch of the tunnel. Increasing the length of the partition piles led to a deterioration in their deformation control effectiveness. The top burial depth of the pile could further improve the control effectiveness of the partition piles. However, when the top burial depth of the pile was excessive and exceeded its critical value, the improvement effect of the burial depth on the partition piles diminished.

Desiccation in soils leads to a loss of water content, increasing soil suction, which in turn, leads to higher shear strength. Drying paths in soils are produced by water uptake by plants or by evaporation due to extreme seasonal changes. This study presents the results of geotechnical centrifuge tests on the effect of water content on deformations and on the failure mechanism of a slope in fine soil. The failure mechanism and the resulting displacements obtained in the experiments were determined using image analysis using the GeoPIV_RG software. The results show a significant effect of soil suction on the failure mechanism. For saturated soils, the slope suffers an abrupt failure mechanism due to shear strength, while for partially saturated soils, this mechanism is modified and the magnitude of deformations diminishes as soil suction increases. Meanwhile, the size of the traction crack decreases as suction levels increase due to the increase in tensile strength produced by the loss of water content. [ABSTRACT FROM AUTHOR]

Scouring around piers is recognised as one of the main causes of bridge failure. Local scour refers to the localised erosion of soil at piers and abutments caused by flow-induced vortices around their base. This paper studies experimentally the response of a bridge pier, supported on a cylindrical embedded foundation, subjected to flood-induced scour. A hybrid two-step methodology is developed to study the hydraulic and the mechanical part of the problem. In the first step, the hydraulic problem of local scour around a pier is experimentally modelled in 1g using a recently developed miniaturised tidal generator. The experimentally generated scour hole is then three-dimensionally scanned to produce a three-dimensional (3D)-printed mould. The latter is used in the second step to reproduce the scour hole in an Ng model, subsequently tested in a drum centrifuge to study the mechanical part of the problem under proper stress scaling. Foundation performance before and after local scour is studied through vertical, lateral monotonic and slow cyclic pushover tests. The effects of general (uniform) scour are also investigated by removing a soil layer of constant thickness. Local scour is shown to have a minor effect on vertical bearing capacity. In stark contrast, the lateral performance is significantly affected, with the foundation moment capacity Mult being reduced by up to 38%. The effect of general scour is even more pronounced, leading to 48% reduction of Mult. The rate of cyclic settlement accumulation is also much more severely affected by general scour as compared to local scour. Overall, the effects of local scour on foundation performance differ substantially to those of general scour, bringing into question the common simplification of ignoring the geometry of the scour hole, making no distinction between local and general scour. [ABSTRACT FROM AUTHOR]

An emerging foundation design scheme, termed a rocking foundation, has the potential to limit loads transmitted to the superstructure, with the potential trade-off of increased settlements and rotations at the foundation level. These soil deformations can be significant for foundations with low vertical bearing capacity factors of safety. In this study, a set of dynamic centrifuge tests was performed to assess the response and to develop design considerations for a rocking foundation embedded in soil with varying degrees of saturation. Foundation design procedures dependent on degree of saturation are developed to predict moment capacities and initial rotational stiffness. Experimental data were used to validate the reliability of these procedures, regardless of the degree of saturation in the underlying soil. As the degree of saturation reduced, the moment capacity of the foundation increased, potentially increasing the load transmitted to the superstructure. Further, foundation settlements and rotations in the saturated soils can be reduced by lowering the water table depth in the underlying soil layer, which is beneficial to the performance of rocking foundations. Consequently, the degree of saturation of the supporting soil should be considered in the foundation design to predict the foundation settlement–rotation response and to assess properly the superstructure ductility demands. [ABSTRACT FROM AUTHOR]

Dual-row retaining walls can be utilised to form embankment structures for protection against tsunamis or as port structures. The dynamic response of these walls involves a complex interaction between the soil and structural elements which is not well understood. In this paper, for the first time a combination of centrifuge and finite-element modelling is used to better understand the mechanical response of the combined wall–soil system. The measured variation of the horizontal stresses leads to bending moment distributions featuring large, singly outward bending or double curvature with significant inward bending of the wall near the ground level. The numerical analyses are used to understand the stress state and highlight the dynamic variations of the vertical effective stress that drive this previously unexplained behaviour. The shear stresses that can develop at the wall–soil interface govern the mechanism by which the vertical effective stresses can vary. Further consideration of the dynamic soil stress state suggests a purer interpretation of the limiting loads in soil–structure interaction problems relative to the traditionally defined dynamic earth pressure coefficients. Combining the altered vertical soil stresses with earth pressure coefficients depending only on the soil friction angle adequately bounds the horizontal stresses that develop in the soil. [ABSTRACT FROM AUTHOR]

Tunnel excavations inevitably induce stress release and soil movement, which may affect the safety and serviceability of nearby existing pile foundations. The effects of tunnel construction on existing piles have been extensively investigated, but little attention was paid to lateral responses of piled foundations to tunnelling. In this study, a series of three-dimensional centrifuge model tests and numerical simulations using an advanced hypoplastic soil model were carried out to investigate the response of an existing vertically loaded 2 × 2 pile group in lateral direction due to advancement of twin stacked tunnels in dry sand. In each test, twin tunnelling was simulated three-dimensionally one after the other, with the first tunnel excavated near the mid-depth of the pile shaft and the second either next to the toe of the pile group (test ST) or below but to one side of the pile group (test SB). In addition, influences of tunnel depths and locations with respect to pile group response are investigated. The most significant lateral movement of the pile cap induced after the twin tunnelling in each test. The induced bending moment and pile deflection can be critical on completion of twin tunnels in test ST. In spite of being vertically loaded pile group, the twin tunnelling caused lateral forces (shearing forces to the pile group) in each test. The calculation charts have revealed that the tunnel depth and location relative to the pile group significantly influence induced pile cap lateral displacement and pile head bending moment. [ABSTRACT FROM AUTHOR]

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Foundation scour can have a detrimental effect on the performance of bridge piers, inducing a significant reduction of the lateral capacity of the footing and accumulation of permanent settlement and rotation. Although the hydraulic processes responsible for foundation scour are nowadays well known, predicting their mechanical consequences is still challenging. Indeed, its impact on the failure mechanisms developing around the foundation has not been fully investigated. In this paper, numerical simulations are performed to study the vertical and lateral response of a scoured bridge pier founded on a cylindrical caisson foundation embedded in a layer of dense sand. The sand stress–strain behaviour is reproduced by employing the Severn-Trent model. The constitutive model is firstly calibrated on a set of soil element tests, including drained and undrained monotonic triaxial tests and resonant column tests. The calibration procedure is implemented considering the stress and strain nonuniformities within the samples, by simulating the laboratory tests as boundary value problems. The numerical model is then validated against the results of centrifuge tests. The results of the simulations are in good agreement with the experimental results in terms of foundation capacity and settlement accumulation. Moreover, the model can predict the effects of local and general scour. The numerical analyses also highlight the impact of scouring on the failure mechanisms, revealing that the soil resistance depends on the hydraulic scenario. [ABSTRACT FROM AUTHOR]

Earthquake-induced soil liquefaction can generate excessive damage to building structures due to significant reduction in soil effective stress. With increasing urbanisation and population growth, the performance of closely spaced buildings, such as those in towns and cities, is of greater concern where structure–soil–structure interaction (SSSI) may occur in conjunction with full or partial liquefaction. This study investigates the seismic performance of isolated and adjacent structures built on shallow foundations on soils of different permeability (i.e. with different amounts of drainage and therefore generating different amounts of excess pore water pressure) using a combination of dynamic centrifuge modelling and finite-element modelling. The results demonstrate that the reduction in free-field ground surface intensity measure (specifically, Housner intensity) due to increasing liquefaction can be correlated to an index which is the normalised integral of excess pore pressure ratio (ru) with depth. This index can express the amount of liquefaction uniquely, even when full liquefaction occurs to only partial depth, or where excess pore water pressures are increased but below the level of full liquefaction at all depths. This underlying correlation means that structural demand reduction (e.g. reduction in inter-storey drift ratio) with liquefaction (SSI effect) can be linked to either: (a) the ru–depth index; (b) the depth of the liquefaction front, which can be estimated from a liquefaction triggering analysis; or (c) the ground surface intensity amplification/attenuation in the free field from the results of one-dimensional free-field soil column analyses. Where there are adjacent structures, a strongly beneficial SSSI effect on co-seismic settlement and a detrimental effect on drift in non-liquefied soil, as observed in this and previous studies, reduced towards a null effect in both cases with increased liquefaction, with liquefaction appearing to isolate the structures from each other (at least for the configuration considered herein). This has also been linked to the amount of amplification/attenuation of surface ground motion in the free field (or amount of liquefaction) as a simple indicator of the likely importance of SSSI effects in liquefiable soil. [ABSTRACT FROM AUTHOR]

In this study, the monotonic and cyclic behavior of an offshore wind turbine with a monopile foundation installed in a sand layer were evaluated in the centrifuge. A simplified offshore wind turbine was modeled, and the lateral load was applied to the tower under displacement control. The monotonic loading test evaluated ultimate lateral load capacity and bending moment profiles under different loading levels. During cyclic loading, variations of moment-rotation responses, cyclic stiffness, and bending moments along the pile were observed. The initial rotational stiffness of the monopile decreased as the loading level increased. In the fatigue limit state (FLS) and service limit state (SLS) loading conditions, no noticeable variation in stiffness was observed with the number of cycles. However, in the ultimate limit state (ULS), the stiffness of the monopile increased during the first few cycles, followed by a decreasing rate of increase, and reached a certain value. The loading rate had a weakening effect on the monopile–soil interaction, which was supported by the bending moments induced in the monopile.

This paper deals with the determination of the load bearing capacity of single and clustered torpedo anchors installed in clay, for vertical and inclined pullout, by means of centrifuge tests. Undrained shear strength profiles were obtained using a T-bar on eight prepared clay beds. These profiles allowed normalization of the force values obtained experimentally in eleven pullout tests on single torpedo anchors and seventeen on clusters of two. The normalized force versus displacement curves for the 45° inclined pullout exhibited higher force values than the 90° vertical pullout. The tests of the clustered units presented the greatest efficiency for pullouts with a spacing of three times the total diameter around the fins of the torpedo anchor. • The non-normalized force observed in pullout tests increases linearly with the increasing average shear strength of the clay bed. • Good agreement between literature data and experimental normalized results regarding the single torpedo anchor in clay. • The single torpedo anchor inclined pullout tests yielded higher forces and normalized displacements (at failure). • The clustered torpedo anchor inclined pullout tests yielded higher forces and normalized displacements (at failure). • Clustered torpedo anchors with a relative spacing distance equal to 3.0 yielded the best performance in all pullout tests conducted (90° and 45°). [ABSTRACT FROM AUTHOR]

Understanding the soil–root mechanical interaction is crucial to advancing the utilisation of vegetation as a nature-based approach to designing more stable slopes and resilient urban forestry against tree windthrown. Although numerical and analytical models for detailed analysis of soil–root interaction exist, these models are seldom validated due to the lack of field data and the significant challenges in quantifying such interactions due to the complex nature of root system. The centrifuge modelling technique is an effective alternative for unravelling the complexities of the hydromechanical behaviour of vegetated soils by recreating prototype stress levels in small-scale physical models and testing them under more controlled conditions. This work presents a critical review on existing centrifuge modelling methods for vegetated soils, paying particular attention on the (i) fundamentals of centrifuge modelling, where principles, scaling laws and applications relevant to modelling vegetated soils are detailed; (ii) methods for modelling soils, including choice of soil material and sample preparation; and (iii) methods for modelling roots by means of natural plants and root analogues, where the replication of root morphology, mechanical properties and capabilities of modelling transpiration effects are discussed. In every topic, the challenges that could further advance the centrifuge modelling of vegetated soils and the possible ways to address them are highlighted. Finally, the prospect for future studies is discussed, highlighting the potential to enhance the understanding of the underlying mechanisms amongst plant roots, soil, water and external loading. • Obeying scaling laws for roots and soils is critical for centrifuge modelling of vegetated soils. • Root modelling should consider morphological, mechanical, and hydrological properties. • 3D-printed root analogues are an alternative to addressing scaling difficulties of real roots. [ABSTRACT FROM AUTHOR]

This paper details a programme of centrifuge tests (100g) on the monotonic and cyclic lateral behaviour of large-diameter semi-rigid monopiles in kaolin clay. Piles of different diameters were used, with a maximum prototype value of 5·9 m. The pile–soil interaction was investigated in terms of the overall load–displacement behaviour, distribution of bending moment and pile deflection, p–y curves and excess pore pressure response. For monotonic loading, procedures were developed to fit the monotonic p–y curves obtained from different pile and soil properties using a hyperbolic tangent model. In addition, an explicit model was derived to assess the development of excess pore pressure within the surficial soil, which was found to approach the vertical effective soil stress at large displacement. For cyclic loading, the accumulation of lateral pile displacement could be accompanied by either increase or decrease in soil resistance, which strongly depends on the loading rate. Strong correlation between the development of peak soil resistance and a dimensionless velocity group was obtained, and was supported by the measured excess pore pressure. Under undrained conditions, the peak soil resistance decreases almost linearly with the accumulation of excess pore pressure. [ABSTRACT FROM AUTHOR]

Discretely spaced reinforced concrete (RC) energy pile rows have been proposed to be installed at the mid-height of soil slopes. Although the pile row can provide mechanical reinforcement to slopes, the piles can also potentially be used to (a) intercept solar energy from roadways for heat storage in the ground to mitigate extremely high carriageway temperatures and (b) to extract shallow geothermal energy for road surface de-icing. In this study, a series of centrifuge model tests was conducted to evaluate the shearing behaviour of unsaturated silt with and without reinforcement by conventional piles and energy piles. Three-dimensional finite-element coupled water–vapour–heat transport analysis was also performed to understand further the effects of pile heating on the responses of temperature and pore-water pressure of the soil. The measured and computed results revealed that the primary effect of pile heating was an increase in soil hydraulic conductivity due to the heat-induced reduction in water viscosity. The heated soil had enhanced water flow and hence developed higher suction. When subjected to translational slip, the silt reinforced by a row of closely spaced RC energy piles exhibited a more ductile shearing response and a lower peak shear resistance, compared to that reinforced by conventional piles. The peak bending moments mobilised in the energy piles in the stable stratum were also smaller. At larger shear displacements, however, the shear resistance converged to a similar value, regardless of the suction and temperature. These findings suggest that piles modified with additional energy transfer functionality can continue to act as reinforcement, potentially preventing soil from a sudden brittle failure, without attracting additional flexural stresses onto the piles. [ABSTRACT FROM AUTHOR]

As a part of studies on cantilever type large-diameter steel tubular pipe retaining walls embedded into stiff ground, single pile lateral loading tests were conducted as a preliminary investigation to understand the influence of embedment depth on the deformation and failure mechanism of rock socketed piles under a constant vertical eccentricity of 6.5 m. Two types of model soft rock grounds, (a) single soft rock layer (SR) and (b) soft rock layer with overlying sand (MS_SR) were made in the centrifuge model. The Influence of embedment depth on the lateral resistance of piles were investigated using three different rock socket depths in both grounds by centrifuge model tests at 50 g centrifugal acceleration. In addition, lateral resistance of piles embedded in Toyoura sand at 80% and 95% relative densities was also investigated. From the loading tests, two different failure modes of the pile foundation under lateral loading were observed, i.e., the ground failure and the structural buckling of pile depending on the embedment depth and ground strength. From the observed results, in stiff ground like soft rock, the lateral resistance changes significantly in a small range of embedment depth. A small increment of embedment depth from 1Φ to 2Φ in SR ground increased the ultimate lateral resistance by about 2.5 times and remarkably enhanced the redundancy against brittle softening and changed the deformation mechanism from brittle to the ductile one. However, the influence of increment of rock socketing from 1Φ to 2Φ in MS_SR ground becomes less significant, especially for the ultimate resistance due to the pile structural failure of all the piles. At the ultimate failure conditions, the rotational fraction dominates the pile top displacement of piles in SR ground. The fraction is about 90% when the ultimate resistance is governed by the rock failure and 65% for the structural buckling of the pile. At the ultimate failure conditions of piles in MS_SR ground about 95% of the pile top displacement is caused by the combination of rotation above the sand surface and translation at the sand surface with almost equal weights. In which, about 80% of the translation at sand surface is caused by the combination of rotation and bending of pile in the overlain sand layer, the ratio between these rotational and bending fractions under ultimate loading conditions (ML ⁓ 0.8Mult) tend to decrease from 4 times to unity, as the dR/Φ increases from 1 to 2. [ABSTRACT FROM AUTHOR]

To evaluate the lateral resistance of rigid monopiles for wind turbines in dense sand under lateral cyclic loading, centrifuge model tests are performed, focusing on the base resistance and degradation of the soil resistance under two-way lateral cyclic loading in the short-term. The slenderness ratio (embedded pile length to diameter) is varied from 3.75 to 8 and the loading frequency is in the range of 0.002 to 0.4 Hz in the prototype scale. Under cyclic loading with a maximum horizontal displacement of 5% of the pile diameter, the build-up of excess pore water pressure is observed, but the maximum value of the average excess pore water pressure ratio is around 50% in the steady-state for dense sand whose relative density is 80%. A simple analytical model for the rigid piles, considering the base resistance, is derived and then used to quantify the significance of the resistance at the pile base and the degradation of the soil resistance under cyclic loading. When the slenderness ratio is less than 5, a significant contribution of the moment resistance at the base is confirmed. The estimation of the degradation of the horizontal subgrade reaction coefficient using the simple analytical model suggests that, through cyclic shear tests for the determination of the deformation properties of the soil in a laboratory, it is possible to estimate the degradation of the soil stiffness and the parameters for the reduced sway-rocking type of foundation model.

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]

Although pile foundations are usually subjected to both lateral and vertical loads, there have been few studies on the pile response under the combined loadings. Moreover, those few studies led to inconsistent conclusions concerning the influence of vertical load on the lateral pile response. This study aims to investigate the effect of vertical load on the monotonic and cyclic lateral response of piles in normally consolidated (NC) and over-consolidated (OC) clay through a series of centrifuge tests. Three-dimensional finite element analyses using an advanced hypoplastic clay model were also performed to gain deep insight into the mechanism. It is revealed that after applying the vertical load and allowing the dissipation of the induced excess pore pressure, the lateral initial stiffness and ultimate capacity of the pile in the NC clay increased by 10.0 and 49.4%, respectively. However, in the OC clay, the application of the vertical load resulted in a 12.6 and 32.4% reduction in the lateral pile initial stiffness and ultimate capacity, respectively, showing a distinct response from that in the NC clay. The above-mentioned distinct influence of vertical load for piles can be attributed to the different evolutions of stress ratio (q/p’) of the soil around the pile in the NC and OC clay, which determines the soil mobilisable shear strength and consequently the lateral pile responses.