Centrifuge Tests Research Articles

Centrifuge tests for earthquake response of shallow soil with locally raised bedrock

ABSTRACT Centrifuge tests were performed to study the dynamic properties of shallow soil with locally raised bedrock (i.e. variable soil depth). The test parameter was the slope of bedrock: 0° (S0), 35° (S35), and 45° (S45). In each test, accelerations were measured along the soil depth, and the results of acceleration, displacement, Fourier transform, and response spectrum were compared. Based on the results, the transfer function (TF), the ratio of response spectrum (RRS), and the site period were estimated. The ground motion and site period of specimens with raised bedrock were smaller than those of the specimen without raised bedrock (i.e. with deeper soil). Further, parametric studies using 2-dimensional finite element (FE) analysis were performed to investigate the effect of design parameters on the response of shallow soil with variable soil depths. For design parameters, the length of raised bedrock and the length of foundation slab were considered. Parametric study results indicated that when the shallow soil region is wide, the results are similar to those of a 1-dimensional soil column. However, when the shallow soil region is narrow, the 2-dimensional response is smaller than the 1-dimensional soil column response. This was also observed in the actual site model.

Macro-element modelling for lateral response of monopiles with local scour hole via hyperbolic hardening relation

Monopile is a popular choice in the foundation supporting offshore wind turbines (OWTs), with local scour significantly impacting their lateral responses. Macro-element model, which encapsulates the response between the monopile and the surrounding seabed soils into a force-displacement relation, has been extensively developed to describe offshore foundations. However, such kind of models specifically targeting monopiles subjected to lateral loading in local scour remain underdeveloped. This work proposes a macro-element model with a succinct hyperbolic hardening relation for laterally loaded monopiles in local scour conditions, using the evolutionary polynomial regression (EPR) machine learning technique for easy and optimal design. First, the finite element model is verified and extended to generate force-displacement responses considering the monopile geometries, soil characteristics, and local scour geometries. These results are then utilised to determine the optimal hyperbolic hardening relation of the macro-element model. Next, the EPR technique is employed to determine the relationship between the hyperbolic hardening relation parameters and the influencing factors. Finally, the macro-element model is successfully evaluated by comparing with measurements from centrifuge tests and numerical solutions by finite element analysis, demonstrating its applicability in practical design and the ability to reproduce FEA results with a significant reduction in computational cost.

Research progress and prospects of seismic performance on underground structure embedded in soft soil foundation

For the complexity and difficulty of seismic research on subway station structure system embedded in soft soil foundation, the seismic research method is quite different from the ground structure. The methods of seismic research on subway station structures in soft soil were summarized, and relevant literature on this field in recent years were sorted out. The advanced progress of theoretical analysis and quasi-static simplification analysis, model test (shaking table test, centrifuge test), numerical simulation (the total stress method, the effective stress method), and dynamic reliability of underground structures were mainly introduced for seismic analysis of subway station embedded in the soft foundation. The advantages and disadvantages of each method and the development direction of this field were proposed briefly in order to better understand seismic analysis of underground structure engineering in soft soil.

Effect of loose sand layers within dense sand on the lateral capacity of extra-extra-large monopiles

A series of 3D finite element analyses were performed to examine the influence of loose sand layers on the monotonic lateral capacity of an extra-extra-large semirigid monopile in dense sand. The SANISAND-04 advanced constitutive model, which was meticulously calibrated, validated and verified using triaxial compression tests, centrifuge tests and a numerical study based on field tests, was adopted for this study. The impact of loose sand layers on monopile response is discussed in terms of its mode of deformation, critical lateral capacity, bending moment, shear force and soil resistance. Results show that the magnitude of the reduction in the monopile lateral capacity is dependent on the thickness and depth of the loose sand layer. Presence of loose sand at the tip of the monopile is also shown to have a significant effect on its lateral capacity.

Seismic resilience assessment of sheet-pile wharves in liquefiable soils using different liquefaction countermeasures

Seismic resilience assessment is essential for maintaining the functionality of sheet-pile wharves in liquefiable soils, preventing significant damages and minimizing losses during earthquakes. This study delves into the seismic resilience of sheet-pile wharves, focusing specifically on the effectiveness of four different liquefaction countermeasure techniques: anchor lengths, cement deep mixing, stone columns, and soil compaction. As such, an advanced two-dimensional (2D) Finite Element (FE) computational framework is established, motivated by a typical large-scale sheet-pile wharf configuration. Within this framework, a recently developed multi-yield surfaces plasticity model is employed, with the modeling parameters calibrated through undrained stress-controlled cyclic triaxial tests and a centrifuge test. Subsequently, the impacts of these liquefaction countermeasures on the seismic resilience of the sheet-pile wharves are systematically investigated. Additionally, the effectiveness of combining longer anchor lengths with the other three mitigation techniques to enhance the seismic resilience of the sheet-pile wharves are examined. It is demonstrated that the synergistic effects of different liquefaction countermeasures can further reduce the liquefaction potential, thereby improving the seismic resilience. Overall, the FE analysis technique and the resulting insights are highly significant for the seismic resilience assessment of equivalent sheet-pile wharves in liquefiable soils, particularly when implementing such liquefaction mitigation countermeasures.

Numerical investigation of punch-through mitigation in stiff-over-soft clays using skirted spudcan

Punch-through failure, a critical issue in offshore engineering, occurs when a foundation penetrates through strong-over-weak soil layers. This paper presents a study on the effectiveness of skirt addition to spudcans in addressing punch-through failure through numerical simulations using the axisymmetric Particle Finite Element Method (PFEM). The validity of the PFEM is verified by simulating a spudcan penetration centrifuge test. Subsequently, PFEM simulations are conducted to analyse the penetration behaviour of both generic and skirted spudcans in stiff-over-soft clays. Results from the simulations reveal the bearing responses of spudcans as they penetrate into clays, elucidate associated failure mechanisms, and underscore the significant of considering the spudcan-driving process in estimating punch-through failure mitigation. Findings indicate that, the skirt effectively mitigates punch-through failure by increasing plug height in the lower soft layer, thus reducing the rate of bearing capacity reduction. Furthermore, critical factors such as skirt length, upper layer thickness, lower layer strength gradient, and the strength ratio between lower and upper layers are assessed through comprehensive parametric studies to evaluate the efficacy of skirted spudcans in mitigating punch-through failure, with eventual goal to provide essential guidance for practical design considerations in offshore foundation systems.

Effect of soil-building interaction on dynamic earth pressure on basement walls

This study investigates dynamic earth pressure on basement walls by considering soil-structure interaction, a factor that is often overlooked by conventional methods like Mononobe-Okabe, Seed and Whitman, and Wood. Superstructure, basement, and soil were numerically simulated in a single model to evaluate the inertial and kinematic effects of buildings on dynamic earth pressure on the basement walls. The numerical model was validated using data from a centrifuge test and an instrumented building. Then, a parametric study was performed, with the height, width, and basement depth of the building varied with six input motions. The results showed that the inertia of the building increased dynamic thrust. Furthermore, the dynamic thrust showed a similar trend with the displacement response spectrum curve of the ground surface acceleration as the building height increased, while the distribution shape transitioned from triangular to inverted triangular. Additionally, wider buildings had increased dynamic earth pressure, while deeper buildings exhibited smaller normalized dynamic thrust.

Numerical simulation of rainfall-induced deformations of embankments considering the coupled hydro-mechanical behavior of unsaturated soils

This paper presents numerical simulations of the deformation response of unsaturated embankments subjected to rainfall infiltration using a coupled hydro-mechanical constitutive model for unsaturated soils. The constitutive model accounts for the influence of degree of saturation on the stress–strain behavior and the influence of void ratio on the water retention behavior. The constitutive model was implemented in the finite difference program FLAC, calibrated using triaxial test data, and validated using measurements of wetting-induced deformations of embankment models from centrifuge tests. The validation demonstrates that the constitutive model can capture the coupled hydro-mechanical behavior of unsaturated soils under wetting conditions. Simulations of the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration indicate that the differential settlement across the top surface of the embankment between the centerline and shoulder increases significantly during rainfall infiltration, which could result in severe damage to overlying transportation infrastructure. As the rainfall infiltration increases, shear strains accumulate and form a potential failure surface at a shallow depth of approximately 2 m from the slope surface. Insights into the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration gained will be useful for considering climate change effects in design and construction of compacted embankments.

Centrifuge tests study on settlement and damage modes of bridge approaches using deep-seated slab

The issue of bridge end bumps is a critical concern in the failure of bridge and bridge approaches. A series of novel centrifuge tests utilizing a ring model box were conducted to investigate settlement and its induced damages at the bridge approach. A new mitigation method, the deep-seated slab, for bridge end bumps was modeled in the test. This study analyzed the decisive role of pavement stiffness, soil modulus, and load cycles on deformation from the perspective of structure-soil interaction under standard traffic load conditions. The test results show that when deep-seated slabs are used, the deformation of the bridge approach follows an exponential decay pattern, eventually stabilizing after approximately one slab length. Furthermore, the upper and lower bridges exhibit distinct damage modes, i.e., the bridge damage by wheel collision at the upper bridge and the pavement damage by wheel impact at the lower bridge. The damage zone on the pavement is approximately 1.7 times the wheel width and the damage zone on the bridge 2.6 times. Finally, a predictive model for the deformation of bridge approaches was proposed, considering the effect of pavement stiffness, subgrade soil modulus, and load cycles. The relationship between the deformation and the three normalized variables conforms to the quadratic polynomial function in matrix form.

A simplified analytical solution of pipeline response under normal faulting

Buried pipelines are one of the critical lifeline structures, and the evaluation of pipeline performance under seismic faulting is essential for protection of pipeline networks. This paper explores an analytical solution incorporating implementation of Winkler models around a beam-type structure using the finite difference method to evaluate the pipeline response under normal faulting. The effectiveness of the analytical solution is assessed by comparing the calculated pipe strains with centrifuge and full-scale laboratory testing measurements. Subsequently, the effect of pipe size, faulting condition, and burial condition on pipe response are investigated. It can be found that a pipeline with a smaller pipe wall thickness or larger pipe diameter is generally more prone to failure. Larger pipe wall thickness or lower D/t ratio should be used for pipe design crossing an active fault. In geotechnically problematic areas, shallow burial depth and less stiff soil with lower modulus are recommended to reduce the probability of exceedance of allowable limit states due to normal faulting. In the end, empirical functions relating to critical D/t ratio are proposed to evaluate the critical states of fault displacement, burial depth, and spread of fault rupture with different diameter-to-thickness ratio considering the controlled limit of failure modes.

Centrifugal Model Test of Multi-Level Slope under Combined Support of Pile-Anchor and Frame-Anchor

The combination support structure of pile-anchor and frame-anchor is widely used in multi-level slope engineering, but the relevant theoretical research lags behind engineering practice. To make up for this deficiency, this paper takes the multi-level slope under combined support of pile-anchor and frame-anchor as the research object, conducts a model test using the geotechnical centrifuge, analyzes the most relevant structural force and slope deformation in engineering, and provides some reinforcement suggestions. The test results show that the combination support structure of pile-anchor and frame-anchor has a good reinforcement effect on multi-level soil slopes, but the deformation range of the slope is greatly affected by the anchoring structure and the anchor rods located within the deformation zone. The structural force condition is significantly different from traditional structures that place the anchor rod anchoring section in a stable soil layer. According to the structural force characteristics, it is recommended that the support design fully consider the positional relationship between the anchor rods and the sliding surface. The design of the frame-anchor support section should strictly control the anchoring depth, so that the potential sliding surface of the slope can be moved backward, thereby increasing the difficulty of sliding. The design of pile-anchor support should ensure that the anchor rods have sufficient length to provide greater anchoring force, thereby improving the bearing capacity of the pile. These research results can enrich and improve the support design theory of multi-level soil slopes, providing guidance for engineering practice.

The installation behavior and capacity of piggy-backed anchors for offshore wind turbine

Piggy-backed anchors present a promising solution to address the limitations of single Drag Embedment Anchors (DEAs). A series of centrifugal model tests were performed on the installation of piggy-backed anchors with varying spacings and attachment points in saturated sand using a magnetometer system. The experimental results validated the proposed theoretical prediction model, which can identify the trajectory, capacity, orientation, and soil failure wedge slip surface. Key findings include: (1). the embedment depth of the second anchor in piggy-backed anchors is significantly greater than that of a single anchor, resulting in the total capacity of piggy-backed anchors exceeding twice that of a single anchor at appropriate spacing; (2). investigating spacing effects shows that capacities are less than twice that of the single anchors when spacing is less than 1Lf due to overlapping failure soil wedges; optimal spacing was found to be 2Lf to 3Lf; (3). piggy-backed anchors with attachment points at the back of fluke exhibit better embedment performance and capacity stability compared to those with attachment points at the padeye. Finally, the prediction model explored the capacity performance of piggy-backed anchors at different loading angles at the final embedment depth, revealing a two-stage failure process compared to single drag anchors.

Optimization of spudcan penetration and influence on caisson foundation

The penetration of spudcans have adverse effects on pre-existing bucket foundations during the blade lifting process of offshore wind power systems, which brings uncertainties for the design and construction of bucket foundations. This study employs the Euler-Lagrange method to explore the influence of spudcan penetration on the adjacent caissons in homogeneous clay within a large deformation finite element (LDFE) model. The LDFE model is validated against centrifuge tests data, with good agreement obtained. Parametric study is then conducted to explore, (i) the influence of the angle between vessel and caissons on the movements of the caisson, and (ii) potential influence factors on the movements of the caisson with an optimal angle between the vessel and caissons, including soil properties, dimensions of caisson and spudcan, and the distance between spudcan and caissons. The results show that 60° is the optimal angle between the vessel and caissons that results in the smallest movement of the caisson, and the caisson foundation inclination angle ranges between 0° and 0.025°, when the pile shoe penetration progress ranges between 0.1 and 1.0. An empirical formula is proposed to predict the rotational angle of caisson with the optimal angle. The study establishes a quantitative approach to assess the influence of the spudcan penetration on existing three-caisson jacket foundations, which provides guidance for the design and construction of offshore wind turbine bucket foundations.

Centrifuge tests on the deformation law of pipelines crossing slopes with different water contents

The deformation law of pipelines crossing landslide areas is an important prerequisite for the scientific design of pipelines. Most current studies find it difficult to simulate the real stress and strain state of pipelines on site. This study conducted two centrifuge model tests to observe pipeline failure indicators (deformation, stress, strain) and slope failure indicators (crack width, soil pressure, landslide displacement) in response to changes in acceleration. The experimental findings align with the results obtained from numerical simulations. The findings indicate that slope failure with high moisture content is more serious. At a moisture content of 15%, the slope has a maximum displacement of 70 mm in close reach to the buried pipeline. The pipeline has a “bimodal distribution” of strain along its axial length. The largest amount of deformation occurs at a distance of L/4 from the center of the pipeline, and the deformation value is 9000 με. Moreover, the maximum deformation in the mid-span is 21 mm, which corresponds to the result accumulated from the numerical simulation. Thus, the pipeline needs to improve the disaster prevention capabilities at the mid-span and L/4 of the pipeline.

Exploring joint orientation effects on rock wedge stability: Experimental and discrete element analysis

The primary objective of this study was to examine and analyze the sliding behavior of rock wedge slopes with the interaction of three joint sets, considering the influence of plunge angles, included wedge angles between joints, and gravity conditions. The discontinuity planes with varying dip directions relative to the wedge's plunge direction were analyzed. The stability assessment of wedge failure was initially reviewed, explicitly focusing on limit equilibrium (LE) analysis and in-house physical tests. Centrifuge tests and discrete element analysis using the 3DEC software were then performed to explore the factors influencing the failure characteristics of the slopes. Additionally, two mitigation strategies were proposed to enhance the stability of the rock wedge slopes. The key findings include the significant impact of plunge angle on wedge slope stability compared to the included wedge angle between joints, the strong effect of gravitational conditions on the collapsing ratio of wedge units, and the higher risk posed by wedge slopes with discontinuity planes dipping out of the slope. The numerical simulation results highlighted the importance of considering joint spacing in slope stability assessments. Mitigation methods such as fixing key wedge blocks were found to effectively reduce the wedge collapsing ratio and improve rock slope stability under more realistic stress levels. This study provides valuable insights for formulating prevention and mitigation strategies, useful for enhancing safety and reducing the potential damage caused by rock wedge failure events.

Three-dimensional numerical analysis of twin tunnelling in two-layered soil strata

Tunnelling induces stress change and displacement in the ground. The excavation of a new tunnel in stratified soil can trigger different patterns of stress redistribution, which may adversely influence nearby tunnels. Research on multi-tunnel interaction has mainly been performed on the assumption of a uniform ground. The effects of different soil stratifications on tunnelling interaction remain poorly understood. In this paper, three-dimensional numerical parametric studies verified by previous centrifuge tests were carried out to analyse the twin tunnelling effects in two-layered soil. An advanced hypoplastic constitutive model that can capture stress-, path-, and strain-dependency of soil behaviour is adopted. Numerical cases investigated include perpendicular twin tunnelling in two sand layers with different relative densities and the location of the interface between the two sand layers. It is revealed that larger settlements and a wider surface settlement trough occur when tunnelling in two-layered soil strata than in a uniform ground. This is because of the wider and larger soil arch induced in two-layered soil strata. The structural response including tunnel deformation, induced bending moment, and induced hoop stress of the existing tunnel can be greater when tunnelling in layered soil strata than in a uniform ground owing to larger stress relief. Moreover, the combination of bending moment and hoop stress can exceed the M−N failure envelope of the structure in layered soil. A conventional simplified assumption of a uniform ground can underestimate of the influence of new tunnel excavation on existing tunnels, resulting in unsafe designs.

Centrifuge modelling of the dynamic response of twin tunnels under train-induced vibration load

The impact of train-induced vibrations originating from underground tunnels has emerged as a significant environmental issue within urban areas. A multitude of studies, encompassing both numerical analyses and experimental investigations, have been undertaken to explore and address this concern. In the experimental tests, the dynamic characteristics of tunnels and soil has generally been studied using physical models with a single tunnel under a 1 g (gravity) environment. However, twin tunnels are commonly constructed in the field, and the stress of the soil is unrealistically simulated in 1 g models. In this paper, the dynamic characteristics of twin tunnels were studied by centrifuge testing. In this paper, results are presented from scaled model tunnel tests conducted at 50 g for four different conditions: a single tunnel, vertical twin tunnels with 60 mm spacing, horizontal twin tunnels with 60 mm and 120 mm spacing. Train load and sweep load were applied by a parallel pre-stressed actuator at the tunnel invert. Accelerometers were employed to gauge the dynamic response of both the tunnels and the surrounding soil. The experimental results illustrate that the interaction between twin tunnels has a clear amplification effect on the dynamic response of the loading tunnel, with the maximum peak particle acceleration (PPA) increasing by up to 43 %. This amplification phenomenon is reduced as the space between the twin tunnels is increased. The experimental results also indicate that the interaction between twin tunnels has a comparatively lower impact on the dynamic response of the surrounding soil when contrasted with the dynamic response of the tunnels themselves.

Numerical analysis of seismic response of a circular tunnel-rectangular underpass system in liquefiable soil

Earthquake-induced liquefaction can lead to uplift displacements of underground structures. While the behaviour of a single underground structure has been extensively studied, the coupled response of a shallow buried circular tunnel adjacent to an underpass with a rectangular section in liquefiable ground remains unclear. In this study, the finite difference method based on the fully coupled effective stress is employed to analyse the seismic response of two adjacent underground structures. Firstly, comprehensive parameters of Hostun sand for P2Psand constitutive model was calibrated. The accuracy of the numerical method is verified through a centrifuge test. Subsequently, comparisons are made between the seismic response of the tunnel-soil-underpass interaction and other cases: free field, tunnel, and underpass. The tunnel-soil-underpass interaction in liquefiable ground is analysed, with a special emphasis on the development of excess pore pressure, acceleration response of the soil, displacement and deformation of the structures. A section deformation coefficient was proposed to depict the tunnel deformation. Finally, horizontal spacing, buried depth of tunnel and underpass are discussed to clarify the interaction of two structures. The optimal relative position of two structures is suggested, providing valuable insights for the spatial design of underground structures in liquefiable ground.

Large-Deformation Modeling of Surface Instability and Ground Collapse during Tunnel Excavation by Material Point Method

Recent rapid urbanization has led to an increase in tunnel construction, escalating the prevalence of ground collapses. Ground collapses, characterized by large deformation and strain-softening, pose a significant challenge for classical numerical theories and simulation methods. Consequently, a numerical framework combining the material point method (MPM) and strain-softening Drucker–Prager plasticity is introduced in this study to more accurately describe the evolution process and failure mechanism of the subgrade during tunnel excavation. The proposed numerical framework was validated against an analytic solution employing a typical ‘dry bottom’ dam model with solid non-linearity and large deformation; some of the results are also compared with those of the SPH method and centrifugal modeling tests to verify the validity of the MPM method in this paper. The validated model was used in this study to conduct a comprehensive analysis of surface instability and ground collapse under varying soil conditions. This included factors such as strata thickness, cohesion, internal friction angle, and a quantitative description of the development of longitudinal subsidence of the surface. The aim was to clarify deformation responses, failure patterns, and excavation mechanisms, providing insights for underground tunneling practices.

Numerical Simulation of Vertical Cyclic Responses of a Bucket in Over-Consolidated Clay

Multi-bucket foundations have become an alternative for large offshore wind turbines, with the expansion of offshore wind energy into deeper waters. The vertical cyclic loading–displacement responses of the individual bucket of the tripod foundation are relevant to the deflection of multi-bucket foundations and crucial for serviceability design. Finite element analyses are used to investigate the responses of a bucket subjected to symmetric vertical cyclic loading in over-consolidated clay. The Undrained Cyclic Accumulation Model (UDCAM) is adopted to characterize the stress–strain properties of clay, the parameters of which are calibrated through monotonic and cyclic direct simple shear tests. The performance of the finite element (FE) model combined with UDCAM in simulating vertical displacement amplitudes is evaluated by comparison with existing centrifuge tests. Moreover, the impact of the bucket’s aspect ratio on vertical displacement amplitude is investigated through a parametric study. A predictive equation is proposed to estimate the vertical displacement amplitudes of bucket foundations with various aspect ratios, based on the cyclic displacement amplitude of a bucket with an aspect ratio of unity.