Centrifuge Test Results Research Articles

Study Method of Pile near Cohesionless Slope under Reversed Lateral Load Considering Sand Strength State and Lateral Deflection of Pile

A p-y curve method of pile near cohesionless soil slope under reversed lateral load is proposed. This method takes into account the failure mode of soil around the pile under reversed lateral load and the interaction mode between pile and soil, and derives the ultimate soil resistance of the pile. Considering the change of the original stress state of soil due to the lateral deflection of the pile foundation, the influence of the relative density, the initial mean effective stress and the lateral deflection of pile foundation on the internal friction angle of the sand is evaluated to further accurately calculate the soil resistance value at each depth. The prediction results of this method are well verified by comparing with the FE results and centrifuge test results. Finally, the influence of the process of lateral deflection of pile on the strength state of soil around the pile and the bearing capacity of pile is studied.

Investigation of the block toppling evolution of a layered model slope by centrifuge test and discrete element modeling

Primary toppling usually occurs in layered rock slopes with large anti-dip angles. In this paper, the block toppling evolution was explored using a large-scale centrifuge system. Each block column in the layered model slope was made of cement mortar. Some artificial cracks perpendicular to the block column were prefabricated. Strain gages, displacement gages, and high-speed camera measurements were employed to monitor the deformation and failure processes of the model slope. The centrifuge test results show that the block toppling evolution can be divided into seven stages, i.e. layer compression, formation of major tensile crack, reverse bending of the block column, closure of major tensile crack, strong bending of the block column, formation of failure zone, and complete failure. Block toppling is characterized by sudden large deformation and occurs in stages. The wedge-shaped cracks in the model incline towards the slope. Experimental observations show that block toppling is mainly caused by bending failure rather than by shear failure. The tensile strength also plays a key factor in the evolution of block toppling. The simulation results from discrete element method (DEM) is in line with the testing results. Tensile stress exists at the backside of rock column during toppling deformation. Stress concentration results in the fragmented rock column and its degree is the most significant at the slope toe.

Cyclic p-y curve model of unidirectional loaded pile in sand considering the characteristic of friction in the strain wedge model

Pile foundations are always subjected to cyclic loads due to the action of wind and waves. A cyclic p-y curve model considering the characteristic of friction force is proposed to study the lateral response of pile foundations under unidirectional cyclic loads in sand. The soil resistance calculated by the improved nonlinear strain wedge model (SWM) is divided into friction resistance and compression resistance according to the different characteristics of soil resistance. The friction resistance depends on the relative displacement between pile and soil, whose direction and magnitude will constantly change in the process of cyclic load, while the compression resistance is analyzed by combining the extended Masing rule and soil degradation model. The validity of the model proposed in saturated sand is verified by centrifuge test results. Furthermore, the influence of cyclic loads on pile deflection, bending moment and soil resistance is analyzed through the results calculated by the cyclic p-y curve model.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Centrifuge investigation of soil–foundation–superstructure interaction under static loading

Extensive efforts have been made to investigate soil–structure interaction (SSI) through numerical simulations whereas little experimental research is reported that tests the applicability of existing numerical models to SSI problems. To provide such experimental data, a unique series of experiments with 3D-printed aluminium framed structures supported by isolated spread footings were conducted in a geotechnical centrifuge. The centrifuge models included equipment and instrumentation that allowed the application of a uniformly distributed static load to the structures as well as the measurement of column loads and foundation movements. The effect of the stiffness of the structure relative to that of the soil was evaluated by varying the number of storeys in the structures and the type/relative density of foundation soils. The variability of soil properties was also studied by modifying the support conditions at single columns at various plan locations of the structure. It is shown that there is appreciable load redistribution in the superstructure, the amount of which depends primarily on the relative structure–soil stiffness and the location of weak and/or rigid foundations. The comparison between the centrifuge test results and the predictions from a simple numerical model with the footings simulated by appropriate linear elastic springs indicates that such a simple numerical model can provide a reasonable prediction of SSI.

Evolution of breakout force with operation period for deeply embedded spudcan in clay

The extraction of spudcan foundation in clay can often be difficult and time-consuming considering the large pull-out force required to overcome the breakout resistance. This is particularly an issue for long operation periods, during which the dissipation of excess pore pressure in the soil will increase the base suction and undrained shear strength of soil. This paper investigates the spudcan breakout force in clay deposits through a coupled pore fluid-effective stress large deformation finite element approach. The complete process of spudcan installation, operation and removal is simulated. The numerical approach is verified against existing centrifuge test results. A good agreement can be obtained between the numerical and experimental results in terms of normalised breakout force and operation period. A series of parametric study is then carried out to explore the effects of installation depth, operational load and soil parameters. Based on the numerical results, a unified relationship can be established between the dimensionless forms of breakout force and operation period, for which a simple algebraic expression is proposed for the convenient assessment of breakout force recovery.

Centrifuge tests of large-diameter steel pipes crossing strike–slip faults

Fault rupture is one of the main hazards for buried continuous pipes, and it has frequently been investigated experimentally in recent decades. In this study, to explore and model the failure mechanisms of continuous steel pipes crossing faults, a series of centrifuge tests of large-diameter steel pipes (D = 1.616 m) crossing strike–slip faults was conducted. Pipe strains are measured using 30 strain gauges, and pipe cross-section deformation is determined using a micro industrial endoscope for the first time; thus, cross-sectional ovalization is accurately derived. Subsequently, factors influencing the responses of steel pipe crossing faults, such as the burial depth, pipe-fault crossing angle, and pipe diameter, are investigated. The centrifuge test results show that the pipe responses, including the pipe strain and deformation, are significantly affected by these factors. Finally, three limit states are considered and compared: 3 % tensile strain, 15 % cross-sectional ovalization, and local buckling limit, and pipe failure behaviors are examined for these cases. The results shown that these tests can serve as benchmarks for theoretical and numerical models.

Calculation Method for Uplift Capacity of Suction Caisson in Sand Considering Different Drainage Conditions

Uplift capacity of suction caissons is one of the main concerns in the design of jackets with multi-caissons supported offshore wind turbine. The uplift movement of suction caissons leads to soil stress variation and increases the difficulty to predict the uplift capacity. In this paper, a calculation method considering soil stress release and differential pressure contribution is proposed to predict the uplift capacity of caisson. Firstly, a series of numerical simulations based on the SANISAND model are conducted to study the uplift responses of suction caisson in sand, and it is verified with centrifuge test results. Considering the soil drainage condition during caisson being pulled out, the fully drained, partially drained and undrained are divided, and an equation is provided to assess differential pressure beneath the caisson lid incorporating the effects of main factors. Based on the above simulation results, a calculation method is proposed to calculate the uplift capacity of caissons. The prediction results are compared with the centrifuge model tests and previous studies, which indicate that the prediction accuracy is much improved. This proposed method contributes to the more accurate assessment of uplift capacity of suction caisson in sand.

Seismic Response of Monopile-Supported OWT Structure Considering Effect of Long-Term Cyclic Loading

Offshore wind turbine (OWT) structures are subject to wave, wind, and seismic loading. Due to the cyclic nature of these loads, OWT foundations can be vulnerable to cumulative deformation and liquefaction triggered by waves and earthquakes. The effects of cumulative deformation and liquefaction on the monopile-supported OWT are not fully appreciated. This paper develops a three-dimensional numerical model for analyzing the seismic performance of large monopile-supported OWT under the long-term effect of cyclic loading. The numerical model was established employing FLAC3D and utilizing SANISAND constitutive model to simulate the soil behavior. The numerical model was validated by comparing its predictions with the results of dynamic triaxial tests and centrifuge tests. A simplified densification and subsidence site model was integrated into the numerical model to facilitate considering the long-term effect of cyclic loading. The numerical model was then used to conduct a comprehensive study to evaluate the influence of long-term cyclic loading on the natural frequency and seismic response of OWT structure. The results demonstrated that the densified subsidence zone around monopile increased the liquefaction resistance. However, the horizontal displacement of pile and the response acceleration of tower-top increased due to soil subsidence around monopile.

A Study on the Interaction Behavior between an Earth-Rock Dam and a New-Typed Polymer Anti-Seepage Wall

Polymer anti-seepage wall has been gradually applied in earth-rock dam reinforcement projects as a new seepage control technique. However, due to all-pervasive properties of the new materials and root-like connection between the materials and soils, the interface characteristics between the polymer wall and the earth-rock dam, as well as the interaction behavior of both, are complex and still not clear, which obstruct studying coordination mechanism of dam and wall under earthquake. Therefore, the interface characteristics and interaction behavior of dam and wall were studied in the article. Firstly, the dynamic shear stress-displacement, shear stiffness and damping ratio of the interface between polymer and soil were investigated by ring shear test. In addition, the viscoelastic constitutive model of polymer materials were researched by dynamic mechanical analysis (DMA) test. Based on tests results, a finite element model of earth-rock dam with polymer wall was established, including a non-linear simulation interface element and viscoelastic polymer constitutive model. Next, the validity of the simulation model was verified based on dynamic centrifuge test results. Then, the interaction behavior and seismic response of the dam with polymer wall were explored by using the verified model. The research results provide a scientific basis for the development and application of new-typed polymer anti-seepage wall in reinforcement engineering.

Novel hybrid MFO-XGBoost model for predicting the racking ratio of the rectangular tunnels subjected to seismic loading

This study proposes a novel hybrid MFO-XGBoost model that integrates the moth-flame optimization (MFO) algorithm and the extreme gradient boosting (XGBoost) to predict the racking ratio of rectangular tunnels subjected to seismic loading. For this purpose, a nonlinear finite difference model of soil-tunnel considering a realistic partial-slip condition is developed and validated against centrifuge test results. Then, 2040 dynamic simulations subjected to 85 ground motions are analyzed to cover a comprehensive suite of soil-tunnel configurations. Based on the generated database, the MFO-XGBoost model is constructed to capture the relationship between various effective parameters and the racking ratio of the rectangular tunnel. The obtained results are compared with those of four existing models to evaluate the performance of the proposed MFO-XGBoost model. The comparison reveals that the proposed MFO-XGBoost model captures well the numerical results of the racking ratio and outperforms other models. Among twelve input variables, parameters with primary and secondary influences are identified. Finally, a web application is built based on the proposed MFO-XGBoost model to calculate the racking ratio of rectangular tunnels, which is computationally more effective compared with alternative procedures.

Nonlinear numerical simulation of dynamic response of pile site and pile foundation under earthquake

AbstractTo study the influence of the nonlinear connection of pile and soil on the dynamic response characteristics of the pile foundation, this article proposes to study the dynamic response of the bridge pile foundation to the slope by combining the centrifugal shaking table test and OPENSEES open source finite element program. This article introduces the pressure-dependent multiyield surface model based on confining pressure. Through the inverse calculation of the similarity ratio of the centrifuge model test, the OPENSEES two-dimensional nonlinear finite element model of the pile group in the slope section can be established. The centrifuge shaking table test is to input the preset seismic wave horizontally at the bottom of the model box. The form of seismic wave is El Centro wave verification of two-dimensional finite element model of the pile group in slope section under earthquake. The reliability of the model is verified by comparing the test and calculated values of dynamic response (residual horizontal displacement and final bending moment) of the pile body under five different peak acceleration seismic wave loading conditions. In the dynamic response experiment of slope pile foundation, in the embedded part below the bedrock surface, the residual horizontal displacement of each pile body is zero. Constrained by the cap beam and tie beam, the displacement of the free section of the pile group at these two positions is basically the same. Through comprehensive analysis, the displacement of P1 and P2 piles is basically the same. The calculated value of the final bending moment of P1 and P2 piles shows the same change trend as the test value, and the test value is slightly larger than the calculated value. The relative errors of the maximum final bending moment of P1 pile under each loading condition are 7.4, 7.8, 12.6, 3.9, and 9.6%, respectively, and the relative errors of P2 pile are 4.6, 3.6, 12.5, 13.6, and 11.5%, respectively. The analysis relative error is caused by the elastic element used in the calculation of the pile body, which is different from the mechanical behavior of the simulated pile body material in the test. Dynamic response of slope site according to the existing centrifuge test results can be seen that the deformation at the slope shoulder of slope site is the most obvious under the earthquake. The inclined interface of soft and hard rock and soil layer will aggravate the dynamic response of the overburden layer on the slope, weakening its ability of seismic energy consumption.

Effect of an undercrossing tunnel excavation on an existing tunnel considering nonlinear soil-tunnel interaction

In heavily urbanised areas, the construction of tunnels underneath existing ones is unavoidable and much caution has to be exercised in both design and construction so as not to undermine the safety of the structure and vehicles. In this paper, a non-linear soil-tunnel interaction model is developed to evaluate the tunnel behaviour that is induced by a new undercrossing tunnel excavation. A barrel interface is proposed between the existing tunnel and surrounding soil, and effects of soil-tunnel interaction are considered by implementing limiting forces; depending on the stress conditions at the tunnel-soil interface, either separation or elastoplastic yielding can thus be modelled. Through validation with the centrifuge test results and instrumentation data from a field case study, it is shown that the proposed model can provide an accurate evaluation of tunnel behaviour. A parametric study also highlights the importance of considering tunnel-soil interaction when analysing the influence of volume loss and structural stiffness on the response of the existing tunnel. Finally, dimensionless design charts are provided as a preliminary evaluation tool in tunnel design.

Effect of Soil Damping on the Soil–Pile–Structure Interaction Analyses in Cohesionless Soils

Pile foundations in earthquake-prone regions must be analyzed and designed considering the dynamic loads. In the fully nonlinear dynamic analyses, the soil nonlinearity can be considered using the modulus degradation curves in the total-stress approach, and the soil damping is controlled by the unloading/reloading rule. Several researchers have investigated the effect of soil damping on the free-field soil response analyses, but the effect on the soil–pile–structure system response has not been studied thoroughly. In this study, the nonlinear elastic method (hyperbolic model) and the elastoplastic Mohr–Coulomb (MC) models were implemented to investigate the effect of soil damping on the pile and structure response. Dynamic soil–pile–structure interaction analyses were performed by simulating two different centrifuge tests published in the literature, and the analysis results were compared with the centrifuge test results. The analyses with the low-intensity input motions showed that the superstructure accelerations and the bending moments in the single pile were estimated with reasonable accuracy. However, the superstructure accelerations might be underestimated under high-intensity motions, especially in the MC model. Additional analyses were performed under six earthquake records. The constitutive models (MC and hyperbolic) may significantly vary the maximum structural acceleration and pile maximum moments (up to 50%). As a result, the responses of the superstructure and the pile in soil–pile–structure interaction problems are highly dependent on the soil constitutive model that must take the damping into account accurately; in turn, due account must be given to the selection of the constitutive model.

Analysis of laterally loaded offshore pile near slope in sand considering slope height

When the pile with embedded depth more than slope height is installed near sand slope, the influence of the slope effect on the lateral behavior of the pile includes not only the distance from pile to slope crest and slope angle, but also slope height. At present, the p−y curve method applied to laterally loaded pile cannot consider this case. Therefore, this paper develops a method based on wedge failure to modify the p−y curve to analyze the lateral behavior of pile installed near slope with limited slope height. Three wedge failure modes in front of the pile are discussed. And the method proposes the concept of an equivalent wedge model to characterize three different wedge failure models in front of the pile near slope and to calculate the ultimate soil resistance. And the method is verified by comparing its results with the results of field test, model test and centrifuge test. Finally, the influence of slope factors and pile factors on load-head displacement curve and maximum bending moment is studied by using this method.

The bearing capacity of A-shaped mat foundations on cohesive soil

ABSTRACT Because of its irregular shape, it is inaccurate to calculate the bearing capacity of the A-shaped mat foundation by a conventional method, which may have potential safety hazards. In this paper, the vertical bearing capacity of the mat is obtained by using the equivalent calculation method (ECM) and the finite element method respectively, and compared with the centrifuge test results. A correction method based on the ECM is proposed. Influences of foundation dimensional parameters on the H, M and H-M combined bearing capacity are studied. Expressions of H-M failure envelopes related to the dimensional parameters are established. A new form of V-H-M envelope expression for mats is suggested according to the effect of vertical load on H-M capacity. The modified ECM and the V-H-M envelope proposed in the paper can be used to quickly evaluate the vertical bearing capacity of the A-shaped mat and its stability under coupling loads.

Centrifuge modeling and numerical analysis on lateral performance of mono-bucket foundation for offshore wind turbines

The lateral loading performance of the mono-bucket foundation for offshore wind turbines is investigated in this study through a series of centrifuge tests and numerical analysis. Six mono-bucket models are considered to assess the influences of aspect ratio under the ultimate condition. The FE model is validated against the centrifuge test results, and the failure mechanism of the foundation is revealed. The parametric study and sensitivity analysis of the soil parameters are performed in the numerical simulations. A modified theoretical calculation method is proposed to estimate the ultimate lateral capacity of the suction bucket foundation by solving the limit equilibrium equations. A correction term is added in the calculation using the “m” method. The position of the rotation center is calculated, demonstrating a better accuracy compared to the previously reported geometric method. The modified calculation method is verified against the field test, the centrifuge test, and the laboratory test. The method is proved to be applicable in estimating the lateral performance of the mono-bucket foundation in a general form. This study aims to provide well-documented data and design references for practical engineers.

Numerical modelling of the effects of foundation scour on the response of a bridge pier

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.

Estimating settlements due to tunnel boring machine excavation

Soft-ground tunnel boring machines (TBMs) are the preferred solution for the construction of long tunnels and linear infrastructure assets, especially in urban areas. TBMs allow the control of tunnel-face stability, minimising the effects on surrounding ground. Unfortunately, existing methods for the assessment of ground surface movements due to TBM tunnelling either utilise complex and computationally expensive numerical analyses or rely on simplistic volume-loss theories, which do not consider the characteristics of the ground and TBM operation. This paper presents a simple formulation to estimate the immediate surface settlement due to the applied TBM support pressure, based on an analogy with the hyperbolic behaviour of stress–strain curves of soils. The variables chosen to describe the ground movement were the maximum surface settlement and volume loss, while the TBM face support pressure is used to describe the tunnel internal support pressure. Uncertainties due to the inherent variability of geotechnical parameters were also considered, resulting in the definition of lower and upper boundaries. Data from a series of centrifuge test results, with and without tunnel-face reinforcement by forepoles, and a real-scale TBM case study were used to validate the proposed model. The analyses show that the proposed model adequately represented the observed settlement data.

Seismic Active Pressure on an Inclined Retaining Wall with Surcharge on a Submerged Backfill

Seismic design of retaining wall supporting submerged retaining wall requires precise estimation of lateral earth pressure considering realistic parameters that constitute hydro-dynamic pressure, frequency inputs and other dynamic properties. The purpose of this study is to establish a closed-form solution for seismic lateral earth pressure acting on a retaining wall with submerged backfill with surcharge. The proposed method includes the dynamic nature by considering time-period, the frequency of the seismic wave, dynamic soil properties. Boundary conditions in the backfill are satisfied due to the propagation of the seismic wave. This study considers amplified seismic waves to estimate inertial forces due to critical soil mass and surcharge. The excess pore pressure generated during the seismic action is also considered in this study. The obtained formulation shows the significant effect of frequency content, damping resistance offered by backfill soil, amplification of seismic wave on seismic earth pressure. The seismic earth pressure obtained from this study is observed to be in a close agreement with centrifuge test results and analytical studies from the literature. This study definitely addresses the lateral pressure distribution on the retaining wall with backfill submergence and surcharge on the surface.