Centrifuge Test Results Research Articles

Stress transfer mechanisms and settlement of a floating pile due to adjacent multi-propped deep excavation in dry sand

Three-dimensional numerical parametric study is conducted to investigate stress transfer mechanisms and settlement of a floating pile due to multi-propped basement excavation by using hypoplastic sand model. Centrifuge test results are adopted to validate the soil model and model parameters. Effects of excavation depth, pile location with respect to basement, working load, sand density and support system stiffness are examined. Downward and upward load transfer mechanisms are observed at different excavation stages, depending on the ratio of excavation depth (He) to pile length (Lp). If excavation-induced reduction of shaft and base resistances are ignored, the pile settlement is underestimated by up to 39%. Negligible pile head settlements are observed when the pile is located at a distance exceeding 2.5 He (excavation depth) from the retaining wall. Basement excavation-induced pile settlement is doubled when the designed factor of safety of pile before excavation reduces from 3.0 to 1.5. By increasing the relative sand density 30–90%, the pile settlement decreases by up to 66%. Excavation-induced pile head settlement is almost insensitive to support system stiffness for shallow excavation (He/Lp < 0.4).

Liquefaction characteristics of offshore wind turbine with hybrid monopile foundation via centrifuge modelling

Some offshore wind farms are built in seismically active areas. The offshore wind turbine (OWTs) are high-rise structures and sensitive to lateral failures during the earthquake. In this study, an innovative hybrid monopile foundation is proposed for OWTs. A series of centrifuge shake table tests is conducted to investigate the seismic response and liquefaction characteristics of the hybrid monopile foundation. Mechanisms of the seismic behavior of soil, lateral displacements of the wind turbine, and structural settlements are evaluated. Centrifuge test results indicate that the liquefaction around the hybrid monopile foundation is weakened compared to the traditional single pile. The soil partially keeps its strength and stiffness during the shaking due to the higher confining stress. The lateral stability of the system is enhanced. OWTs with the hybrid monopile foundation tends to settle more during the earthquake due to the soil-structure interaction and the static bearing induced soil shearing. Two types of hybrid monopile foundations are constructed with different weights and materials, which are predominant influence factors for their seismic response. This study aims to investigate the seismic behavior of the hybrid monopile foundation for OWTs during earthquake events and provide references for practical designs.

Ultimate bearing capacity of vertically loaded strip footings on sand overlying clay

Conventional methods toward addressing the bearing capacity of strip footings on sand overlying clay may yield unreliable solutions. In this technical note, the bearing capacity of footings on sand overlying clay is evaluated using discontinuity layout optimization (DLO). A set of design charts that can be directly applied to the classical bearing capacity formulation is developed. Subsequently, a simplified regression-based approach, relevant to a wide range of geometric and strength parameters, is proposed. The model uncertainty for the proposed method is characterized. Validations are performed with the results of centrifuge tests and previously proposed analytical solutions.

Seismic response of pile–raft system embedded in spatially random clay

This note examines the seismic response of a pile–raft system using the random field finite-element method. The numerical method is validated by recently published centrifuge test results. The randomness in the maximum shear modulus of soils in the system is considered by three-dimensional random fields in order to investigate the statistical characteristics of the maximum pile bending moment response. The computed pile bending moment results indicate that the deterministic results can be linked up with the random analysis results by an amplification factor. Therefore, a reliability-based geotechnical design can be applied.

Stress transfer from rocking shallow foundations on soil-cement reinforced clay

Equivalent-static pushover analyses with a three-dimensional (3D), nonlinear, finite-difference model are used to investigate the static and seismic stresses imposed on soil-cement grid reinforcements in soft clay profiles by overlying structures supported by shallow footings. The goal is to evaluate the potential stress concentrations in the soil-cement grid during foundation rocking and the potential for large foundation settlements associated with the local crushing of the soil-cement. The numerical analyses are first validated using data from dynamic centrifuge experiments that included cases with and without large foundation settlements and localized crushing of the soil-cement grids. The experimental and numerical results indicate that the stresses imposed on the soil-cement grid by the overlying structures must account for foundation rocking during strong shaking and stress concentrations at the soil-cement grid intersections. The numerical analyses provide reasonable predictions of the structural rocking loads and the zone of the expected crushing or lack of crushing, but underestimate the accumulation of foundation settlements when the seismic demands repeatedly exceed the soil-cement strength. The simulated moment-rotation and uplift behavior of the footings under monotonic lateral loading are reasonably consistent with the dynamic centrifuge test results. Parametric analyses using the validated numerical model illustrate how the stress transfer varies with the area replacement ratio, the thickness of the top sand layer, the properties of the bearing sand layer, and the relative stiffness of the soil-cement and the surrounding soil. A design model for estimating the stresses imposed on a soil-cement grid by rocking foundations was developed and shown to provide a reasonable basis for assessing whether or not local damage to the soil-cement grid is expected.

Centrifuge model testing of a loess landslide induced by rising groundwater in Northwest China

Considerable artificial irrigation in the Heifangtai area has caused the continued rise of groundwater levels, which has resulted in several large-scale landslides in the loess platform area. Considering the No. 13 landslide of Jiaojia in the Heifangtai area as a geological archetype, a series of centrifuge tests was conducted to study the deformation and failure characteristics of the slope due to the continuous rise in the groundwater level. Meanwhile, the pore water pressure, earth pressure, deformation, and evolution characteristics of the slope were monitored and recorded in real time. Furthermore, isotropically consolidated undrained triaxial testing (ICU) was conducted to investigate the static liquefaction of loess specimens, which was related to the slope instability. The centrifuge test results showed that a series of parallel and steep tension cracks at the top of the slope were present before the landslide, which was caused by the rising groundwater. The ICU experimental results showed that as the pore water pressure increased, strong strain softening and static liquefaction of saturated loess soils were found, which was related to the sudden destruction of the slope.

The Comparison of Laboratory Tests and Numerical Analysis of Pressure and Tension Bearing Capacities of the New System of Microbulb and Micropile System on Clay in Shiraz, Iran

Abstract In recent years, the implementations of Micropiles have been increased extensively in large project constructions due to the unique advantages of this method comparing to concrete piles. The results of numerical analysis and centrifuge tests show that the efficiency and importance of micropiles can be considered as an effective solution. This research is designed to compare the newly designed system of microbulb with micropile applying a numerical modeling with finite element software Plaxis 3D foundation v1.6. We also used the FHWA code for a comparative study between micropiles and microbulbs. Then the effective factors were analyzed in order to decide which method is better between these two methods of micropile and microbulb. The results showed that the new microbulb system is an appropriate solution for improvement of mechanical properties of the soil, increase of bearing capacity and settlement decrease.

Model of Consecutive, Steady-State Underflow for Vertical Tailing Silos

A continuous tailing discharge model is proposed for solving the problems of high fluctuation in underflow concentration and low, unstable actual discharge concentration in vertical tailing silos. On the basis of the mass balance equation, a mathematical model for continuous tailing discharge is derived, and a partial differential equation related to the height of the tailing silo in the compression area and tailing slurry concentration is obtained. The effective solid stress equation and the solid flux equation can be obtained from the results of centrifuge tests and intermittent sedimentation tests. The volume concentrations of underflow corresponding to various heights of tailing surfaces are simulated by using fluid mechanics software under dynamic balancing conditions. The results of the simulations are highly similar to those calculated by the derived mathematical model of continuous tailing discharge, and the results of the industrial test are also closely related to the results of the differential equation, thereby verifying the accuracy of the proposed discharge model.

Evaluation of EPS wall effectiveness to mitigate shallow foundation deformation induced by reverse faulting

A comprehensive study is presented to illustrate the effect of an expanded polystyrene sheet (EPS) wall for absorbing fault rupture and its effects on shallow embedded foundations. Numerical modeling has been thoroughly validated through centrifuge test results. Various embedment depths and fault dip angles were considered to specify the critical foundation position on the soil surface. Furthermore, the impact of vertical trenching of an EPS wall, next to a rigid foundation, was investigated to mitigate surface foundation deformation. Results indicated that, due to high compressibility and low shear resistance that is not time dependent, an EPS wall panel protects the foundation by absorbing fault induced shear strains in the soil stratum. The effectiveness of such walls depends mainly on the geometry, location and mechanical characteristics of the walls, foundation location, bedrock fault offset, and fault dip angle.

Evaluation of liquefaction potential of marine sandy soil with piles considering nonlinear seismic soil–pile interaction; A simple predictive model

An elastoplastic, dynamic, finite-difference method was applied to study the effects of nonlinear seismic soil–pile interaction on the liquefaction potential of marine sand with piles. The developed model was well validated using the centrifuge test. The results showed that acceleration, bending moment, and excess pore water pressure complied well with centrifuge test results. The effect of different affecting parameters on liquefaction potential was investigated using parametric study. Using a sensitivity analysis, the pile embedment parameter was shown to be the most influential parameter. Finally, applying the evolutionary polynomial regression technique, a new model for predicting the liquefaction potential was presented.

Seismic response of non-connected piled raft foundations

ABSTRACT Connected piled raft foundations (CPRFs) have proven to support heavy structures under vertical load. However, when subjected to seismic loading, significant load transfer from the raft beneath to the pile head may cause huge shear stress and bending moments in the piles. Thus, this paper investigates the behaviour of non-connected piled raft foundations (NCPRFs) subjected to earthquake loading. For this purpose, a three-dimensional finite element model was presented and validated against centrifuge test results. This numerical model was then employed to perform a series of numerical analysis. According to the results, pile slenderness ratio and raft thickness can be considered as essential factors in evaluating the dynamic response of NCPRF. The comparison of CPRF and NCPRF has also indicated that using NCPR system leads to significantly less shear force and bending moments along the piles, which results in less required reinforcement and more economical design in comparison with CPR systems.

Effect of Flexibility Ratio on Seismic Response of Cut‐and‐Cover Box Tunnel

Equivalent linear time history analyses are conducted to calculate the seismic response of various types of cut‐and‐cover single box tunnels. A finite‐element numerical model is calibrated against the results of centrifuge tests. The calculated tunnel responses compare favourably with the measurements. A validated model is then used to quantify the seismic response of box tunnels. The flexibility ratio (F) is illustrated to have a governing influence on the tunnel response. It is shown that the previously developed relationship between F and the racking ratio (R) is applicable for a wide range of F up to 20. It is also shown that an increase in F accompanies corresponding increase in R, the spectral acceleration in the tunnel lining, and the shear stress along the tunnel lining‐soil interface. The thrust in the tunnel lining is also revealed to increase with F, although the calculated value is significantly lower than the pressure on yielding walls. Additionally, the surface settlement is shown to increase with an increase in F.

Pipeline–soil–water interaction modelling for submarine landslide impact on suspended offshore pipelines

The submarine landslide is one of the major geohazards in deep-water oil and gas developments. The impacts of glide blocks or out-runner blocks, which carry the geotechnical properties of the parent soil mass before the landslide, on pipelines normal to the direction of slide, are investigated in this study. A computationally efficient numerical modelling technique is developed using a computational fluid dynamics approach, incorporating a strain-rate and strain-softening dependent model for the undrained shear strength of clay sediment, to simulate the lateral penetration of a pipe in a clay block. The role of water in the cavity and channel formed behind the pipe during the lateral penetration on drag force is successfully simulated. Numerical simulations for varying depths of the pipe explain the change in soil failure mechanisms in which the channel behind the pipe and berm play a significant role, especially at shallow depths. As the cavity behind the pipe may not be completely filled with soil, the limitations of smooth/rough and bonded/unbonded interface conditions, as used typically in pipe–soil interaction analysis, are discussed. Based on a comprehensive parametric study, calibrated against centrifuge test results, a set of empirical equations is proposed to calculate drag force for practical applications. The effects of inertia on drag force are examined.

Recent centrifuge modelling of offshore geotechnical problems at IWHR

Centrifuge modelling has been proven to be an efficient and reliable approach for examining offshore geotechnical problems. This study reports the two series of centrifuge tests to understand the behaviour of spudcan penetration in a “soft-stiff-soft clay” stratigraphy and the behaviour of gravity anchor subjected to a lateral loading. A hydraulic system has been adopted to apply the large compressive and tensional load on the spudcan and gravity anchor, respectively. Load cells were installed on the base of the spudcan to directly measure the stress acting on the spudcan base. A 2D laser scanner was adopted to monitor the horizontal, vertical movement and tilting of the gravity anchor. The influence of the relative soil stiffness on the spudcan pentration behaviour and the soil deformation and interaction with the gravity anchor are discussed based on the centrifuge test results.

Estimating the Effects of Tunnelling on Preexisting Jointed Pipelines

Tunnel excavation inevitably results in ground movements and changes in soil stress, leading to additional stress on and settlement of nearby buried pipelines. This article focuses on the response of jointed pipelines to twin tunnelling. The relationship between the relative pipe‐soil displacement and the relative pipe‐soil stiffness was first determined. Based on this analysis, a series of numerical parametric studies encompassing 7776 conditions were performed to investigate the responses of a jointed pipeline to twin tunnelling. The results are used to estimate a regression equation for the relationship between the relative pipe‐soil stiffness and the normalized maximum joint rotation angle. This equation can be used for the direct calculation of the maximum joint rotation angle that will result from single or twin tunnelling and for the assessment of the tunnelling‐induced risk to jointed pipelines. The applicability and reliability of the regression equation are validated by comparing the calculated values with the results of earlier centrifuge tests.

Seismic behavior of suction caisson for offshore wind turbine to generate more renewable energy

Increasing population growth requires more sustainable development of energy. Wind turbines convert wind energy (as renewable energy) to electrical energy. This non-contaminated energy has an inexhaustible energy source. One of the vital parameters in such structures is the choice of foundation type. Suction caissons are now used extensively worldwide for offshore wind turbine. Considering the presence of a number of offshore wind farms in earthquake areas, the study of seismic behavior of suction caisson is necessary for better design. In this paper, the results obtained from three suction caisson models with different diameter (D) and skirt length (L) in saturated sand were compared with centrifuge test results. All models are analyzed using 3D finite element method taking account of elasto-plastic Mohr–Coulomb constitutive model for soil which is available in the Abaqus library. The earthquake load applied to the base of models with a maximum acceleration of 0.65 g. The results showed that numerical method is in relative good agreement with centrifuge results. The settlement and rotation of foundation decrease by increasing the skirt length and foundation diameter. The sand soil outside the caisson is prone to liquefaction due to its low confinement.

Implementation of a hydromechanical elastoplastic constitutive model for fully coupled dynamic analysis of unsaturated soils and its validation using centrifuge test results

Prediction of unsaturated soil behavior during earthquake loading has received increasing attention in geotechnical engineering research and practice in recent years. Development of a fully coupled analysis procedure incorporating a coupled hydromechanical elastoplastic constitutive model for dynamic analysis of unsaturated soils has, however, been limited. This paper presents the implementation of a coupled hydromechanical elastoplastic constitutive model into a fully coupled dynamic analysis procedure and its validation using a centrifuge test. First, the fully coupled finite element equations governing the dynamic behavior of unsaturated soils with the solid skeleton displacement, pore water pressure, and pore air pressure as nodal unknowns are briefly presented. The closest point projection method is then utilized to implement the coupled hydromechanical elastoplastic constitutive model into the finite element equations. The constitutive model includes hysteresis in soil–water characteristic curves, cyclic elastoplasticity of the solid skeleton, and the coupling mechanisms between the SWCCs and the solid skeleton. Finally, the analysis procedure is validated using the results from a dynamic centrifuge test on an embankment constructed of compacted unsaturated silt subjected to base shaking. Reasonable comparisons between the predicted and measured accelerations, settlements, and deformed shapes are obtained.

Investigation of geometric effects on three-dimensional tunnel deformation mechanisms due to basement excavation

Numerical parametric study is conducted to investigate geometric effects of basement excavation on existing tunnels. Centrifuge test results are adopted to validate numerical models. If basements with a same excavation area are constructed above a tunnel, the most preferable excavation geometry is short-rectangular basement, followed by cylindrical, square, and long-rectangular basements. When excavation diameter of cylindrical basement reaches eight times of the final excavation depth, three-dimensional effects of basement-tunnel interaction at basement centerline are negligible. If three-dimensional effects in a small basement are ignored, the heave and transverse tensile strain of tunnels are overestimated by up to 160% and 50%, respectively.

Assessment of Monopile-Gravel Wheel Foundations under Lateral-Moment Loading for Offshore Wind Turbines

This paper evaluates the lateral performance of a monopile reinforced by a gravel wheel for offshore structures via centrifuge tests and three-dimensional finite-element (FE) modelings. The gravel wheel comprises a ring frame placed on the pile head and filled with large particles to potentially utilize gravel or crushed stone in offshore areas. The results of centrifuge tests and FE analyses demonstrate that the lateral loading capacity of the monopile increases when combined with a gravel wheel, and the improvement depends on the diameter and thickness of the wheel. By means of FE methods, the interaction between the pile and surrounding soils and gravel fill are illustrated to explain how the gravel wheel contributes to the lateral resistance of the hybrid system. Furthermore, an equivalent layer method adopting the conventional p-y curves is suggested to predict the lateral response of the hybrid foundation. This method is validated by comparisons with the centrifuge tests results. Finally, a case study of the monopile-gravel wheel foundation indicates that the gravel wheel is less efficient in configurations where the ultimate capacity of the hybrid system is dictated by the bending capacity of structures rather than the strengths of soils.

Numerical analysis of earthquake-induced deformation of liner system of typical canyon landfill

Integrity of composite liner system is closely related to the serviceability of Municipal Solid Waste (MSW) landfill and should be taken into account in the seismic design of landfill. Moreover, extensive application of geomembrane (GM) and geosynthetic clay liner (GCL) in composite liner system generates soil-geosynthetic interaction problem. In this study, a numerical model is proposed to analyze earthquake-induced deformation of liner system with a full consideration of the dynamic landfill-liner-foundation interactions. Rate-dependent exponential model for GM and equivalent linear model for GCL are developed. Normal and tangential behaviors of geosynthetic interface within liner system are defined with physical constitutive relationship and solved with appropriate numerical technology. The present model is validated against experimental results of centrifuge test. A series of two-dimensional dynamic finite element analyses of typical canyon landfill are performed to provide an insight into the mechanism of deformation within liner system. Results indicate that tension of GM is mainly influenced by non-uniform friction between its upper and lower surfaces, while shear strain of GCL is strongly connected to outward slippage of landfill. Resistance of the composite liner system plays an important role in the distribution of force and deformation in this dynamic interaction problem, which deserves special attention in engineering practice.