Geotechnical Centrifuge Tests Research Articles

Numerical study of geosynthetic-reinforced soils above voids – validation on centrifuge tests

ABSTRACT Understanding the load transfer and deformation mechanisms within geosynthetic-reinforced embankment overlying voids is fundamental for improving the current design methods. Based on geotechnical centrifuge tests, the numerical study highlights the cavity opening procedures within reinforced systems. Two procedures were studied: the increasing-diameter and gradual movement ones. For the gradual opening process, a cubic function is found to be well-suited, while a combination of cubic and quadratic functions is more appropriate for a progressive opening process. The study sheds light on the understanding of the load distribution over the cavity expansion. The soil arch formation occurs when the height reaches 2.5 times the sinkhole width. A triangular shape is found to be suitable for describing a progressive opening process, while an inverse triangular shape is more appropriate for a gradual opening process. Appropriate functions are utilized to fit the deformed geometry of the soil surface and of the deflected geosynthetics.

Research on the density of underwater sand and gravel fillings based on geotechnical centrifugal tests

The density of underwater dumping materials plays a major role in cofferdam engineering designs. However, it is difficult to accurately measure the density of underwater dumping materials. This study investigated the influence of particle size distribution of granular materials on the density of underwater filling materials. Considering the underwater dumping project of a hydropower station dam in the Han River as the prototype condition, several underwater dumping density centrifugal tests were conducted on sand gravel and sand mixtures. Based on the CKY200 large-scale geotechnical centrifuge, a single-embankment vertical-blockage underwater filling test device in a centrifuge field was developed. The results demonstrated the relationship between the compactness of the dumping body and the grading of loose particles, depth of dumping, and height of the upper loading. This indicates that the underwater filling material has a higher compactness and can reach a dense state. When the grading is poor, the compactness decreases. The greater the depth of filling, the greater the compactness. When vertical compression was applied to the upper part of the backfill, the compactness of the backfill, particularly in shallow areas, was significantly improved. These results provide insights for the design and construction of underwater dumping water conservation systems.

Load transfer mechanisms for capped and uncapped non-displacement piles above tunnels: a centrifuge modelling study

This paper presents results from geotechnical centrifuge tests of tunnelling in dry dense sand beneath loaded non-displacement piles, focusing on the effect of a pile cap (representative also of a raft or grade beam) in contact with the soil surface on load transfer mechanisms. Experiments included loading tests to ascertain the foundation capacity and load-displacement response in the presence/absence of an underlying model tunnel. Individual ‘reference’ pile response is compared for cases with and without a pile cap, including pile displacements and load distributions between the head, shaft, and base; the case of ‘friction’ piles with a compressible base are also considered. Results show that uncapped piles with relatively large service loads experience ‘geotechnical failure’ (i.e. large settlements or a significant increase in settlement rate with tunnel volume loss) in order to mobilise base or shaft resistance. Pile caps are shown to reduce settlements and prevent geotechnical failure of both reference and friction piles; they also improve the post-tunnelling foundation performance under loading. The paper provides evidence to enable engineers to consider the beneficial role of shallow elements in contact with the surface on the performance of pile foundations both during and subsequent to tunnelling.

Influence of erosion voids and traffic loads on buried large-diameter reinforced concrete pipes

Geotechnical centrifuge tests were conducted to examine the influence of invert voids and surface traffic loads on 1400 mm diameter reinforced concrete pipes buried with a shallow soil cover depth of 700 mm. Void formation beneath the pipe was simulated during centrifuge testing. The test results revealed that before void formation, the surface load directly above the middle of the pipe caused a significant increase in not only the circumferential bending moments but also the longitudinal bending moments, the latter of which was considerable and could not be ignored. Void formation beneath the middle of the pipe led to a reduction in both the circumferential bending moments and longitudinal bending moments at all measuring positions, i.e., crown, springline, and invert. The most significant reduction occurred at the invert, and there was even a reversal in the sign of the invert longitudinal bending moment. A comparison was made between centrifuge tests with erosion voids and surface loads at different horizontal positions, which had a marked influence even when the positions differed by half a pipe length. Joint rotation played an important role in relieving large bending moments of pipe barrels in a jointed pipeline when the void and surface load were located at the joint.

Failure Mechanism and Deformation-Based Design of Narrow Geosynthetic Reinforced Soil Walls

In recent years, the working performance of mechanically stabilized earth (MSE) walls has shown their outstanding stability and capacity to accommodate large deformation. The behavior and failure mechanisms of conventional MSE walls have been carefully examined. In cases where space is limited, such as in mountainous regions, in coastal regions, and for road expansion, the conventional MSE wall can be modified by adjusting the length of reinforcement to conform the construction area. For narrow geosynthetic reinforced soil (GRS) wall, the modification and arrangement of reinforcement components, including reinforcement tensile strength, vertical spacing, and aspect ratio, play key roles in the behavior of reinforced earth walls and can also lead to differences in the distribution of lateral earth pressure compared with conventional MSE walls. In this study, a series of geotechnical centrifuge tests are conducted to clarify the failure behaviors, distribution of lateral earth pressure, and deformation progresses of narrow GRS walls. Among the investigated variants, it is verified that improved reinforcement strength leads to a significant decrease in horizontal wall displacement. The relationship among lateral earth pressure, zero-earth-pressure zone, and horizontal displacement can be applied to predict the deformation of a narrow GRS wall.

Formulation and Performance of Model Concrete in Reduced-Scale Physical Model Tests.

The utility of geotechnical centrifuge tests depends on how correctly they predict the physical and mechanical behaviour of concrete. In this study, a model concrete material that consisted of α-gypsum plaster, fine silica sand, and water was developed. An orthogonal test design was used to evaluate the effect of the mix proportion on the model concrete performance. The physical (i.e., flowability and bleeding rate) and mechanical (i.e., compressive and flexural strength) characteristics were considered as indices. Various mix ratios resulted in remarkable relative contributions to model concrete performance, and each raw material dosage exhibited positive or negative synergy. The water-plaster ratio (W/P) and aggregate-plaster ratio (A/P) strongly influenced the mechanical and physical characteristics, respectively. Multiple linear regression analysis (MLRA) was carried out to determine a forecast model for various small-scale test demands. Finally, the applicability and outlines of the presented forecasting method in proportioning design were evaluated by typical use of model concrete in small-scale model tests.

Serviceability performance of buildings founded on rubber–soil mixtures for geotechnical seismic isolation

ABSTRACT Base isolation is a low-damage seismic design strategy that can be used for constructing resilient structures. Geotechnical seismic isolation (GSI) is a new category of emerging base isolation techniques that has attracted global interest in the past decade. Research on GSI based on rubber-soil mixtures (RSM) has focused on structural performance under earthquake actions, whilst there are concerns over the serviceability limit states (SLS) requirements in relation to (i) human comfort under strong winds and (ii) ground settlement under gravity, which may induce cracking and durability issues in structures. This article presents the first study on the serviceability performance of buildings constructed with the GSI-RSM system. The finite element model of a coupled soil-foundation-structure system has been validated by data recorded from geotechnical centrifuge testing. The numerical estimates of ground settlement have also been compared with analytical predictions. It is concluded that the GSI-RSM system can satisfactorily fulfill the SLS requirements.

Failure of levees induced by toe uplift: Investigation of post-failure behavior using material point method

Levees are essential structures in flood defense systems, and their failures can lead to devastating consequences on the surrounding territories. One of the failure mechanisms mostly controlled by the foundation soil stratigraphy is the instability of the land side slope, triggered by the development of high uplift pressures in the foundation. This complex phenomenon has been investigated experimentally with centrifuge tests or large-scale tests and numerically with the limit equilibrium method (LEM) and the finite element method (FEM). In this work, we applied a multiphase formulation of the material point method (MPM) to analyze the development of toe uplift instability mechanism, from the onset of failure to large displacements. The numerical model is inspired by an experiment carried out in a geotechnical centrifuge test by Allersma and Rohe (2003). The comparison with the experiment allows for understanding critical pore pressure triggering large displacements in the foundation soils. Moreover, we numerically evaluated the impact of different values of foundation soils' hydraulic conductivity on the failure mechanism. The results show that hydraulic conductivity mainly influences the time of failure onset and the extension of shear localization at depth. Finally, the advantages of using large displacement approaches in the safety assessment of earth structures are discussed. Unlike FEM, there are no issues with element distortions generating difficulties with numerical convergence, allowing for full post-failure reproduction. This capability permits precise quantification of earth structure damages and post-failure displacements. The ensuing reinforcement systems' design is no longer over-conservative, with a significant reduction in associated costs. ©2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Simulation of cyclic laterally-loaded piles in undrained clays accounting for soil small-strain characteristics

The performance of pile foundations subjected to cyclic lateral loading is central to offshore geotechnical engineering. Although non-linearity and path-dependence of soil mechanical behavior at small strains are widely recognized, the influences of these local behavior on global response of pile foundations are less explored. This work presents numerical analyses of cyclically-loaded piles in undrained clays that explicitly account for soil small-strain characteristics. By using intergranular strain (IGS) elasticity method, a soil model proposed by the authors for undrained clays during cyclic loading is extended to include non-linear small-strain behavior. Via comparing against element tests, the constitutive relation is shown capable of representing the observed variation of soil stiffness and damping with strain magnitudes, as well as the dependence of such non-linearity on stress paths. Based on the above soil model, finite element simulations are performed to interpret the response of laterally loaded piles from geotechnical centrifuge tests. The impacts of soil small-strain behavior on pile force–displacement relations, hysteric loops, and the ability of a pile–soil system to dissipate energy are investigated. This work highlights that the sensitivity of these foundation responses to small-strain non-linearity of soils can vary considerably, depending on pile geometry features and head constraint conditions.

Soil-structure interaction of a laterally loaded hybrid foundation for offshore wind turbine

With the ramping up of the capacity of the offshore wind turbine (OWT), the water depth, and the weight of the superstructure, the need of foundation structure with higher performance but reasonable cost demands innovation from geotechnical and structural engineering. The novel hybrid foundation takes advantage of the mature monopile technology, and innovatively integrates a loading wheel-bucket component around the monopile in the upper layer of soil. The addition of the integrated hybrid component is proven to enhance the overall performance of the foundation by previous research. The soil-structure interaction (SSI) of the hybrid foundation is the key to optimize the design of the structure. In this paper, a series of finite element (FE) analysis of the hybrid foundation under monotonic load are performed. The results of the FE analysis are compared with that of the geotechnical centrifuge tests. The soil pressure distributions on the pile and the hybrid component are investigated. The variation of the soil pressure profile with the increase of rotation angle is also studied. The soil resistance on the skirt of the bucket component is illustrated in the analysis. The FE models also prevail the failure mechanism of the hybrid foundation.

On seismic protection of wind turbine foundations founded on liquefiable soils

Wind turbine structures can be susceptible to damage from earthquake loading due to large overturning moments at the foundation level. As the wind energy market expands to seismically active parts of the world, it becomes important to evaluate the seismic behaviour of these structures. Especially, earthquake-induced liquefaction can be important when wind farms are located on sites with marginal soil conditions such as loose, saturated sands or non-plastic silts. Reduction of stiffness during liquefaction can cause excessive foundation settlement and/or rotation. Thus, suitable mitigation methods should be investigated. This paper discusses the seismic behaviour of the wind turbine on liquefiable soil and the effectiveness of the stone column-based liquefaction mitigation method. Two geotechnical centrifuge tests were conducted with and without stone columns for a model wind turbine on a circular raft foundation. The results show that stone columns can significantly reduce both the settlement and rotation of the raft.

Laboratory-scale geotechnical centrifuge test system design and consolidation behavior analysis

Laboratory-scale geotechnical centrifuge test system design and consolidation behavior analysis

Probabilistic site characterisation using monitored spudcan installation data

Predicting spudcan penetration is essential for reducing risk during the installation of an offshore mobile jack-up platform. However, predictions made prior to installation often deviate from those recorded in practice, mainly because inherent uncertainties in seabed soils are not adequately considered. In this paper, the characteristics of the seabed are quantified using monitored spudcan installation data by developing a Bayesian framework coupled with Metropolis algorithm-based Markov chain Monte Carlo (MCMC) simulation. Applied to sand overlying clay conditions multiple readings from the load–penetration curve are used to derive statistical interpretations of seabed parameters such as sand thickness, sand friction angle and the undrained shear strength profile of clay. Illustrative examples demonstrate the proposed method using data from geotechnical centrifuge tests and a monitored offshore jack-up. The accuracy of the method is also retrospectively examined against a database of 66 centrifuge tests. The results show that the proposed method can provide consistent soil parameters with reduced uncertainty regardless of the influence of the updating sequence, number of MCMC chains, scale factors and the prior distributions applied. This method offers the offshore industry a new methodology to statistically interpret seabed conditions by incorporating monitored data. Potentially it could be combined with spatial interpolation methods to infer ground conditions and more accurately predict future spudcan installations.

Improving Semianalytical Predictions of Long-Term Basement Heave in Over-Consolidated Clay

A series of geotechnical centrifuge tests to investigate the long-term heave behavior of basements in over-consolidated clay was performed, and the profiles of the slab displacement and underslab pressure were measured simultaneously. The same prototypes were simulated using Plaxis 2D with the clay stratum represented by the small-strain hardening soil model. The results were compared, and good agreement was found in terms of the profiles of slab-soil contact pressures. The data indicated that existing semianalytical methods of long-term heave predictions can be improved by assuming a quadratic distribution of slab-soil contact pressure. Example calculations for the relaxation ratio method and the relative stiffness method are presented.

Role of Footing Embedment on Tunnel–Foundation Interaction

This technical note investigates the effect of footing embedment depth on tunnel–structure interaction using geotechnical centrifuge testing. A 2-story framed building on separate footings, either resting directly on the surface or embedded in the soil, and subjected to tunneling-induced displacements is modeled. Measurements of the displacements of the footings and underlying soil, ground deformations, and structural distortions are presented. Results show that footing embedment increases foundation differential settlements and horizontal displacements, thereby causing a greater level of distortion within the frame. Furthermore, the embedded footings result in a larger magnitude of ground displacements and shear strains of the soil. Finally, modification factors and relative stiffness parameters are presented, indicating a greater effect of the embedment on horizontal deformations than the angular distortion of the bays.

Centrifuge experimental study of behaviour of basal-reinforced rubble mound over soft soil

Large settlement and lateral deformation during construction of a rubble mound, such as a breakwater, over a soft clayey foundation are a challenge. Different mechanisms contribute to excessive settlement of marine clay, including consolidation, the size of the rubble particles, the penetration of individual rock particles and the flow of sludge through the rock pores. Placing a geotextile layer or sand mattress can restrict the amount of material loss caused by these mechanisms. To accurately assess the performance of a geotextile under a rubble mound, eight geotechnical centrifuge tests were conducted. The shear strength of the foundation and the type of the separator layer were the variables considered. The results indicate that the use of a geotextile as basal reinforcement reduced settlement at the breakwater crest and increased stability. In addition, the horizontal displacement of the rubble mound and the clayey foundation decreased remarkably.

Centrifuge study on the use of protective walls to reduce tunnelling-induced damage of buildings

Tunnel excavation in urban areas causes ground movements that could damage existing nearby piled structures. In practice, to protect structures from tunnelling-induced damage, a stiff protective wall can be constructed between the tunnel and the adjacent piled structure. In this paper, results from four hybrid geotechnical centrifuge tests (where data are coupled between the centrifuge and numerical models) are used to quantify the effect of protective walls on reducing the impact of tunnelling on an adjacent framed building with four piles. Two protective walls with different embedded depths are considered: a ‘shallow’ wall with its toe at the tunnel axis depth and a ‘deep’ wall with its toe below the tunnel invert. Compared to the ‘no-wall’ case, the ‘deep’ protective wall is shown to significantly reduce uneven pile settlements, structural distortions, and load transfer (through the building) between piles; the ‘shallow’ wall is shown to have little benefit. Data from the instrumented walls and piles are used to explain the dominant mechanisms at play and investigate how the load is redistributed within the piles.

Centrifuge application of fibre Bragg gratings: pile axial loads and wall bending moments

One challenge for geotechnical centrifuge testing of soil–structure interaction problems is the reliable measurement of induced structural strains/forces. This paper presents a novel application of fibre Bragg grating (FBG) sensors for strain measurement within geotechnical centrifuge tests. FBG sensors have several advantages for centrifuge testing, in particular their small size and minimal self-weight. This paper gives an overview of recently developed installation and calibration procedures for FBG sensors within buried centrifuge model structures. The effect of thermal expansion/contraction of the materials (including both the fibre and structures) is considered and assessed. The precision and reliability of the FBG sensors are demonstrated using verification tests. The application of the FBG sensors is considered for two geotechnical problems, namely, pile jacking and a ‘retaining wall’ adjacent to a tunnel (acting as a protective wall to prevent an adjacent structure from tunnelling-induced ground movement). The results demonstrate that the FBG sensors can provide reliable measurements of pile axial strains/forces and protective wall bending moments. The paper provides evidence to support the routine adoption of FBG sensors for strain/force measurement of structures in geotechnical centrifuge modelling.

Modeling of laterally cyclic loaded monopile foundation by anisotropic undrained clay model

Improved design methods for laterally loaded monopile foundation are crucial for the offshore wind turbine (OWT) sector, as established approaches no longer apply to such a pile foundation characterized by a relatively small length-to-diameter ratio. Numerical modeling is the primary approach to improve and validate design methods for monopiles and a central part of it is the soil constitutive model. This work presents a model that captures the essential mechanical characteristics of cyclically loaded undrained clay yet retains simplicity demanded by practicing engineers. This objective is realized by simplifying undrained clays as single-phase materials governed by total stresses and utilizing kinematic hardening plasticity. Via a new kinematic hardening rule, strength and stiffness anisotropy of clay and their cyclic deterioration are accommodated. The model is evaluated at different scales of simulating laboratory soil tests and analyzing monopile subjected to monotonic and repeated lateral loading in geotechnical centrifuge tests. The latter validation provides an encouraging demonstration of how monopile foundation can be numerically analyzed based on soil data acquired mainly from in situ site characterization and routine laboratory tests.

Dynamic response of stand-alone and adjacent footing on saturated sand

This paper presents results from geotechnical centrifuge tests performed at the Japanese Institute of Occupational Safety and Health (JNIOSH) in Tokyo, Japan. A range of shallow footing configurations with different bearing pressures founded on saturated sand in a laminar box on a shake table was studied using harmonic excitations. The free-field condition was also studied as a reference. The study aimed to address the influence of stand-alone and closely adjacent shallow foundations on the dynamic response of the soil-foundation-structure system. A partially drained response of the soil and a minor dilative response for large values of shear strain were observed for the free-field case. In the case with footings, the settlement was found to be largely due to the footing penetrating the supporting sand. A dilative response of the soil beneath the footings was observed for the stand-alone configurations at the beginning of the base motions, this was not observed in the case of adjacent footings. Adjacent footings also tilt inwards consistently while stand-alone footings did not present a clear tilt pattern. In terms of the acceleration recorded in the near-surface soil beneath the footings, a reduction was observed after a few cycles in the free-field case. This reduction was considerably smaller in the case with footings with no significant differences between the response of the stand-alone and that of the closely adjacent footings.