Centrifuge Model Tests Research Articles

Post-liquefaction deformation mechanisms of stone column-improved liquefiable sloping ground under cyclic loadings

Liquefaction-induced large deformations in sloping ground caused heavy damage to buildings and infrastructures during earthquakes, and its evaluation and mitigation challenge. In this study, a series of soil element tests using hollow cylinder apparatus (HCA) were conducted to investigate the relationship between residual volumetric strain and residual shear strain of medium dense to dense saturated sand with moderate initial static shear stress. The soil element tests indicate that the developments of residual volumetric strain and residual shear strain are dominated by the Post-liquefaction Deformation Potential (PLDP) of soil, which is well correlated to the maximum cyclic shear strain developed during cyclic loading. Then, the applicability of PLDP to characterize the post-liquefaction deformation response in gently sloping ground was investigated by centrifuge model tests without and with stone column improvement. The model tests of medium dense and dense sand slopes proved the applicability of PLDP preliminarily. The mitigation mechanisms against settlement and lateral spreading in gentle slopes by densification and drainage effects induced by stone columns were also observed and discussed. The present study provides the conceptual term of PLDP for evaluating post-liquefaction deformations of natural and stone column-improved gently sloping grounds, which helps to develop mitigation techniques for liquefiable sloping ground subjected to earthquake loadings.

Failure Behavior and Surrounding Soil Stress Responses of Suction Anchor in Low-Strength Muddy Clay

Anchorage failure of a suction anchor is more likely to occur in low-strength muddy clay. This paper focuses on the failure behaviors of suction anchors and muddy clay stress responses. The centrifugal model test was used to study the loading processes of suction anchors with various pulling angles. Firstly, the multi-stage developing process of anchoring force was analyzed according to the test results. Numerical modeling was used to validate the test results. The displacement of the suction anchor and muddy clay soil were analyzed using the numerical results. Then, the numerical and testing results were compared to analyze the horizontal soil pressure responses around the suction anchors. It was found that the change in loading direction affected the distribution and development of soil stress. The horizontal soil resistance played a crucial role in improving the bearing capacity. The soil stress variation and anchor displacement revealed that the suction anchors exhibited multi-attitude coupling movement during the inclined pulling. The vertical pulling suction anchor showed shear–slip failure behaviors, while the inclined pulling suction anchors showed compression–shear–slip coupling failure behaviors. The results of this study provide insight into the interaction mechanism between suction anchors and muddy clay, serving as a reference for the design and application of suction anchors.

Dynamic centrifuge tests on the synergistic mechanism of pile-anchor structure retaining rock slopes

Pile-anchor reinforced slopes show to have better seismic resistance performance than other traditional supporting structures. However, their bearing mechanisms are not yet fully understood. In this study, dynamic centrifuge model tests are carried out to study the dynamic response and synergistic mechanism of rock slope reinforced by pile-anchor composite structures under seismic loading. The centrifuge model is devised by following the scaling principle, and the properties of the model materials are determined by laboratory tests. Some parameter studies are carried out considering the anchor elastic modulus, pile bending stiffness, and pile embedding depth. The test results are discussed in terms of the failure mechanism of the slope to help better understand the synergistic mechanism of the pile-anchor structure in seismically active areas.

Cyclic p-y curve model of piles subjected to two-way load considering the collapse and densification of sand

A cyclic p−y curve model is proposed to study the lateral response of piles in sand under two-way cyclic loads such as wind and wave. The collapse and densification of the soil around the pile caused by the repeated formation of the gap between the pile and soil during the cyclic loading are considered in the proposed model. Based on the nonlinear strain wedge model, the cyclic p−y curve is preliminarily constructed considering the friction characteristics. A collapse and recompression model of sand is established to account for the variation of the relative density of the caved soil and the characteristics of the pile-soil gap. The densification of sand caused by recompression is evaluated using the stiffness hardening index model. The cyclic p−y curve is modified by considering the collapse and densification of sand and its rationality is verified by comparing the results of calculation and centrifugal model test. Furthermore, the strengthening by the caved soil, the characteristics of the cyclic p−y curve, and the soil resistance are analyzed.

Pumping-induced reactivation of a pre-existing normal fault: insights from a centrifuge model test

Pumping-induced normal fault reactivation and the resultant ground fracture have been observed in faulted basins worldwide, but the process and mechanism are poorly understood thus far. In this study, we conducted centrifuge model tests to investigate and analyze these issues. Two simplified faulted models, Models 1 and 2, were developed based on an actual event that occurred in the Beijing Plain, China. Our model tests reproduced the pumping-induced normal fault reactivation, characterized by localized hanging wall subsidence with new fault scarp formation in the models. The monitoring results showed that water table decline drastically accelerated uneven subsidence. Although the deformation pattern did not respond to variations in the material properties of the hanging wall and footwall, the magnitude of the reactivated normal faulting was influenced. The maximum vertical offset in Model 2 was much larger than that in Model 1 due to a larger compression modulus in the hanging wall and footwall in Model 2. Furthermore, the reactivation mechanism was revealed based on Anderson’s faulting theory. Normal faulting occurs once the maximum principal stress becomes vertical and the intermediate and minimum principal stresses become horizontal. Groundwater pumping increases the effective stress, leading to the addition of vertical stress and the resultant normal fault reactivation. Our findings provide a better understanding of human interactions with the Earth’s surface and are helpful for mitigating faulting-caused disasters.

A Study on the Lateral Load Capacity of a Novel Hybrid Monopile via a Centrifuge Model Test

Large-diameter monopiles that can safely support the lateral loads caused by wind and waves have been widely used for the foundations of offshore wind turbines. However, when the penetration depth is insufficient as a result of the presence of thick soft ground or it is difficult to penetrate rock, the lateral load capacity of the monopiles may be insufficient, leading to structural instability of the wind turbine system. To address this problem, hybrid monopiles have been proposed, which include appendages such as suction buckets attached around the monopiles installed on the seabed. Such suction buckets are arranged in the form of a tripod at 120° intervals with respect to the center of the monopile. These increase the bending resistance of the monopile by sharing the lateral load applied to it. Although the proposed monopiles were presented as conceptual foundation types, their actual support mechanisms and bearing capacity improvement effects must be verified experimentally. In this study, a centrifuge model test was conducted to identify the support mechanism of hybrid monopiles and the degree of improvement in their bearing capacity compared to the existing large-diameter monopiles. The experiment results showed that an appendage composed of suction buckets dispersed the load acting on the monopile, thereby significantly increasing its bearing capacity.

Evaluation of the Behavior of Geogrid-Reinforced Soil Walls Subjected to Rainfall Through Centrifuge Model Tests

Evaluation of the Behavior of Geogrid-Reinforced Soil Walls Subjected to Rainfall Through Centrifuge Model Tests

Large deformation finite element analyses of axially loaded helical piles in cohesive soil

Large deformation finite element (LDFE) analysis is suitable for research on soil-pile interaction. The objectives of this study were to develop an LDFE model for axially loaded double-helix piles and to examine the factors that control their failure modes. The model was implemented with the re-meshing and interpolation technique with a small strain approach. Three types of helical piles in clay modeled as a Tresca material were simulated in an axisymmetric setup. The model was calibrated against the results of centrifuge model tests. Analyses show that the failure mode changed gradually from a cylindrical shear mode (CSM) to a transitional failure mode and then an individual bearing mode (IBM) with increasing inter-helix spacing. Current results suggested that there might not be a unique inter-helix spacing ratio that distinguishes CSM from IBM. Further, the effects of helix embedment depth, inter-helix spacing, and undrained soil shear strength on the failure modes were assessed by parametric analyses. Finally, the failure modes and helix-bearing factors, shown to depend on these parameters, were summarized in several charts for potential practical applications. Results suggested that the lower helix's bearing factors remained unchanged, but the upper helix's bearing factors changed with the embedment depth and failure modes.

Limit Equilibrium Analysis of Landfill Instability Based on Actual Failure Surface

Slope stability is one of the key engineering problems in the whole lifetime of landfills. In this paper, combined with the wedge limit equilibrium analysis, a landfill stability analysis method based on the actual failure surface is proposed, and the model is verified according to the data of centrifuge model tests. It is found that this method can more accurately calculate the factor of safety (FS) of the slope of the landfill and evaluate the stability of the slope. Finally, sensitivity analysis was conducted to evaluate the effects of the actual failure surface angle, water level, soil parameters, and the presence or absence of a dam on the factor of safety.

Centrifuge modeling of slope failure induced by elevated gas pressure in wet municipal solid waste landfill

Understanding triggering mechanisms of slope failure is of great importance to the stability analysis and safety warning of waste landfill. This paper presents a centrifuge model test on slope failure induced by elevated gas pressure in wet landfill. The formation process of liquid level and gas pressure in the landfill is simulated by means of liquid and gas injections under a centrifugal acceleration of 66.7 g. The injected liquid contains surfactants, which allow it to generate foam in the waste pores when mixed with the injected gas. The pore gas and liquid pressures under two-phase flow condition are monitored separately to clarify the instability process. It is found that the continuous gas injection makes the pore gas pressure increase to peak values of 83.0 kPa–100.8 kPa, which are higher than the peak liquid pressures of 61.3 kPa–75.6 kPa. The formation of high gas pressure zone is attributed to the low gas permeability, which is affected by high liquid saturation as well as foam generation. The slope failure occurs when the pore gas pressure increases to the peak value and the corresponding shear strength decreases to the critical value. Although the gas injection raises the liquid level, the factor of safety of landfill slope will be overestimated if only considering the effect of liquid pressure. According to the response curves of displacement to gas pressure rise, the critical ratios of gas pressure to earth pressure are determined to be 0.74–0.84, which fall within the range of the prototype landfill. The difference between foam and air on pore pressure distribution is also calculated and discussed.

Unsaturated slopes behavior under antecedent intermittent rainfall patterns: centrifuge and numerical study

Antecedent rainfall is a prime factor for rainfall-induced landslides on unsaturated slopes. The effects of the intermittent behavior of antecedent rainfall on landslide initiation are uncertain. The work described here had the objective of showing the influence of antecedent intermittent rainfall patterns to predict landslide initiation. Soil slope models prepared from silty sand were tested in centrifuge model testing. At first, soil slopes experienced different antecedent rainfall patterns, namely, uniform gap, decreasing gap, and increasing gap, before they were exposed to continuous rainfall until the failure was initiated. The seepage and deformation behaviors of instrumented slopes were evaluated and back-analyzed with soil–water–air coupled hydromechanical finite element analysis using calibrated material parameters and suitable boundary conditions. The evolution of porewater pressure, displacements, and deviatoric strains was found to provide comparable responses. The analysis of incremental velocity clearly showed that times for landslide initiation follow the order of decreasing gap, uniform gap, and increasing gap antecedent rainfall patterns. The study identified that not only cumulative rainfall, but also antecedent intermittent rainfall patterns have a significant effect as a triggering agent and suggested incorporating it as a parameter for landslide early warning mechanisms.

Centrifugal model test study on instability mechanism by rain of widened roadbed slope

To study the instability mechanism of the slope of the widened highway roadbed during rainfall, a rainfall simulation device was developed for geotechnical centrifugal model testing. Settlement and widening tests for the old roadbed, and settlement and simulation rain tests for the widened roadbed were performed in sequence. During the test, the settling process of the old and the new roadbeds and the instability process of the embankment slope by rainfall were observed by the settling monitoring device installed on the centrifuge manipulator system. The test results showed that the major consolidation of the old roadbed was completed in the first year of operation; after the roadbed was widened, the new settlement of the old roadbed was much smaller than the new settlement of the widened roadbed, resulting in the differential settlement between the new and old roadbeds. Longitudinal cracks formed by the differential settlement on the road surface provided infiltration channels for rainwater. After rainfall, the rainwater first infiltrates downward at the foot of the slope and expands the vertical joints of the loess to form vertical infiltration channels, causing settlement at the foot of the slope. After that, the rainwater infiltrated laterally into the interior of the roadbed and beyond the slope toe, increasing further settlement. The settlement area expanded continuously, eventually causing the roadbed slope to become unstable. According to the research results, the key to preventing rainfall damage to the subgrade was to protect the slope foot, block the rainfall infiltration channel, and reduce the settlement of the foundation.

Mechanism of Seismic Earth Pressure on Braced Excavation Wall Installed in Shallow Soil Depth by Dynamic Centrifuge Model Tests

Mechanism of Seismic Earth Pressure on Braced Excavation Wall Installed in Shallow Soil Depth by Dynamic Centrifuge Model Tests

Liquefaction-induced Soil-Pile Dynamic Interactions in Inclined Ground

Damage from earthquakes is relatively larger on inclined ground rather than on flat ground. Therefore, awareness on the importance of evaluating disaster risks on inclined ground is increasing. In particular, when piles are installed in the ground, the dynamic interaction between the ground and the pile is considerably complex, making it difficult to clearly determine the characteristics of the slope and pile. Therefore, this study numerically analyzed using centrifuge model test data with a laminar shear box and interpreted the lateral boundary of the numerical analysis section as a circulatory boundary. The excess pore water pressure, horizontal displacement of the pile, and input and response accelerations were compared and analyzed using the centrifuge model test in the numerical analysis. Consequently, the proposed numerical analysis technique could well predict the dynamic behavior of a pile installed in inclined ground.

Theoretical framework for predicting accumulation of soil berms and peak sliding resistance for tolerably mobile foundations

Tolerably mobile subsea foundations are designed to slide on the seabed to accommodate flowline thermal expansion and contraction, and are a potential alternative to conventional (fixed) foundations. During the periodic sliding events that occur during operation, soil berms form at the extremities of the foundation footprint. The size of the berm increases throughout the life-cycle of the foundation, leading to increasing peak sliding resistance. This may hinder mobility of the foundation and overstress the pipeline connections that the foundation is designed to support. Equally, the berms may be relied on to reduce sliding and thus minimize settlement of the foundation, which can also overstress pipeline connections. This paper analyses the mechanism leading to berm accumulation and its mobilisation, also addressing periodic remoulding and reconsolidation of the sediment in the berm. A framework is proposed to predict the accumulation of soil berms and the resulting peak sliding resistance, and is validated by eight centrifuge model tests performed on a kaolin clay and a calcareous silt.

Numerical Investigation of Existing Tunnel Deformation Induced by Basement Excavation Considering the Unloading Ratio

Basement excavation may induce deformations of the adjacent tunnels. The response of existing tunnels to basement excavation considering the critical unloading ratio is rarely studied. In this study, a three-dimensional numerical model is established to investigate basement–tunnel interaction. Then, the numerical model is validated by simulating the centrifuge model test. Thereafter, the influences of basement geometry and tunnel location relative to the basement on the vertical deformation of the tunnel are studied. The results show that the vertical deformation of the tunnel increases linearly with the unloading ratio, which describes the degree of excavation depth above the tunnel. But there exists a critical unloading ratio of 0.6, beyond which the vertical deformation of the tunnel increases significantly. On this basis, an empirical model is proposed to predict the vertical deformation of the tunnel considering the unloading ratio.

Seismic performance of sheet-pile wall supporting liquefiable backfill: Blind predictions of centrifuge model tests using Ta-Ger constitutive model

Blind numerical simulations of seven centrifuge tests with “identical” models of a sheet-pile wall supporting medium dense liquefiable backfill and subjected to seismic excitation were performed as part of the LEAP 2020 project. The analyses were conducted with FLAC using the Ta-Ger soil constitutive model. The Ta-Ger model parameters were calibrated against available laboratory DSS data on Ottawa sand with similar relative density with a focus on capturing at an element level: i) the liquefaction triggering resistance, ii) the post-liquefaction rate of shear-strain accumulation, iii) overburden effects on liquefaction triggering resistance and iv) realistic shear stress-strain responses. A single numerical model was built in prototype scale and analyses were performed by only varying the input seismic motion recorded at the base of each of seven centrifuge tests. Comparisons of numerical predictions with measured centrifuge test responses indicate that the analyses successfully capture the primary mechanisms of the system response. These included liquefaction in the free-field and development of negative pore pressures behind the wall, with accurate predictions of outward wall displacement for the majority of the tests. Most centrifuge tests and numerical predictions consistently exhibit a systematic linear trend of increase in wall displacements with spectral acceleration at the predominant frequency of the system. The numerical analyses overpredicted wall displacements only for centrifuge tests not following this trend, indicating that the variations maybe due to experimental variations from their specifications that were not considered in the blind predictions.

Centrifuge modelling of blast effects in dry sand: Coriolis effect and soil arching

Centrifuge model tests are conducted to investigate the dynamic response of dry sand under blast loading. The characteristics and propagation mode of blast waves in dry sand are studied. The Coriolis effect on blast-induced cratering is carefully scrutinised, and both the theoretical and experimental results are provided and agree with each other. In the explosion-induced cratering process, the sand ejecta is subjected to horizontal and vertical Coriolis forces simultaneously; the former directly determines the horizontal motion offset, while the latter affects the particle motion by altering the flight time, and the Coriolis effect on cratering can only be observed apparently for soil ejecta with a relatively small launch angle. Redistribution of the static earth pressure (blast-induced arching effect) in deep-buried, fully confined explosion events under hypergravity is observed. The friction between sand particles is significantly enhanced by the hypergravity to serve as the supporting arch springing. Conceptual analysis is conducted to further reveal the mechanism of the blast-induced arching effect based on the trapdoor test, from the three aspects of displacement mode, stress development and post-detonation stress distribution.

Selection of efficient earthquake intensity measures for evaluating seismic fragility of concrete face rockfill dam

This study explores the crucial role of intensity measure (IM) selection for probabilistic seismic demand model development of concrete face rockfill dam (CFRD) subjected to earthquake ground motions. Nonlinear dynamic analyses are performed for CFRD with an advanced hysteretic soil model using a finite element analysis program to develop a database of earthquake-induced dam crest settlements. The numerical model is validated through centrifuge model test measurements. A selection process was carried out to evaluate the IMs based on the goodness of fit, as well as their efficiency, practicality, and proficiency. Eventually, a range of optimal IMs are selected. Conditional probability function is employed to develop fragility curves for CFRD employing both scalar and vector IMs. Five settlement ratio-based limit states are defined herein. The results show that vector fragility surfaces produce enhanced predictions of the dam damage compared with the scalar fragility curves that are most often employed.

On the determination of cyclic shear stress for soil liquefaction triggering in centrifuge model test

As the dynamic shear stress acting on soil column could not be measured directly during earthquakes, the simplified procedure proposed by Seed and Idriss is used to calculate the earthquake-induced cyclic shear stress based on acceleration record of strong motion station for liquefaction triggering evaluation. However, when this procedure is applied to physical modeling such as centrifuge model test, it will result in overestimation of the cyclic loading because the acceleration is monitored in liquefiable soil deposit and the acceleration time history will be distorted due to the softening process, and the whole time history is counted regardless the triggering of liquefaction or not. To address this problem, a centrifuge model test with long duration shaking was conducted to observe the difference of acceleration response between the soil deposit and the sidewall at the same elevation. And a series of stress-controlled undrained cyclic triaxial tests of the same sand used in centrifuge model test were carried out to obtain the cyclic strength at different elevations in model ground. This study shows that the cyclic loading before the time of liquefaction occurrence instead of the whole time history corresponds to the cyclic strength well, and also the calculation based on acceleration record of sidewall is closer to the cyclic strength than that in the soil. Thus a method is proposed to determine the cyclic shear stress in dynamic centrifuge model test, which takes the acceleration record before the time of liquefaction triggering at the sidewall of laminar container instead of that of the liquefiable soil as the source data to calculate the cyclic shear stress ratio.