Numerical analysis on seismic performance of underground structures in liquefiable interlayer sites from centrifuge shaking table test

When an underground structure passes through a liquefiable soil layer,the soil liquefaction may pose a significant threat to the structure.A centrifuge shaking table test was performed to research the seismic response of underground structures in liquefiable interlayer sites,and a valid numerical model was obtained through simulation model test.Finally,the calibrated numerical model was used to perform further research on the influence of various distribution characteristics of liquefiable interlayers on the seismic reaction of underground structures.The key findings are as follows.The structure faces the most unfavorable condition once a liquefiable layer is located in the middle of the underground structure.When a liquefiable layer exists in the middle of the structure,the seismic reactions of both the underground structure and model site will increase with the rise of the thickness of the liquefiable interlayer.The inter-story drift of the structure in the non-liquefiable site is much smaller than that in the liquefiable interlayer site.The inter-story drift of the structure is not only associated with the site displacement and the soil-structure stiffness ratio but also closely associated with the slippage of the soil-structure contact interface under the condition of large deformation of the site.

Centrifugal model tests and numerical simulations for barrier dam break due to overtopping

The present study focuses on the breaching process and failure of barrier dams due to overtopping. In this work, a series of centrifugal model tests is presented to examine the failure mechanisms of landslide dams. Based on the experimental results, failure process and mechanism of barrier dam due to overtopping are analyzed and further verified by simulating the experimental overtopping failure process. The results indicate that the barrier dam will develop during the entire process of overtopping in the width direction, whereas the breach will cease to develop at an early stage in the depth direction because of the large particles that accumulate on the downstream slope. Moreover, headcut erosion can be clearly observed in the first two stages of overtopping, and coarsening on the downstream slope occurs in the last stage of overtopping. Thus, the bottom part of the barrier dam can survive after dam breaching and full dam failure becomes relatively rare for a barrier dam. Furthermore, the remaining breach would be smaller than that of a homogeneous cohesive dam under the same conditions.

Failure behavior of soil-rock mixture slopes based on centrifuge model test

The stability of soil-rock mixtures(SRMs) that widely distributed in slopes is of significant concern for slope safety evaluation and disaster prevention. The failure behavior of SRM slopes under surface loading conditions was investigated through a series of centrifuge model tests considering various volumetric gravel contents. The displacement field of the slope was determined with image-based displacement system to observe the deformation of the soil and the movement of the block during loading in the tests. The test results showed that the ultimate bearing capacity and the stiffness of SRM slopes increased evidently when the volumetric block content exceeded a threshold value. Moreover, there were more evident slips around the blocks in the SRM slope. The microscopic analysis of the block motion showed that the rotation of the blocks could aggravate the deformation localization to facilitate the development of the slip surface. The high correlation between the rotation of the key blocks and the slope failure indicated that the blocks became the dominant load-bearing medium that influenced the slope failure. The blocks in the sliding body formed a chain to bear the load and change the displacement distribution of the adjacent matrix sand through the block rotation.

Centrifuge model tests on pile-reinforced slopes subjected to drawdown

Piles are generally an effective way to reduce the risk of slope failure.However,previous approaches for slope stability analysis did not consider the effect of the piles coupled with the decrease of the water level(drawdown).In this study,a series of centrifuge model tests was performed to understand the deformation and failure characteristics of slopes reinforced with various pile layouts.In the centrifuge model tests,the pile-reinforced slopes exhibited two typical failure modes under drawdown conditions:across-pile failure and through-pile failure.In the through-pile slope failure,a discontinuous slip surface was observed,implying that the assumption of the slip surface in previous stability analysis methods was unreasonable.The test results showed that drawdown led to instability of the piles in cohesive soil,as the saturated cohesive soil failed to provide sufficient constraint for piles.The slope exhibited progressive failure,from top to bottom,during drawdown.The deformation of the piles was reduced by increasing the embedment depth and row number of piles.In addition,the deformation of soils outside the piles was influenced by the piles and showed a similar distribution shape as the piles,and the similarity degree weakened as the distance from the piles increased.This study also found that the failure mechanism of unreinforced and pile-reinforced slopes induced by drawdown could be described by coupling between the deformation localization and local failure,and it revealed that pile-reinforced slopes could reduce slope deformation localization during drawdown.

Effectiveness of Fiber Bragg Grating monitoring in the centrifugal model test of soil slope under rainfall conditions

Centrifugal model testsare playing an increasingly importantrolein investigating slope characteristics under rainfall conditions. However, conventional electronic transducers usually fail during centrifugal model tests because of the impacts of limitedtest space, high centrifugal force, and presence of water, with the result that limited valid data is obtained. In this study, Fiber Bragg Grating(FBG) sensing technology is employed in the design and development of displacement gauge, an anchor force gauge and an anti-slide pile moment gauge for use on centrifugal model slopes with and without a retaining structure. The two model slopes were installed and monitored at a centrifugal acceleration of 100 g. The test results show that the sensors developed succeed in capturing the deformation and retaining structure mechanical response of the model slopes during and after rainfall. The deformation curvefor the slope without retaining structure shows a steepresponse that turns gradualfor the slope with retaining structure. Importantly, for the slope with the retaining structure, results suggest that more attention be paid to increase of anchor force and antislide pile moment during rainfall. This study verifies the effectiveness of FBG sensing technology in centrifuge research and presents a new and innovative method for slope model testing under rainfall conditions.

Centrifugal model test on a riverine landslide in the Three Gorges Reservoir induced by rainfall and water level fluctuation

Frequent soil landslide events are recorded in the Three Gorges Reservoir area,China,making it necessary to investigate the failure mode of such riverside landslides.Geotechnical centrifugal test is considered to be the most realistic laboratory model,which can reconstruct the required geo-stress.In this study,the Liangshuijing landslide in the Three Gorgers Reservoir area is selected for a scaled centrifugal model experiment,and a water pump system is employed to retain the rainfall condition.Using the techniques of digital photography and pore water pressure transducers,water level fluctuation is controlled,and multi-physical data are thus obtained,including the pore water pressure,earth pressure,surface displacement and deep displacement.The analysis results indicate that:Three stages were set in the test(waterflooding stage,rainfall stage and drainage stage).Seven transverse cracks with wide of 1–5 mm appeared during the model test,of which 3 cracks at the toe landslide were caused by reservoir water fluctuation,and the cracks at the middle and rear part were caused by rainfall.During rainfall process,the maximum displacement of landslide model reaches 3 cm.And the maximum deformation of the model exceeds 12 cm at the drainage stage.The failure process of the slope model can be divided into four stages:microcracks appearance and propagation stage,thrust-type failure stage,retrogressive failure stage,and holistic failure stage.When the thrust-type zone caused by rainfall was connected or even overlapped with the retrogressive failure zone caused by the drainage,the landslide would start,which displayed a typical composite failure pattern.The failure mode and deformation mechanism under the coupling actions of water level fluctuation and rainfall are revealed in the model test,which could appropriately guide for the analysis and evaluation of riverside landslides.

A modified generalized scaling law for the similitude of dynamic strain in centrifuge modeling

Soil strain is the key parameter to control the elasto-plastic deformation and even the failure processes.To overcome the defect that the strain of the model soil is always smaller than that of the prototype in Iai′s generalized scaling law(GSL),a modified scaling law was proposed based on Iai′s GSL to secure the same dynamic shear strain between the centrifuge model and the prototype by modulating the amplitude and frequency of the input motion at the base.A suite of dynamic centrifuge model tests of dry sand level ground was conducted with the same overall scaling factor(λ=200)under different centrifugal accelerations by using the technique of“modeling of models”to validate the modified GSL.The test results show that the modified GSL could achieve the same dynamic strain in model as that of the prototype,leading to better modeling for geotechnical problems where dynamic strain dominates the response or failure of soils.Finally,the applicability of the proposed scaling law and possible constraints on geometry scaling due to the capability limits of existing centrifuge shaking tables are discussed.

Centrifuge model test of an irrigation-induced loess landslide in the Heifangtai loess platform, Northwest China

The Heifangtai platform in Northwest China is famous for irrigation-induced loess landslides.This study conducted a centrifuge model test with reference to an irrigation-induced loess landslide that occurred in Heifangtai in 2011.The loess slope model was constructed by whittling a cubic loess block obtaining from the landslide site.The irrigation water was simulated by applying continuous infiltration from back of the slope.The deformation,earth pressure,and pore pressure were investigated during test by a series of transducers.For this particular study,the results showed that the failure processes were characterized by retrogressive landslides and cracks.The time dependent reductions of cohesion and internal friction angle at basal layer with increasing pore-water pressure were responsible for these failures.The foot part of slope is very important for slope instability and hazard prevention in the study area,where concentration of earth pressure and generation of high pore-water pressures would form before failures.The measurements of earth pressure and pore-water pressure might be effective for early warning in the study area.

Centrifugal model test and numerical simulation of vertical earth pressure on soft foundation box culvert

To obtain the vertical earth pressure on a soft foundation box culvert and investigate the interaction of the soil-culvert-foundation system, both a centrifugal model test and a numerical simulation were conducted and the comparisons with the current methods to determine the load on a culvert were completed. The results of the model test and numerical analysis are in satisfactory agreement, which shows that the direction of the shear stress between the culvert and the adjacent embankment depends on the differential settlement between them. A vertical earth pressure concentration appears on the culvert with a rigid piles foundation because of a downward shear stress. The ratio of the load on a soft foundation culvert and the overburden pressure above the culvert raises first and then decreases as the backfill height increases. In order to reduce the load on a culvert, it is suggested to limit the stiffness difference of the foundations under the culvert and embankment and to use a light backfill over the culvert.

Effect of the sloping seabed on 3D soil-spudcan interaction using a material point-finite element(MPM-FEM)model

The sloping seabed affects the bearing capacity and failure mechanism of soil,which may compromise the stability and safety of offshore structures such as jack-up platforms.This paper employs a coupled model combining the material point method and finite element method(MPM-FEM)to analyze the impact of sloping seabeds on the three-dimensional soil-spudcan interaction.The MPM-FEM model implements the B¯approach to solve the challenge of volumetric locking due to the incompressibility constraints imposed by yield criterion.It is validated against the centrifuge results.The effects of sloping seabeds on penetration resistance,soil flow pattern,lateral response,stress distribution,and failure mechanism are discussed.The soil mainly undergoes overall failure when the ratio of penetration depth to spudcan diameter(i.e.D P/D)is between 0 and 0.25.As the slope angle increases,the soil on the side of lower slope is expelled further,resulting in an asymmetric stress distribution and a larger horizontal sliding force of soil.When D P/D increases to 0.75,the soil transitions to localized plastic flow failure,and the range of soil flow affected by the spudcan penetration decreases.The results show that,when the slope angle increases,the lateral displacement and stress distribution on the lower slope of a sloping seabed is significantly larger than that of a horizontal seabed,impacting the spudcan and surrounding soil behavior.The study suggests that the seabed slope significantly affects the range of soil flow and failure at shallow penetration,indicating that the slope angle should be taken into account in the design and installation of offshore jack-up rigs,particularly in areas with sloping seabeds.

Modelling Design and Test of Steam Centrifugal Compressor

This paper discusses the modeling design and accuracy analysis of a single stage steam centrifugal compressor for China′S first seawater desalinization plant, and gives a brief presentation on the closed loop vapour test platform designed and constructed for qualification of the compress or for performance and operational reliability, and results of tests run with the compressor. Both tests and actual operation show the modeling design is successful, and the compressor achieved and exceeded what was expected in performance. The seawater desalinization plant has run with excellent performance and reliability for several years for the national seawater desalinization demonstration program. This proves it is practical and practicable to develop and design China′s steam centrifugal compressors in series from air centrifugal compressors by maintaining equal specific volume ratio at both inlet and outlet of the compressor.

Centrifuge model tests of the formation mechanism of coarse sand debris flow

Using the self-developed visualization test apparatus, centrifuge model tests at 20 g were carried out to research the macro and microscopic formation mechanism of coarse sand debris flows. The formation mode and soil-water interaction mechanism of the debris flows were analyzed from both macroscopic and microscopic points of view respectively using high digital imaging equipment and micro-structure analysis software Geodip. The test results indicate that the forming process of debris flow mainly consists of three stages, namely the infiltration and softening stage, the overall slide stage, and debris flow stage. The essence of simulated coarse sand slope forming debris flow is that local fluidization cause slope to wholly slide. The movement of small particles forms a transient stagnant layer with increasing saturation, causing soil shear strength lost and local fluidization. When the driving force of the saturated soil exceeds the resisting force, debris flow happens on the coarse sand slope immediately.

Centrifuge Model Tests and Numerical Simulations of the Impact of Underwater Explosion on an Air-Backed Steel Plate

Damage and threats to hydraulic and submarine structures by underwater explosions(UNDEXs)have raised much attention.The centrifuge model test,compared to prototype test,is a more promising way to examine the problem while reducing cost and satisfying the similitude requirements of both Mach and Froude numbers simultaneously.This study used a systematic approach employing centrifuge model tests and numerical simulations to investigate the effects of UNDEXs on an air-backed steel plate.Nineteen methodical centrifuge tests of UNDEXs were conducted.The shock wave pressure,bubble oscillation pressure,acceleration and the strain of the air-backed steel plate were recorded and compared with numerical studies using the finite element analysis(FEA)commercial software ABAQUS.By implementing empirically derived and physically measured pressures into the numerical models,the effects of the shock wave and bubble oscillation on the steel plate were investigated.Generally,the numerical results were in agreement with the experimental results.These results showed that the peak pressure of an UNDEX has a significant effect on the peak acceleration of the steel plate and that the impulse of the UNDEX pressure governs the peak strain of the steel plate.

Centrifuge model test and numerical interpretation of seismic responses of a partially submerged deposit slope

Partially submerged deposit slopes are o ften encountered in practical engineering applications.Howeve r,studies on evaluating their stability under seismic loading are still rare.In order to understand the seismic behavior of partially submerged deposit slopes,centrifuge shaking table model tests(50 g) were employed.The responses of horizontal accelerations,accumulated excess pore pressures,deformation mode,and failure mode of the partially submerged deposit slope model were analyzed.In dynamic centrifuge model tests,EQ5 shaking event was applied numerically.The results indicated that in the saturated zone of the deposit slope,liquefaction did not occur,and the measured horizontal accelerations near the water table were amplified as a layer-magnification effect.It was also shown that the liquefaction-resistance of the deposit slope increased under multiple sequential ground motions,and the deformation depth of the deposit slope induced by earthquake increased gradually with increasing dynamic Ioad amplitude.Except for the excessive crest settlement generated by strong shaking,an additional vertical permanent displacement was initiated at the slope crest due to the dissipation of excess pore pressure under seismic loading.The result of particle image velocimetry(PIV) analysis showed that an obvious internal arc-slip was generated around the water table of the partially submerged deposit slope under seismic loading.

Centrifugal Model Tests on Railway Embankments of Expansive Soils

Based on the centrifugal model tests on railway embankments of expansive soil in Nanning Kunming railway,the author studied several embankments under different physical conditions. The stress and strain states and settlement of the embankments were analyzed, and the obtained results can be used as a reference to field construction.

Centrifugal Model Tests on the Settlement of Railway Embankment on Deep,Completely Decomposed Granite Soil

Settlement control of high-speed railways is a key technology in embankment engineering. In order to reveal the engineering characteristics of the deep, completely decomposed granite soil in the Hainan East Ring Railway, four groups of centrifuge model tests were conducted. We studied the settlement properties, under the embankment action, of untreated subsoil, subsoil treated by dynamic compaction, and subsoil reinforced with cement-mixed piles. In particular, we examined the relationship between settlement and time, including the settlement during and after construction. The results show that the Weibull model can describe the relationship between embankment settlement and time well, and that the post-construction settlements of the subsoil meet the requirements of the relevant code. Among the two foundation treatment measures, dynamic compaction is more effective than reinforcement with cement-mixed piles. The tested pressure on the contact surface between embankment and subsoil was obviously different from the commonly used calculated values.

CENTRIFUGAL TEST STUDY ON THE BEHAVIOR OF SOFT CLAY AT SEA BOTTOM UNDER THE ACTION OF WAVE FORCE

The importance of studying the behavior of the soil at the sea bottom under the action of wave force has arisen with the development of offshore engineering.In this paper,the behavior of the soft clay under the action of wave forces is studied by performing centrifugal tests.The soil profile and the wave characters were simulated in the centrifugal model cell according to the typical environmental conditions of the oil fields in the Bohai gulf.Test results show that the soft clay layer will be seriously softened near the upper surface under the maximum wave height and slightly affected in the deeper layer,and that no liquefaction was recorded in the silty sand sublayer during the test.It is proven that the centrifugal test is a valid technique for simulating the interaction between soil and wave.

Numerical modeling of centrifuge cyclic lateral pile load experiments

To gain insight into the inelastic behavior of piles, the response of a vertical pile embedded in dry sand and subjected to cyclic lateral loading was studied experimentally in centrifuge tests conducted in Laboratoire Central des Ponts et Chaussees. Three types of cyclic loading were applied, two asymmetric and one symmetric with respect to the unloaded pile. An approximately square-root variation of soil stiffness with depth was obtained from indirect in-flight density measurements, laboratory tests on reconstituted samples, and well-established empirical correlations. The tests were simulated using a cyclic nonlinear Winkler spring model, which describes the full range of inelastic phenomena, including separation and re-attachment of the pile from and to the soil. The model consists of three mathematical expressions capable of reproducing a wide variety of monotonic and cyclic experimentalp-y curves. The physical meaning of key model parameters is graphically explained and related to soil behavior. Comparisons with the centrifuge test results demonstrate the general validity of the model and its ability to capture several features of pile-soil interaction, including： soil plastification at an early stage of loading, ＂pinching＂ behavior due to the formation of a relaxation zone around the upper part of the pile, and stiffness and strength changes due to cyclic loading. A comparison of the p-y curves derived from the test results and the proposed model, as well as those from the classical curves of Reese et al. （1974） for sand, is also presented.

Influence of pile spacing on seismic response of piled raft in soft clay: centrifuge modeling

In order to study the infl uence of pile spacing on the seismic response of piled raft in soft clay, a series of shaking table tests were conducted by using a geotechnical centrifuge. The dynamic behavior of acceleration, displacement and internal forces was examined. The test results indicate that the seismic acceleration responses of models are generally greater than the surrounding soil surface in the period ranges of 2–10 seconds. Foundation instant settlements for 4×4 and 3×3 piled raft (with pile spacing equal to 4 and 6 times pile diameter) are somewhat close to each other at the end of the earthquake, but reconsolidation settlements are greater for 3×3 piled raft. The seismic acceleration of superstructure, the uneven settlement of the foundation and the maximum bending moment of pile are relatively lower for 3×3 piled raft. Successive earthquakes lead to the softening behavior of soft clay, which causes a reduction of the pile bearing capacity and thus loads are transferred from the pile group to the raft. For the case of a 3×3 piled raft, there is relatively smaller change of the load sharing ratio of the pile group and raft after the earthquake and the distribution of maximum bending moments at the pile head is more uniform.

Numerical Analysis and Centrifuge Modeling of Shallow Foundations

The influence of non-coaxial constitutive model on predictions of dense sand behavior is investigated in this paper. The non-coaxial model with strain softening plasticity is applied into finite-element program ABAQUS, which is first used to predict the stress-strain behavior and the non-coaxial characteristic between the orientations of the principal stress and principal plastic strain rate in simple shear tests. The model is also used to predict load settlement responses and bearing capacity factors of shallow foundations. A series of centrifuge tests for shallow foundations on saturated dense sand are performed under drained conditions and the test results are compared with the corresponding numerical results. Various footing dimensions, depths of embedment, and footing shapes are considered in these tests. In view of the load settlement relationships, the stiffness of the load-displacement curves is significantly affected by the non-coaxial model compared with those predicted by the coaxial model, and a lower value of non-coaxial modulus gives a softer response. Considering the soil behavior at failure, the coaxial model predictions of bearing capacity factors are more advanced than those of centrifuge test results and the non-coaxial model results;besides, the non-coaxial model gives better predictions. The non-coaxial model predictions are closer to those of the centrifuge results when a proper non-coaxial plastic modulus is chosen.