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 modeling of buried continuous pipelines subjected to normal faulting

Seismic ground faulting is the greatest hazard for continuous buried pipelines.Over the years,researchers have attempted to understand pipeline behavior mostly via numerical modeling such as the finite element method.The lack of well-documented field case histories of pipeline failure from seismic ground faulting and the cost and complicated facilities needed for full-scale experimental simulation mean that a centrifuge-based method to determine the behavior of pipelines subjected to faulting is best to verify numerical approaches.This paper presents results from three centrifuge tests designed to investigate continuous buried steel pipeline behavior subjected to normal faulting.The experimental setup and procedure are described and the recorded axial and bending strains induced in a pipeline are presented and compared to those obtained via analytical methods.The influence of factors such as faulting offset,burial depth and pipe diameter on the axial and bending strains of pipes and on ground soil failure and pipeline deformation patterns are also investigated.Finally,the tensile rupture of a pipeline due to normal faulting is investigated.

Centrifuge modeling of PGD response of buried pipe

A new centrifuge based method for determining the response of continuous buried pipe to PGD is presented. The physical characteristics of the RPI's 100 g-ton geotechnical centrifuge and the current lifeline experiment split-box are described: The split-box contains the model pipeline and surrounding soil and is manufactured such that half can be offset, in flight, simulating PGD. In addition, governing similitude relations which allow one to determine the physical characteristics, (diameter, wall thickness and material modulus of elasticity) of the model pipeline are presented. Finally, recorded strains induced in two buried pipes with prototype diameters of 0.63 m and 0.95 m (24 and 36 inch) subject to 0.6 and 2.0 meters (2 and 6 feet) of full scale fault offsets and presented and compared to corresponding FE results.

Uplift mechanism for a shallow-buried structure in liquefi able sand subjected to seismic load: centrifuge model test and DEM modeling

Based on a centrifuge model test and distinct element method(DEM), this study provides new insights into the uplift response of a shallow-buried structure and the liquefaction mechanism for saturated sand around the structure under seismic action. In the centrifuge test, a high-speed microscopic camera was installed in the structure model, by which the movements of particles around the structure were monitored. Then, a two-dimensional digital image processing technology was used to analyze the microstructure of saturated sand during the shaking event. Herein, a numerical simulation of the centrifuge experiment was conducted using a two-phase(solid and fl uid) fully coupled distinct element code. This code incorporates a particle-fl uid coupling model by means of a "fi xed coarse-grid" fl uid scheme in PFC3D(Particle Flow Code in Three Dimensions), with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate the uplifts of the shallow-buried structure and the sharp rise in excess pore pressure. The corresponding micro-scale responses and explanations are provided. Overall, the uplift response of an underground structure and the occurrence of liquefaction in saturated sand are predicted successfully by DEM modeling. However, the dynamic responses during the shaking cannot be modeled accurately due to the restricted computer power.

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.

A Comparative Study of the Seismic Performances and Failure Mechanisms of Slopes Using Dynamic Centrifuge Modeling

In this study, dynamic centrifuge model tests were performed for sand slopes under different earthquake ground motions and slope angle to characterize the seismic performance of slopes. Four groups of tests under varying seismic input amplitude were conducted. Under the action of increasing earthquake intensity, the rigidity of the soil decreases and the damping ratio increases, both of the dynamic response and the predominant period of slopes are increased. Three types of seismic waves with the same seismic intensity were applied in the model tests. It shows that the variability in the ground motion leads to the acceleration response spectra of the slopes being completely different and the Northridge seismic wave with low-frequency component is closest to the predominant period of the slope model. In addition, the effect of slope angle on the seismic performance of slopes were also clarified. The results reveal how the slope angle affects the acceleration recorded on the ground surface of the slope, both in terms of the peak ground-motion acceleration(PGA) amplification factor and the predominant period. Finally, the permanent displacement of the model slopes under different earthquake intensities were further analyzed. Based on the nonlinear growth of the permanent displacement of the slope, the test results demonstrated the failure process of the slope, which can further provide a basis for theperformance-based seismic design of slopes.

Geotechnical centrifuge model tests for explosion cratering and propagation laws of blast wave in sand

This paper presents the explosion cratering effects and their propagation laws of blast waves in dry standard sands using a 450 g-t geotechnical centrifuge apparatus.Ten centrifuge model tests were completed with various ranges of explosive mass,burial depth and centrifuge accelerations.Eleven accelerometers were installed to record the acceleration response in sand.The dimensions of the explosion craters were measured after the tests.The results demonstrated that the relationship between the dimensionless parameters of cratering efficiency and gravity scaled yield is a power regression function.Three specific function equations were obtained.The results are in general agreement with those obtained by other studies.A scaling law based on the combination of the π terms was used to fit the results of the ten model tests with a correlation coefficient of 0.931.The relationship can be conveniently used to predict the cratering effects in sand.The results also showed that the peak acceleration is a power increasing function of the acceleration level.An empirical exponent relation between the proportional peak acceleration and distance is proposed.The propagation velocity of blast waves is found to be ranged between 200 and 714 m/s.

Centrifuge model test and field measurement analysis for foundation pit with confined water

The similarity law of centrifuge test was developed for the seepage field and stress field of a foundation pit with confined water by analyzing control equations,and a similarity index and a similarity coefficient of centrifuge test were obtained.Based on the deep foundation pit of the Huangxing Road Station of the Shanghai metro line M8,the deformation stability of the pit was tested.Finally,a comparative study was conducted on the test results of the pit deformation and the field measurement results.Comparison results show that the pit deformation regularity of the test is basically identical with that of the field measurement,and the difference in pit deformation between the test and the field measurement is within 50%.The centrifuge model test can effectively simulate the displacement response of the ground and retaining structure during dewatering and excavation for the pit with confined water,which provides a reliable basis for the design and construction of the pit with confined water.

Centrifuge modeling of tunneling-induced ground surface settlement in sand

Stress changes in the soil induced by tunnel excavation may cause excessive ground settlement.However,high-quality experimental data on ground settlement due to tunnel excavation are limited.In this study,centrifuge tests are conducted to investigate the threedimensional ground surface settlement,considering different intersection angles and cover-to-tunnel diameter ratios.The results indicate that the major influence zone along the longitudinal direction on the ground surface settlement is±1.25D,where D is the tunnel diameter.When the monitoring section is perpendicular to the tunneling direction,the transverse ground settlement due to the tunnel excavation is symmetrical with respect to the tunnel centerline.In contrast,an asymmetric ground settlement profile is observed when the monitoring section intersects the tunneling direction at an angle of 60.Applying a Gaussian curve to fit the ground surface settlement curve,the width parameter,K(i.e.,the distance between the tunnel centerline and the inflection point of the settlement trough to the tunnel burial depth),varies from 0.33 to 0.39.The ground surface settlement induced by twin tunnel excavation can be captured reasonably by superimposing two identical Gaussian curves.When the cover to tunnel diameter ratios(C/D)are 1.5 and 2.7,the maximum ground surface settlements are 0.67%of D and 0.35%of D,respectively.It is clear that the maximum ground surface settlement decreases with an increase in the C/D ratio.

Evaluation of seismic behavior of box culvert buried in the ground through centrifuge model tests and numerical analysis

Japan has yet to establish a seismic design for road box culverts(RBCs)because past major earthquakes did not damage them.In recent years,structures with enlarged sectional dimensions(the purpose of which is to optimize the internal space in RBCs)have become common.However,the unknown seismic strength of such large RBCs makes RBC seismic designs increasingly necessary.Several seismic design methods have been developed for and applied to rectangular underground structures,e.g.,cut-and-cover tunnels,which are structurally similar to RBCs.Although these methods are applicable to RBCs,it is uncertain whether they can be applied directly because there are currently no evaluation results from model tests on RBCs,which have unique structural features,e.g.,no haunch at the bottom of the sidewalls.Therefore,we verify a seismic behavior of an RBC and develop a method for evaluating it in order to establish a seismic design for RBCs.We conducted centrifuge model tests subjected to seismic force and a numerical analysis using an elastoplastic finite analysis method,in addition,we validated this analysis by comparing the test and analytical results.The test results show RBCs will most likely develop rocking rotation when the ground strain exceeds approximately 0.08%.A comparison of the numerical and experimental results shows that this analysis can estimate the shear deformation behavior with approximately 90%accuracy in square cross-section cases.Meanwhile,this analysis has tendency to underestimate the axial forces in each case and to overestimate the bending moments of some members in the case of wide cross-sections.

Centrifuge modelling of tunnelling below existing twin tunnels with different types of support

Tunnel excavation below existing tunnels produces ultimate and serviceability problems to the existing tunnels.The behaviours of induced stresses on the existing tunnels haven’t yet been fully recognized.In this study,a centrifuge model test was adopted to investigate the effects of new tunnelling on two existing overlying tunnels.One existing tunnel model simulated a prototype composite lining tunnel and the other simulated a prototype segmental lining tunnel.The volume loss produced by new tunnel excavation was modelled by an in-flight actuator system.The surface settlements,the existing tunnels settlements,the soil pressures on existing tunnels,the bending movements of existing tunnels,and the joint behaviours of existing tunnels were monitored.The volume of surface settlement trough was much smaller than the soil volume moving into the tunnel,due to the heave of the tunnel bottom and the dilation of sand during shearing.The maximum settlement of the segmental lining model was larger than that of the composite lining model as the equivalent bending stiffness of the composite lining model was larger than that of the former.Due to new tunnel excavation,the soil pressures on different positions of the existing tunnel behaved differently,and the bending movements of the existing tunnels decreased.Moreover,the joint deformation of existing tunnel caused by new tunnel excavation could be classified into three types:(1)translation,(2)rotation,and(3)combination of both.

Physical modeling and visualization of soil liquefaction under high confining stress

The mechanisms of seismically-induced liquefaction of granular soils underhigh confining stresses are still not fully understood. Evaluation of these mechanisms is generallybased on extrapolation of observed behavior at shallow depths. Three centrifuge model tests wereconducted at RPI's experimental facility to investigate the effects of confining stresses on thedynamic response of a deep horizontal deposit of saturated sand. Liquefaction was observed at highconfining stresses in each of the tests. A system identification procedure was used to estimate theassociated shear strain and stress time histories. These histories revealed a response marked byshear strength degradation and dilative patterns. The recorded accelerations and pore pressures wereemployed to generate visual animations of the models. These visualizations revealed a liquefactionfront traveling downward and leading to large shear strains and isolation of upper soil layers.

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 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.

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.

New approach to assess sperm DNA fragmentation dynamics： Fine-tuning mathematical models

Background： Sperm DNA fragmentation（sDF） has been proved to be an important parameter in order to predict in vitro the potential fertility of a semen sample. Colloid centrifugation could be a suitable technique to select those donkey sperm more resistant to DNA fragmentation after thawing. Previous studies have shown that to elucidate the latent damage of the DNA molecule, sDF should be assessed dynamically, where the rate of fragmentation between treatments indicates how resistant the DNA is to iatrogenic damage. The rate of fragmentation is calculated using the slope of a linear regression equation. However, it has not been studied if s DF dynamics fit this model. The objectives of this study were to evaluate the effect of different after-thawing centrifugation protocols on sperm DNA fragmentation and elucidate the most accurate mathematical model（linear regression, exponential or polynomial） for DNA fragmentation over time in frozen-thawed donkey semen.Results： After submitting post-thaw semen samples to no centrifugation（UDC）, sperm washing（SW） or single layer centrifugation（SLC） protocols, sD F values after 6 h of incubation were significantly lower in SLC samples than in SW or UDC.Coefficient of determination（R-2） values were significantly higher for a second order polynomial model than for linear or exponential. The highest values for acceleration of fragmentation（aSDF） were obtained for SW, fol owed by SLC and UDC.Conclusion： SLC after thawing seems to preserve longer DNA longevity in comparison to UDC and SW. Moreover,the fine-tuning of models has shown that sDF dynamics in frozen-thawed donkey semen fit a second order polynomial model, which implies that fragmentation rate is not constant and fragmentation acceleration must be taken into account to elucidate hidden damage in the DNA molecule.

Tunnel flexibility effect on the ground surface acceleration response

Flexibility of underground structures relative to the surrounding medium, referred to as the flexibility ratio, is an important factor that influences their dynamic interaction. This study investigates the flexibility effect of a box-shaped subway tunnel, resting directly on bedrock, on the ground surface acceleration response using a numerical model verified against dynamic centrifuge test results. A comparison of the ground surface acceleration response for tunnel models with different flexibility ratios revealed that the tunnels with different flexibility ratios influence the acceleration response at the ground surface in different ways. Tunnels with lower flexibility ratios have higher acceleration responses at short periods, whereas tunnels with higher flexibility ratios have higher acceleration responses at longer periods. The effect of the flexibility ratio on ground surface acceleration is more prominent in the high range of frequencies. Furthermore, as the flexibility ratio of the tunnel system increases, the acceleration response moves away from the free field response and shifts towards the longer periods. Therefore, the flexibility ratio of the underground tunnels influences the peak ground acceleration （PGA） at the ground surface, and may need to be considered in the seismic zonation of urban areas.

Identification and imaging of soil and soil-pile deformation in the presence of liquefaction

A simple identification technique is developed to visualize the dynamic deformation mechanisms of centrifuge models of saturated soil and soil-pile systems using the measurements provided by sparsely distributed sensors. Crosscorrelation analyses are employed first to assess the variation of shear wave velocity profile with time as soil experiences stiffness reduction and degradation during dynamic excitations. The corresponding time-dependent modal configurations are determined using the finite-element technique. These configurations are used along with recorded motions to evaluate optimal time histories of displacement and strain fields based on a spectral motion reconstruction. Visualizations of the response of infinite slope and soil-pile centrifuge models revealed salient and complex multi-dimensional deformation patterns, especially at high pore pressure ratios. The developed technique provides an effective tool to visualize and analyze the dynamic response of centrifuge, shake-table and field soil systems.

Innovative mitigation method for buried pipelines crossing faults

A new remediation technique is proposed to mitigate large deformations imposed on buried pipeline systems subject to permanent ground deformation.With this technique,low-density gravel(LDG)with high porosity,such as pumice,is used as backfill in the trench containing the pipe near an area susceptible to PGD.This countermeasure decreases soil resistance,soil-pipe interaction forces and strain on the pipe as the pipeline deformation mechanism changes to a more desirable shape.Expanded polystyrene geofoam has been introduced to decrease the density of the pipeline backfill;however,LDG is more efficient regarding workability during construction,environmental effects,durability,fire safety,and cost-effectiveness.A series of centrifuge model experiments in which the pipelines were subjected to reverse faulting was conducted to evaluate the proposed method.During faulting,the axial and bending strain and pipe deflection were measured.A comparison of the responses of the remediated pipeline and the pipeline without remediation indicates that the proposed technique substantially mitigates the effects of large deformation.