Model Tests and Numerical Simulations of Liquefaction and Lateral Spreading

This paper describes the specifications developed by and distributed to all of the centrifuge test facilities involved in LEAP-UCD-2017. The specified experiment consisted of a submerged medium dense clean sand with a 5-degree slope subjected to 1 Hz ramped sine wave base motion in a rigid container. This document describes the detailed geometry, sensor locations, methods of preparation, quality control, shaking motions, surface markers, and surface survey techniques.

Ottawa F-65 sand (supplied by US Silica, Ottawa, Illinois) was selected as the standard sand for LEAP-UCD-2017. Between December 2017 and February 2018, each LEAP research team sent 500 g samples of sand to UC Davis for grain size analysis and minimum and maximum dry density testing. The purpose of this testing was to confirm the consistency of the sand used at various test sites and to provide updated minimum and maximum density index values. The variation of measured properties among the different samples is similar to the variation measured during repeat testing of the same sample. Modified LEAP procedures to measure index densities are used to confirm consistency of the sands, and the results from these procedures are compared to results from ASTM procedures. The LEAP procedures give repeatable results with median index densities of ρmin = 1457 kg/m3, ρmax = 1754 kg/m3. Relative densities calculated with facility-specific index densities varied by less than 4%, so we conclude that average index densities from all the sites may be used for analysis of the results. The LEAP procedures are easier to perform than the ASTM procedures and do not require specialized equipment; therefore, continued use of the LEAP procedure for frequent quality control purposes is recommended. However, the values from ASTM procedures are expected to be more consistent with values adopted in liquefaction literature in the past; therefore, we recommend using the median ASTM values for analysis of LEAP data. Index densities from ASTM procedures (ρmin = 1490.5 kg/m3, ρmax = 1757.0 kg/m3) produce relative densities that are 4 –10% smaller than the index densities from the LEAP procedures.

This paper presents the results of soil characterization and element tests of Ottawa F65 sand. The data presented is intended to be used as calibration material for the prediction exercise conducted as part of the Liquefaction Experiments and Analysis Project (LEAP 2017). The databank generated includes soil specific gravity tests, particle size analysis, hydraulic conductivity tests, maximum and minimum void ratio tests, and cyclic triaxial stress-controlled tests. An effort was made to ensure the consistency and repeatability of the test results by reducing the sources of variability in the sample preparations and increasing the number of tests. The uniformity of the soil was evaluated by conducting tests on samples from five different batches. The results showed that the sand is uniform among the five batches. Due to significant variability in previously reported maximum and minimum void ratio results, the effects of the test operator were studied by comparing test results obtained from three different operators. For the triaxial tests, a constant height dry pluviation method was used for sample preparation. To eliminate the effect of the human error in maintaining a constant drop height and to ensure consistency of the sand fabric between different samples, a device was developed to facilitate the sample preparation. The cyclic triaxial experiments were performed using three different soil densities, and a liquefaction strength curve was obtained for each density based on a 2.5% single amplitude axial strain criteria. The developed databank in this study was made publicly available for the community through DesignSafe.

This paper compares experimental results from every facility for LEAP-UCD-2017. The specified experiment consisted of a submerged medium-dense clean sand with a 5-degree slope subjected to 1 Hz ramped sine wave base motion in a rigid container. The ground motions and soil density were intentionally varied from experiment to experiment in hopes of defining the slope of the relational trend between response (e.g., displacement, pore pressure), intensity of shaking, and density or relative density. This paper is also intended to serve as a useful starting point for overview of the experimental results and to help others find specific experiments if they want to select a subset for further analysis. The results of the experiments show significant differences between each other, but the responses show a significant correlation, R2 ~ 0.7–0.8, to the known variation of the input parameters.

This paper describes how the LEAP-UCD-2017 data is organized in DesignSafe; it is intended to help users, archivers, and curators find or organize data of interest. Several key files, folders, and documents included in the archive are discussed: (1) an Excel format data template used to document much of the data and metadata for each model (sensor data, cone penetrometer data, and surface marker data as reported by the experimenters), (2) processed sensor data files with time offsets and zero and calibration corrections that facilitate comparison of consistently formatted data from various model tests, (3) plots of processed data for quick overview and comparison of results among experiments, and (4) photographs taken during construction and testing.

A cone penetrometer was specifically designed for the LEAP project to provide an assessment of centrifuge model densities independent from mass and volume measurements. This paper presents the design of the CPT and analyses of the results. Due to uncertainty in the specifications about how to define zero depth of penetration, about 20% of the CPT profiles were corrected to produce more accurate results. The procedure for depth correction is explained. After these corrections, penetration resistances at the representative depths of 1.5, 2, 2.5, and 3 m (prototype depth) are correlated to the reported specimen dry densities by linear regression. Using the inverse form of the linear regression equations, the density of each specimen could be estimated from the penetration resistance. Kutter et al. (LEAP-UCD-2017 comparison of centrifuge test results. In Model Tests and Numerical Simulations of Liquefaction and Lateral Spreading: LEAP-UCD-2017, 2019b) found that the density determined from penetration resistance was a more reliable predictor of liquefaction behavior than the reported density itself. Finally, the centrifuge tests at different LEAP facilities modeled the same prototype in different containers using different length scale factors (1/20 to 1/44); thus the ratio of layer thickness to cone diameter was different in each experiment. It appears that the penetration resistances are noticeably affected by container width and, to a lesser extent, resistance is affected by the length scale factor.

The experimental results of LEAP (Liquefaction Experiments and Analysis Projects) centrifuge test replicas of a saturated sloping deposit are used to assess the sensitivity of soil accelerations to variability in input motion and soil deposition. A difference metric is used to quantify the dissimilarities between recorded acceleration time histories. This metric is uniquely decomposed in terms of four difference component measures associated with phase, frequency shift, amplitude at 1 Hz, and amplitude of frequency components higher than 2 Hz (2 + Hz). The sensitivity of the deposit response accelerations to differences in input motion amplitude at 1 Hz and 2 + Hz and cone penetration resistance (used as a measure reflecting soil deposition and initial grain packing condition) was obtained using a Gaussian process-based kriging. These accelerations were found to be more sensitive to variations in cone penetration resistance values than to the amplitude of the input motion 1 Hz and 2 + Hz (frequency) components. The sensitivity functions associated with this resistance parameter were found to be substantially nonlinear.

This paper summarizes the guidelines provided to the numerical simulation/prediction teams that participated in the LEAP-2017 prediction exercise. These guidelines are developed for the Phase 1 of the simulations that focused on the use of cyclic triaxial element tests for calibration of constitutive models that the participating teams used in their numerical simulations/predictions.

This paper presents a summary of the element test simulations (calibration simulations) submitted by 11 numerical simulation (prediction) teams that participated in the LEAP-2017 prediction exercise. A significant number of monotonic and cyclic triaxial (Vasko, An investigation into the behavior of Ottawa sand through monotonic and cyclic shear tests. Masters Thesis, The George Washington University, 2015; Vasko et al., LEAP-GWU-2015 Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019) and direct simple shear tests (Bastidas, Ottawa F-65 Sand Characterization. PhD Dissertation, University of California, Davis, 2016) are available for Ottawa F-65 sand. The focus of this element test simulation exercise is to assess the performance of the constitutive models used by participating team in simulating the results of undrained stress-controlled cyclic triaxial tests on Ottawa F-65 sand for three different void ratios (El Ghoraiby et al., LEAP 2017: Soil characterization and element tests for Ottawa F65 sand. The George Washington University, Washington, DC, 2017; El Ghoraiby et al., LEAP-2017 GWU Laboratory Tests. DesignSafe-CI, Dataset, 2018; El Ghoraiby et al., Physical and mechanical properties of Ottawa F65 Sand. In B. Kutter et al. (Eds.), Model tests and numerical simulations of liquefaction and lateral spreading: LEAP-UCD-2017. New York: Springer, 2019). The simulated stress paths, stress-strain responses, and liquefaction strength curves show that majority of the models used in this exercise are able to provide a reasonably good match to liquefaction strength curves for the highest void ratio (0.585) but the differences between the simulations and experiments become larger for the lower void ratios (0.542 and 0.515).

This paper presents comparisons of 11 sets of Type-B numerical simulations with the results of a selected set of centrifuge tests conducted in the LEAP-2017 project. Time histories of accelerations, excess pore water pressures, and lateral displacement of the ground surface are compared to the results of nine centrifuge tests. A number of numerical simulations showed trends similar to those observed in the experiments. While achieving a close match to all measured responses (accelerations, pore pressures, and displacements) is quite challenging, the numerical simulations show promising capabilities that can be further improved with the availability of additional high-quality experimental results.

This paper describes the numerical sensitivity study requested prior to the December 2017 LEAP workshop. Several but not all of the simulation teams participated in this sensitivity study. The results of the sensitivity study are used to begin to map out the simulation response surfaces that relate residual displacement to PGAeff and relative density. The simulation response surfaces are compared to the corresponding response surfaces determined by nonlinear regression of the centrifuge test data. The definition of the experimental response surface allows a means to objectively reduce the influence of outliers in the experiment dataset. The residuals between the experiments and the regression surface are used to quantify the uncertainty associated with experiment-experiment variability. Some metrics for assessing the comparison between simulations and experiments are explored; it is suggested that differences in the logarithm of displacement are more meaningful than arithmetic differences. As expected, some models predicted the average displacement well and some predicted triggering of liquefaction and the shape of the response function better than others. LEAP-UCD-2017 is not a final assessment of simulation procedures; instead, the results can be used to improve simulation specifications and calibration procedures and as a stimulus for more careful review of simulation results before they are submitted.

As part of the LEAP project the seismic response of a liquefiable 5° slope was modelled at a number of centrifuges around the world. In this paper the two experiments conducted at Cambridge University are discussed. The model preparation is detailed with particular emphasis on the sand pouring, saturation and slope cutting process. The presence of the third harmonic in the input motion is shown and its significance discussed. The potential for wavelet denoising to filter random electrical noise from the pore pressure traces is illustrated. CPT strength profiles are highlighted and a possible softer layer in one of the tests is discussed. Whilst the specifications called for one dense and one loose test, the likelihood that both Cambridge tests were loosely poured is assessed. The PIV technique is used to obtain the displacements of the slope during the test. Finally, the correspondence between the PIV displacements and physical measurements of the marker movements is compared.

Three centrifuge experiments were performed at the University of California, Davis, for LEAP-UCD-2017. LEAP is a collaborative effort to assess repeatability of centrifuge test results and to provide data for the validation of numerical models used to predict the effects of liquefaction. The model configuration used the same geometry as the LEAP-GWU-2015 exercise: a submerged slope of Ottawa F-65 sand inclined at 5 degrees in a rigid container. This paper focuses on presenting results from the two destructive ground motions from each of the three centrifuge models. The effect of each destructive ground motion is evaluated by accelerometer recordings, pore pressure response, and lateral deformation of the soil surface. New techniques were implemented for measuring liquefaction-induced lateral deformations using five GoPro cameras and GEO-PIV software. The methods for measuring the achieved density of the as-built model are also discussed.

Three centrifuge tests were conducted at Ehime University for the LEAP-2017 exercise. The experiment consisted of a submerged clean sand, with target relative densities of either 65 or 80%, with a 5 degree slope in a rigid container. Models were prepared along with the specifications and each model was subjected to a 1 Hz ramped sine wave base motion twice. This paper provides overview of the models and some details of the effects on the pore pressure responses of relative density and shaking histories of the models. Typical residual deformation obtained by PIV analysis and liquefaction resistance estimated based on the model response and laboratory cyclic shear tests are also shown.

In the framework of the LEAP 2017 exercise, two dynamic centrifuge tests on a gentle slope of saturated Ottawa-F64 have been performed at the IFSTTAR centrifuge. These tests were conducted in parallel with other tests performed in nine other centrifuge centers. The objectives were to compare the experimental results, e.g., effect of the experimental procedure or of test parameters on the results, and to provide a database for numerical modeling. In this framework, all the tests were performed on the same prototype geometry and the first base shaking was a 1 Hz ramped sine with an effective amplitude of 0.15 g at the prototype scale. Among some of the tests performed in the various centrifuge facilities, several parameters were modified, such as the density and the second input motion, in order to study their impact on the slope behavior when it is subjected to base shaking. This paper details the procedure followed at the IFSTTAR center for the buildup of dense sand and medium loose sand models and the deviations from the specifications provided by the leaders of the LEAP-UCD-2017 program. The main deviations are, for both tests, the absence of the measurement of the saturation degree and, for the first, the characterization of the soil properties with CPT test and bender element measurements during the test. Some results of the tests performed are briefly presented such as the acceleration, pore pressure buildup, and the deformation of the slope surface.

Since the earthquakes of Niigata (Japan, 1964) and Alaska (USA, 1964), the dangers of liquefaction have been highlighted and research into liquefaction has been actively performed. Particularly, as part of the provision and verification of liquefaction data through physical modeling by using centrifuge and numerical prediction, the Liquefaction Experiments Analysis Project (LEAP) was launched. The purpose of the recent LEAP-UCD-2017, in which nine facilities participated, was to evaluate the uncertainty and repeatability of response in a previous study for LEAP-GWU-2015. The ground models were prepared in a rigid box with a 5° sloping model with relative densities of 85 and 50% by using Ottawa sand. The models were subjected to nondestructive and destructive motions based on a ground motion consisting of a tapered 1 Hz sine wave. This paper describes not only the experimental procedure in detail but also the difference in the sensor response of the ground model corresponding to the relative density of 85 and 50% during liquefaction. Moreover, it provides data for permanent horizontal–vertical displacement through liquefaction and for validation of the numerical model.

As part of the LEAP-UCD-2017 exercise, 24 centrifuge tests were conducted at 9 centrifuge facilities around the world; among them, 3 tests were carried out in the facilities of the Disaster Prevention Research Institute at Kyoto University. The main objective of the tests is to characterize the median response and uncertainty of the dynamic response of a uniform-density sloping deposit of clean sand, subjected to a ramped sinusoidal wave; this chapter introduces specific details of the model preparation, testing procedures, and achieved response. Additionally, a brief discussion related to the uncertainties in the density estimation is presented.

Liquefaction Experiments and Analysis Projects (LEAP) aim to use simple centrifuge modeling tests to validate and calibrate the numerical modeling results. In LEAP-UCD-2017 project, the design and specification of the model are quite uncomplicated than that of the earlier stage of LEAP. The model stored in a rigid container was construted by medium dense sand with 5 degrees of slope and subjected to a 1-Hz sinusoidal wave of base motion. Models were built, tested and cross-validated by many different research institutes. This paper describes in detail the result of experiments carried at National Central University (NCU), Taiwan (R.O.C.) This paper describes in details the unique part of the experiments at NCU.

The Liquefaction Experiments and Analysis Projects (LEAP) is an international effort to use experimental data from physical modeling at different (international) centrifuge facilities to validate soil liquefaction numerical models and analysis tools. The goals of LEAP-2017 experimental efforts are to assess the repeatability potential at each facility, the reproducibility of centrifuge tests among different facilities, and the sensitivity of the experimental results to variation of testing parameters and conditions. A number of tests of the same (sloping deposit) centrifuge model were repeated at Rensselaer Polytechnic Institute in 2015 and 2017. This paper focuses on two specific tests to assess and demonstrate repeatability at this facility.

Three dynamic centrifuge tests with different densities (corresponding to loose, medium dense, and dense deposits) were conducted at Zhejiang University for LEAP-UCD-2017 for an exercise of repeatability and reproducibility. The same model used in LEAP-GWU-2015 representing a 5-degree slope consisting of saturated Ottawa F-65 was repeated in 2017, but more rigorous protocols and new techniques (CPT and high-speed cameras) were included in Zhejiang University experiments. Test facilities and detailed modeling and testing procedures are presented; uncertainties in input parameters are also discussed. Preliminary results associated with selected experiment are presented.

In accordance with the Liquefaction Experiments and Analysis Projects (LEAP)-UCD-2017 guidelines, two stress-dependent bounding surface constitutive models, Manzari-Dafalias and PM4Sand, were calibrated for Class-B prediction of centrifuge experiments of a sloped ground surface model of uniformly deposited Ottawa F-65 sand. Different calibration techniques and objectives were chosen for each material model. It was shown that both models were capable of simulating the behavior of cohesionless soils under liquefaction.

Within the framework of the LEAP-UCD-2017 exercise, Type B simulations of centrifuge tests were conducted assuming a hypoplastic constitutive model for sand. Differently from the most common elastoplastic approach, the hypoplasticity does not decompose the strain rate into elastic and plastic parts and does not use explicitly the notions of the yield surface and plastic potential surface. The process followed to calibrate the constitutive model is presented in detail. The initial state of stresses in the analyzed mesh, the key parameters used in the dynamic simulation phase, and a comparison of the simulation with some experimental results are reported. All the simulations were performed using the model parameters calibrated by using the laboratory test data. Finally, a sensitivity analysis of computed displacement to soil density and ground motion intensity show the influence of such factors on the seismic soil response of liquefiable soils.

Simulations by effective stress analysis were carried out for nine centrifuge experiments of a gentle sloped ground. The constitutive equation is a hyperbolic model as a stress–strain relationship and a bowl model as a strain–dilatancy relationship. The experimental results largely vary depending on density and input acceleration. The analyses results can explain such a tendency of variability.

Increasing the confidence in estimates of liquefaction-induced deformations from numerical models requires careful validation of both constitutive models and numerical simulation approaches. This validation should be performed with proper consideration of the intended use of the simulation results and the uncertainty inherent in comparing numerical simulations to laboratory or physical models. This paper describes the comparison of a set of calibrated numerical simulations to the results of centrifuge experiments with a submerged, liquefiable slope. These simulations were performed as part of the 2017 LEAP simulation exercise using the constitutive model PM4Sand and the numerical platform FLAC. The simulation results were compared to those from the centrifuge to identify which aspects of the experimental response the simulations were adequately able to capture. This paper provides a description of the simulation approach including the constitutive model and numerical platform. The process of calibrating PM4Sand to the results of laboratory experiments is described. This is followed by a description of the system-level simulations and a comparison of the simulation results with the experimental results. A sensitivity study was performed to examine trends in the simulation response and a discussion of the findings from this study is presented.

As a part of the LEAP-UCD-2017 project, type B simulations were performed to predict the results of nine selected centrifuge model tests using a two-dimensional effective stress analysis program incorporating the cocktail glass model. In Phase I of the numerical simulation, three parameter sets of the cocktail glass model were determined for three different dry densities. In Phase II, numerical models of the finite element method were made based on the data of the centrifuge model. At that time, for each centrifuge model test, we decided which parameter set of the cocktail glass model to apply by referring to the dry density of the analyzed ground. There was a tendency that the response values of the numerical simulation were slightly larger than the response values expected from the difference between the achieved dry density of the centrifuge model and the dry density on which the applied parameter set depends.

This paper presents numerical simulations (modified Type-B) related to LEAP-UCD-2017 (Liquefaction Experiments and Analysis Projects) dynamic centrifuge model tests for a liquefiable sloping ground conducted by various institutions. The numerical simulations are performed using a pressure-dependent constitutive model implemented with the characteristics of dilatancy and cyclic mobility. The soil parameters are determined based on a series of available stress-controlled cyclic triaxial tests during the Type-A simulation phase for matching the liquefaction strength curves of Ottawa F-65 sand. The computational framework for the dynamic response analysis is discussed. Computed results are presented for the selected centrifuge experiments (modified Type-B simulations). Measured time histories (e.g., displacement, acceleration and excess pore pressure) of these experiments are reasonably captured. Comparisons between the numerical simulations and measured results showed that the pressure-dependent constitutive model as well as the overall employed computational framework have the potential to predict the response of the liquefiable sloping ground, and subsequently realistically evaluate the performance of an equivalent soil system subjected to seismically induced liquefaction.

Fugro participated in the Liquefaction Experiment and Analysis Projects (LEAP) by performing numerical simulations using two different constitutive models implemented in the software FLAC. Fugro developed a calibration framework based on soil-specific laboratory test data considering multiple elements of dynamic response such as liquefaction triggering criteria (i.e., number of cycles to a specified strain and pore pressure ratio threshold) as well as post-triggering strain accumulation rate. The calibrated model parameters were subsequently used to obtain “Type B (blind)” predictions of the centrifuge experiments with the opportunity to refine after the centrifuge results were provided (Type C simulations). Overall, Fugro’s blind predictions captured the centrifuge test responses well with small refinements needed during Type C simulations. Estimated deformations were within a factor of about two compared to the observed. The overall good comparison provides confidence in the proposed calibration framework, which can be implemented and used for different sand types and project conditions.

This study reports the results of type-B predictions for dynamic centrifuge model tests of a liquefiable sloping ground conducted at various centrifuge facilities within a framework of the LEAP-UCD-2017. The simulations are carried out with a finite strain analysis program, called “FLIP TULIP,” which incorporates a strain space multiple mechanism model based on the finite strain theory (including both total and updated Lagrangian formulations). The program can take into account the effect of geometrical nonlinearity as well as material nonlinearity’s effect. Soil parameters for the constitutive model are determined referring to the results of laboratory experiments (e.g., cyclic triaxial tests) and some empirical formulae. This chapter describes the parameter identification process in details as well as the computational conditions (e.g., geometric modeling, initial and boundary conditions, numerical schemes such as time integration technique). Type-B prediction results are compared with the centrifuge test results to examine the applicability of the program and constitutive model.

Under the LEAP-UCD-2017 project framework, simulations of centrifuge shaking table tests on gently sloping ground are conducted in this study. A unified plasticity model for large post-liquefaction shear deformation of sand is used in the simulations. The model is able to provide a unified description of sand behaviour under different states from the pre- to post-liquefaction regimes. The model parameters are calibrated against undrained cyclic triaxial test results. Using the calibrated parameters, a series of Type-B simulations are performed to predict the results of nine centrifuge tests conducted at various facilities with different settings without previously knowing the test results. Using the same simulation setup, a sensitivity study is conducted to investigate the influence of soil density and input motion on the response of the slope.

Numerical simulations of LEAP-UCD-2017 were performed to validate the numerical modeling approach and provide insight to capabilities and limitations of the adopted constitutive model. This chapter focuses on using an extended version of the SANISAND constitutive model implemented in FLAC3D program at UBC. The constitutive model was calibrated based on the available laboratory element tests on Ottawa F65 sand. It was then used for simulation of the centrifuge tests on a mildly sloping liquefiable ground of the same soil subjected to dynamic loading. The study covered the Types B and C simulations and the sensitivity analyses. Type B simulations were successful in capturing some aspects of measurements from the experiments. A simplified approach for changing the soil permeability was adopted in Type C simulations, and the improved simulation results were again compared with those measured in the experiments. In the numerical sensitivity analyses, the model appeared to provide reasonable trends for simulation of different sample densities, and ground motion intensities and frequency contents.

LEAP-UCD-2017 is a blind prediction exercise for numerical modelers with relatively simple centrifuge tests. However, the application of numerical simulation should be discussed from both modeler’s and practitioner’s viewpoints. The authors applied one of the most common dynamic analysis programs in Japan (FLIP ROSE) for this exercise. The program, FLIP ROSE, has been continuously updated for 20 years with voluntary supports from practitioners. Using this scheme, the authors discuss several issues including the possible variation in parameter determination in practice and the effect of artificial Rayleigh damping on predictions. Although only a limited number of cases of centrifuge tests were analyzed, the application of a common analysis program by practitioners pointed out some issues to be addressed by numerical modelers.

The seismic deformation and soil-structure interaction analyses completed to date (i.e., preliminary design) for the project would benefit from more rigorous validation and calibration of the constitutive models and numerical procedures used in future analyses. The results from the Liquefaction Experiments and Analysis Project (LEAP) are expected to benefit future phases of the project (i.e., detailed design).

Significance of calibration procedure consistency for geotechnical earthquake engineering is emphasized. The material parameters should be calibrated consistently for specimen preparation during the tests. Particularly, the laboratory-based parameters should not be mixed up with those for in-situ conditions.

Two suggestions are provided for expanding the scope of LEAP project for the next stage. The first suggestion is to perform undrained cyclic shear tests with addditional static deviator stress to represent more realistic stress conditions in soil-structure systems during earthquakes. The other suggestion is to perform centrifuge model tests with non-homogeneous soil layer such as mildly sloping ground of liquefiable sand overlain by a less permeable capping crust layer, leading to a unlimited flow failure of surface crust. Effects of void ratio redistribution and seepage flow will be studied in detail in this type of cetrifuge tests.

The post-liquefaction residual strength of soils often plays a key role in the seismic stability analysis of embankment dams and other earth structures. A commonly held view is that back analyses of field case histories of liquefaction flow failures offer at present the most practical approach for estimating the residual strength of liquefied soils. Thus, it is common in engineering practice to estimate this strength parameter based on empirical correlations. However, the available data for such correlations are very limited and represent only a small range of conditions encountered in practice. Thus, use of the correlations in practice generally involves considerable uncertainty. A case history is presented that illustrates questions that are often encountered in practice and that would benefit from additional research, perhaps by means of centrifuge testing and numerical modeling.

If the same prototype soil and water are used for a centrifuge model, as it is generally accepted, the seepage velocity in the centrifuge is increased by N times earth gravity produced by the centrifuge environment. The increased seepage velocity produces a time scaling conflict between diffusion time and dynamic time. Many centrifuge researchers prefer not to scale soil particle size; thus, the viscous fluid is used to resolve the time conflict. The purpose of viscous fluid is to maintain the same seepage velocity as in the prototype condition. In other words, the hydraulic conductivity of model soil in centrifuge environment is same as in the prototype condition. For a parametric study, especially, a comparison of test results produced by multiple centrifuge facilities such as the LEAP project, the soil permeability should be consistent. Viscous fluid should be prepared carefully and accurately measured fluid viscosity used for each centrifuge test should be reported because it could affect the soil permeability significantly. It is ideal that the permeability of soil with viscous fluid is measured in the laboratory following ASTM standards before each test. The permeability test should be conducted in such a way that it validates the Darcy’s flow behavior for the hydraulic gradients (and seepage velocity) anticipated in the centrifuge test. Permeability of the sample using the viscous fluid should be reported for each test conducted by each facility. It is unlikely that the permeability will match perfectly between each test but numerical validation can account for the variation of the soil permeability.

In dynamic centrifuge modelling, to resolve conflicts in scaling of time between dynamic and diffusion events, viscous fluids are used to reduce a model’s permeability. Recently, with the ease of its handling, methylcellulose (MC) solution is commonly used (Stewart et al. 1998). The author conducted constant head permeability tests of Toyoura sand by varying the fluid viscosity, paying attention to transient changes in its permeability and found that the permeability of sand with MC solution continuously decreased with the fluid passing, whereas it remained constant for purified water. Then, by modelling of models of the liquefaction model in centrifuge testing, the effect of permeability reduction is monitored and was confirmed that the effect is relatively minor. However, this is thought to be one of the sources of epistemic errors in centrifuge modelling.

Under undrained cyclic loading, sand experiences decrease in effective stress, which can result in liquefaction. Test results show that large cyclic shear strain is generated at zero effective stress during undrained cyclic loading. This post-liquefaction shear strain has been observed to progressively increase in amplitude with increasing number of loading cycles until it eventually stabilizes at a bounded value. However, there has been no clear explanation on why and how this cyclic shear strain is generated. The fabric mechanism behind this post-liquefaction shear strain phenomenon is briefly discussed in this study.