Liquefaction Experiments And Analysis Projects Research Articles

Large post-liquefaction deformation of sand: Mechanisms and modeling considering water absorption in shearing and seismic wave conditions

Large deformation of sand due to soil liquefaction is a major cause for seismic damage. In this study, the mechanisms and modeling of large post-liquefaction deformation of sand considering the significant influence of water absorption in shearing and seismic wave conditions. Assessment of case histories from past earthquakes and review of existing studies highlight the importance of the two factors. Based on the micro and macro scale mechanisms for post-liquefaction shear deformation, the mechanism for water absorption in shearing after initial liquefaction is revealed. This is aided by novel designed constant water-absorption-rate shear tests. Water absorption in shearing can be classified into three types, including partial water absorption, complete water absorption, and compulsory water absorption. Under the influence of water absorption in shearing, even a strongly dilative sand under naturally drained conditions could experience instability and large shear deformation. The mechanism for amplification of post-liquefaction deformation under surface wave load is also explained via element tests and theoretical analysis. This shows that surface wave–shear wave coupling can induce asymmetrical force and resistance in sand, resulting in asymmetrical accumulation of deformation, which is amplified by liquefaction. A constitutive model, referred to as CycLiq, is formulated to capture the large deformation of sand considering water absorption in shearing and seismic wave conditions, along with its numerical implementation algorithm. The model is comprehensively calibrated based on various types of element tests and validated against centrifuge shaking table tests in the liquefaction experiments and analysis projects (LEAP). The model, along with various numerical analysis methods, is adopted in the successful simulation of water absorption in shearing and Rayleigh wave-shear wave coupling induced large liquefaction deformation. Furthermore, the model is applied to high-performance simulation for large-scale soil-structure interaction in liquefiable ground, including underground structures, dams, quay walls, and offshore wind turbines.

Predictions of seismic response for sheet-pile wall in liquefiable deposit within LEAP 2022 using the CycLiq constitutive model

The Liquefaction Experiments and Analysis Projects (LEAP) organized centrifuge tests and corresponding numerical predictions for the seismic response of sheet-pile wall in liquefiable deposits in 2022. Within LEAP 2022, two versions of the CycLiq constitutive model, i.e., the original (CycLiq) and modified (CycLiq-M) versions, were calibrated and used for numerical predictions. CycLiq-M was first calibrated using cyclic direct simple shear (CDSS) element tests on the test material, Ottawa F65 sand, and then used for Type-A predictions of another set of CDSS tests for which the results were priorly unknown, validating the model's performance at the element level. Type-B predictions of centrifuge shaking table model tests on sheet-pile wall in liquefiable deposit was then performed using both versions of the calibrated model, without prior knowledge of test results. The results show that the properly calibrated CycLiq model can make reasonable predictions for soil acceleration, excess pore pressure, settlement, and especially sheet-pile wall displacement. In particular, the CycLiq-M version with enhanced cyclic resistance simulation capability exhibits significant improvement in prediction accuracy. Using numerical simulation, the influence of soil-container friction in centrifuge model tests was assessed.

Stochastic analysis of the seismic performance of sheet-pile walls supporting heterogeneous liquefiable ground

This paper presents the findings of a stochastic analysis conducted to evaluate the effects of the spatial soil variability and temporal base motion variability on the seismic response of sheet pile walls supporting liquefiable ground. A random finite element analysis is used to model the centrifuge experiment performed during the 2020 phase of the Liquefaction Experiments and Analysis Project (LEAP). The relative density of the main soil deposit in the LEAP-2020 experiments ranged from 50 to 75 %. In this study the spatial variability of each centrifuge model was determined from the inflight cone penetration tests (CPT) performed before the shaking phase in each experiment. The vertical spatial correlation length was directly determined from the CPT measurements, while the mean and coefficient of variation of the soil relative density were obtained from an empirical relationship correlating the cone tip resistance with the relative density. The variability observed in the achieved base motions for the LEAP-2020 centrifuge experiments was modeled by generating synthetic base motion time histories that matched the variability observed in the response spectra of the achieved base motions. The results obtained from this analysis shed light on the effects of the base motion variability and soil density on the excess pore pressure development and the triggering of soil liquefaction. It also provides an understanding on the sensitivity of the wall lateral displacement to such variabilities. The observed variability in the simulation response is compared with the variability observed in the centrifuge experiments. This comparison is used to evaluate the capability of the current numerical simulation tools in modeling the variability in the wall response.

LEAP-ASIA-2019: Validation of centrifuge experiments and the generalized scaling law on liquefaction-induced lateral spreading

A round-robin centrifuge model test for an identical saturated sloping deposit with various initial conditions was conducted in the framework of liquefaction experiments and analysis project (LEAP) with 10 international institutes. To pursue two main objectives: (1) the validation of the generalized scaling law (GSL); and, (2) the development of additional experimental data that fill the gaps in the previous LEAP, each institute was assigned two tests, identical in prototype scale: a model following the generalized scaling law and a model following conventional centrifuge scaling law. The test results show a significant match in the dynamic behavior of these models, which validates the applicability of the GSL under given conditions. However, in this study, a deviation is observed when the surface ground deformation becomes larger (i.e., 250 mm). The trend surface that correlates peak amplitude of input acceleration (PGAeff), the relative density of the ground (Dr_qc(2.0 m)), and surface lateral displacement (Ux) is updated and confirmed with new data sets.

Verification of generalized scaling laws: Two centrifuge tests of a liquefiable sloping deposit

Two centrifuge experiments were conducted at Rensselaer Polytechnic Institute (RPI) to evaluate and assess the validity of the generalized scaling laws. The experiments were performed within the framework of the Liquefaction Experiments and Analysis Project (LEAP) and consisted of testing a saturated sloping deposit subjected to a tapered base input acceleration. The two tested models reflected consistent soil conditions but were built based on different scaling principles. The first model observed the conventional scaling laws for centrifuge physical modeling. The second model reflected the generalized scaling laws. The two tested models exhibited consistent response before liquefaction. The generalized scaling model showed a higher susceptibility to liquefaction and had a higher rate of pore pressure buildup.

Centrifuge modeling of the dynamic response of a sloping ground – LEAP-UCD-2017 and LEAP-ASIA-2019 tests at Kyoto University

The Liquefaction Experiments and Analysis Project (LEAP) is a joint international project that pursues the verification, validation and uncertainty quantification of numerical liquefaction models. As part of this project, eight centrifuge models were developed at the facilities of Kyoto University, to simulate the dynamic behavior of a submerged, uniform-density, 20 m long, 4 m deep and 5° sloping deposit of Ottawa F-65 sand. The results are intended to compose part of a reliable database in the development of current and future V&V processes of liquefaction models.This paper presents the main characteristics of the model preparation and testing process, and an analysis of: (1) the validity of the “Generalized Scaling Law” for centrifuge modeling (2) the consistency and repeatability of the tests by means of the development of an additional repeatability-test, and numerical modelling, and (3) the importance of the viscosity of the fluid in liquefaction modelling.

Stress-strain behavior and liquefaction strength characteristics of Ottawa F65 sand

The stress-strain-strength response of soils is of significant interest to development and calibration of realistic constitutive models that can be used in numerical simulation of geotechnical engineering problems. An extensive characterization of Ottawa F65 sand along with various monotonic and cyclic tests conducted during the course of the Liquefaction Experiments and Analysis Projects (LEAP) are presented here. The specimens in these tests were prepared using a meticulous sample preparation technique to facilitate their consistency and repeatability. Monotonic drained and undrained triaxial tests shed light on the steady-state (critical state) of Ottawa sand, while stress-controlled and strain-controlled cyclic triaxial and direct simple shear tests provide key information on the cyclic stress-strain behavior and liquefaction strength of this soil. The triaxial tests identify the liquefaction strength of the soil at different densities, while the direct simple shear tests evaluate the effect of overburden pressure on the cyclic response. The results of these experiments are also compared to the experimental results available in the literature. The data obtained from the cyclic triaxial and direct simple shear tests were further analyzed by plotting the computed shear modulus degradation and damping curves. The results show how the soil stiffness degrades as cyclic shear stress is applied for soil at different densities and confining stresses. It can be seen that the rate of shear modulus degradation increases with the increase of confining stress and decreases with the increase of soil density. The damping curves consistently show an increasing in damping ratio until a shear strain of about 0.05%, followed by a plateau at about 20–25% damping ratio for shear strain between 0.05 and 0.5%, and ending with a decrease in damping until reaching a final damping ratio of about 10%.

Numerical analysis of LEAP centrifuge tests on sloping liquefiable ground: Influence of dilatancy and post-liquefaction shear deformation

The Liquefaction Experiments and Analysis Projects (LEAP) provides an international collaboration platform to assess key aspects related to seismic induced soil liquefaction. Within this scope, the current study investigates the influence of dilatancy and post-liquefaction shear strain on the seismic response of mildly sloping liquefiable ground, based on simulations of LEAP 2017–2019 centrifuge tests. A unified plasticity model for large post-liquefaction deformation, which has been implemented in the OpenSees finite element framework, is used in the numerical simulations. The model parameters are calibrated rigorously against element tests performed for Ottawa F65 sand used in LEAP 2017–2019, especially for liquefaction resistance and post-liquefaction shear strain observed in undrained cyclic hollow cylinder torsional shear tests. The calibrated model is used to simulate a series of centrifuge shaking table tests on sloping ground constructed at different soil densities. Good agreement between simulation and test results is achieved, validating the constitutive model and numerical simulation method. In depth analysis on the influence of dilatancy and post-liquefaction shear strain on the seismic response of the slope model is then conducted. The results show that these two factors can significantly affect the seismic acceleration, excess pore water pressure, and lateral displacement response of sloping ground, and therefore must be appropriately reflected in seismic liquefaction analysis oriented constitutive models.

The effects of base motion variability and soil heterogeneity on lateral spreading of mildly sloping ground

Centrifuge modeling has been used to observe some key characteristics of liquefiable soils during seismic motions. If carefully conducted, the results of centrifuge tests can be used for validation of constitutive models and numerical modeling tools. However, a thorough evaluation of numerical models requires knowledge of the soil properties and the uncertainties associated with these properties. Moreover, the base excitations achieved in centrifuge tests often vary from the target base excitation, and a fair evaluation of the quality of blind prediction of a centrifuge test requires an account of the uncertainties associated with the achieved base motion. This paper presents an analysis of the effects of the inherent variability present in the soil density and base motion on the lateral spreading of mildly sloping ground. The analysis combines fully-coupled non-linear finite element modeling with Monte Carlo simulation. The stochastic analysis is based on the variabilities observed in the density of the soil specimens and in the achieved base motions of the centrifuge tests conducted for the Liquefaction Experiments and Analysis Projects (LEAP): LEAP-GWU-2015 and LEAP-UCD-2017. The two-surface plasticity model for sand is used to model the soil response. The model is calibrated against element tests performed on Ottawa-F65 sand to determine its liquefaction strength. The finite element model is pre-validated through deterministic simulation of the centrifuge experiments conducted during the LEAP project. The sources of variability are identified and their magnitudes are quantified based on the results obtained from the LEAP centrifuge experiments. First, the effects of the soil spatial variability are presented. Then, the effects of the base motion variability are discussed. Finally the combined effects of the variability in the soil density and base motion are evaluated. The results obtained from the stochastic analysis are compared with the variability in the soil response observed in the centrifuge experiments. The results obtained from this study show that by carefully modeling the different sources of variability, the stochastic analysis was able to model the observed variability in the lateral displacement of the centrifuge experiments. The results obtained confirm the observation that the lateral displacement of liquefiable soil is more sensitive to the base excitation variability than the variability in the soil density.

Influence of the relative density and K0 effects in the cyclic response of Ottawa F-65 sand - cyclic Torsional Hollow-Cylinder shear tests for LEAP-ASIA-2019

The Liquefaction Experiments and Analysis Project (LEAP) is a joint international project that pursues the verification, validation and uncertainty quantification of numerical liquefaction models. As part of this project, twenty-three Hollow Cylinder Cyclic Torsional Shear Tests were conducted in the installations of the Disaster Prevention Research Institute at Kyoto University, focusing on the influence of the relative density and the K0 effects in the cyclic response of Ottawa F-65 Sand. The results are intended to compose part of a reliable database in the development of current and future V&V processes of liquefaction models.The paper presents the characteristics of the model preparation, test development, and a brief discussion of (1) the influence of the relative density and the K0 effects, (2) the volumetric strain due to post-cyclic reconsolidation, and (3) the repeatability and consistency of the tests.

Study of the effects of container boundary and slope on soil liquefaction by centrifuge modeling

Two centrifuge model tests were conducted as part of the Liquefaction Experiments and Analysis Project (LEAP), and three extra tests were used to compare with the LEAP tests. The two models selected from LEAP-GWU-2015 were prepared in rigid box with a slope angle of 5 degrees. One of the extra models was prepared in a laminar box with a slope angle of 5 degrees and has the same model properties as the models from LEAP-GWU-2015. The other two extra models were prepared as flat ground in rigid box and laminar box respectively and have the same model properties. The effects of the container boundary and the dip direction of the slope models on soil liquefaction were investigated by assessing the acceleration, excess pore water pressure, and ground surface deformation. From the test results for the model in the laminar box, it is concluded that the soil contained in a laminar box have different dynamic characteristics compared with a rigid box soil. For the models in a rigid box, the soil on the high side of the slope has larger excess pore water pressure value and the excess pore water pressure changes are greater than the soil on the low side of the slope during shaking. The interaction between the slope dip and the direction of input motion affects dynamic performance during a seismic event.

Stress-strain response of the LEAP-2015 centrifuge tests and numerical predictions

The Liquefaction Experiments and Analysis Projects (LEAP) is an international effort to produce a set of high quality test data and to then use it in a validation exercise of existing computational models and simulation procedures for soil liquefaction analysis. A validation effort (LEAP-GWU 2015) was undertaken using a benchmark centrifuge model of a sloping deposit tested in a rigid-wall container. This article presents and discusses the shear stress-strain responses and effective stress paths of the LEAP-GW 2015 centrifuge tests and numerical predictions (including an assessment of the effects of the rigid boundaries).

Physical Modeling of Soil Liquefaction: Repeatability of Centrifuge Experimentation at RPI

Numerical models are commonly used in practice to predict the occurrence and propagation of liquefaction and lateral spreading. Nevertheless, there are still only a few good-quality data sets to validate these models. An international collaboration between physical and numerical modelers has been initiated through the Liquefaction Experiments and Analysis Projects (LEAP) to establish a standard method for validation of numerical models. One of the main goals of LEAP is to provide numerical modelers with high-quality experimental data that can be used in a series of prediction exercises. In this framework, a small-scale centrifuge test of a sloping deposit was conducted at Rensselaer Polytechnic Institute (RPI) in January 2015 and provided comprehensive data sets for evaluation of soil liquefaction numerical tools. The same experiment was repeated in September 2015 to assess the robustness and repeatability of the physical modeling methods at RPI. This paper serves a double objective: (1) to study the response and liquefaction potential of sloped deposits experimentally and (2) to establish the repeatability of experimental data to be used in numerical simulations.

LEAP-GWU-2015 experiment specifications, results, and comparisons

LEAP (Liquefaction Experiments and Analysis Projects) is an effort to formalize the process and provide data needed for validation of numerical models designed to predict liquefaction phenomena. For LEAP-GWU-2015, one project within LEAP, an experiment was repeated at 6 centrifuge facilities (Cambridge University, Kyoto University, University of California Davis, National Central University, Rensselaer Polytechnic Institute, and Zhejiang University) and the results were shared and archived for the purposes of validation of numerical models. This paper describes the specifications for the LEAP-GWU-2015 experiment and compares the experimental results from the six facilities. The specified experiment was for uniform medium dense sand with a 5 degree slope in a rigid container subject to a ramped, 1Hz sine wave base motion. The experiment was meant to be relatively simple to enable different facilities to produce comparable experiments. Although it cannot be claimed that identical experiments were precisely replicated on different centrifuges, it is argued that the results are similar enough that each experiment lends veracity to the set of results. A benefit of variability between experiments is that the variety enables a more general validation. Important lessons with regard to specification of future experiments for validation of numerical models are summarized. LEAP-GWU-2015 has demonstrated an approach that is a useful reference for future validation studies.

Physical modeling of soil liquefaction: Overview of LEAP production test 1 at Rensselaer Polytechnic Institute

Liquefaction induced ground failures during seismic events have been of major concern to the engineering community. Laboratory, full-scale, and centrifuge experimentation has provided valuable insights on the mechanics and propagation of liquefaction. Concurrently, advances in the computational field have led to refined numerical tools to simulate the liquefaction phenomenon. In an effort to better evaluate the modeling capabilities, the Liquefaction Experiments and Analysis Project (LEAP) aims at providing numerical modelers with high quality experimental data that can be used in a series of prediction exercises. For this purpose, in early 2015 several centrifuge tests were conducted at different facilities all over the world and were followed by numerical simulations. This paper presents the centrifuge test that was conducted at the Center for Earthquake Engineering Simulation at Rensselaer Polytechnic Institute and discusses the modeling techniques that were employed in order to gain a variety of information on the soil response.

Prediction of LEAP centrifuge test results using a pressure-dependent bounding surface constitutive model

In accordance with the “Liquefaction Experiments and Analysis Projects (LEAP )” guidelines, class-A, B and C predictions are performed for the response of centrifuge tests on a sloped configuration of Ottawa-F65 soil in a rigid container. The objective of this paper is to discuss important aspects on the calibration process of a pressure-dependent bounding surface constitutive model for the simulation of centrifuge experiments. The constitutive model used in this study is based on the theory of Critical State Soil Mechanics that has shown to be suitable to represent the behavior of cohesionless soils. Results of class-A predictions match experiment data reasonably well. An attempt is made to find a single set of material parameters to be used for class-B and class-C prediction of all six centrifuge tests.

Numerical Predictions for Centrifuge Model Tests of a Liquefiable Sloping Ground Using a Strain Space Multiple Mechanism Model Based on the Finite Strain Theory

This paper presents the results of numerical simulations for dynamic centrifuge model tests of a liquefiable sloping ground performed by various institutions within a framework of Class A, B, and C prediction phases of the LEAP (Liquefaction Experiments and Analysis Project). The simulations are performed by using a strain space multiple mechanism model based on the finite strain theory (including both total and updated Lagrangian formulations), in which both material and geometrical nonlinearity are considered. In the simulation, dynamic response analyses are carried out following self-weight analyses with gravity. The soil parameters of the constitutive model are determined based on the results of laboratory soil tests (e.g., cyclic triaxial tests) and some empirical formulae. The identification process of the parameters is explained in details besides the computational conditions (e.g., geometric modeling, initial and boundary conditions, numerical schemes such as time integration technique). In addition to the numerical results of the Class A prediction using a target input motion, those of the Class B and C predictions using recorded motions in the centrifuge model tests are also presented. Comparison between these predictions and measured results has revealed that the constitutive model parameters for effective stress analyses should be calibrated to well capture the shape and trend of liquefaction resistance curves, and subsequently estimate the damage of soil systems due to liquefaction with higher accuracy.