The LEAP international collaboratory is introduced and its key objectives and main accomplishments during the planning phase of the US-LEAP (LEAP-2015) are presented. The main theme of LEAP-2015 was lateral spreading of sloping liquefiable soils. A summary of the results of the laboratory element tests performed on the selected soil (Ottawa F-65) is presented. The numerical simulations submitted by several predictors at different stages of the project are compared with the measured responses of sloping deposit specimens tested in a rigid box at six different centrifuge facilities around the world. The comparisons are presented for three rounds of simulations labeled here as types A, B, and C simulations. The type A simulations involved the response of the soil specimen to a prescribed base excitation with a maximum amplitude of 0.15g (Motion #2). Comparisons of the numerical simulations with the experimental results show that a sub-set of type A simulations were in reasonably good agreement with the responses measured in the reference centrifuge experiment. The predictors subsequently assessed the performance of their type A simulations by comparing them to the measured responses, made the necessary adjustments in their models, and conducted a type B simulation of the response of the same soil specimen subjected to an amplified base excitation with a maximum amplitude of 0.25g (Motion #4). In these type B simulations, the achieved base motions were used and the simulations showed an improved correlation with the experimental results. The predictors also conducted a type C simulation of the original test (Motion #2) using the base motions achieved on the six centrifuge facilities. The results showed very good agreement with the experimental results.

The Liquefaction Experiment and Analysis Project (LEAP), an international research collaboration among researchers from the US, UK, Japan, China and Taiwan, is a validation campaign to assess the capabilities of existing numerical/constitutive models for liquefaction analysis by using laboratory experiments and centrifuge tests. The nature, goals, and scope of the project are presented in this paper. The goals of the planning phase of the project that is currently ongoing in the US are briefly discussed. The main components of the validation campaign and their corresponding challenges are outlined. © 2015 Taylor & Francis Group, London.

As part of the Interagency Performance Evaluation Task Force (IPET) investigation into levee breaches in New Orleans following Hurricane Katrina, centrifuge modelling was undertaken of representative levee cross-sections on the 17th Street, Orleans and London Avenue Canals. Two mechanisms were observed leading to breaching of the levee in the models, both of which stemmed from a water-filled crack that formed in front of the flood wall. Depending on the foundation conditions and geometry of the levee and flood wall, the crack led either to a rotation of the flood wall landwards, with uplift and sliding on the top of the sand towards the landward toe of the levee, or to a translational (sliding) failure in the clay layer commencing from the bottom or toe of the flood wall. In the Orleans models no breach ensued, although it was clear these sections were close to failure. The centrifuge model tests identified, at an early stage in the IPET investigation, the importance of the ‘gap' mechanism affecting the stability of the flood walls, and confirmed that levee geometry and flood wall depth of penetration, together with the underlying soil profile, were critical to the performance of the system under flood loading.

Physical modeling was performed to investigate the performance of New Orleans levees during Hurricane Katrina. Centrifuge models of the levees and floodwalls on the London Ave Canal and the 17th Street Canal are presented in this paper. The focus of this paper is to present the techniques used to model the New Orleans levees and simulate the flood event during the hurricane. The centrifuge model tests identified the importance of crack formation in relation to the stability of the floodwall. Findings from this study are extremely valuable, as they provide detailed insight into the mechanisms that led to the failure of the levees.

Two series of centrifuge model tests were conducted using Nevada sand. Four saturated models placed in a mildly inclined laminar box and simulating a 6-m-thick deposit were shaken inducing liquefaction effects and lateral spreading. The sand was deposited at a relative density, Dr =45 or 75%; two of the 45% models were subjected to overconsolidation or preshaking. The second series involved in-flight measurements of static cone tip penetration resistance, qc , simulating the standard cone penetration test (CPT) 36-mm cone. Values of qc increased with Dr , overconsolidation, and preshaking. A normalized resistance, qc1N , was assigned to each of the four liquefaction/lateral spreading models. Increases in Dr , overconsolidation, and preshaking decreased liquefaction and ground deformation, but relative density alone captured these effects rather poorly. Conversely, qc1N predicted extremely well the liquefaction and lateral spreading response of the four models, confirming Seed’s hypothesis to explain the s...

Hurricane Katrina was one of the worst natural disasters in U.S. history. The effects of the hurricane were particularly devastating in the city of New Orleans. Most of the damage was due to the failure of the levee system that surrounds the city to protect it from flooding. This paper presents the results of centrifuge models conducted at Rensselaer Polytechnic Institute and the U.S. Army Corps of Engineers simulating the behavior of the levees at London Avenue North and South that failed during Hurricane Katrina. Those levees failed without being overtopped by the storm surge. Also included are the results of a centrifuge model of one levee section at Orleans Canal South, which did not fail during the hurricane. The key factor of the failure mechanism of the London Avenue levees was the formation of a gap between the flooded side of the levee and the sheetpile. This gap triggered a reduction of the strength at the foundation of the protected side of the levee. The results are fully consistent with field observations.

Centrifuge modeling of the 17th Street Canal and Orleans Canal North levees was performed in this study. During hurricane Katrina the levees on the 17th Street Canal failed, leading to breaches in the outfall canal in the city. Two mechanisms were observed in the centrifuge modeling that could cause a breach. First, a water-filled crack formed in front of the floodwall as the water in the canal rose above the top of the levee. The levees on the 17th Street Canal, which were supported on clay foundations, failed when this cracking led to a translational (sliding) failure in the clay layer commencing at the toe of the floodwall. The levees at Orleans Canal North, where failure did not occur, were also modeled to demonstrate that the model tests could successfully simulate failure and nonfailure conditions. The centrifuge model tests identified the importance of the crack formation in relation to the stability of the floodwall. These tests also confirmed that levee geometry, floodwall depth of penetration, and the underlying soil profile were all critical to the performance of the system under flood loading.

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. Cross-correlation 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.

Earthquake induced liquefaction continues to be a major threat to many engineered structures around the world. Analysis of liquefaction becomes particularly difficult for two-dimensional (and 3D) problems such as dam/foundation systems. Predominantly, analyses for such systems are performed utilizing some type of finite element or finite difference procedure. Verification or validation of the analyses relies on very limited field performance data with reduced knowledge of the full scope of system conditions or loading conditions.Research reported in this paper represents a portion of ongoing work to obtain a database of information useful for numerical model calibration and to gain a better understanding of the complex dynamics of liquefying foundations under earth dams. Specifically, a highly instrumented model of an earth dam with clay core founded on a liquefiable foundation subjected to earthquake loading is being studied. Properties of the liquefiable foundation are varied to determine the related effects on the overlying earth dam. In this paper, results from three centrifuge physical models will be presented. The models are identical, with the exception of the location (depth) of a liquefiable layer in the foundation, and are subjected to the same dynamic excitation. Results and discussion related to the significance of the liquefiable layer location within the foundation and damage to the earth dam are presented. Published by Elsevier Ltd.

Earthquake induced liquefaction is a major concern for embankment dam safety. The U.S. Army Corps of Engineers (USACE) has responsibility for the safe performance of over two hundred dams in high seismic hazard areas of the U.S. The USACE periodically evaluate the safety of these dams against expected seismic events in the light of new hydrologic or seismic data or changes in the state-of-the-art design or construction criteria. Those dams that are found deficient are retrofitted through appropriately designed remediation methodologies. One of the major seismic safety problems for the dams is liquefiable foundation deposits. This paper is focused on the USACE experience in dynamic stability evaluation and remediation of earth dams with liquefiable foundations. Several major earth dam remediation case histories are discussed. Furthermore, the current state-of-the-art in retrofit ground improvement for liquefiable dam soil deposits is provided.

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

One of the major sources of earthquake-induced damage to earth embankments has been liquefaction of loose, saturated, sandy foundation soils that often prevail throughout the coastal and marine environments. Dynamic analysis of such embankment/foundation systems primarily relies on some type of finite element or finite difference procedure. Verification or validation of such analyses requires either high quality field performance data, which is virtually nonexistent, or data from physical model testing. Research reported in this paper represents a portion of ongoing work to obtain such a database. Specifically, a model of an earth dam founded on a marine-like saturated sandy deposits subjected to earthquake loading is being studied. Several properties in the foundation deposit are varied to determine the related effects to the earth dam. In this paper, results from three centrifuge physical models, studying the effect of loose layer depth in the foundation is presented. The significance of the depth of a loose liquefiable layer and resulting damage to the earth dam is discussed.

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress-strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.

The prediction of liquefaction and resulting displacements is a major concern for earth structures located in regions of moderate to high seismicity. Conventional procedures used to assess liquefaction commonly predict the triggering of liquefaction to depths of 50 m or more. Remediation to prevent or curtail liquefaction at these depths can be very expensive. Field experience during past earthquakes indicates that liquefaction has mainly occurred at depths less than about 15 m, and some recent dynamic centrifuge model testing initially appeared to confirm a depth or confining-stress limitation on the occurrence of liquefaction. Such a limitation on liquefaction could greatly reduce remediation costs. In this paper an effective stress numerical modeling procedure is used to assess these centrifuge tests. The results indicate that a lack of complete saturation and densification at depth arising from the application of the high-acceleration field are largely responsible for the apparent limitation on liquefaction at depth observed in some centrifuge tests.Key words: liquefaction, dynamic centrifuge modeling, numerical modeling, depth limitation.