A viscoplastic constitutive model for plastic silts and clays for static slope stability applications

A viscoplastic model for representing plastic silts and clays in geotechnical static slope stability applications is presented. The PM4SiltR model builds on the stress ratio-controlled, critical state-based, bounding surface plasticity model PM4Silt and is coded as a dynamic link library for use in the finite difference program FLAC 8.1. PM4SiltR incorporates strain rate-dependent shear strength, stress relaxation, and creep using a consistency approach combined with an internal strain rate and auto-decay process. The model does not include a cap, and as such cannot simulate strain rate-dependent consolidation under increasing overburden stress. Six parameters control the viscous response for PM4SiltR, while the parameters controlling the nonviscous components of the response are the same as for PM4Silt. Single element simulations are presented to illustrate the influence of viscoplasticity on the constitutive response in direct simple shear loading and undrained creep. Single element responses are shown to be consistent with observed experimental results. Simulations of a hypothetical tailings dam constructed using the upstream method are performed to illustrate use of PM4SiltR at field scale. Results of field-scale simulations show PM4SiltR can model undrained creep and progressive failure leading to delayed slope instability after relatively minor changes in loading conditions at field scale

LEAP-2017 Simulation Exercise: Calibration of Constitutive Models and Simulation of the Element Tests

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)

LEAP-2017: Comparison of the Type-B Numerical Simulations with Centrifuge Test Results

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

A borehole array data–based approach for conducting 1D site response analyses I: Damping and V S randomization

One-dimensional site response analysis (1D SRA) remains the state of practice to estimate site-specific seismic response, despite the ample evidence of discrepancies between observations and 1D SRA-based predictions. These discrepancies are due to errors in the input parameters, intrinsic limitations in the predicting capabilities of 1D SRAs even for sites relatively compliant with the 1D SRA assumptions, and the inability of 1D SRAs to model three-dimensional (3D) wave propagation phenomena. This article aims at reducing 1D SRA mispredictions using small-strain damping profiles factored by a damping multiplier (Dmul) and randomized shear-wave velocity (VS) profiles. An approach for conducting 1D SRAs for site-specific site response assessment is developed to reduce the 1D SRA errors in magnitude and variability. First, sites from a database of 534 downhole sites are classified as 1D- or 3D-like, depending on the substructure conditions inferred from observed transfer functions. Second, data from the 1D-like sites are compared against predictions from 1D SRAs conducted using various trials of Dmul and VS standard deviations (Formula presented.) for VS randomization. Third, Dmul and (Formula presented.) are selected based on their combined ability to reduce the root mean square error (RMSE) in SRA predictions. Results indicate that 1D SRAs conducted with Dmul = 3 and (Formula presented.) lead to an overall minimum RMSE and thus provide more accurate site response estimates. The use of these parameters in forward SRA predictions is discussed in a companion paper

Centrifuge Testing of Liquefaction-Induced Downdrag on Axially Loaded Piles : Data Report for SKS03

Earthquake shaking can cause significant soil settlements, especially if the shaking causes liquefaction. Soil settlements will induce drag loads that can significantly increase the axial loads in a pile foundation and/or cause significant pile settlement (Figure 1). The liquefaction-induced downdrag on piles is affected by the complex interplay and timing of a variety of processes including the development and dissipation of pore water pressures, soil settlement, sand boils and gaps that provide vents for high excess pore pressures. Since it has not been possible to accurately model all these complex processes, simplifying assumptions are used to account for downdrag in the current design procedures. A series of centrifuge tests were designed to investigate the complex processes and the validity of the simplifying assumptions. This report describes the details of the second (SKS03) of the two model tests performed under this project. Sinha et al. (2021b)describes the previous centrifuge test series (SKS02). In SKS03, the soil profile consisted of (from top to bottom in prototype dimensions) 1 m of coarse sand, a 2 m clay crust, about 4.7 m of loose sand, 1.3 m of silt, 4 m of medium dense sand and 8 m of dense sand. Three 635 mm diameter piles were embedded about 15 m into the deposit, with their tips embedded about 1.9 m into the deeper dense sand. The three piles were loaded by lumped masses clamped just above the pile head; the static loads were different on each pile (500 kN, 1500 kN, and 2400 kN). The piles were instrumented with several strain gauge bridges designed to measure the axial load distribution in the piles. The base of the model was shaken with multiple earthquake ground motions with peak horizontal accelerations ranging from 0.08 g to 0.61 g. In addition to earthquake shaking, a pile load test was performed on one of the piles.As in SKS02, drag loads were observed to increase from earthquake shaking. Most of the pile settlement occurred during shaking, and very minimal settlement happened post shaking. Among all piles, the heavily loaded piles suffered the most settlement. Higher drag loads were observed on lightly loaded piles as compared to the heavily loaded piles. As expected, the neutral plan was found to be relatively deep for the lightly loaded pile and shallow for the heavily loaded pile

Use of Photron Cameras and TEMA Software to Measure 3D Displacements in Centrifuge Tests

Snapshots recorded from multiple cameras viewing the same dynamic event from different angles can be processed and used for the dynamic tracking of 3D displacements of multiple targets placed on the model. This report describes the first combined use of new high-speed Photron cameras and the TEMA Classic 3D software at the Center for Geotechnical Modeling (CGM) at University of California, Davis (UCD). The cameras and their mounting, as well as the target markers, lighting, camera calibration, and camera triggering are described, followed by a discussion on the software options selected for the analysis of videos recorded for a centrifuge model test conducted on the 9 m-radius centrifuge. The results presented show that this method is effective and reliable in obtaining the positions, displacements, velocities, and accelerations of the targets. Recommendations are made for improvements in future applications.Obtaining the 3D displacements of targets requires multiple cameras to take snapshots(images) of the target from different view angles and a software to perform the image analysis.The Photron High Speed Camera system available at CGM UCD is equipped with six MH6monochromatic cameras that can record videos up to 10,000 frame per second and with amaximum resolution up to 1920 × 1400 pixels (only applicable for frame rates less than 1000 fps).Multiple trigger methods are available to trigger the cameras to start recording. The ResDAQsoftware at the CGM was modified to synchronize the Photron’s image acquisition systemsynchronized with the DAQ system. Triggers (CAMERATrigger and SNAPSHOTTrigger) weredeployed within the shaker controller to ensure the image acquisition and DAQ coincided with thedynamic shaking experiment. The CAMERATrigger triggers the Photron image acquisition systemto start saving recordings at the beginning of the shaking, i.e., when the motion file is sent to theshaker to control the shaking of the servo-hydraulic shaking table. The SNAPSHOTTrigger systemenables taking snapshots at a variable rate which is especially useful in a dynamic test when imagesare needed to be taken at a fast rate during shaking, and slower rate during reconsolidation. TheTEMA CLASSIC 3D software offers a library of tracking algorithms (correlation, quadrant, virtualpoints, center of gravity, etc.) that can be used to process images and track multiple targetssimultaneously to obtain their 3D position. Depending upon the plane of motion and number ofcameras used, it can obtain the 2D as well as the 3D motion of the object.Using cameras and image analysis to obtain 3D movements of the model comprises several steps. These steps in order of implementation include: planning the marker locations, preparation of model surface, designing and producing the markers, positioning the markers, mounting the cameras, providing appropriate lightning, recording, and synchronizing videos, calibrating the cameras for lens distortion, determining the camera location and orientation, and finally using image processing to obtain 3D movements. Placing well-designed camera target markers at key locations makes it easier for TEMA to track them in the recorded images. A larger size target marker should be used for distant objects (away from the camera). Target markers should be placed on moving parts of the model (such as soil, pile, model container, and the centrifuge bucket) to enable calculation of their relative motion. The material used to fabricate the markers should not produce glare in the videos taken. Having proper lighting is key, especially at high frame rates. Sufficient light, uniform light, and no reflections are desired. At least two camera views must overlap for each target of interest such that the recorded images can be later processed to obtain its 3D position. The camera pairs should be mounted on a stiff beam, properly positioned, and focused to monitor the important parts of the model surveyed with target markers. When displacements are important in the direction of the view angle of the camera, the cameras should be moved apart to increase the stereo angle. It is further advised to take practice videos using the actual lighting, frame rate, and shutter speed to confirm the image quality and the field of view. This report outlines and describes all the steps in detail through an example implementation on a centrifuge model test featuring a layered liquefiable deposit with three embedded piles (SKS03). Two pairs of high-speed Photron cameras were placed in the model to monitor movements in the north and south section of model. The camera beam, light beams, and camera holder system were designed to be modular to make it easy to position and orient the cameras in any direction within the model. Three strips of LED lights (1000 lumens/foot) produced sufficient lighting to run the cameras at 1600 fps and a 4000 Hz shutter speed. Quadrant target markers and square grid markers were designed and placed throughout the model (on the soil surface, the piles, the container, and the centrifuge bucket). The model was shaken with multiple earthquake motions and videos of the model with target markers were recorded.The snapshots recorded during and post shaking were processed in TEMA to obtain the 3D dynamic position of target markers. Soil and pile movements were obtained relative to the container by subtracting the average movement of the container top ring from their absolute movements. Movements obtained in the center section of the model independently from the north pair and the south pair cameras were identical. The obtained movements had a precision of 0.15 mm with smaller noise likely due to beam vibration, lighting variability, reflections from reflection from moving targets. Pile settlements obtained from the image analyses matched with the hand measurements taken using a depth gage. It was also possible to differentiate the marker positions obtained from the image analysis to obtain a reasonable estimate of the accelerations of the objects. The natural frequency of the camera beam (60 Hz) was found to be smaller than the applied shaking (in the order of 100 Hz). The vibration of the camera beam introduced noise in the obtained movements and as such installing the cameras on a stiffer beam would have reduced these vibrations. Results obtained on soil and pile movement showed that this method is effective and reliable in obtaining positions, displacements, velocities, and accelerations of the targets, and thus promising for use in future applications.The use of cameras makes the model instrumentation relatively easier, cleaner (i.e., no LVDTracks and cables running across the model) and provides more model space for performing otherimportant investigations. It offers contactless sensing, which reduces the potential disturbance ofthe model. At the same time, the video recordings offer an immense amount of data which can beprocessed to get 3D displacements at any point within the model. The high-speed Photron camerasand TEMA software are found to be a great addition to the Center for Geotechnical Modeling atthe University of California, Davis, towards simplifying the model instrumentation whileadvancing the sensing capabilities in centrifuge tests, and they overall make an important steptowards the future of contactless model instrumentation and monitoring

A borehole array data–based approach for conducting 1D site response analyses II: Accounting for modeling errors

Site response estimates from one-dimensional (1D) site response analyses (SRAs) carry inaccuracies due to modeling and parametric errors. Modeling errors are due to the condensation of the three-dimensional (3D) wave propagation phenomenon to the vertical propagation of a horizontally polarized wave through a soil column, and parametric errors are due to the incomplete knowledge of the distributions of soil parameters, leading to the selection of nonoptimal input parameters for a site of interest. While parametric errors are traditionally handled using different soil parameters (e.g. alternative shear-wave velocity profiles), modeling errors are generally neglected. This paper proposes an approach for conducting linear elastic 1D SRAs to improve site response predictions and account for modeling errors. First, ground-motion data from borehole array sites are collected, processed, and screened for appropriateness (e.g. expected shear strains lower than 0.01%, signal-to-noise ratio higher than 3). Second, 1D SRA predictions in terms of transfer functions and amplification factors are compared against observations, and the discrepancies are quantified as residuals. Finally, the residuals are partitioned into a model bias term (Formula presented.), a site term (Formula presented.) with standard deviation (Formula presented.), and a event- and site-specific remaining residual (Formula presented.) with standard deviation (Formula presented.). Values for (Formula presented.) and (Formula presented.) for forward predictions are recommended. The sensitivity of the site response residuals to region, site type (1D- or 3D-like), and the applicability of findings to outcropping applications are discussed, and an example application for a hypothetical project site is presented

Centrifuge Testing of Liquefaction-Induced Downdrag on Axially Loaded Piles: Data Report for SKS02

Earthquake-induced liquefaction can cause soil settlement at pile interfaces, which can induce negative skin friction resulting in additional load (known as drag load) and drag the pile downwards (Figure 1). Despite significant research on the effects of liquefaction on structures and the seismic response of piles, there is still a knowledge gap in the evolution and assessment of liquefaction-induced downdrag on piles mainly related to the complex interplay and timing of the different mechanisms during/post liquefaction such as excess pore pressure generation/dissipation patterns, sequencing and timing of settlements, presence of interface gaps and ejecta, location of the initial neutral plane, and settlement around the tip. This has led to simplifying assumptions in current design procedures, which might result in over-conservatism in drag load estimation. Commonly used numerical tools lack the ability to model these mechanisms, while the absence of experimental data hinders the development and validation of new models. A series of centrifuge tests were planned to investigate the factors affecting the magnitude of liquefaction-induced drag load and pile settlement. This report describes the results for the first test series (SKS02). The soil profile included 1 m of coarse sand layer, underlain by 4 m of clay crust and 9 m of liquefiable soil over deeper dense soil. The test involved two medium diameter (D) piles, with their tip embedded to the depth of 0D and 5D in the dense sand. The model was shaken with multiple scaled Santa Cruz earthquake motions with peak horizontal accelerations ranging from 0.025 g to 0.4 g. With multiple shakings, drag loads were observed to increase on the piles. Higher drag loads were observed on deeply embedded (5D) piles as compared to the shallow embedded (0D) pile. While significant settlements occurred in soil during and post shaking, the piles recorded considerably smaller settlements. Most of the pile settlement occurred during shaking and very small settlements happened during the reconsolidation phase. It was observed, that with multiple shakings, the overall drag load on the piles saturated and could become as large as the one interpreted from considering the negative skin friction on the pile in the liquefiable soil taken equal to the positive interface drained shear strength.&nbsp

Geotechnical Investigation of Pore Pressure Behavior of Muddy Seafloor Sediments in an Arctic Permafrost Environment

Herschel Island, Yukon, Canada, is made of ice-rich permafrost and is affected by high rates of coastal erosion, likely to increase with decreasing summer sea ice extent. During an interdisciplinary expedition to Herschel Island in July 2014, geotechnical investigations were carried out in shallow water environments of up to 20 m water depth and at different beaches. The free-fall penetrometer BlueDrop was deployed at 299 positions. Apart from obtaining vertical profiles of sediment strength and the pore pressure response upon impact, the pore pressure evolution over a period of one hour after deployment was investigated. The focus area for these tests was Pauline Cove, located at the south-eastern side of the island, being sheltered by a spit from the open Beaufort Sea and affected by a number of old and young retrogressive thaw slumps, delivering large amounts of mud. The sediment resistance profiles revealed up to three distinct layers of sediment strength, expressing different consolidation states, or possibly changes in sediment composition. This stratification was supported by the pore pressure results, including pore pressure evolution “on-the-flight” during penetrometer penetration as well as pore pressure evolution at maximum penetration depth with the penetrometer being at rest. The sediment surface layer 1 was characterized by a thickness of 5–20 cm depending on the respective location, low sediment resistance and predominantly hydrostatic pressure. It most likely has frequently been reworked by wave action, and exhibited similar geotechnical signatures as fluid mud. Layer 2 reached sediment depths of 30–60 cm, showed an increase in sediment resistance and distinct subhydrostatic pore pressures during penetration, while pore pressures increased in an asymptotic manner to suprahydrostatic (160–180% of hydrostatic pressure) over an observation period of 30–50 minutes. Based on comparison to other examples from the literature, it was hypothesized that layer 2 was composed of overconsolidated mud. Layer 3 featured a significant increase in sediment resistance as well as pore pressure during penetration. As soon as the probe came to rest, the pressure decreased significantly to subhydrostatic conditions, before swinging back to being suprahydrostatic and then slowly dissipating. A similar behavior has been associated to silty sands and high bulk densities. Here, it may suggest a change in sediment composition, likely influenced by coarser nearshore and beach sediments, representing also a denser sediment matrix. The pore pressure results will complement the geological and geotechnical characterization of the coastal zone of Hershel Island, and contribute to the investigation of erosion and deposition processes. Copyright © 2015 by ASM

Effects of Excess Pore Pressure Redistribution in Liquefiable Layers

Existing simplified procedures for evaluating soil liquefaction potential or for estimating excess pore pressures during earthquakes are typically based on undrained cyclic tests performed on saturated soil samples under controlled loading and boundary conditions. Under such conditions, the effect of excess pore pressure (ue) dissipation and redistribution to neighboring soil layers cannot be accounted for. Existing simplified procedures treat liquefiable layers as isolated soil layers without any boundary conditions even if dense and loose layers are very thin, permeable, and adjacent to each other. However, redistribution is likely to increase and decrease ue in the neighboring dense and loose layers respectively. Until now, no procedure short of fully coupled numerical analysis is available to estimate the importance of redistribution. This paper presents an approximate analytical procedure for assessing the effects of ue redistribution in (1) soil layers that would have liquefied if they were undrained, and (2) soil layers that would have not liquefied even if undrained. It is found that a layer that is initially assumed liquefied under undrained conditions might not even liquefy accounting for the ue redistribution to neighboring layers. On the other hand, a layer initially assumed to not liquefy can develop significant ue and can even liquefy due to pore pressure migration from the neighboring layers. Thus, accounting for redistributed ue is important for liquefaction consequence assessment quantification, particularly in systems that span the depth of these effects like deep foundations. Migration of u toward the tip of a pile can reduce its capacity, even if the tip is embedded in a dense sand layer. On the other hand, if redistribution can result in the reduction of ue in initially assumed liquefied layers, risks associated with liquefaction might be avoided. A criterion is also developed to evaluate the thicknesses of a layer below which redistribution could prevent liquefaction even if the layer is deemed liquefied according to the existing liquefaction-triggering procedures. Finally, the proposed procedure is illustrated by application to selected shaking events of centrifuge tests involving liquefaction of layered soil profiles. The predictions from the procedure matched the centrifuge test results reasonably