College of Engineering

Piles are designed to transfer superstructure loads using positive skin friction and tip resistance while undergoing acceptable settlements. However, when liquefaction-induced soil settlement occurs, it can drag the pile downward and result in negative skin friction and drag load. In such cases, estimating the drag load and pile settlement becomes important for pile design. A series of centrifuge model tests were performed to study liquefaction-induced downdrag on piles. The tests included four heavily instrumented piles installed in two different soil layered profiles with their tip embedment zero, three and five times their diameterin the dense sand. Loads on the piles were varied to study their effect on drag load and pile settlement. Results are presented describing the mechanism behind the development of liquefaction-induced downdrag, the magnitude of drag load, and pile settlement. Finally, recommendations are made for the design of piles in liquefiable soils

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.

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. 

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.

Earthquake-induced liquefaction typically causes soil settlement which may lead to downdrag in axially loaded piles. The drag load generated may overstress the pile or cause significant foundation settlements. Despite significant research progress on the effects of liquefaction on structures and the seismic response of piles, there is still a knowledge gap in the assessment of liquefaction-induced downdrag. This paper discusses different factors that govern this mechanism and presents a parametric study performed using the AASHTO-recommended neutral plane method using displacement-based t-z spring analyses on a simplified profile where liquefiable layer depth and thickness, reconsolidation strains in dense and loose sand, tip conditions, and pile types (L/D ratios) are varied. The results obtained from this preliminary analysis draw some important conclusions regarding the performance of large, medium, and slender piles and are used to design centrifuge model tests to further investigate and understand the complex mechanisms under more realistic conditions

Modeling and simulation of earthquake soil-structure interaction (ESSI) require a number of sophisticated modeling and simulation approaches to reduce modeling uncertainty and improve the accuracy of results. The superstructure can be supported by either a shallow or deep foundation and in dry, partially saturated, or fully saturated soil. An interface element is thus required to accurately model the interaction of dry as well as partially saturated soil with the foundation. The current modeling techniques mostly assume a hard normal contact behavior i.e. normal contact stiffness is constant with penetration. However, more physical contact stress expected between the soil-foundation interface is non-linear. The normal contact stiffness increases with penetration until the soil surface becomes hard. At this stage, any further penetration can be assumed to be of hard contact. In this thesis, a soft contact formulation is presented to model the non-linear stiffness at the soil-foundation interface. The cyclic shear behavior of the soil-structure interface is highly non-linear and sophisticated. It includes hysteresis, hardening, softening (dilation), and particle breakage. Depending upon the normal stress or confinement, the shear behavior of the interface can have hardening until a peak shear strength is attained and then soften to the critical or residual shear strength. In this thesis, apart from the most popular Elastic Perfectly Plastic shear model, two additional shear models with nonlinear hardening and non-linear hardening/softening are proposed with minimum modeling parameters to model the monotonic as well as cyclic shear behavior at the soil-foundation interface. In partially or fully saturated conditions, during dynamic events (seismic shaking) pore fluid pressures in the soil adjacent to foundations will change dynamically. Moreover, for strong shaking, the structure might rock, and foundation-soil interface might develop gaps and create suction pressure pulling the water up in tension. A coupled element is developed to model the changes in dynamic pore-fluid pressures and effective stress at the soil-foundation interface for submerged conditions. An extensive verification for all the components of the proposed elements is also performed. 

This paper details the development of a new 1‑D combined finite difference and finite element procedure for calculating in-depth pavement temperatures, which has been implemented in the CalME design method.  The model is driven by a database of known surface temperatures rather than raw climatic inputs and an energy balance at the surface, since it was noted that the pavement structure had little impact on the surface temperature (if the surface properties remain constant).  The model runs quickly, enabling direct simulation of in-depth temperatures while performing Monte Carlo based simulation of pavement reliability.  The disadvantages to this approach are that it requires the surface temperatures to be developed independently and that it does not have a coupled moisture model for the prediction of freeze/thaw conditions.  The paper also details some anomalies in the output of the Enhance Integrated Climatic Model (used in the new AASHTO mechanistic-empirical design method), that were found during development.  Finally, the paper collects various published values for thermal properties of pavement materials, to aid in the implementation of thermal models.