Fountain Valley, CA, United States
Fountain Valley, CA, United States

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Taiebat M.,University of British Columbia | Jeremic B.,University of California at Davis | Dafalias Y.F.,University of California at Davis | Dafalias Y.F.,National Technical University of Athens | And 2 more authors.
Soil Dynamics and Earthquake Engineering | Year: 2010

To predict the earthquake response of saturated porous media it is essential to correctly simulate the generation, redistribution, and dissipation of excess pore water pressure during and after earthquake shaking. To this end, a reliable numerical tool requires a dynamic, fully coupled formulation for solid-fluid interaction and a versatile constitutive model. Presented in this paper is a 3D finite element framework that has been developed and utilized for this purpose. The framework employs fully coupled dynamic field equations with a u-p-U formulation for simulation of pore fluid and solid skeleton interaction and a SANISAND constitutive model for response of solid skeleton. After a detailed verification and validation of the formulation and implementation of the developed numerical tool, it is employed in the seismic response of saturated porous media. The study includes examination of the mechanism of propagation of the earthquake-induced shear waves and liquefaction phenomenon in uniform and layered profiles of saturated sand deposits. © 2009 Elsevier Ltd. All rights reserved.

Law H.,Earth Mechanics Inc. | Wilson P.,Earth Mechanics Inc.
NCEE 2014 - 10th U.S. National Conference on Earthquake Engineering: Frontiers of Earthquake Engineering | Year: 2014

Site-specific seismic coefficients are developed for use in pseudo-static analyses of tall walls and slopes for the Fourth Bore Caldecott Tunnel project in Oakland, CA. The site is located about 1.9 km from the Hayward fault, with expected peak ground acceleration for the 1500-year return period design earthquake in excess of 1 g. Due to the unusually tall walls and high ground acceleration, seismic coefficients are developed based on a rational approach incorporating effects of wavelength and wall height as well as coherency of spatially varying ground motions. Two-dimensional site response analyses are conducted for a suite of ground motion time series in order to develop average horizontal acceleration time series for the potential sliding mass over a range of wall/slope heights. The peak accelerations from those time series are found to decrease with increasing wall/slope heights. This is attributed mainly to reduction in the high frequency content of the overall sliding mass ground motions. Furthermore, the use of peak acceleration values is considered conservative in the context of a pseudo-static limit equilibrium approach, because the seismic coefficient is traditionally applied as a constant force on the potential sliding mass. In reality, the actual peak acceleration only occurs for a brief instant. In order to avoid this undue conservatism, a Newmark sliding block approach was implemented to determine yield acceleration values which would limit the predicted displacement levels to a certain threshold value (about 10 cm) which suggests overall wall/slope stability. After incorporating the above siteresponse effects as well as the Newmark deformations, seismic coefficients of 25% to 33% of the PGA were identified for use in pseudo-static analyses for design of wall/slope reinforcements.

Wilson P.,Earth Mechanics Inc. | Elgamal A.,University of California at San Diego
Soil Dynamics and Earthquake Engineering | Year: 2015

Dynamic lateral earth pressure is recorded during ten shake table testing events. In these tests, peak input acceleration at the base of the retaining wall varied in the wide range of 0.13-1.20. g in order to include scenarios of relevance to recently observed strong earthquake excitations. The results shed light on the influence of soil cohesion and the effect of small wall movements on the magnitude and distribution of earth pressure. In accordance with field practice, a commonly encountered dense sand backfill with a small percentage of fines (SP-SM) is used. Inside a large soil container, earth pressure is measured against a rigid wall (backfill height H=1.7. m) that is allowed to undergo limited translation/rotation due to the imparted dynamic excitation (up to 10. mm or 0.006. H at 1.2. g base acceleration). In this particular series of experiments, favorably low dynamic pressures were recorded at backfill accelerations of up to about 0.7. g in light of: (i) the relatively high soil strength (including cohesion) that precluded a limit equilibrium type failure in the backfill, and (ii) the high soil stiffness coupled with the small value of observed wall translation/rotation (as much as 3. mm or 0.0018. H at ground surface). In tests with instants of very high acceleration (in the range of 1. g), the corresponding dynamic earth pressure is found to be of much significance for practical applications. Lateral thrusts recorded during these instants of strong shaking compare well with limit equilibrium predictions that include the soil cohesion intercept. Exclusion of the cohesion intercept results in substantial over-prediction of the measured lateral forces. © 2014 Elsevier Ltd.

Wilson P.,Earth Mechanics Inc. | Law H.,Earth Mechanics Inc.
Geotechnical Special Publication | Year: 2012

This paper presents a case study of the evaluation of seismic soil-pile performance in a sloping river embankment containing soft, saturated low-plasticity fine-grained soil. Topics addressed include: i) liquefaction susceptibility of the site soils according to SPT- and CPT- based analysis procedures; ii) cyclic simple shear tests performed in order to assess the fine-grained soil response under earthquake conditions; iii) conducted non-linear numerical time history analyses of the embankment-foundation response to design ground motions; iv) comparison of 3D and 2D model results of ground-pile response in order to determine useful scaling factors for incorporating 3D effects (e.g., soil arching) in 2D models; and v) evaluation of driven timber and reinforced concrete piles for ground improvement as well as stiffening the drilled shaft foundations as strategies for helping the bridge meet the performance-based design criteria under lateral spreading conditions. © 2012 American Society of Civil Engineers.

Wilson P.,Earth Mechanics Inc. | Elgamal A.,University of California at San Diego
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2010

Passive earth pressure is recorded in two different tests, using a 6.7-m long, 2.9-m wide soil container. In these tests, sand with 7% silt content is densely compacted behind a moveable test wall to a supported height of 1.68 m (5.5 ft). Lateral load is applied to the vertical reinforced concrete wall section, which displaces freely along with the adjacent backfill in the horizontal and vertical directions. The recorded passive resistance is found to increase until a peak is reached at a horizontal displacement of 2.7-3% of the supported backfill height, decreasing thereafter to a residual level. In this test configuration, a triangular failure wedge shape is observed, due to the low mobilized wall-soil friction. Backfill strength parameters are estimated based on this observed failure mechanism. From these estimates, along with triaxial and direct shear test data, theoretical predictions are compared with the measured passive resistance. Using the test data, a calibrated finite-element model is employed to produce additional load-displacement curves for a wider range of practical applications (e.g., potential bridge deck displacement during a strong earthquake). Hyperbolic model approximations of the load-displacement curves are also provided. © 2010 ASCE.

Wilson P.,Earth Mechanics Inc. | Lee A.,Earth Mechanics Inc. | Law H.,Earth Mechanics Inc.
Geotechnical and Structural Engineering Congress 2016 - Proceedings of the Joint Geotechnical and Structural Engineering Congress 2016 | Year: 2016

Bridge abutments supported on cast-in-drilled-hole (CIDH) piles embedded in mechanically stabilized earth (MSE) retaining walls are becoming increasingly common in California. Pile behavior under lateral (e.g., earthquake) loading for this relatively new abutment system is currently not well established. To that end, three-dimensional finite element modeling is undertaken for a representative pile-supported MSE abutment with wrap-Around MSE walls retaining the approach embankment on the two longitudinal sides and beneath the bridge deck. In this configuration, seven 0.9-m (3-foot) diameter CIDH piles in a single row extend 7.6 m (25 feet) above native subgrade inside the MSE wall, and connect to the abutment footing. The piles are located close to the wall face, at a longitudinal clearance of 0.46 m (18 inches). Lateral pile top loading is applied to the finite element models in the longitudinal and transverse directions separately, in two sets of analyses. Under longitudinal pile top loading, the response of the single row of piles is found to be dictated primarily by the properties of the MSE reinforcement material. In contrast, the response under transverse loading is found to be governed significantly by the soil resistance in between the seven piles. For use in repetitive design iterations, simplified beam-spring analysis procedures are developed, similar to those used for ordinary level-ground lateral pile analysis. Design recommendations based on the simplified models are found to compare well with results from the finite element analyses. © ASCE.

Shamsabadi A.,Office of Earthquake Engineering | Kapuskar M.,Earth Mechanics Inc.
Transportation Research Record | Year: 2010

This paper investigates the behavior of skewed highway bridges subjected to earthquake loading with strong velocity pulses. The behavior of bridges with skewed abutments in the longitudinal direction is strongly coupled by transverse loading. The interaction between skewed bridge abutment foundations and backfill has a strong impact on dynamic bridge response. While bridge structures may remain in the linear range during seismic loading, local nonlinear behavior at the abutment-embankment interface can cause significant nonlinear bridge structure response. Skewed bridge models are presented incorporating soil-abutment-structure interaction using nonlinear abutment springs. As a case study, a global three-dimensional nonlinear finite element model of the skewed and seismically instrumented Painter Street overpass in Rio Dell, California, with monolithic abutments was developed. The model used nonlinear foundation-soil interaction based on approach soil properties from geotechnical tests. A soil continuum finite element analysis was performed using constitutive hardening soil material to evaluate abutment backfill passive resistance considering backwall skew. The bridge response resulting from seismic ground motion records was compared with structure response data. The computer models represented fairly well the overall seismic response of the skewed bridge. Bent pile foundations had much less impact on overall bridge response than the abutments. Near-fault ground motions with high-velocity pulses generated asymmetrical impulsive loading and large displacements in transverse directions leading to significant rotation and residual deck displacement. These permanent structure displacements can control the design of the abutment seat width and shear keys and could exceed column displacement capacity and impose additional moment at the column not considered in current design.

Law H.,Earth Mechanics Inc. | Lam I.P.,Earth Mechanics Inc. | Wilson P.,Earth Mechanics Inc.
Structures Congress 2014 - Proceedings of the 2014 Structures Congress | Year: 2014

Soil-structure interaction modeling techniques are presented for bridges supported on large gravity caissons at deep-water sites. The fundamental mode of vibration of these massive caisson foundations tends to be in the short period range, in contrast with the longer period superstructure response. This results in an overall system with complex behavior during an earthquake, which needs to be carefully considered in the analysis and design. Modeling techniques with varying levels of complexity are demonstrated based on project experience from three bridges in highly seismic regions including: Second Tacoma-Narrows Bridge (WA), Strait of Messina Bridge (Italy) and South Park Bridge (WA). Use of numerical model pushover analyses to develop nonlinear distributed soil springs is shown to help overcome several of the limitations of the more basic lumped linear stiffness matrix approach. The relative importance of wave scattering effects and the evaluations of earth pressure distributions considering seismic loading scenarios for assessment of required reinforcements are also addressed. © 2014 American Society of Civil Engineers.

Wilson P.,Earth Mechanics Inc. | Law H.,Earth Mechanics Inc.
Structures Congress 2015 - Proceedings of the 2015 Structures Congress | Year: 2015

Project-specific earthquake ground motions are developed for design of the Sixth Street Viaduct replacement project in Los Angeles, California. The replacement bridge, selected as a result of a design competition, is a twelve span tied-arch structure nearly 1 km in length, with a friction-pendulum base isolation system to mitigate high seismic loading. Following state of the art procedures used on recent major toll bridge projects in California, the design ground motions are developed by: performing seismic hazard analysis to develop target rock response spectra at two performance levels (100-yr and 1000-yr return period); selecting ten sets of seed motion time histories based on the controlling source information (3 sets for 100-yr and 7 sets for 1000-yr); performing spectrum matching to the target rock spectra; conducting free field site response analysis to develop depth-varying motions at each support location; and performing pile-soil interaction time history analyses to develop kinematic motions for each bridge support. To develop the target rock motion spectra, comparisons are made between seismic hazard analyses conducted using the widely adopted NGA-West1 and the newly released NGA-West2 relationships, as well as procedures recommended by the California Department of Transportation (Caltrans). Long period motion, beyond the range of typical applications (T of up to 10 seconds), is also addressed due to the friction-pendulum base isolation system. Since the friction-pendulum bearing size is determined based on non-linear time history analyses, careful consideration is given to the cross correlation of the two component horizontal design acceleration time histories. This helps to minimize unintentional over or undershoot in shaking in directions other than the two orthogonal axes for which the reference rock motions are developed.

Elgamal A.,University of California at San Diego | Wilson P.,Earth Mechanics Inc.
Geotechnical, Geological and Earthquake Engineering | Year: 2012

At high levels of seismic excitation, passive earth pressure at the abutments may provide resistance to excessive longitudinal bridge deck displacement. Full scale tests and Finite Element (FE) simulations are performed in order to investigate this resistance. The static passive earth pressure force-displacement relationship is recorded in two tests, by pushing a model wall into dense sand with silt (c- φ) backfill. Based on an analysis of the recorded data, a calibrated FE model is developed and used to generate passive force-displacement relationships for a range of typical backfi ll soil properties and abutment heights. In an additional testing phase, the wall-backfi ll system is subjected to shake table excitations in order to document the corresponding dynamic earth pressure forces and mechanisms. At high g-levels of excitation, the instantaneous passive resistance is shown to also depend on the inertial backfill forces caused by ground shaking. Based on the testing and simulation results, simplifi ed abutment models are presented, which include the experimentally observed backfi ll inertial effects along with the static passive force-displacement resistance. Numerical FE simulations are fi nally used to demonstrate the infl uence of these abutment models within the overall bridge system confi guration, and highlight the salient dynamic response characteristics as a function of the various modeling parameters. © Springer Science+Business Media B.V. 2012.

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