No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Modified design procedures for bridge pile foundations subjected to liquefaction-induced lateral spreading
Abstract
Effective-stress nonlinear dynamic analyses (NDA) were performed for piles in the liquefiable sloped ground to assess how inertia and liquefaction-induced lateral spreading combine in long- and short-duration motions. A parametric study was performed using input motions from subduction and crustal earthquakes covering a wide range of durations and amplitudes. The NDA results showed that the pile demands increased due to (a) longer duration shakings, and (b) liquefaction-induced lateral spreading compared to nonliquefied conditions. The NDA results were used to evaluate the accuracy of the equivalent static analysis (ESA) recommended by Caltrans/ODOT for estimating pile demands. Finally, the NDA results were used to develop new ESA methods to combine inertial and lateral spreading loads for estimating elastic and inelastic pile demands.
... The effect of motion duration on the interaction of inertial and kinematic loads is particularly important in highly seismic regions like the Pacific Northwest of the United States, where the probabilistic seismic hazard includes significant contributions from the Cascadia Subduction Zone, which is expected to produce a long-duration Magnitude 9 earthquake. Khosravifar et al. [22] and Nasr and Khosravifar [23] studied the effects of ground motion duration on inelastic pile demands on relatively stiff large diameter shafts in liquefied soils and found that inelastic pile demands are amplified in long-duration earthquakes due to incremental yielding in the plastic hinge. Dickenson et al. [24] examined the effects of long-duration motions on the seismic performance of a wharf structure at the Port of Los Angeles in a testbed study and found that plastic hinges in piles (0.6 m concrete piles) formed generally once the ground displacements passed a threshold of approximately 0.3 m. ...
... Using the probability of pulse motions per Hayden et al. [50]; two of the four selected crustal motions contained velocity pulses. Additional details on the selection of ground motions and the matching process are provided in Khosravifar and Nasr [23]. It should be noted that while significant duration (D 5-95 ) is used in subsequent plots as an indicator of motion duration, specifically in the case of 2011 Tohoku motions, significant duration is a poor indicator of significant energy due to multiple sections of strong shaking that may be separated in time as shown by Walling et al. [51]. ...
Nonlinear dynamic analyses were performed to evaluate the effects of ground motion duration on the dynamic response of a pile-supported wharf subjected to liquefaction-induced lateral ground deformations. The numerical model was first calibrated using recorded data from a well-instrumented centrifuge test, after which incremental dynamic analyses were conducted using a suite of spectrally matched motions with different durations. The nonlinear dynamic analyses were performed to evaluated three loading scenarios: combined effects of inertial loads from the wharf deck and kinematic loads from ground deformations, deck inertial loads only in the absence of liquefaction (with minimal kinematic loads), and kinematic loads only in the absence of deck mass inertia. The analysis results were evaluated to provide insights on the relative contribution of inertial and kinematic demands on the response of the wharf with respect to motion duration. It was found that the contribution of peak inertial and peak kinematic loads to the maximum total demand increases only slightly with motion duration and intensity. The response of the wharf was found to be primarily governed by kinematic demands when subjected to long-duration motions for the type of foundation analyzed in this study which is commonly used in the port industry.
... The primary motivation for employing the proposed framework in this project was to validate recent studies performed by Khosravifar & Nasr (2018), regarding decoupled simplified SSI analysis results and how to enhance them for being closer to a more realistic and less conservative approach of deep foundation designs. Assuming 100% of the inertial component and 100% of the kinematic component during a seismic event involving a pile group foundation is considered a significantly conservative approach. ...
... The inertial component of 50% may be referred to an inertial coefficient β which considers the lack of simultaneity in estimating the structural demand of piles with limited or no nonlinear behavior. Khosravifar & Nasr (2018) obtained results regarding a back-calculated coefficient β for crustal and subduction zones (i.e., near field and far field seismic input motions, respectively), from which values β values of 0,60 (near field) and 0,75 (far field) were obtained from a given statistical trend study. ...
During a seismic event, a coupled behavior known as soil-structure interaction (SSI) may be highly relevant if the given structural stiffness is large in comparison to the underlying soil rigidity. Regarding a foundation's structural seismic design, some method-ologies are usually employed in professional practice, which depend on the considered degree for controlling the foundation's non-linear (inelastic) structural behavior. SSI could be analyzed either by a fully coupled analysis or a decoupled SSI study. This research provides a framework applied for deep foundations as a means to realize an enhanced decoupled SSI analysis through an inertial coefficient β, which considers the lack of simultaneity between the inertial and kinematic interaction effects and in estimating the structural demand of piles with limited or no nonlinear behavior. Numerical and analytical methods were employed in addition of a comprehensive site investigation, involving a deltaic-estuarine environment in the city of Guayaquil, Ecuador.
Design earthquake ground motions for dynamic analyses are typically specified as outcrop motions, which may have to be modified for input at the base of a FLAC model. Often a ‘deconvolution’ analysis using a 1-D wave propagation code, such as the program SHAKE, is performed to obtain the appropriate input motion at depth. This seemingly simple analysis is often the subject of considerable confusion. In this paper the theory and operation of the program SHAKE and input requirements of FLAC are reviewed, and the application of SHAKE for adapting design earthquake motions for FLAC input is described. Numerical examples illustrating typical cases are presented, and several questions that commonly arise are addressed.
The national seismic hazard maps for the conterminous United States have been updated to account for new methods, models, and data that have been obtained since the 2008 maps were released (Petersen and others, 2008). The input models are improved from those implemented in 2008 by using new ground motion models that have incorporated about twice as many earthquake strong ground shaking data and by incorporating many additional scientific studies that indicate broader ranges of earthquake source and ground motion models. These time-independent maps are shown for 2-percent and 10-percent probability of exceedance in 50 years for peak horizontal ground acceleration as well as 5-hertz and 1-hertz spectral accelerations with 5-percent damping on a uniform firm rock site condition (760 meters per second shear wave velocity in the upper 30 m, VS30). In this report, the 2014 updated maps are compared with the 2008 version of the maps and indicate changes of plus or minus 20 percent over wide areas, with larger changes locally, caused by the modifications to the seismic source and ground motion inputs.
A semiempirical approach to estimate liquefaction-induced lateral displacements using standard penetration test ~SPT! or cone penetration test ~CPT! data is presented. The approach combines available SPT- and CPT-based methods to evaluate liquefaction potential with laboratory test results for clean sands to estimate the potential maximum cyclic shear strains for saturated sandy soils under seismic loading. A lateral displacement index is then introduced, which is obtained by integrating the maximum cyclic shear strains with depth. Empirical correlations from case history data are proposed between actual lateral displacement, the lateral displacement index, and geometric parameters characterizing ground geometry for gently sloping ground without a free face, level ground with a free face, and gently sloping ground with a free face. The proposed approach can be applied to obtain preliminary estimates of the magnitude of lateral displacements associated with a liquefaction-induced lateral spread.
Earthquake ground motions in the near-fault region frequently have intense, double-sided pulses in the velocity-time series that can be very damaging to structures. Many of these velocity pulses are attributed to the effects of forward directivity, which occurs when a fault ruptures toward a site. However, pulses are not always observed in the forward directivity region, and some pulses cannot be explained by forward directivity. The relative contribution of pulse-type motions to the overall seismic hazard should be considered when selecting records in a suite of design ground motions for a site in the near-fault region. This study uses a new scheme to classify records from an enhanced database of records from shallow crustal earthquakes that have the closest site-to-source distances less than 30 km with moment magnitudes greater than 6.0 as either pulse or nonpulse motions. The resulting database of 673 records from 52 earthquakes contains 141 pulses, including 74 explained well by forward directivity. Logistic regression is used to develop a simple model to estimate the proportion of pulse motions as a function of closest site-to-source distance and epsilon of the seismic hazard. The resulting relationship can be used to estimate the number of pulse-type motions that should be included within a suite of ground motions to represent the proper contribution of pulse motions to the seismic hazard. Guidance is also provided for selecting pulse records. (C) 2014 American Society of Civil Engineers.
Liquefiable soil-structure interaction material models are developed and implemented in the open- source finite-element modeling platform OpenSees. Inputs to the free end of the materials include the ground motion and mean effective stress time series from a free-field soil column. Example simulations using a single element attached to a soil element demonstrate key features. The models are then used to analyze centrifuge experiments of a single pile in a level liquefiable profile and a six-pile group in a sloping liquefiable profile that resulted in lateral spreading. Measured displacements and mean effective stress time series are used as inputs to isolate the response of the material models from predictive uncertainties in free-field ground motion and excess pore pressure. The predicted pile response agrees reasonably well with measurements. The cyclic mobility behavior of sand in undrained loading is shown to be an important mechanism affecting bending moments in the piles; neglecting the dilatancy component of the sand's response (i.e., ignoring the cyclic mobility behavior) can result in underprediction of the demands imposed on the piles. (C) 2013 American Society of Civil Engineers.
Inelastic response of extended pile shafts subjected to liquefaction-induced lateral spreading is investigated using nonlinear dynamic analyses (NDA) covering a range of soil, pile and ground motion conditions. Each soil-structure scenario was analyzed for three cases: a baseline case with soil liquefaction and superstructure inertia; a case with liquefaction, but without superstructure inertia (i.e., superstructure mass removed); and a case without liquefaction (i.e., pore pressure generation eliminated), but with superstructure inertia. Results show that the combined effects of lateral spreading and superstructure inertia produce larger demands (often by more than 50%) than are produced by either loading case alone, such that the combined demand cannot be enveloped by analyzing the two load cases separately. The results of these parametric analyses provide a database that is used in subsequent development of an equivalent static analysis (ESA) design procedure.
The seismic design of extended pile shafts for the combined effects of dynamic shaking and liquefaction-induced lateral spreading is investigated using nonlinear dynamic finite element analyses (NDA). Results of NDA parameter studies are used to illustrate how inertia and lateral spreading loads combine during shaking. The NDA results are used to evaluate equivalent static analysis (ESA) methods. Implications for design practice are discussed.
An equivalent static analysis (ESA) procedure is proposed for the design of extended pile shafts subjected to liquefaction-induced lateral spreading during earthquake loading. The responses of extended pile shafts for a range of soil, structure and ground motion conditions were examined parametrically using nonlinear dynamic finite element analyses (NDA). The results of those parametric analyses were used to develop and calibrate the proposed ESA procedure. The ESA procedure addresses both the nonliquefaction and liquefaction cases, and includes criteria that identify conditions which tend to produce excessive demands or collapse conditions. The ESA procedure, its limitations, and issues important for design are discussed.
This paper presents a kinematic analysis of a single pile embedded in a laterally spreading layered soil profile and discusses the relevancy of conventional analysis models to this load case. The research encompasses the creation of three-dimensional (3D) finite-element (FE) models using the OpenSees FE analysis platform. These models consider a single pile embedded in a layered soil continuum. Three reinforced concrete pile designs are considered. The piles are modeled using beam-column elements and fiber-section models. The soil continuum is modeled using brick elements and a Drucker-Prager constitutive model. The soil-pile interface is modeled using beam-solid contact elements. The FE models are used to evaluate the response of the soil-pile system to lateral spreading and two alternative lateral load cases. Through the computation of force density-displacement (p-y) curves representative of the soil response, the FE analysis (FEA) results are used to evaluate the adequacy of conventional p-y curve relationships in modeling lateral spreading. It is determined that traditional p-y curves are unsuitable for use in analyses where large pile deformations occur at depth. DOI: 10.1061/(ASCE)GT.1943-5606.0000468. (C) 2011 American Society of Civil Engineers.
In saturated clean medium-to-dense cohesionless soils, liquefaction-induced shear deformation is observed to accumulate in a cycle-by-cycle pattern cyclic mobility. Much of the shear strain accumulation,occurs rapidly during the transition from contraction to dilation near the phase transformation,surface,at a nearly constant low shear stress and effective confining pressure. Such a stress state is difficult to employ as a basis for predicting the associated magnitude of accumulated permanent shear strain. In this study, a more convenient,approach,is adopted,in which,the domain,of large shear strain is directly defined by strain space parameters. The observed cyclic shear deformation is accounted for by enlargement and/or translation of this domain in deviatoric strain space. In this paper, the model,formulation,details involved,are presented,and,discussed. A calibration phase,is also described,based,on data from,laboratory sample,tests and dynamic,centrifuge experiments,for Nevada sand at a relative density of about 40%. DOI: 10.1061/ASCE1090-02412003129:121119 CE Database subject headings: Liquefaction; Constitutive models; Cyclic plasticity; Soil dynamics; Centrifuge models.
Seismic input to nonlinear dynamic analyses of structures is usually defined in terms of acceleration time series whose response spectra are compatible with a specified target response spectrum. Time domain spectral matching used to generate realistic design acceleration time series is discussed in this paper. A new and improved adjustment function to be used in modifying existing accelerograms while preserving the nonstationary character of the ground motion is presented herein. The application of the new adjustment wavelet ensures stability, efficiency and speed of the numerical solution and prevents drift in the resulting velocity and displacement time series. [DOI: 10.1193/1.3459159]
Effects of inertial and kinematic forces on pile stresses are studied based on large shaking table tests on pile-structure models with a foundation embedded in dry and liquefiable sand deposits. The test results show that, if the natural period of the superstructure, Tb, is less than that of the ground, Tg, the ground displacement tends to be in phase with the inertial force from the superstructure, increasing the shear force transmitted to the pile. In contrast, if Tb is greater than Tg, the ground displacement tends to be out of phase with the inertial force, restraining the pile stress from increasing. With the effects of earth pressures on the embedded foundation and pile incorporated in, pseudo-static analysis is conducted to estimate maximum moment distribution in pile. It is assumed that the maximum moment is equal to the sum of the two stresses caused by the inertial and kinematic effects if Tb Tg. The estimated pile stresses are in good agreement with the observed ones regardless of the occurrence of soil liquefaction.
This paper intended to evaluate the behavior of saturated sand and sloped ground subjected to flow failure with seepage of pore water in the ground after earthquake and the resultant liquefaction. Triaxial compression tests of sand with constant deviator stress but changing of pore pressure and volume of the specimens were conducted in this study. It was revealed that the relation between the volume change and the amount of shear strain during deformation depended on the initial density of the sand but it did not much depend on shear stress and initial confining stress levels. Based on this test results and numerical analysis of the seepage of pore water in liquefied ground, a methodology was proposed to predict the deformation of inclined ground due to liquefaction.
Reducing seismic risk to highway mobility: assessment and design examples for pile foundations affected by lateral spreading
- S A Ashford
- M H Scott
- D Rayamajhi
Ashford, S. A., Scott, M. H. and Rayamajhi, D. 2012. Reducing seismic risk
to highway mobility: assessment and design examples for pile foundations affected by lateral spreading. ODOT Report.
Seismic design of pile foundations for liquefaction effects
- R W Boulanger
- D Chang
- S J Brandenberg
- R J Armstrong
- B L Kutter
Boulanger, R. W., Chang, D., Brandenberg, S. J., Armstrong, R. J. and
Kutter, B. L. 2007. Seismic design of pile foundations for liquefaction
effects. 4th International Conference on Earthquake Geotechnical
Engineering, Netherlands, pp. 277-302.
User's technical manual for LPile
- Ensoft
Ensoft. 2016. User's technical manual for LPile 2016.
Geo-congress 2012: state of the art and practice in geotechnical engineering
- A Khosravifar
- R W Boulanger
Khosravifar, A. and Boulanger, R. W. 2012. Design of extended pile shafts
for liquefaction effects. In Hyrciw R. D., Athanasopoulos-Zekkos A.
and Yesiller N. (eds). Geo-congress 2012: state of the art and practice in
geotechnical engineering. Geotechnical Special Publication No. 225,
Reston, VA: ASCE Geo-Institute, pp. 1690-1699.
Open system for earthquake engineering simulation user manual
- S Mazzoni
- F Mckenna
- M H Scott
- G L Fenves
Mazzoni, S., McKenna, F., Scott, M. H. and Fenves, G. L. 2009. Open system for earthquake engineering simulation user manual. Berkeley:
University of California.
Geotechnical design manual
Oregon Dept. of Transportation. 2014. Geotechnical design manual. Tech.
Services Branch, Salem, OR.
Guide specifications for LRFD seismic bridge design
- Aashto
AASHTO. 2014. Guide specifications for LRFD seismic bridge design. 2nd
edn, with 2014 Interim. Washington, DC: AASHTO.