Assessment of Liquefaction-Induced Foundation Soil Deformations

Although there exist some consensus regarding seismic soil liquefaction triggering assessment of free field soil sites, estimating the liquefaction triggering potential beneath building foundations still stays as a controversial and a difficult issue. Assessing liquefaction triggering potential under building foundations requires the estimation of cyclic and static stress state of the soil medium. In the recent studies (e.g. Unutmaz 2008), the cyclic stress ratio corrected for Kα and Kσ effects under and adjacent to building foundations subjected to cyclic loading are to be estimated with the help of a series of 2-D and 3-D numerical simulations for different generic cases. A representative and a maximum cyclic stress ratio terms of the soil-structure-earthquake interaction system, denoted as CSRSSEI,rep and CSRSSEI,max respectively was defined as a function of i) ratio of the pre-earthquake fundamental period of the structure and soil (σ), ii) free field spectral acceleration at the fixed-base period of the structure(SA), iii) the peak ground acceleration of the free field soil sites (PGA), and iv) aspect ratio of the structure (h/B). In this paper, the results of the previously mentioned numerical findings have been verified by using case histories documented after 1999 Kocaeli Earthquake, where significant foundation displacements were observed due to liquefaction of the underlying foundation soils. The foundation soil profiles of these case histories generally consist of silty soils, sand-silt mixtures and silt-clay mixtures. Overburden and procedure corrected SPT-N values vary in the range of 2 to 5 blows/30 cm in the upper 5 meters and gradually increases up to a maximum value of 25 blows/30 cm beyond depths of 5 to 8 m’s. Overlying structures are mainly 3 to 4 storey, residential buildings with no basements. As the concluding remark, the proposed simplified procedures are shown to predict cyclically-induced foundation settlements accurately within an accuracy factor of two (i.e.: predictions fall within 1:2 and 2:1 limits of the measured settlements)

Standard Penetration Test-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Potential

This paper presents new correlations for assessment of the likelihood of initiation (or “triggering”) of soil liquefaction. These new correlations eliminate several sources of bias intrinsic to previous, similar correlations, and provide greatly reduced overall uncertainty and variance. Key elements in the development of these new correlations are (1) accumulation of a significantly expanded database of field performance case histories; (2) use of improved knowledge and understanding of factors affecting interpretation of standard penetration test data; (3) incorporation of improved understanding of factors affecting site-specific earthquake ground motions (including directivity effects, site-specific response, etc.); (4) use of improved methods for assessment of in situ cyclic shear stress ratio; (5) screening of field data case histories on a quality/uncertainty basis; and (6) use of high-order probabilistic tools (Bayesian updating). The resulting relationships not only provide greatly reduced uncertainty, they also help to resolve a number of corollary issues that have long been difficult and controversial including: (1) magnitude-correlated duration weighting factors, (2) adjustments for fines content, and (3) corrections for overburden stress

Global Shear Wave Velocity Database for Probabilistic Assessment of the Initiation of Seismic-Soil Liquefaction

Engineering practitioners commonly use penetration-based methods (SPT & CPT) for assessment of seismic liquefaction triggering hazard. On the horizon, shear wave velocity (Vs) may offer engineers a third tool that is lower cost and provides more physically meaningful measurements. Development of the shear wave velocity liquefaction method has been hampered by a paucity of published velocity profiles; particularly in deeper soil deposits (\u3e10m) and deposits subjected to high cyclic stress ratios (CSR \u3e 0.3). A review of the literature reveals that most historic liquefaction sites fitting this depth and CSR criteria are located in Asia, though most of these sites remain untested for Vs. To remedy this scarcity of data, we set out to assemble a global Vs dataset by acquiring new data in Japan, Taiwan, China, India, and the United States (US). These data are merged with the exiting catalog of published velocity data. To acquire new field data, we use the recently developed continuous swept-sine wave spectral analysis of surface waves test (CSS-SASW). The CSS-SASW test has proven to be extremely reliable at rapidly gathering high signal-to-noise dispersion data sufficient to invert 20-40 meter Vsprofiles. So far, we have acquired new velocity profiles at nearly 300 liquefaction-evaluation sites throughout Asia and the US, mostly at sites previously tested by conventional penetration methods. This new dataset represents the majority of the worlds documented sites of liquefaction occurrence since instrumental recording. To correlate the global shear wave velocity data set with likelihood of initiation of seismic-soil liquefaction, we utilize high-order probabilistic tools (Bayesian updating) developed for structural reliability. A multi-parameter limit-state function for liquefaction triggering is modeled and evaluated based on the means, distributions and uncertainties of each model-variable. Each case history is then sub-divided into ‘quality’-ranking categories based on the conjugate-uncertainties of CSR and Vs1. A low-pass cut-off of the coefficient of variation is used filter-out poorly constrained sites. Finally for the probabilistic analysis, the Bayesian updating procedure is used to iteratively compute coefficients for the limit-state function that minimize model error. The intended outcome of this effort is a new evaluation of the Vs-liquefaction- triggering boundary in light of a global data set and modern limit-state probabilistic tools

CPT-Based Probabilistic and Deterministic Assessment of In Situ Seismic Soil Liquefaction Potential

This paper presents a complete methodology for both probabilistic and deterministic assessment of seismic soil liquefaction triggering potential based on the cone penetration test (CPT). A comprehensive worldwide set of CPT-based liquefaction field case histories were compiled and back analyzed, and the data then used to develop probabilistic triggering correlations. Issues investigated in this study include improved normalization of CPT resistance measurements for the influence of effective overburden stress, and adjustment to CPT tip resistance for the potential influence of thin liquefiable layers. The effects of soil type and soil character (i.e., fines adjustment) for the new correlations are based on a combination of CPT tip and sleeve resistance. To quantify probability for performance-based engineering applications, Bayesian regression methods were used, and the uncertainties of all variables comprising both the seismic demand and the liquefaction resistance were estimated and included in the analysis. The resulting correlations were developed using a Bayesian framework and are presented in both probabilistic and deterministic formats. The results are compared to previous probabilistic and deterministic correlations

Field Case Histories for SPT-Based In Situ Liquefaction Potential Evaluation

This report provides documentation of the field performance case histories and site response analyses described in the report entitled SPT-Based Probabilistic and Deterministic Assessment of Seismic Soil Liquefaction Initiation Hazard, by Cetin, et al. (2000). Table 3-5, taken from that report, summarizes the interpreted field performance case histories from the original database of Seed et al. (1984) that were found to conform to the standards of Data Classes A, B or C, and so were used in the studies of Cetin et al. Table 3-7, also taken from that report, summarizes the cases that were deleted from the overall database of Seed et al. (1984), as not conforming to the defined standards of data classes A, B, or C. Table 3-6, summarizes the interpreted field case histories from earthquakes that occurred subsequent to 1984 that were found to conform to the standards of Data Classes A, B or C, and so were utilized in the studies by Cetin et al. (2000). Broader and more detailed documentation of all of these case histories, including in many cases the interpretations of other researchers, is presented in Appendix A. Appendix B summarizes the seismic site response analyses performed by Cetin et al. (2000) in interpreting 53 of the field case histories from Appendix A. Finally, Table 3-9, also taken from Cetin et al. (2000), summarizes a suite of proprietary field performance case histories from alluvial sites in the 1995 Hyogoken-Nambu (Kobe) Earthquake, and these are further documented in as much detail as is currently allowed in Appendix C. In all of these Tables and Appendices, the interpretations presented are those of the research team. A more detailed description of some of the details of the methods and procedures used to evaluate and analyze these field performance case histories is presented in Cetin (2000), though the final evaluations presented in this report in some cases post-date that dissertation, and are the most recent interpretations undertaken under these current studies. This report is intended only as a concise summary of a vast amount of data. Details regarding interpretation and processing of these data are presented in Cetin et al. (2000) and Cetin (2000)