Raymond B. Seed’s research while affiliated with Xi’an Jiaotong-Liverpool University and other places

Seismic analyses embankment dams and levees depend on many important factors in order to reliably predict expected performance. The main factors for reliable forward seismic analyses include (1) seismic hazards evaluation, (2) characterization of subsurface conditions, (3) foundation and embankment characterization and materials parametrization to represent dynamic strengths and stiffnesses, (4) suitable characterization of any structural project elements, (5) appropriate applications of concepts, relationships, and constitutive models, and (6) engineering evaluation of seismic analysis results, considering current modeling capabilities and constraints. Significant progress in seismology currently allows practitioners to evaluate and predict seismic hazard levels at project sites with greater confidence, and to better characterize and develop suitable seismic "input" motions. Similarly, recent advancements in numerical modeling and the engineering use of appropriate relationships, constitutive models, protocols, and analytical platforms have led to advances in our ability to perform project-specific seismic analyses for dams and other critical buildings and infrastructure potentially susceptible to seismic soil liquefaction hazard and/or cyclic softening and strain softening of soils. When it comes to performing geotechnical and geophysical site investigations for seismic analyses, sometimes the importance of selection of appropriate methods, and key details of execution of these methods, are underestimated. This can lead to improper execution due to a lack of understanding of method-specific limitations for some sets of site-specific conditions. Most of these method-specific considerations are addressed in ASTM procedures. However, due to lack of awareness regarding intricacies or details of many geotechnical and geophysical investigation methods, incomplete or misleading data can be produced. In some cases, the resulting data can lead to incorrect seismic analysis results. An evaluation of current state of practice protocols has been performed based on experience from a number of USACE projects. These project-specific and site-specific experiences provided insights regarding details involved in performing and interpreting different investigative methods such as the Standard Penetration Test, Cone Penetration Test, Becker Penetration Test, Dynamic Penetration Test, borehole and surface geophysical tests, soil sampling methods, and laboratory tests. A set of guidelines have been developed that may be useful in selection and performance of appropriate methods, considering (1) project-specific objectives of seismic analyses, (2) project site conditions and the selection of suitable field investigation methods and details of their execution in field, and (3) suitable data interpretation and application to analytical model parameterization. If properly implemented, these guidelines may help to prevent misleading interpretation and characterization of in-situ conditions, and may increase reliability in performing forward seismic analyses to evaluate expected seismic performance.

Becker Penetration Tests (BPT) and instrumented Becker Penetration Tests (iBPT) are regularly performed to characterize the density of embankment dams and foundations that consist of sandy gravel to gravelly soils and rockfill, where other methods such as the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT) are difficult to perform. An embankment dam constructed in the 1950s was recently evaluated using BPT/iBPT and sonic borings, as well as earlier large diameter in-situ ring density tests and BPTs, original construction records, and observations and in-situ testing as part of a dam modification project. Three commonly used BPT and iBPT methods for conversion of BPT/iBPT blow counts to equivalent SPT N60 values were evaluated. Wide differences in the resulting equivalent SPT N60 values between (1) the BPT-based Harder and Seed (1986) method and the two instrumented BPT-based methods of (2) Sy and Campanella (1994) and (3) DeJong et al. (2017) and Ghafghazi et al. (2017) were observed in this project. This finding is consistent with several other projects that were reviewed by the project team. These would result in significantly different estimates of expected seismic performance of the dam. The availability of 34 in situ large diameter ring density tests, and corollary laboratory tests performed to determine in situ relative density (DR) from the ring density tests, as well as the well-documented emplacement and compaction history of the dam embankment, presented a valuable opportunity to examine and evaluate the three BPT interpretation methods. In the current project, the interpreted equivalent SPT N60 values and the resulting inferred in situ relative density (Dr) values were compared with in situ relative density values that were determined based on (1) thirty four large diameter in-situ ring density tests, and (2) construction history. Based on an evaluation of origins and development of the three BPT and iBPT interpretation methods listed above, and further analyses of available data, a fourth alternative "end bearing" method to determine site-specific equivalent SPT N60 values from BPT was developed. This proposed method systematically and transparently analyzes the collected BPT instrumentation data and provides equivalent SPT N60 values considering in-situ soil characteristics; a significant improvement to commonly used procedures. In addition to site characterization for seismic analyses, this method can be used for deep foundation design and capacity verification and in selecting the most appropriate method for BPT to SPT N60 conversion. The end bearing method also supported selection of the Harder and Seed (1986) method as the most appropriate method for this current dam re-evaluation project.

Probabilistic seismic hazard analyses and nonlinear seismic deformation analyses are routinely performed to evaluate the expected seismic performance of high hazard embankment dams. The State of Practice is rapidly evolving from simplified methods to use of fully-coupled seismic pore pressure generation and nonlinear seismic deformation analyses using either finite difference or finite element analysis frameworks. An embankment dam constructed in the 1950's was recently re-evaluated using current State of Practice methods for seismic hazard characterization, site and embankment characterization, and seismic deformation analyses. Even though the dam shells and transition zones were constructed with lifts of 2 to 12 feet thickness using potentially liquefiable materials, with only one dozer pass for leveling, a re-evaluation of the seismic performance of the dam performed in the late-1980's showed limited to negligible seismic deformation potential. The current approaches used, with more recent ground motions, site characterization, and numerical modeling techniques indicate significantly different seismic deformation estimates. The updated probabilistic seismic hazard analyses included development of input motions using scenario-based target spectra, and site characterization included selecting an appropriate Becker Penetration Testing (BPT) (conventional or instrumented) to Standard Penetration Testing conversion method based on an evaluation of three common methods currently in use, and then comparing the results with in-situ relative density testing and construction history, and an alternative fourth method for instrumented BPT data analysis. Analysis cross-sections included discretization of looser soil layers that were neglected in previous studies. The coupled liquefaction triggering and nonlinear seismic deformation analysis methods using FLAC were selected based on demonstrated predictive ability to evaluate seismic deformations in well-documented seismic dam performance case histories. Finally, the analyses were continued through post-shaking deformations, and the stability of the deformed crest with reduced upstream shell support was evaluated by evaluating deformed meshes from FLAC, considering capabilities of FLAC for large deformations, and performing limit equilibrium analyses, a critical step in performing suitably comprehensive overall deformation analyses. An assessment of the potential final post-earthquake deformed geometry was performed through a set of pre-and post-earthquake deformed shape analyses that predict the potential runout distance of the predicted upstream flow slide, and the geometry and stability of the remaining crest conditions. These updated analyses using current State of Practice approaches and methods demonstrate that the recent evolution of procedures for seismic analyses of an embankment dam can be very important, in order to properly assess the expected seismic deformations and performance of a major embankment dam.

Analyses of the expected seismic performance of earthen dams subject to potential liquefaction hazard are routinely performed for high hazard dams. The State of Practice has increasingly evolved from simplified methods to the use of fully-coupled seismic pore pressure generation and seismic deformation analyses using either finite difference or finite element analysis frameworks. Many of the key factors involved in these relatively complex analyses are treated differently in commonly used constitutive models, and in different analysis frameworks. The Upper and Lower San Fernando Dams seismic performance case histories during the 1971 San Fernando earthquake were re-analyzed in these current studies by means of fully nonlinear seismic deformation analyses, using (1) a suite of four analytical models, (2) a suite of three cyclic pore pressure generation relationships, and (3) a suite of three post-liquefaction residual strength relationships. The results of these analyses have shown that a successful implementation of non-linear deformation analyses would involve (1) how cyclic pore pressure generation and liquefaction triggering is accounted for in different constitutive modeling approaches, (2) treatment of critical state and dilatant behaviors in soils during shaking, (3) evaluation and implementation of post-liquefaction residual strength Sr, (4) modeling of potential strain softening of the non-liquefiable (e.g. clayey) soil layers, (5) continuation of the analyses through post-shaking conditions, (6) dealing with numerical difficulties associated with large deformations and displacements within the continuum analysis framework, and (7) suitable engineering assessment of the analytical results. Failure to suitably accomplish any of these can result in potentially misleading and/or potentially unconservative findings. When these are suitably accomplished, seismic deformation analyses were found to be capable of producing excellent engineering insights and a good basis for engineering decision making and/or mitigation design.

This study serves as an update to the Cetin et al. (2000, 2004) [1,2] databases and presents new liquefaction triggering curves. Compared with these studies from over a decade ago, the resulting new Standard Penetration Test (SPT)-based triggering curves have shifted to slightly higher CSR-levels for a given N1,60,CS for values of N1,60,CS greater than 15 blows/ft, but the correlation curves remain essentially unchanged at N1,60,CS values less than 15 blows/ft. This paper addresses the improved database and the methodologies used for the development of the updated triggering relationships. A companion paper addresses the principal issues that cause differences among three widely used SPT-based liquefaction triggering relationships.

Probabilistic and deterministic seismic soil liquefaction triggering methodologies are proposed in Cetin et al. [1]. This manuscript: i) presents the protocols, which need to be followed for the correct use of this methodology for forward engineering (design) assessments, ii) guides the engineers through the procedure, and iii) discusses the “tricks” alongside the protocol. An illustrative soil profile shaken by a scenario earthquake is presented, through which consistent estimations of representative SPT blow-counts along with fines content are discussed. Additionally, the estimation of CSR input parameters are illustrated. Last but not least the uncertainty estimations of these input parameters are presented along with the probability and factory of safety for the assessment of liquefaction triggering. • A simplified methodology and its use to assess liquefaction triggering hazard of a soil site under an earthquake scenario event. • The consistent and unbiased mean estimates of input parameters of SPT blow-counts(N1,60), fines content (FC), vertical effective (σ'v) and total (σv) stresses, maximum ground acceleration (amax), stress reduction (or non-linear shear mass participation) factor (rd) and moment magnitude (Mw) along with their uncertainties are discussed. • Outlined methodology enables engineers to estimate the probability of- and factor of safety against- seismic soil liquefaction triggering for design problems.

The preceding companion paper presented the updating of the seismic soil liquefaction triggering relationship of Cetin et al. [1], and compared the resulting updated relationship with the earlier version. In this second paper, a detailed cross-comparison is made between three triggering relationships: (1) Seed et al. [2], as slightly updated by the NCEER Working Group (Youd et al. [3]), (2) Boulanger and Idriss [4], and (3) Cetin et al. [5]. Differences between these three triggering relationships, and the apparent causes of them are examined. Also studied are the impacts of these differences on levels of conservatism with regard to evaluation of liquefaction triggering hazard, and the resulting risks for engineering projects.