Evaluation of the State of Practice Regarding Nonlinear Seismic Deformation Analyses of Embankment Dams Subject to Soil Liquefaction Based on Case Histories
- Author(s): Chowdhury, Khaled Hossain
- Advisor(s): Seed, Raymond B
- et al.
Evaluation of the State of Practice Regarding Nonlinear
Seismic Deformation Analyses of Embankment Dams
Subject to Soil Liquefaction Based on Case Histories
Khaled Hossain Chowdhury
Doctor of Philosophy – Civil and Environmental Engineering
University of California, Berkeley
Professor Raymond B. Seed, Chair
Nonlinear seismic deformation analysis (NDA) is an important analytical tool used to (1) evaluate seismic safety of existing dams, (2) to design seismic mitigation of dams, (3) to evaluate and implement reservoir restrictions and other interim safety measures while seismic mitigation design and implementation are pending, and (4) for design of new embankment dams to ensure the seismic safety of dams that serve important purposes such as flood control, water supply, power generation, and tailings impoundments, especially with regard to risk exposures of downstream populations and facilities.
Nonlinear seismic deformation analyses of embankment dams are complex. They require (1) proper site characterization, (2) development of ground motions considering the recent State of Knowledge protocols, (3) liquefaction triggering relationships, (4) post-liquefaction strength relationships, (4) modeling of behaviors of non-liquefiable soils, (5) constitutive models, (6) accounting for volumetric recompression settlement, and (7) engineering evaluation of analysis results. These inter-dependent aspects of seismic deformation analyses, when applied through nonlinear analytical tools such as FLAC, may not always provide correct predictive answers if the concepts, relationships, and models are not implemented properly.
Evaluation of different aspects of the current State of Practice in seismic deformation analyses was performed in these current studies. The approach taken was to apply suites of combinations of (1) four different analytical or constitutive models, (2) three liquefaction triggering relationships, (3) three post-liquefaction residual strength (Sr) relationships, and (4) various additional analysis protocols to back-analyses of a series of three well-documented seismic performance field case histories.
The three field performance case histories were (1) seismic site response and performance of the Port Island vertical strong motion array in the 1995 Kobe, Japan earthquake, (2) the performance of the Lower San Fernando Dam during the 1971 San Fernando earthquake (a large deformations or flow failure case history), and (3) the performance of the Upper San Fernando dam (a moderate deformations case history) during the 1971 San Fernando earthquake.
Approaches and implementation protocols for the current studies included (1) evaluating different modeling schemes, (2) performing seismic deformation analyses with different numerical modeling schemes, (3) identifying the accuracy and reliability of different modeling schemes, and their advantages and limitations, based on three well documented case histories, (4) identifying advantages and limitations of continuum-based numerical modeling schemes in predicting deformations in embankment dams, and (5) developing improved analytical approaches to improve performance of seismic deformation modeling for forward analyses.
The lessons learned from the NDA are important. A careful implementation of the different concepts, relationships, and models successfully predicted the performance of both the moderate deformations observed in the Upper San Fernando Dam, and the large deformations or flow failure observed in the Lower San Fernando Dam in the 1971 San Fernando earthquake. Six out of nine NDA performed for the USFD successfully predicted magnitudes and principal mechanisms of this moderate deformations case history. Four out of six NDA performed for the LSFD successfully predicted magnitudes and mechanisms of this large deformations or flow failure case history.
Lessons learned from evaluation of the current State of Practice regarding seismic deformation analyses of embankment dams subject to liquefaction are important. These lessons were developed based on insights from different NDA performed in the current studies and also considering the current State of Practice guidance and protocols.
The back-analyses in the current studies demonstrated an ability to produce very good engineering “predictions” of both observed mechanisms of displacements and distress, as well as magnitudes of deformations and displacements.
Accomplishing this appears to require the following:
1. Suitable analytical or constitutive models.
2. Calibration of these models with respect to cyclic (seismic) pore pressure generation with suitable liquefaction triggering relationships, including both Kα and Kσ relationships.
3. Use of suitable post-liquefaction residual strength (Sr) relationships.
4. Suitable procedures and protocols for transition to Sr behaviors in potentially liquefiable soils.
5. Suitable treatment of potential cyclic softening, and strain softening, behaviors in sensitive clayey soils.
6. Suitable characterization of geometry and stratigraphy, and suitable evaluation of material properties and behaviors.
7. Suitable development and application of appropriate seismic “input” motions.
8. Appropriate evaluation and interpretation of the analysis results, with an understanding of the models and relationships employed, and also the intrinsic limitations of the continuum analysis methods employed with regard to accurate analyses of very large deformations and displacements.
9. And engineering judgment.