Date

2015-05-15

Author

Çetin, Kemal Önder

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Within the scope of this paper, seismic assessment stages followed during the design and performance assessments of geo-structures will be presented. Some of these stages involve i) seismic hazard assessments, ii) pseudo-static stability evaluations and the determination of seismic coefficient k, iii) allowable permanent deformations, which will be discussed herein, with the emphasis in similarities and differences in available national and international codes, regulations and guidelines. For earthfill and rockfill dams, there exists a consensus regarding the selection of a 50 % probability of exceedance in 100 years hazard level for operation basis earthquake levels. The dam subjected to this shaking level is expected to behave elastically or almost elastically. However, for risk class "high" and "very high" dams, international codes suggest a seismic assessment which adopts a 10,000 year return period hazard level for safety evaluation earthquake, whereas national guideline only recommends a 2475 year return period level. This relatively unconservative selection is somewhat compensated later by the selection of a seismic coefficient k value in the range of 0.4-0.6 of PGA, surprisingly independent of the failure block geometry, level of shaking, expected permanent damage, dam height and stiffness, etc. For retaining structures, until recently, 475 year return periods establish the design practice. However after AASHTO (2006), 1000 year return periods are adopted as the basis for design. Recommendations are listed by the author to achieve unbiased combinations of these assessment parameter choices. As the concluding remark, consistent with international literature, 1.0-1.5 meters are judged to be acceptable permanent displacement levels, which are expected to not jeopardize the stability of the dam, as a general and crude guideline. Permanent displacements exceeding 3.0 meters can be concluded as large displacements which may trigger serious slope stability problems. It should be noted that at these large displacement levels, numerical assessment results in the form of permanent displacements may be unconservatively biased due to limitations in boundary conditions and constitutive models. The 1.5 to 3 meter range is defined as the gray zone, for which additional more sensitive analyses are required. Similarly, for conventional gravity and semi-gravity type retaining systems, 5-10 cm permanent displacement are judged to be acceptable. These recommendations are consistent with current state of knowledge, however use of them as a unique decision making criterion without considering the characteristics of the geostructures, the site and safety evaluation earthquake scenario, is not recommended.

Subject Keywords

Geo-structures, Earthfill and rockfill dams, Retaining structures, Seismic hazard assessment, Deterministic analysis, Seismic coefficient

URI

https://hdl.handle.net/11511/78492

Collections

Department of Civil Engineering, Conference / Seminar