Design parameters for seismic safe design of dams
The site specific reports for determination of seismic parameters involve estimation of seismic parameters either using the deterministic seismic hazard analysis (DSHA) method or the probabilistic seismic hazard analysis (PSHA) method. Since both the approaches have their own strengths and weaknesses, they are required to be used in combination to arrive at the most appropriate engineering decisions.
The site specific seismic study for a river valley project requires an understanding of the seismic scenario with regard to dam site, which includes geological setting of the area, tectonic features and the history of earthquake occurrence in the region. The study enables evaluation of design ground motion based on identifiable seismic source zones and appropriate ground motion attenuation laws.
Ground motion can be characterized by peak values of expected acceleration, velocity, and/ or displacement. Ideally, all factors affecting ground motion should be considered for evaluation of these parameters; but this is not practical. Generally, one source factor (magnitude) and a single transmission path factor (distance) only are considered. The local site effects are often disregarded, or limited to simple distinction between rock and alluvial sites and possible consideration of near-field effects. Empirical relations derived from available earthquake data(attenuation relations) relate ground motion parameters to distance from the source and to magnitude.
The site specific seismic design parameters will depend on many variable factors. It is an extremely difficult task to determine the exact parameters in view of uncertainties related to:
- choice of earthquake parameters (namely magnitude, distance, focal depth and mechanism) related to MCE and DBE
- choice of ground motion attenuation relationship for computing spectral accelerations and
- the elastic modulus, shear modulus and damping characteristics of the material used in construction.
In view of above, it is important to exercise a judicious balance in various steps involved in the study.
A. Determination of Site-specific Seismic Coefficients:
With the knowledge of the natural period of the dam section, the response spectra can beused to obtain the site-specific seismic coefficients. The type and preliminary section of thedam, the computational formula, and other assumption made in the computation of naturalperiod should be furnished in the study.
Horizontal seismic coefficient: The horizontal seismic coefficient values (αh) shall be computed as per the DBE level of response spectra using effective peak ground acceleration(EPGA) criteria. For a given return period (475 years for DBE condition) and desired damping(5% damped spectrum for the concrete/masonry dam and 10 % damped spectrum for earthen and rockfill dams), the EPGA is determined by dividing the corresponding short period spectral acceleration value by 2.5. The short period spectral acceleration corresponds to 0.2 second spectral ordinate of the selected damped spectrum (USACE EM 1110-2-6053). The horizontal seismic co-efficient is then arrived at by taking 2/3rd of the EPGA value. The horizontal seismicco-efficient (αh) thus obtained shall be compared with αh values arrived through the application of IS-1893 (1984) (i.e. 0.06 for Zone-II, 0.12 for Zone-III, 0.15 for Zone-IV and 0.24for Zone-V) and the higher of the two values shall be adopted. For the calculation of horizontal seismic co-efficient, the PGA value should be obtained directly from the actual response spectral amplitudes and not from the normalized spectra.
Vertical seismic coefficient: Vertical seismic co-efficient (αv) shall be taken as 2/3rd of the horizontal seismic co-efficient.