Seismic coefficients for slope stability analysis of dam/embankment

[georam]

I would be interested to find some information or discussion about the use of seismic coefficient
in the pseudo static stability analysis of dam and embankment.
Given that a horizontal peak ground acceleration(pga) and a vertical pga are given, based on the
location where the structure is to be built, how do you come up with a seismic coefficient in the
stability analysis. If only one of the two pga’s are known (pgah or pgav), how to estimate the
value of the other ?
Do you only use the pgah, pgav or should you use a combination of the two. What combination is
used ? Some suggestions include pgav = 2/3 pgah. Seismic coefficient in combination,
horizontal = 2/3 pgah, vertical = 1/3 pgav or 4/9 pgah.
Seed 1979, Marcuson 1992 recommended to use seismic coefficient as roughly 1/3 to 2/3 of
pgah.
Looking at the whole spectrum of what people discussed, you can practically pick up any number
and it can be justified ! Any comments ?
Others look at the dam safety guidelines, and usually they find very vague statements.
To throw more confusion, the values provided for the pgah or pgav are even in doubt with
introduction of “Continental Plates theory”, versus “historical data used to determine the pga.
Which is correct ? Which is the recommended practice ?

Regards

 

[flamby]

The value of Vertical acceleration recommended is 2/3rd of that in horizontal direction as per Indian Code (2002) for structures and buildings (other than dams). The section pertaining to dams is not yet out.

Interestingly, the earlier revision 1982 has recommended the value of 0.25 to 0.75, but a value of 0.50 was recommended in general for all structured including dams.

I have never seen any value higher than 0.75 been used anywhere.

For large dams, it is a practice to engage seismic research institutes of repute, who can recommend specific values for the region in question based on study of past records of all the seismic events.

And you have to use both the values. Vertical ground motion increases or decreases the weights of water and dam participating, while horz. force generates hydrodynamic forces and inertia.

 

[pigdog]

Georam,
I think the "seismic coefficient" method has been modified. I found a reference here in the forums by searching on "seismic" as a key word.
"IBC 2000 Seismic Force Locating", in may of 2002 pointed towards the USGS and other sources for this information. The Army Corp site also specifically addresses dams.

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em.htm

search on "seismic"
roger