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# Seismic coefficient for slope stability

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## Seismic coefficient for slope stability

(OP)
﻿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

### RE: Seismic coefficient for slope stability

Hello,
The ratio between the peak values of horizontal and the vertical components of the ground acceleration at a location is a function of various parameters. The ratio is usually derived by statistical analysis carried out as part of a seismic hazard study.
For example, a figure of 2/3 for vertical/horizontal has been accepted for various USA sites, while Ambraseys (EESD, 1988) gave a figure of 1/2 for various European sites.
Try a literature survey to determine the ratio appropriate to the location of interest to you. If none is available, consult an expert to identify another location, with similar seismological characteristics to the location of interest and for which a widely accepted is available, and use this ratio for your work.
I hope that this note is of some use.
Regards,

### RE: Seismic coefficient for slope stability

(OP)
Dear paul01ste

Thanks for your input. The selection of the factor to be applied to the seismic coefficient is just the one that I am wondering about. Why in US we use factor 0.67, and in Europe a factor of 0.5  Is the european more conservative? I also recall, that the factor may even vary to as little as 0.33. Whay can't there be a consensus ?

Thanks for your info, anyway.
Regards

### RE: Seismic coefficient for slope stability

The reason you can't have one value for correlation between PGAH and PGAV is because it does vary as you move from Zone 3 to Zone 4, and then Near-Source.  I think vertical accelerations of 1.5 times PGAH were measured during the Northridge earthquake.  My guess is once you get 15 or 20 kilometers away from your controlling source, you could probably safely use 2/3.

### RE: Seismic coefficient for slope stability

This is originally an old post and don't know if the original poster will be monitoring it anymore but......pseudo static analysis is applying a static factor to slope stability to somehow pretend to simulate a transient load.  Thus the seismic coeffiecient does not relate to the pga in any real sense.  Duration and arias intensity are more meaningful.  Seismic coefficiets have been derived and recommended for analysis based on Newmark analysis (displacement analysis) of slopes and "back calculating" coefficients that result in acceptable slope displacements.  Newmark analysis uses actual earthquake records to "shake" the slope and calculates slope displacements. Currently there is a big push to coax consultants away from pseudostatic analysis and back towards Newmark analysis if there is a real seismic issue for a slope.  A computer program that uses Newmark analysis is available from the USGS for a reasonable cost (USGS Open File Report 03-005).
Check out the following publications for the latest opinion from California
1. Guidelines for Evaluating and Mitigating Seismic Hazards in California, CGS Special Publication 117 (previously CDMG)
2. Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Landslide Hazards in California, ASCE/SCEC publication(available free on the SCEC website)

Hope this helps.

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