Seismic Earth Pressure
Seismic Earth Pressure
(OP)
Curious to know if many people are using the 2008 Sitar/Al Atik methods for seismic earth pressure estimations rather than the classic Mononobe-Okabe, or if any agencies are allowing/recommending the newer method?





RE: Seismic Earth Pressure
http://www.soilstructure.com/
RE: Seismic Earth Pressure
RE: Seismic Earth Pressure
I am setting Kh = larger of 0.375(Sds/2.5) or 0.375(PGA). And I should mention it is based on the 2013 Mikola & Sitar method. I use this same Kh value for slope analysis as well as retaining structures. I look for F.S. of 1.15 when I add seismic Kh.
http://www.soilstructure.com/
RE: Seismic Earth Pressure
RE: Seismic Earth Pressure
If you get your PGA by any other method, our values will not match. For example 5% probability being exceeded in 50 years, 10% probability being exceeded in 50 years, Section 11.8.3 ASCE 7, etc.
http://www.soilstructure.com/
RE: Seismic Earth Pressure
The problem I see is with back slopes and toe slopes where the conclusions are too simplified.
In your example, where is the site coefficient? Just wondering.
RE: Seismic Earth Pressure
How does 2012 AASHTO implement the Sitar method? I don't have that code.
Thanks
RE: Seismic Earth Pressure
AASHTO also uses Kh = As/2 as a general rule assuming some movement can be tolerated. (As = PGA x site coefficient) They also use inertial forces in addition to the soil pressure.
I do not necessarily agree with the 0.40g criteria and think they have gone too far but the 0.40g exemption covers most states. The IBC has gone the other way and requires seismic analysis on all walls now.
AASHTO is less conservative than IBC, who would have thought that...
RE: Seismic Earth Pressure
Can you incorporate a slope into the Simplified M-O equation as it seems this approach is for level backfill only?
Is there a pseudo-static approach that will calculate these parameters?
It seems the Standard M-O will handle seismic earth pressure analysis of slopes with relatively high friction angles 35 degrees +, however under 30 degrees this method does not converge on many slopes.
Has anyone tried to model the Equivalent Fluid Pressures for a vertical cut using a slope stability analysis, and using horizontal pressures on the face of the vertical cut?
RE: Seismic Earth Pressure
Couple of thoughts about Coulomb and M-O analysis for cohesionless soils:
In a static analysis, the backslope can not be greater than the soil phi angle.
In a seismic analysis, the backslope can not be greater than the phi angle minus Tan-1(Kh) (from memory).
The relationship is relative, low phi angle -> low slopes, high Kh -> low slopes. There is no one value such as 6 degrees or 30 degrees but it is the relationship between slopes, phi angle, and Kh that governs the inability to solve.
A trial wedge solution can work for a retaining wall (not considering that a slope may not be safe by itself) if the slope breaks at some point or there is a cohesive component to be considered. When the equation solutions do not work, other equilibrium methods have to be employed.
I have not messed with the slope stability method in a while but it can be a method to come up with a static or seismic thrust on a wall assuming that slopes are not infinite and/or cohesionless. All methods will not solve if there is only soil friction and an infinite condition to contend with.
RE: Seismic Earth Pressure
Good points and much appreciated. I have read some about the trial wedge approach but not had hands on. We have actually redesigned the wall for non-yielding and therefore at-rest pressure. I had the idea to calculate the static at-rest, and add to it the seismic difference of the M-O equation to determine total earth pressure, but from research this approach was not recommended (or at least from what I could read).
From Wood (1973), Whitman (1991), and others, the at-rest seismic portion is 2*kh*gamma for earth pressure. I am not sure how this is going to go over as my at-rest static was almost equal to the at-rest seismic for 22 degree soil and ~0.6g PGA (0.3Kh), and level backfill. We shall see what the review brings... wish me luck.
RE: Seismic Earth Pressure
One thing you may want to watch out for is using Ko (at-rest) is based on limited or restrained soil movement. Reducing the PGA of 0.60 to a Kh = 0.30 is based on a structure's ability to move and deform in response to excessive load. These tend to be conflicting theories so probably not a correct approach. Same problem as adding Ko pressure to M-O pressure increment, apples and oranges.