Seismic Earth Pressure

[moe333]

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?

 

[FixedEarth]

I have been using it on my reports and software since November 2013. Some agencies I have to explain it to them in more detail under plan check comments but no resistance to it at all.

http://www.soilstructure.com/

 

[moe333]

Fixed Earth; Are you basing delta-Kae on max. dynamic earth pressure or max dynamic wall moment?

 

[FixedEarth]

moe333

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/

 

[moe333]

Is the PGA you reference the PGA-M (Geometric mean MCE PGA) in the 2013 CBC? That would always be larger than Sds/2.5. It appears the 2013 CBC requires PGA-M to be used for all seismic-soil related analyses. Do you have a different interpretation?

 

[FixedEarth]

I am basing my calculations on the paper by Mikola & Sitar in 2013 and no other interpretations are involved. This paper is available online. I get my PGA based on 2% probability of being exceeded in 50 years from a USGS website using site co-ordiantes. Then I return to Mikola & Sitar's 2013 study which says Kh is equal to (3/8)*PGA and Seismic thrust is equal to 3/8KhGammaH^2. Then if my wall is 6 ft or higher(per IBC 2012), I give separate static and seismic earth pressure parameters. I require F.S. sliding of only 1.15 if Kh is added & 1.50 minimum if static earth pressures are only applied.

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/

 

[Doctormo]

FixedEarth - How do you justify using a paper vs. published codes? Not saying you are wrong but there are some codes such as 2012 AASHTO which are based on the Sitar work but not the same as you are using.

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.

 

[moe333]

Doctormo,
How does 2012 AASHTO implement the Sitar method? I don't have that code.
Thanks

 

[Doctormo]

AASHTO still uses M-O but based on the Sitar work concludes that seismic analysis is not required for retaining walls if As < 0.40g. It also allows for other solutions such as trial wedge, GLE, etc that will solve if the slope breaks.

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...

 

[ocgeo]

In the M-O standard equation, you cannot analyze a slope greater than 6 degrees, with Kh = 0.28, and phi = 22 degrees.

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?

 

[Doctormo]

ocgeo,

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.

 

[ocgeo]

Thanks Doctormo.

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.

 

[Doctormo]

ocgeo

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.