Retaining Wall Backfill Pressures Due to Seismic Forces

[charlierock]

I am a soils engineer who has never worked in a seismic zone 4 before. I now have a project in zone 4 which has some retaining walls to 10 feet high. For granular retaining wall backfill, I generally give an active pressure of 30 pcf for level backfill and increase that value for sloping backfill. Will these pressures increase during a seismic event and how do I provide design values to the structural engineer for the active pressures due to a seismic load??

 

[Focht3]

Are you licensed in the state where the project lies? Does someone at your firm have relevant seismic design experience? Where is the project located (city, state only)? What will the wall serve? (basement, bridge abutment, etc.?)

Please don't misunderstand - I don't mean to offend - but I'm a bit concerned about a Geotechnical Engineer of Record providing design recommendations on a Seismic Zone 4 project when s/he apparently has no relevant seismic design experience. In my view, it's more important to recognize one's limitations than one's strengths -

Wall design involves a lot more than selecting an equivalent fluid pressure based on the backfill material to be specified.

Please explain what's going on; perhaps I'm overreacting. At least I hope so...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[koodi]

Retaining walls have held up well in earth quakes, no failures that I know of. The CBC only requires earthquake design for walls over 12 ft in height that are state owned and what not. In most instances, the engineer designing the retaining wall will take the earthquake into account if and how he so chooses.

In my experience, it would be preferable if the soils engineer limited his report to the results of his tests, resulting properties of the soil, global stability, and magnitude of expected seismic event. Less recommendations and opinions. I have not seen a soils report that specified increased EFP for earthquake loadings. Not to say that subsequent communtications with the soils engineer have not been an invaluable resource, when such designs have been warranted.

 

[VoyageofDiscovery]

Hi charlierock,

Look into Bolton Seed and Robert Whitman "Design of Earth Retaining Structures for Dynamic Loads" where they make use of Mononobe-Okabe analysis.

HTH

Regards

VOD

 

[Focht3]

I think the Seed & Whitman reference (M-O analysis) is appropriate -



[blue]koodi[/blue]:

I'm very troubled by your comments. Do you expect the geotechnical engineer to provide recommendations in a vacuum? I found the following deeply disturbing:

In my experience, it would be preferable if the soils engineer limited his report to the results of his tests, resulting properties of the soil, global stability, and magnitude of expected seismic event. Less recommendations and opinions.

That's a darn foolish thing to say, in my opinion. It's akin to a geotechnical engineer telling you to provide him the area calculations and "appropriate" constants, and s/he will do the HEC-2 analyses. Or, "Run the analyses, I'll decide what they mean." How much respect would you have for a fellow "professional" that had such little regard for your expertise? And what would you think of their judgment?

The proper term is geotechnical engineer, not "soils engineer." That term was replaced in the late 1960's.

What do you think geotechnical engineering is, a "cookbook" procedure?

Very disturbing.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[charlierock]

Thanks to all...The Mononobe-Okabe equation was one of the first I looked into for this problem. This site has ROCK on the surface which is an important detail I previously left out. I am the assistant to the chief engineer for this project and the chief believes that during a seismic event, the horizontal forces of the earthquake will travel thru the rock and shake the wall & wall backfill together as if they were one...he believes the wall and backfill (properly compacted granular backfill) will move together during the seismic event and no additional forces should be included in the design of the wall whether the backfill is flat or sloping.

 

[Focht3]

That assumption may or may not be correct - depending on the problem geometry and construction details...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[koodi]

Focht's last statement is correct: soils engineering is a calculated risk.

Charlierock's last statement is incorrect. Seismically induced forces will occur. It is unlikely the wall and backfill will move together.

However, since you have not specified the type of wall? It is definitely indeterminate what those forces and motions might be.

It is uncomfortable putting "geotech" in specifications. "Geotech" has come to mean a specific legal title. A soils report could come from a civil engineer, engineering geologist, or a geophysicist--all of whom are not necessarily "Geotechnical Engineers." Please limit discussion to professional topics.

 

[Focht3]

Hmmm,

[blue]koodi[/blue] opined,
Please limit discussion to professional topics.


I am. Surely competency is a "professional topic"?

Since I'm licensed to practice civil engineering in California - the only state, to my knowledge, with a geotechnical engineer title act - I know "a bit" about the subject. (I see that your posts focus on bridge issues - and discuss Caltrans specs. It's reasonable to assume you primarily design bridges in California.) And I count quite a few California-licensed GEs as friends and former associates. I've also had the opportunity to be involved in projects in many areas of the U.S. as well as Saudi Arabia, Hong Kong, Spain, the South Pacific and Mexico. And, of course, my native Texas.

The mere presence of a title act does not change the simple fact that the term "soils engineer" was replaced by "geotechnical engineer" more than thirty years ago. It was changed for good reason - part of which had to do with the problems of many "soils engineering" firms being unable to get professional liability insurance. It was part of an effort to change the practice - to get away from the very kind of thinking that [blue]koodi[/blue] has espoused. If you limit the recommendations of the geotechnical engineer, and something foreseeable goes wrong, who shoulders the blame - and the costs?

I am well aware that some - perhaps most - of the state engineering license acts in the U.S. permit engineers of many different stripes to practice geotechnical engineering. And many engineers not trained as geotechnical engineers do, with varying degrees of success (or failure, depending on your perspective.) Most engineers that were not trained as geotechnical engineers have limited competency - that is, they are competent in a very narrow area of geotechnical engineering. When confronted with an unusual problem, they are stumped - or don't recognize it at all. They make very serious mistakes based on their limited knowledge.

California permits licensed engineering geologists to perform certain engineering activities, including recommendations for foundation design. These do not require the supervision of a licensed engineer. I have seen some really awful, even dangerous, recommendations - design parameters that clearly show that the geologist didn't understand what he was doing. Simply because the legislature (via the practice acts) permits something does not mean that it's a good idea...

That's exactly what I found yesterday. The engineer (and builder) were told by both the owner and the developer that other builders were no longer using slab-on-grade foundations on the north side of a certain street because a number of houses had experienced damaging differential foundation movement. The north side borders a creek, and has very steep slopes (locally ~1:1 H:V). Yet the engineer - industrial, by education, with a long career in the military supervising base maintenance - failed to heed the warning. And he failed to get a geotechnical engineer involved. Any competent geotechnical engineer would have insisted on piers. What's the worst part? The owner was willing to pay the additional cost.

To insist on "less recommendations and opinions" is both foolish and unprofessional, in my opinion. You are running the very serious risk of not being told about a potential problem that is outside your area of education, training, competence and/or experience. After all, that geotechnical engineer - whose opinion you don't value - could save you from getting sued some day.

{[blue]Focht3[/blue] descends from his favorite soap box}



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[koodi]

Hey Focht3,
Take it easy. Thank you for writing in and all, but geez try not to make it so personal. I hold the geotechnical engineer's recommendations and opinions in the highest regards--and will not make a move without his report and consultation. I would never attempt to do the geotechnical engineer's job, because I am neither competent nor qualified to do so. "Less recommendations and opinions" was a poor choice of words. I was trying to say that the soils report should be concise and based on concrete data, "less fluff" would have been better.

Hopefully, in future, Focht3 (whose opinion I hold in high regard) will write in about the seismic analysis, perhaps in another thread that is not so off topic.

 

[Focht3]

I'm sorry if you felt that my comments were personal. They were not intended to be -

Clear, concise and to the point - all elements of good writing. I, too, hate sloppy reports. And sloppy thinking! I'll go along with the concept of "less fluff" -


My site visit yesterday will make a good case history; the owner has lots of photos and thoroughly documented the design and construction process. Unfortunately, it will have to wait a few years...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[charlierock]

It is a gravity rock wall with 3" minus granular backfill. The wall is 10 feet high. Why will the horizontal forces in the wall be different than the horizontal forces in the backfill?

 

[cap4000]

I have never worked in a Zone 4 but I can tell the Seismic
Forces can be more than just the soil active pressure. These pressures get COMBINED and are additive with each other. Get yourself the Design CD's from either Allan Block or Keystone. The M-O Equation is the recognized standard for your situation. Good Luck.