Earth Pressure with Narrow Backfill Zone
Earth Pressure with Narrow Backfill Zone
(OP)
Hello all, this is my first post. Looks like a great resource.
Anyway, I have a situation where a retaining wall is being constructed near a building foundation (about 4 to 10 feet away). I have been asked to provide lateral earth pressures under static and seismic conditions. The backfill between the retaining wall and foundation wall will be free-draining gravel (crushed stone).
Does anyone have experience with lateral pressures where there is a narrow backfill zone.
Any help would be appreciated.
Thank you.
Anyway, I have a situation where a retaining wall is being constructed near a building foundation (about 4 to 10 feet away). I have been asked to provide lateral earth pressures under static and seismic conditions. The backfill between the retaining wall and foundation wall will be free-draining gravel (crushed stone).
Does anyone have experience with lateral pressures where there is a narrow backfill zone.
Any help would be appreciated.
Thank you.





RE: Earth Pressure with Narrow Backfill Zone
Your post did not clearly explain your situation -
Is this an existing building foundation? How high is the new wall? What is the elevation of the foundation bearing surface relative to the new wall?
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.
RE: Earth Pressure with Narrow Backfill Zone
Has anyone had experience determining what the lateral pressures (static and seismic) would be in this situation?
Thank you.
RE: Earth Pressure with Narrow Backfill Zone
RE: Earth Pressure with Narrow Backfill Zone
Perhaps Geo69 could confirm the proposals for the building foundations and the type of founding soils? The soils beneath the building would also determine whether there is a reduced lateral earth pressure on the adjacent proposed retaining wall.
RE: Earth Pressure with Narrow Backfill Zone
This is my current understanding of the situation:
The (soil nail) retaining wall will be constructed first. The height of the wall will vary from 14 to 26 feet from a constant foundation grade. A wall foundation will be constructed at the same grade as the proposed soil nail wall in front of the soil nail wall. The distance between the two will vary from 4 to 10 feet.
You should design the soil nail wall (static case) as if there will be no backfill between it and the foundation wall, as the foundation wall will not be there during the soil nail wall construction. The foundation wall should be designed to withstand construction conditions - think of it as a bin that will contain whatever backfill you put between it and the soil nail wall. NAVFAC DM 7.2 page 77 shows static design pressures for compacted cohesionless fill (c=0) behind a retaining structure. A discussion of earthquake loading is presented starting on page 76.
Hope this is helpful.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.
RE: Earth Pressure with Narrow Backfill Zone
Just for future reference, you may want to post this kind of question in the 'Earth Retention Engineering' (Geotechnical) forum next time.
Welcome to eng-tips.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.
RE: Earth Pressure with Narrow Backfill Zone
Yes, I should have used the earth retention forum.
I looked in DM 7.02 and am not sure which figure you are referring to; I may have an older version. Anyway, the foundation wall has been designed using at-rest pressures. The soil nail wall will also be designed as a permanent structure and will be built during excavation for the building. The structural engineer is wanting to use a reduced static and dynamic lateral pressure after the building is built and the space between the soil nail wall and building is backfilled. His thinking is that the soil nail wall will still be retaining the soil behind it and that the building will not "feel" the effect of the entire at-rest pressure because a portion of the wedge of soil which exerts the pressure will be retained by the soil nail wall. They are looking for the reduced pressure to save in steel and concrete for the building foundation.
RE: Earth Pressure with Narrow Backfill Zone
http://www.ce.utexas.edu/em2000/papers/AMSanad.pdf
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"Earth Pressures on Unyielding Retaining Walls of Narrow Backfill Width", Canadian Geotechnical Journal, No. 6, Dec 2001.
RE: Earth Pressure with Narrow Backfill Zone
Based on calculations, I've previously estimated wall pressures for this case by indicating that the soil pressure will increase down to a depth of two times the distance between the foundation wall and retaining wall. The pressure will stay constant below that depth. I've done some calculations using both active and at-rest pressures that indicate this. I've compared these calculations to the pressure based on a silo and they appear to have some agreement.
This is one of those things that remind me geotechnical engineering is an art as well as a science!
RE: Earth Pressure with Narrow Backfill Zone
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RE: Earth Pressure with Narrow Backfill Zone
Another alternative is to use a Culman's Diagram, with wedges stoping at the reaining wall.
My concern with the soil nail wall is if the nails will be bonded far enough outside the failure wedge if the retaining wall is in the failure wedge.
RE: Earth Pressure with Narrow Backfill Zone
That's the approach (area reduction) I usually look at first when I have a similar situation. Another example of that situation is where you are supporting dirt in front of a close building but are not going below the adjacent building.
RE: Earth Pressure with Narrow Backfill Zone
Have you ever checked out Redi-Rock. Its massive size and weight allow you to build closer to the property line and go higher without using geogrid in alot of cases. They also have a natural rock look which is aesthetically pleasing. Blocks range from 500-2000 lbs. each.