How Far Back to Hold Building

[marinaman]

If there is a building proposed to be built on the fill side of a newly proposed cast-in-place concrete contilevered retaining wall, how far back from the back side of the wall would the building need to be held to not load the proposed retaining wall?

The wall retains 9' of soil and the grade behind the wall is level.

The bottom of footing of the proposed building, located on the fill side of the retaining wall, is 2'-4" below grade, thus the distance from the bottom of the proposed building footing to the top of the proposed retaining wall footing is 6'-8".

The proposed building is a brick clad, wood framed, two story townhome building.

Thanks for taking the time to render an opinion!

 

[msquared48]

9 to 10 feet back should be OK. The heel will be impacted, but this should not be a problem.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jdgengineer]

I have always used the USS Sheet Pile Design Manual when determining the lateral pressures caused by surcharge loading. A copy was posted to this site, and can be found here.

http://www.slideruleera.net/Steel-Piling.html

See page 16, for line loading apply a surcharge to retaining walls. I would think somewhere in the range of twice the distance away from the height of the wall the surchage loading becomes fairly insignificant.

The question comes though, if this is both a newly proposed wall and a newly proposed residence -- why not just design the wall for the surcharge loading?

 

[marinaman]

Thanks msquared48.

The soil retained by the cantilevered retaining wall probably would helps confine the soil that the proposed building will be founded upon, if the building was too close to the proposed wall, and because of that, I wanted to be sure I had the architect set his building back far enough away from the retaining wall so that my retaining wall is not laterally loaded by the proposed building weight.

While I was thinking about overturning, sliding of the proposed retaining wall has my attention too!

 

[jdgengineer]

marinaman, unless this is an already constructed retaining wall that you are trying to avoid adding load to, I think you'll want to sharpen the pencil.

I'm assuming the architect wants to probably utilize as much space as he can within setback requirements for the house (unless this is a really large lot).

It's not uncommon to apply surcharge loading to a retaining wall (i.e. think partial basement) and it's something that can be designed pretty easily.

 

[marinaman]

jdg, I appreciate your comments!

Actually, my architect has a great deal of space to place these buildings.

This architect is pretty darn knowledgeable. He and I discussed this project at length the other day. I told him I could design the wall for the surcharge, or, he could place the buildings far enough back from the wall to where we do not have to worry about surcharge. He wants some area between the wall and face of building anyway, so that folks can have a little yard right there. Also, given the high visibility of this retaining wall (parallel to a road) he told me he wanted me to go with the most conservative approach. I told him that would mean to move the building back, away from the wall, until it no longer would load the proposed wall. He likes that idea and wants me to tell him how far back to place the building.

I agree with you jdg....we need to use our space as best we can, but in this instance, my client wants to be outside of the area that would load this wall.

Seems like, from what I can gather from your previous post, If we place the building (2) x (6'-8") back from the back face of wall (13'-4") then the building is outside the area of loading this wall.

 

[jdgengineer]

In my opinion it would be largely outside of the influence of the wall at that point. It will still be loading the wall to some degree but in my opinion no all that significantly. See the USS sheet pile manual I referenced. It has a procedure to calculate the surcharge pressure. You can then check that at different distances away from the wall to see when you are satisfied that it is far enough to not impact the wall design.

 

[GeoPaveTraffic]

There are two elements to be considered hear. The first, the affect of the building on the wall, has been well discussed above.

The second, the affect of the wall on the building. Retaining walls are generally designed and built around active earth pressures. To develop active earth pressures, the wall must rotate (deflect). When the wall deflects the soil backfill will move, some of this movement will be vertical and some will be horizontal.

For these reasons, I would not want my building with the a 1:1 slope from the back bottom of the retaining wall to intersect any portion of the footing.

Mike Lambert

 

[jdgengineer]

GeoPaveTraffic -- That is a good point. Now my understanding was that it didn't take a whole lot of movement to move the soil from "at-rest" pressure to "active" pressure. If you backfilled/compacted the fill prior to constructing the house, wouldn't most of the deflection already have occured?

 

[GeoPaveTraffic]

Generally takes 1 to 5% of the wall height in horizontal movement.

Will it happen after backfill and before the building is constructed? Who knows. Depends on how much time occurs and the type of soil.

If granular soil, my be ok. If clay, maybe not.

Mike Lambert

 

[jdgengineer]

Thanks for the insight GeoPaveTraffic, it's good to hear things from the geotech point of view.

Where I practice in California our Geotechs typically require granular soil to be used as the backfill. Site soil can only be used per approval from the Geotech.

 

[Ron]

If the fill is granular, a 45 deg line should be drawn from the bottom of the retaining wall footing to the footing for the building. That's about what Mike McCann noted.

 

[jdgengineer]

Ron - were does e 1:1 relationship for granular backfill come from? I know it is pretty standard language in a lot of the geotechnical reports we get. Does the Bousinesq-Newmark method (essentially the procedure of the sheet pile method I posted) not apply to granular backfill? I need to brush up on the equations a but I thought i remember they were fairly universal assuming the soil behaves as a homogenous elastic material.

 

[marinaman]

To All - This is a very good discussion. I appreciate everyone's input.

Ron - To add to JDG's comment, if the backfill soil was slightly more cohesive than granular fill, I'm assuming the angle of influence to the adjacent building footing would be steeper?

 

[fattdad]

The Rankine active wedge is formed by a line that begins at the toe of the wall and extends upward and toward the ground surface using the horizontal angle (45+phi/2).

If the building is beyond this active wedge there should be limited influence on the retaining wall. This is not completely true, but close. You'd have to do a Coulomb construction based on the loading of the building as the actual building loads will affect the stresses acting on the Rankine failure plane.

If you are considering elastic conditions and you want to know how far back the building should be to limit any stress from acting on the retainign wall, you'd have to use the Boussinesq equations AND DOUBLE THE CALCULATED LOADS. At some point you may want to say close enough as you may end up with a building that's a few dozen feet away and the calcualted effect is just a few psf.

If you have cohesivs soil, discount cohesion (it'll fatigue over time). I'd expect a lower friction angle and greater stresses on the wall (or a further distance requirement).

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

One, too, must not forget footing embedment. If the footing is taken down below the "active zone" line as discussed by fattdad, the building may be closer than where the line breaks out at the ground surface. A footing taken to 6 ft may be closer to the wall than a footing taken only 3 ft below grade.

 

[dicksewerrat]

I would keep the bldg back 3 picnic table lengths or 18 feet. You want some room to sit around , room for a fence to keep the pets up on the yard and room to turn the lawnmower around. This gives you a 3to1 and no issues with the bldg and wall. And now you have a usable space.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com