Footing Close to Retaining Wall
Footing Close to Retaining Wall
(OP)
I have a structure (3-story) proposed at about 2 to 2.5 meters from a retaining wall. I am concerned about normal deflections of walls during active loading conditions that will provoke settlements on strucuture (Poisson's constant effect). Is designing for at rest condition a solution to provide a more rigid wall, or should we consider the installation of tie backs? Or, what I prefer, recommending a proper buffer distance.
Anyone with experience on this condition?
Anyone with experience on this condition?





RE: Footing Close to Retaining Wall
1. regardless of how you design it, a cantilever retaining wall will rotate significantly and will therefore allow some settlement of the footings.
2. What happens in 20 years time when they want to remove the retaining wall (and dont have any drawings) they rmove it and the whole footing collapses.
You need to bring the footing down to a level below the zone of influence of the retaining wall. This zone of influence is usually taken as a 45 degree line from the bottom of the retaining wall footing (but this may vary depending on local soils, regulations e.t.c.). You can do this by either cheap blinding concrete under the footing or by using piles/caiisons. Moving the building outside this zone of influence may also be an option.
Avoid soil anchors ('tie backs') as much as possible as these tend to be expensive.
Regards
csd
RE: Footing Close to Retaining Wall
Tiebacks, not only costly, probably wont produce enough force to hold the wall with the new structure in place. Tiebacks are mainly designed as a little added insurance in retaining a wall.
Rakers are one option that you can take. Rakers of a large enough size could be a viable solution to your problem, although these too are costly.
The best solution for you is a deep foundation. Depending on the depth of the existing retaining wall, Caissons or mini-piles WILL suffice.
I'll be more than happy to answer any of your questions, please feel free to ask.
Not bad for a 20 yr old, huh?
Joe
Project Engineer
RE: Footing Close to Retaining Wall
It sounds like you have a few options under consideration: 1) reconstruct the wall taking into account the new loads (using ko), 2) modify the existing wall to provide additional lateral support through tiebacks or other restraint, or 3) relocate the proposed structure to avoid interacting with the wall.
Each option no doubt has it's costs. Another option to consider is deep foundations (a variant on option 3). Modifying the existing wall will depend on the quality of the information you have on the existing wall.
I would suspect that the existing wall was not structurally designed to withstand lateral bracing loads, so I would probably rule out that option just to be safe.
Good luck.
Jeff
RE: Footing Close to Retaining Wall
For the case that this was a run-of-the-mill office building that's 10 ft from an as-yet-designed retaining wall, I'd consider using Bousinesq lateral load forumla (you have to double it as it's half-space) in conjunction with Ko earth pressure loading. I'd include the full foundation loads and also the design floor loading also.
For the case that you use deep foundations for the structure, I'd likely still design the wall for at-rest loading and consider whether there is a contribution from the floor loading onto the wall.
Maybe my reply sounds conservative - don't know. As it stands, I don't have the full picture. . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: Footing Close to Retaining Wall
Thank you all for your advice.
RE: Footing Close to Retaining Wall
RE: Footing Close to Retaining Wall
f=d
¡papá gordo ain’t no madre flaca!
RE: Footing Close to Retaining Wall
projectJOE said, "Tiebacks, not only costly, probably wont produce enough force to hold the wall with the new structure in place. Tiebacks are mainly designed as a little added insurance in retaining a wall." Tiebacks can be costly, but I disagree with the rest of his statement. Tiebacks could easily hold the lateral pressure caused by the building foundations BUT this is not a good idea. Raker braces are an even worse idea. As projectJOE and others have said, deep foundations would also work. However, the cheapest solution is probably deeper spread footings as I mentioned above.
RE: Footing Close to Retaining Wall
Jeff
RE: Footing Close to Retaining Wall
Footings near sheetpile walls can be problemmatic, as the wall profile will deflect as well as the top rotating. Ths allows more settlemnt of nearby structures than a rigid wall.
Be sure you have high quality well compacted backfill and be sure the retained side is well drained.
RE: Footing Close to Retaining Wall
I'm doing the same thing right now. My wall is a large block segmental wall w/ grid and my bldgs are 3-story high-end ski homes. Anyway, i'm dropping the footings to get away from the wall and bear on native materials.
-Ham
RE: Footing Close to Retaining Wall
I must politely disagree.
Just because a cantilever retaining wall is rigid doesnt mean that it wont move as a rigid body under load. The retaining wall relies on the passive and bearing resistance of the soil and movement is required for these to mobilise.
In my opinion, the use of Ko is conservative for a cantilever wall, but I would agree with using Ko if tiebacks are used.
With this potential movement in mind, I would never rely on a cantilever retaining wall to take pressure from footing loads.
I agree with your comments regarding the sheetpiles and the backfill.
csd
RE: Footing Close to Retaining Wall
1.) The cast in place wall profile is relatively stiff, and unlike a sheetpile wall will not significantly deflect outward(buldge), which could induce settlement. If the structure is overloaded, failue modes include rotation, bending or shear with rotation being the most likely.
2.) Permenant retaining walls are generally designed on the basis of Ko to reduce rotation to a negligable amount. Further, permenant retainig walls are generally designed without passive pressure, so that should excavation of some of the fill occur in front of the wall, the wall would not be jepordized.
3.) Residential foundation loads are relatively light and can be conservatively estimated. For a 4 meter wall a footing 2.5 meters away and 1-2 meters deep will not have a significant impact on the lateral load on the wall.
4.) The calculations for 3.) can be easily run to determine if additional efforts are nessecary. Before utilizing deep foundations or tiebacks, the engineer should check if they are needed. I have not run the numbers and can not say for sure, but it would appear to me that the first step is to see what effect the foundations have on the wall.