Footing Close to Retaining Wall

[Gimbli]

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?

 

[csd72]

I would not recommend that you use the retaining wall to support your footing loads for 2 reasons.

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

 

[projectEST]

Like csd72 stated, that retaining wall was only built to hold that existing earth in place. With the added weight of the structure the wall would be under much more strain and stress than it was designed for.

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

 

[jdonville]

Gimbli,

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

 

[fattdad]

I'm not sure from the original post that it's an existing retaining wall. The original post also does not state how high the adjacent retaining wall is or will be. Not sure about the rest of the world, but for me I'd also want to know in advance of further discussion what type of 3-story building you are building. Is it an office, warehouse, shopping center, medical office building or archive building? Any idea of the structural loads?

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!

 

[Gimbli]

The wall is not existing, it is proposed as part of the project. The structure, as told, will be about 2 to 2.5 meters from the retaining wall and the wall is proposed to be some 4 meters high (exposed height). The wall footing will be cast over in situ soil and the proposed building over upgrading fill.

Thank you all for your advice.

 

[Gimbli]

And the proposed 3-story building is made out of pre-cast concrete elements and will be an apartment building.

 

[fattdad]

I think I'd design the retaining wall for at-rest conditions and take into account the lateral load from the perimeter footing (times two). That is these are the stresses I'd give to the structural engineer. They may determine that it's more cost effective to lower the footing out of the influence zone and just build a retaining wall for active earth pressures. Ultimatly, I guess I'd do both and have the structural engineers figure out which is better for the client.

f=d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

Do not support the building with a cantilevered concrete retaining wall, especially a precast building. When you excavate for the retaining wall, excavate back a few more feet and install the first row of building foundations at the same level as the retaining wall footings. Footings farther away from the retaining wall, behind the first row of new footings, can gradually rise up to their normal elevation. Then, in the future, if someone wants to demolish the retaining wall, they may need to retain the dirt laterally under the building slab, but they will not need to underpin (or repair) the building.

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.

 

[jdonville]

I agree with PEInc. Deeper spread footings is the way to go. I had assumed that the wall was existing.

Jeff

 

[DRC1]

A permenant concrete wall should be designed for Ko wether or not the wll footing is present. I would not want to mix tie backs with a cantilevered concrete wall. I would check how much surcharge you actually get against the wall at 2- 2.5 meters. assuming that is clear distance, it probably does not have a significant effect.
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.

 

[DarthSoilsGuy]

Hey,
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

 

[csd72]

DRC1,

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

 

[DRC1]

My understanding of the wall is that it is a cast in place cantilevered wall, I assume tied to a cast in place footing with a significant heel. I further understand the wall to be built during the course of the construction of the building. For such a 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.