Restraining an Existing Cantilevered Wall

[dsumerfi]

An owner wants to construct a 1-story wood-framed building against an existing 10' cantilevered site wall that is restraining 6' of soil. The wall has been there for at least a decade. I have the design criteria for the existing wall, which used 35 pcf active pressure. The wall is appropriately drained. There is no reinforcing at the front face of the wall. Is it necessary to strengthen the front face of the wall so that it meets the positive-moment demand that would be appropriate for a new restrained retaining wall with at-rest pressures? Or is it reasonable to say that the wall has already "rotated" or "stabilized" into its active pressure condition, and that the new structure won't really be restraining any future movement?

I should also mention that we're in a region of moderate seismicity, SDC C, so any increased retaining pressure from a seismic event would put the midspan of newly-restrained wall into positive moment where there is no reinforcing (this is the part that makes me uncomfortable).

Thanks!

 

[Ron]

You didn't state your location, so I will assume from your statements that you are in the US and subject to the International Building Code or a state variant of such.

The wall apparently was not designed for a surcharge load such as the building which will be placed adjacent to it. Further, from the info given, the soil conditions are likely a phi angle of around 30 degrees and a unit weight of around 120 pcf.

The wall will likely need to meet current code requirements, which have changed since it was constructed. Assume the current requirements and then bring the wall to that standard, which might require significant remediation.

 

[RFreund]

When you say "against" do you mean on the low side of the wall (i.e. the existing wall will now be a basement wall) or on the highside of the wall? If on the high side, how close?

EIT

http://www.HowToEngineer.com

 

[dsumerfi]

Ron, yes IBC applicable. I think you're missing the point of the question though, which is more about the long term movement of the wall (maybe this is my fault for not explicity stating that the new structure is on the lower side of the wall). If the roof diaphragm of the new lower structure butts into the face of the wall, does the wall need to be considered "restrained" if the active pressure has already been engaged and in place for a significant period of time? Is there a likelyhood for future movement, even though there is no additional surcharge on the retained side?

The new building is on the toe side, which is why I'm concerned with converting the wall from a cantilevered condition to a restrained condition. I understand there will be additional load on the toe, and will account for that. But being a 1-story wood framed structure, I don't anticipate the additional bearing pressures on the toe from the light-framed structure will be very large.

If this were new construction, it would be obvious - it's a restrained retaining wall and should be designed for the higher at-rest pressures. But the wall is already there and has been allowed to stabilize into its "active pressure" condition. So the real question is: Is future movement possible in the existing wall? If there is no future movement, there is no "restrained condition" and therefore no strengthening necessary. If there is future movement, which is now restrained by the new roof diaphragm, then checking the wall for its restrained condition seems appropriate.

 

[hokie66]

I don't know that you can confidently say that a cantilevered retaining wall will ever stop leaning. Have you checked to see if it is leaning now? Was the outer face battered?

When you say that the wall is "appropriately drained", what do you mean and where does the drainage daylight? Will it be into the new building? What happens if the drainage clogs, which is common? Is the wall waterproofed on the earth side?

It is generally not good practice to incorporate an existing retaining wall as a bearing wall in a new structure. I would look at isolating the new structure from the existing wall.

 

[FixedEarth]

The toe bearing pressure during Retwall design is usually close to the allowable bearing capacity. One of ways two ways to approach this- you can get the field footing dimensions, assume the soils values Ron mentioned and compute your allowable bearing capacity. Then with the RetWall geometry, determine your toe bearing pressure. This will tell you how much additional bearing capacity reserve you have on the toe side. If you are maxed out, you can consider a corbel pier that is cantilevered or deep foundation support that just misses the existing toe.

Is future movement possible in the existing wall? - Not likely from the backfill side since there are no new active pressures or new surcharges imposed.

If there is future movement, which is now restrained by the new roof diaphragm, then checking the wall for its restrained condition seems appropriate. - If there is a future movement it will be because you surpassed the allowable bearing capacity on the toe and will induce tilt and differential settlement. (The heel bearing pressure is unaffected but there is now higher bearing pressure on the toe). May be a geotechnical engineer can assist you with some of these calculations.

 

[dsumerfi]

Fixed,

Thanks for your response. Regarding the future movement, I agree with you, there is no new active pressures or surcharges on the retained portion which is why I don't think it's necessary to check if the existing cantilevered wall is adequate for a restrained condition...unless there's a seismic event. After thinking about it some more, I have a rational approach in mind which is seems reasonable to me, not unnecessarily or prohibitively conservative, and most closely approximates the realistic existing conditions:

1. Check the cantilevered wall in its current condition for active retained pressures and additional toe surcharge from new structure dead and live loads. Assuming that is adequate (may possibly need to extend toe to ensure bearing pressure < allowable):
2. Design the roof diaphragm of the lower structure to restrain the wall ONLY for the seismic surcharge (and seismic component of wall self-weight).
3. For D+EQ load combination, superimpose the cantilevered wall moment diagram (DL only) and the restrained wall moment diagram (EQ surcharge only) to determine if any positive moment is present in the wall under D+EQ loading. If positive moment exists somewhere in the mid-height of the wall, design strengthening as appropriate. If no positive moment exists, that means the seismic surcharge will not overcome the "prestress" that is currently in the wall from the retained soil in the cantilever condition.

This seems a reasonable analysis method. Any thoughts?

 

[FixedEarth]

Dsumerfi-

1- O.K.
2- An upper restraint is not needed for seismic earth pressure. Seed & Whitman(1970) found that existing cantilevered retaining walls with adequate static F.S. against Sliding & Overturning, performed well during a seismic event.
3- So the positive moment will not come from seismic earth pressure- it will be there only if you place a new upper restraint on the existing retaining wall from your lower structure.

One other thing, if the new footing for the lower structure is placed within 2.5 proposed footing widths of your existing wall footing, you will create an overlapping soil stress zone. This is the case even if the existing toe bearing pressure is below the allowable bearing capacity.

 

[RFreund]

FixedEarth - What are you getting at with the comments regarding the overlapping soil stress zone? Are your referring to footings at different elevations?

If you have 1 footing that has a bearing pressure below the allowable bearing capacity then place a new footing next to it that is also below the allowable bearing capacity and both footing are at the same elevation, are you saying this will create an issue?

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

Yes there will be overlap of vertical stresses if the two footings are close to each other, even if at the same elevation. See attached.