New wall footing adjacent to existing wall footing with different elevations

[CLT49er]

I have an existing building with a new addition to be constructed adjacent to it (both single story). The existing building has exterior walls consisting of unreinforced CMU with brick veneer (cavity appears to be grouted solid below grade). The exterior walls have reinforced pilasters supporting the gravity loads and large windows between the pilasters. So essentially the lower part of the wall is nonbearing except at the pilasters. The building has no control joints or expansion joints of any kind and there has been unsightly cracking due to this.

The existing wall has deep wall foundations. Typically I would place the new wall foundation at the same elevation (bottom) as the existing wall foundation. However, in this case I am concerned about excavating next to the existing wall and turning it into an unreinforced CMU retaining wall during construction. In addition, water was found at 3ft below grade in some of the soil borings so dewatering during excavation could also be an issue. Lastly, I do not want to make the existing cracks in the wall worse by excavating too deep. Due to the location of this project and the small budget I do not trust that local contractors will be capable of properly bracing this wall during excavation. The existing building will be occupied.

I was thinking that since the existing wall is backfilled on both sides I could place the new foundation near the existing grade so that the no excavation is required. The new wall is not a bearing wall and is only supporting its own weight. Both the existing building and the new addition are a single story.

I think that the surcharge from the new foundation would be resisted by the development of passive pressure on the other side of the existing unreinforced foundation wall and that there would be essentially no bending in the existing foundation wall. I was thinking about requiring that the contractor core through the existing slab on grade to verify that there are no voids on that side of the wall. If voids are found then the under slab grouting will be required. Does anyone here see any potential problems with doing this?

If unsuitable soils are encountered and undercutting is required I was thinking about limiting the excavation width to just a few feet so that the existing wall can span horizontally. The hole will be filled with flowable fill or lean concrete. This would have to be done in segments similar to underpinning. The new foundation could then be constructed on top of the concrete.

 

[SlideRuleEra]

IMHO, you have come up with a good compromise solution to several problems. I believe it will work.

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[dik]

If you have no idea of the construction of the existing wall, then you should found your new wall at the same footing elevation. Take lots of photos of the existing wall to show any distress. I would never bear a new footing so it produced a loading on an adjacent wall. Another homeowner?

Dik

 

[CLT49er]

It is the same owner and it is not a home. I have the existing drawings and it is unreinforced masonry other than at the pilasters. The foundation wall is supposed to be filled on both sides with soil according to the drawings. There are no signs of distress in the floor that would suggest there are any voids but this would be verified during construction by drilling through the slab. The cracking in the masonry walls is occurring at the corners of the building and is due to expansion and contraction since the building has no joints at all. This footing is not loading a basement wall.

Normally I would not place a new footing higher than an existing but in this situation I am concerned that excavating might cause more damage. I have called out temporary shoring on drawings before only to make a site visit and it not be there....

 

[BAretired]

Maybe screw piles would be an option worth considering.

BA

 

[CLT49er]

Under the new footing? Do you think there would be any impact to the existing wall during the drilling process? The rest of the new building addition is on wall footings so I would be concerned about differential settlement between the wall on screw piles and the intersecting wall footings. Maybe cut a joint through the footing in addition to the wall?

 

[BAretired]

Piles would be centered under the new wall. They would miss the existing footing and continue down below the elevation of the existing footing. I would be less concerned about differential settlement with screw piles than with placing new footings on existing fill. But there is no perfect solution.

BA

 

[CLT49er]

I was kind of thinking of having the existing fill undercut and replaced with a lean concrete. The existing fill would just have to be excavated in short lengths along the existing wall so that the wall would be able to span horizontally between the sides of the cut. Kind of like underpinning. That way you sort of end up with a really big footing that happens to bear at the bottom of the existing footing. It would just take more concrete than normal and a more time to excavate. I was thinking it might be cheaper than screw piles.

 

[JAE]

I'd use helical piling as BAretired suggests. The concern here is that you might have a net downward settlement of the new footings, causing a companion settlement in the existing. This risk might be clarified by discussing with a geotechnical engineer. Even if you don't have a geotech on the project, sometimes calling your favorite local geotech and just asking the hypothetical questions gets you some understanding.

Use the helical piling under the new wall and use a continuous grade beam to span between helical piling - offsetting piling in 1 ft. staggered offsets to add rotational stability (per the IBC).
The vertical load would then be transferred to a deeper strata of soil and perhaps minimize any settlement effects on the existing building. And you minimize excavations adjacent to existing elements and minimize that risk as well.

Your option makes sense but is open, in my view, to further distressing the existing wall. That may be OK to your client if they own that existing building and understand the possible risk of further damage. If they don't own it - might be careful.


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[SlideRuleEra]

CLT49er - Any solution has to balance risk to the existing structure versus risk to the new structure. That balance is going to be a judgement call. Your proposed solution minimizes risk to the existing structure, too me, that is priority one. A geotech can likely provide info to help make an informed decision.

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[ajk1]

The solution shown in your sketch is not acceptable because you would be placing the new footing within the zone of the backfill that was used to the original wall (rule of thumb is excavation line of 7 vertical to 10 horizontal excavation line, or something near that), which should not normally considered as compacted and of proper material to bear a new footing on. The screw pile solution suggested by BARetired seems one of the most practical and acceptable, and probably cost effective solutions.

 

[civeng80]

From your drawing it seems that the soil to the left would have been disturbed to construct the existing footing, so placing a footing higher than the existing would be risky. Maybe bored piers or even segmented excavation to the existing footing level would be my option.

 

[rwpe]

You could cantilever a thickened slab or create beams at some spacing within the slab to allow the foundation to be pulled back from the wall. I would assume that the portion of the CMU wall from the slab up would need to remain as located. The wall would be supported on the beam element at the slab level. As a suggestion, pull the foundation 5'-0" back from the existing wall. At the slab, design 12 inch deep beams at 6'-8' on center to support a point load from the wall. At the bottom of the wall, add a bond beam to span between the slab beams.

If this is not feasible, I agree with the pile ideas, but I would push for using micro-piles supporting a grade beam.