New wall footing adjacent to existing wall footing with different elevations
New wall footing adjacent to existing wall footing with different elevations
(OP)
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.
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.






RE: New wall footing adjacent to existing wall footing with different elevations
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RE: New wall footing adjacent to existing wall footing with different elevations
Dik
RE: New wall footing adjacent to existing wall footing with different elevations
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....
RE: New wall footing adjacent to existing wall footing with different elevations
BA
RE: New wall footing adjacent to existing wall footing with different elevations
RE: New wall footing adjacent to existing wall footing with different elevations
BA
RE: New wall footing adjacent to existing wall footing with different elevations
RE: New wall footing adjacent to existing wall footing with different elevations
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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RE: New wall footing adjacent to existing wall footing with different elevations
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: New wall footing adjacent to existing wall footing with different elevations
RE: New wall footing adjacent to existing wall footing with different elevations
RE: New wall footing adjacent to existing wall footing with different elevations
If this is not feasible, I agree with the pile ideas, but I would push for using micro-piles supporting a grade beam.