New Foundation over Old Foundation

[McQSE]

I have a project where they want to build a new light industrial building in the same location the old building was located. The old building was damaged by a tornado. They want to leave the old foundation in place, add a minimum of 3 feet of fill, and then build the new foundation. The new building is larger than the old building so some of the new footings will be located over the old foundation and some will not. The new foundation will be monolithic with about a 6" slab and about 24" deep grade beams and footings. It will be the same construction company that built the original building.

What are your thoughts on this? Assuming adequate compaction, I can't think of any problems.

 

[MiketheEngineer]

Might the compaction over the old slab be different than around the new area? Could this be a problem??

 

[SlideRuleEra]

This is one of those times when there "could" be problems... but most likely things will go smoothly. Sounds like you have a reasonable plan. Perhaps you could have a geotech firm take a few hand auger soil samples around the existing foundation to (inexpensively) get an idea of what is underground (also, this will demonstrate that you did not "forget" about the potential for soil problems). If nothing turns up, proceed with your plan.

http://www.SlideRuleEra.net

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[msquared48]

Differential settlement of the foundation would be a concern to me here. This is basically a combination of preloaded and non-preloaded areas. Just asking for problems here.

Mike McCann
MMC Engineering

 

[lovethecold]

Are you wanting to leave in the old footings while removing the foundation walls, floor slab, and old fil? Or leaving it all in. I see where you could be going both ways. I can definately see the differential settlement that msquared48 is talking about, but that would depend on the type of soils and how the fill was placed and compacted. Backfill around the outside of the old building may not have been compacted to the spec for footings and floor slab, especially if fat clay was used. Backfill is frequently not tested where I am at.

Three feet of additional fill could also be problematic. In my neighbor, that can frequently add up to greater than 1 inch of settlement, and sometimes that much calculated differential settlement.

Like SlideRuleEra alluded to, there may not be problems. I would have the contractor show proof that testing was performed and that it is adequate for the new specs. It may be that the new footings could simply be overdug and replaced with an engineered fill, and compacted appropriatley to help minimize the differential settlements. Have a reputable geotechnical engineer from the area do some borings and look over the plans.

Hope this helps.

- EIT Geotechnical Engineer. I am here hoping to help, and learn a little along the way.

 

[McQSE]

Thanks for all the input. I agree that differential settlement could be a problem.

The original foundation is a monolithic slab-footing foundation so it will all be left. The area surrounding the original building is a gravel parking area for the 18 wheeler trailers the company manufactures. The 3 feet of fill is needed to bring the building up to the required flood elevation. The old building is too low.


The surrounding parking area looks good. There are no signs of any rutting due to the trailers. They have some heavy supply trucks in there.

Thanks again for your input.

 

[msucog]

all in all it's a coin toss...could be with a two headed coin though. either drill it to see what might be there or go with it as long as the owner has a full understanding that all bets are off. i suspect that if the original building is in good shape and if the new fill is well compacted and if the new building has similar loads over similar areas, then it's "probably" not that big of a deal. i say "probably" because it has no monetary effect on me sitting here on the other side of the screen. i suppose it's also possible that the original foundation was goo and will perform very "poorly" with the new area fill loads and building loads. until you drill, who the heck knows...even after you drill, it's an "odds are" kind of game.

good luck.

 

[dirtfiend]

It seems to me there is potential for significant differential settlement depending on the underlying soil conditions, the depth of influence of the new loads and the consolidation of the soils under the old footings. I would suggest finding a geotech to help.

 

[kslee1000]

I think you can minimize the effect of differential settlement by 1. Use different types of foundation at area above the old foundation, and areas outside of it. 2. Use simple steel framing with metal wall/roof pannels rather than concrete and masonry materials. 3. Provide adequate brace and joint details to accomodate anticipated movements.

Most importantly, the owner shall be informed that due to his decision on not to remove the old foundation (a potential saving), he could be facing quite a few unpleasant repairs until meaningful settlement has stopped.

 

[Bluefoot]

1. Obtain the orginal as-built drawings and compare the old footprint/load conditions with the new ones;
2. Consider leaving slabs/footing units in place, provided slabs cut to smaller sizes (2x2m);
Subexcavate at areas where you have concerns of uniformity of the substructure/subsoil condition;
3. Cut footings from 3' below the the proposed underside of footing or slab, in your case, add 3' of imported fill would be OK;
4. Surcharge the whole footprint area (extended at least 1.2 m outside the footprint) prior to the foundation work.