New Bank Slab on Grade Built on Top of Old Foundations 4

[cap4000]

I have a Bank Project and the Bank CEO does not want to remove the old footings and foundations under the new 5 inch concrte slab. Its 4000 square feet in plan with old brick and concrete footings buried throughout. The site has a nice and clean loamy sand soil for its new foundations. Is uneven or differential settlement the only engineering issue to be concerned about. I believe the new slab will crack due to the buried debris. Any tips will be greatly appreciated.

 

[cvg]

is the bank CEO an engineer? If not why is he advising you on the design? Yes, the footings should be removed and yes settlement and cracking may be an issue. Since the entire building is resting on the foundation, why would you want to skimp and risk it to save a few dollars?

 

[jheidt2543]

Make a deal with the Bank CEO, you won't advise him on interest rates for real estate and car loans and he won't do advise you on engineering.

Everybody that ever remodeled their basement thinks they are an engineer or contractor. You can bet he'll be the first to ask for your E&O Insurance carrier's phone number if the floor settles!

 

[hokie66]

I wouldn't immediately discount leaving the old footings in place. It would depend on the type of new building and a lot of other factors. This is where engineering judgment comes in instead of a hard and fast approach of ripping out everything which is preexisting. If all of our ancestors had taken this approach, archaeologists would be out of work. (I know, they didn't have lawyers to worry about.)

 

[MiketheEngineer]

BTW - this subject has been discussed a few times on these forums. You might want to a search...

 

[fattdad]

Once again agreeing with the fellow Hokie. I'd likely just remove enough of the existing foundations to provide a minimum 12 inches of subbase aggregate beneath the new slab. Not sure there'd be any real reason to remove all of the existing foundation.

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

Is the proposed structure similar to what has already been there? Do you have the foundation details, including soil borings? Was there any problem of differential settlement? Re-evaluate the foundation, and see whether it can take the new loads adequately. If not, do the engineering and don't let the CEO dictate.

 

[Mickney]

plumbing and other internal utilities may be difficult (or costly) to install if they are required to penetrate the old foundations.

 

[oldestguy]

I'd go ahead as the CEO wants, but I'd get an OK to do some improvement.

It is amazing what you can build over if you run a fast roiling surcharge over the site, along with compaction at slab grade. They may need fill on site the site. Or dig a hole, run the surcharge (no need to have it sit a long time) Fill the hole when done.

Then I'd say we gotta do these things:

Design the slab so that any area 10' x10' has nothing under it. That means maybe more than a 5" slab. Maybe some thickened edges.

Then, since not all is that secure, make sure that every wall has joints at least every 10 feet, a joint at each door and above and below each window.

I'd also ask for a release from possible claims later.

Without these modifications, walk away.

 

[Bluefoot]

Are the new and old building footprints the same and structurally comparable? Many factors may affect the decision to where to remove and where not to, but by the end of the day, a detailed review and inspection (geotechnically and structurally) may cost the banker a lot more.

 

[InDepth]

There are several ways to approach this. I would consult the following texts (see below):

1) Reuse the foundations (if you are confident)
2) Make a matt - provides settlment redundancy
3) Provide piles that avoid the debris.

*I would be very weary of any old fill.

Reuse of foundations for urban sites: a best practice handbook (X414)

http://www.ciria.org/acatalog/X414.html

C653 Reuse of Foundations
Tim Chapman Sara Anderson Jan Windle

http://www.ribabookshops.com/site/viewtitle.asp?pid=8071