New Building Expansion Foundation Next to Existing 1993 & 2006 Footings

[Eng_Struct]

Hi Group,

I am considering extending existing column spread footing for a new column as part of the expansion. As shown in the sketch the existing spread footings are within 10ft of each other but are at complete different founding elevation. The original construction drawings require bearing of 6000 psf and it is hard for me to believe that within 10ft the footing depth would have needed to be increased by 5ft to get the required bearing.

They was also an addition in 2006 that utilized caisson at founding depth of 19ft. We do not have geotech reports from any of the previous construction hence it is difficult to tell what might have been the reason for going with caisson instead of spread footing. The loads from 2006 addition are not any different than the 1993 construction.

Lastly, based on the available bearing capacity, I can make the foundation for new column work by combining new existing footing with the existing as shown. I wonder if I have to consider any surcharge acting on the existing caisson. In my mind, lateral pressure from the footing should be a concern since there will be equal amount of passive pressure to counter act the surcharge. Perhapse the only thing I need to check is if the bearing pressure combined from the caisson and new spread footing will exceed the allowable bearing at the base of caisson footing.

 

[SlideRuleEra]

I would consider doing this (green)... not an ideal solution, but best I can come up with. Will tell you "why" if you are interested.

 

[Eng_Struct]

Thanks, SlideRuleEra!

Are you thinking along the lines of separating the new from the existing? This will put the new footing too close to that caisson and I am not sure what implications this will have.

 

[SlideRuleEra]

StrucEng92 - Any solution is going to be a compromise, just need to find the best one. Sketching an elevation view of the existing area makes it easier (for me) to evaluate the situation:

My main "rule" for retrofits is not do anything to (loaded) existing foundations... unless absolutely necessary. Even excavating down 5 1/2 feet in the crowded location is tricky.

After more consideration, I believe the best approach to investigate is putting an independent footing at the location shown on my plan sketch, but support the footing with micropiles. Also, if bottom of this micopile-supported footing can be raised (to minimize excavation depth), do so.

There are other compromises, say a new caisson, but I believe the above is best.

FWIW, I see why the 2006 engineer used a caisson... to avoid excavation and the same problems with congestion that you have now.

 

[Eng_Struct]

Solutions like caisson/micropiling/helical piling will require consideration as to how close the installing equipment can get to the existing building. From discussions with a few contractors, I may be able to get as close as 16" with the rig to existing face of the building.

Also, the new footing location will receive a diagonal brace as well. My understanding is that micropiles and/or helical piles are not greatest when it comes to lateral capacity. I will have to look what works for my demands.

Thanks again!