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RE: Enlarging a footing

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Gerard-

 

No soils report? To me, that’s the easiest (and probably the cheapest) solution. You could go to a Geotech and tell him you only need specific information, like a letter saying you can use 2,000 PSF (or whatever) on an existing, already settled, foundation. Sure, he might have to poke his finger in the soil and run a couple of tests to reduce his “pucker factor”, but he probably doesn’t have to run a full blown report. IIRC, soil doesn’t actually “fail”, but it may consolidate excessively producing undesirable settlement. You would probably be coming up with the best solution of all (not disturbing the existing foundation) with this approach. Of course, I’m not a Geotech (nor do I play one on TV), so you need to consult with one to get more definitive information. I just know that if this were my job and the owner balked at getting a soils report, I would remind him (in writing) that I am not responsible for geotechnical performance nor does my insurance cover failures related to geotechnical issues. At that point, they usually spring for a report.

 

With regards to the various structural solutions, I think either is a judgment call. If you widen the footing, make sure your new reinforcement is developed into the existing well enough. I’m not sure if there will be room to do this.

 

Regards,

 

T. William (Bill) Allen, S.E. (CA #2607)

V/F (949) 248-8588

San Juan Capistrano, CA

http://members.cox.net/ballense/

 

-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
Sent:
Monday, September 08, 2003 10:43 AM
To:
seaint(--nospam--at)seaint.org
Subject: RE: Enlarging a footing

 

Robert’s step by step account for the load re-distribution is what I was thinking as well. Once the existing footing deflects beyond it’s current deformation, the new footings start to help out and do the work.

 

The footing in question is about 40 feet long and the width increase is necessary because there is no soils report and I have to assume 1000 psf soil pressure by code. It is very likely the soil’s capacity is more than twice this amount based on back calculating the loads and original footing sizes.

 

This footing is being enlarged because it is under a shearwall and I am strengthening the wall system for a larger seismic demand. This is also a voluntary action by the owner, his goal is to take care of the easily correctable deficiencies with a minimal disruption to daily operations within the building.

 

Yes, I was adding new full depth concrete on either side. I think that compressed air sandblasting and/or bush hammering would be suitable as Jim suggested. I’m not sure if this classifies as “a lot” – I think this about 1 or 2 days work (for 1-2 persons) for this amount of footing – but I could be wrong.

 

Thanks for the responses so far, keep them coming if you have more to add.

 

-gerard