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RE: Enlarging a footing

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I always specify sandblasting or bush hammering to 1/4" amplitude.  But it can be spendy if you have a lot of footing to do.  Developing transverse bending moment stresses is not especially nice, either.  And you never know what the difference in true bearing values are with freshly compacted soil adjacent to a footing that has been there and settled for a length of time.
 
Jim Persing, PE
-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
Sent: Monday, September 08, 2003 9:09 AM
To: seaint(--nospam--at)seaint.org
Subject: Enlarging a footing

I am trying to enlarge an existing strip footing by increasing its width. I have proposed to place new concrete on either side of the existing footing thus doubling its width. The existing footing is 2ft widex2ft-6in deep and I have called for shear friction dowels on each side (2 rows, ea. Side) to share the bearing with the new concrete on either side.

 

The plan checker is questioning the surface preparation of the existing concrete and how that is going to be achieved. He wants underpinning – since the existing footing has not failed or shown any signs of settlement, I think increased bearing is an appropriate solution rather than deepening the foundation. Underpinning would require shoring and an increase in cost and possible disruption of service for the tenant. Underpinning would also change the distribution of shear and moment in the existing footing, it’s capacity may not be up to snuff.

 

Can anyone recommend a dry method of surface preparation to enlarge the footing as I described? The concern is hydraulic methods of surface preparation would saturate the soil.

 

Thanks,

-gerard