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RE: Strengthening soils under existing footing?

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I had a similar situation when the Owner wanted to support  a new storage mezzanine from columns supporting only a roof. I had suspected that the soil was better than the original boring logs, so I had a geotech firm drill borings at each footing and we found the soil capacity to be double what the original logs said. I checked the existing footings for strength and they were OK.
Other ideas:
cement or chemical grouting depending upon the type of soil. 
underpinning the footings with mini piling 
The exact details will depend upon the type of soil that you have.
Jim K.
 -----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)]
Sent: Thursday, July 29, 2004 12:31 PM
To: seaint(--nospam--at)
Subject: Strengthening soils under existing footing?

Has anyone considered a way to strengthen the soils under an existing footing without disturbing much of the surrounding?  I am envisioning pressure grouting through a 30" x 30" footing to the soil below to effectively increase the bearing area.  Hmm...
The original design drawings ca. 1975 state a 4000psf soil capacity.  The owner wants to add a penthouse/roof structure, but the soil bearing pressure is limiting the ability to do so.  Its hardly practical to widen the footings and add new columns centered on the new improved/footing.
Is grouting effective in this case, and is it even cost effective?  Any thoughts?
Thanks in advance,
Jim Wilson, PE
Stroudsburg, PA

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