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RE: Foundation Pinning

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Gail,
 
Underpinning houses, or even providing a deep foundation using items like "helical piers," is something that is quite common here in the Powder River Basin and Gillette, Wyoming.  I have, in fact, done several remediation projects which involved A.B. Chance Helical Pier installation for underpinning the foundation.
 
As far as procedure, we don't rely on the pier installing contractor for anything other than providing a helical pier to be installed to achieve a specified load.  I.M.R., Inc., out of Colorado, has done extensive testing and data collection on helical piers as well as assisting in developing the "Torque vs. Capacity Ratio."  As the design professional, I use this information to select an appropriate pier size.
 
However, prior to this, I do a load analysis of the house, tracing loads to the tops of the foundation walls.  I then analyze the foundation walls as concrete beams with supports at whatever spacing necessary to maintain the structural integrity of the foundation wall and it's reinforcing (which is usually never a problem) and provide the most economic use of helical piers, based on their size (see paragraph above).  On top of that, we use a common rule of thumb to provide piers at all building corners, to allow for adequate support of all walls at the intersection, in the event that a contractor did not install corner bars for the foundation wall reinforcing.
 
Lastly, if the house is on expansive soils, we provide a geotechnical engineer specified amount of void space below the existing footers.  We work with the installer for means of installation and costs.  Furthermore, we don't release the drawings without first seeing a soils report to verify the soil strata below the building.  That way, when it comes time to install, we know what depth we are looking at in order to achieve a quality soil in which we will achieve our desired torque capacity.  There is a possiblity that you have a thin layer of quality dense soil (only a couple of inches) before you enter a very loose soil that has next to nil blow counts.
 
I have seen foundation remediation prices here in town reach a contractor dollar figure of over $60K (this doesn't include the engineering services).  Remedial foundation work for helical pier installation is a pricey endevour.  If an owner can realize the need to begin with, they will be left with a much less expensive installation option on new construction.  That isn't always the case, as you are finding out.
 
Dave Maynard, PE
Gillette, Wyoming
-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Saturday, May 07, 2005 8:30 AM
To: seaint(--nospam--at)seaint.org
Subject: Foundation Pinning

I recently turned down a request to work on a foundation underpinning project in Alexandria, Virginia (several houses in a subdivision built on mud from the Potomac, fill, and who knows what else.)
 
There were  estimates from three different contractors on the number and location of piles required.  These estimates were completely different, by like orders of magnitude.  The project was already headed for litigation,  and some of the parties involved did not have pleasant personalities, so I backed out.
 
But I am curious,  in locations where this is commonly done (Houston comes to mind),  what is the accepted procedure for determining the pile placement and particularly the number of piles.   Does the design have to be done by a registered engineer?
 
Does it make a different whether the foundation is post-tensioned?
 
Gail Kelley