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Re: Downdrag on cast in place drilled piers (caissons)

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I have been EOR of projects where  a bentonite slurry for drilling fluid was used due to problems with caving soils. However, the bentonite drilling fluid is now considered hazardous waste in California, it must be collected and hauled off site for proper disposal. As you can imagine this is no longer a cost effective alternative for drilling contractors. What is used now is a polymer drilling fluid that does not have the hazardous waste issue of bentonite. If I remember correctly the polymer fluid is mixed with Clorox after coming out of the hole which through chemical reaction makes the mixture benign enough to go into the storm drain.
While you did not mention it in your example, you must drill an oversize hole, install a smaller diameter sleeve, reinf. and pour concrete then pull the sleeve. This is how I visualize having a void around the caisson shaft for placement of the bentonite slurry.
This is an interesting idea, I'll have to look into the coefficient of friction between a drilling fluid left in place and the concrete surface...and of course convince the soils engineer.
----- Original Message -----
Sent: Monday, September 22, 2003 11:38 AM
Subject: RE: Downdrag on cast in place drilled piers (caissons)




Yes.  But it is more commonly done through highly expansive upper soil stratas in order to avoid uplift on the caisson.  The hole is drilled and a bentonite slurry is placed between the caisson and the soil.  It provides bracing of the caisson laterally, but will not provide a surface for uplift.  Expansive soil volumes vary seasonally with moisture content.  As the expansive strata expands, the slurry is squeezed out at the top into a reservoir, and as the soil shrinks the slurry fills the void space again.  And if there is any caving, the coefficient of friction is almost nothing. 

Harold O. Sprague

-----Original Message-----
From: Mark Baker [mailto:shake4bake(--nospam--at)]
Monday, September 22, 2003 1:13 PM
Subject: Downdrag on cast in place drilled piers (caissons)


Has anyone had any experience with the installation of a sleeve throughout the fill depth in contact with a caisson to eliminate the effect of downdrag on the caisson shaft.


I am entertaining the idea of a 24" dia. pvc pipe sleeve, have for now ruled out the use of sonotube.




Mark D. Baker