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RE: Concrete mix for placement in water

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Paul-
Pump the water out just before the concrete is placed.  Keep the concrete
pump hose submerged below the top of the concrete surface as you pour (not
too deep or it is a bear to remove, just 12 inches or so).  This forces the
water (which is lighter than the wet concrete mix) to the top.  Pour
continuously.  I have typically not changed the mix design because placing
the concrete by tremie methods keeps forces the water out.  With a 24" pier,
(3" clear all around and approximately 1-1/2" for steel, you should still
have 14 to 15 inches interior to put your water pump and your concrete pump
hose down.  I typically carry the spiral reinforcement to the bottom of the
main reinforcement.  It is cheap and makes the cage much easier to handle
and place.

If you have trouble with the bottom of the hole being soft, over drill the
hole about a foot or so and backfill that extra depth with drain rock or
post hole mix then tamp with the hydraulics on the drill rig (I assume your
piers are primarily friction piers since you have the clay at the bottom).  

If the bottom of the hole is caving moderately, I've found adding a few
sacks of post hole mix and stirring with the auger in the bottom of the hole
sometimes helps to stabilize it.  Obviously, if the hole continues to cave,
you will probably have to case.

Having the post hole mix and/or drain rock on site BEFORE the pour begins
can make the job go much smoother.

Regards,
Bill Cain, S.E.
Oakland  CA



	-----Original Message-----
	From:	Paul Feather [SMTP:pfeather(--nospam--at)san.rr.com]
	Sent:	Tuesday, September 12, 2000 9:18 AM
	To:	SEAOC List
	Subject:	Concrete mix for placement in water

	We have a project with 24" dia. drilled caisson foundation.  The
soils report did not indicate a water table problem, yet we are finding
water in many of the caisson locations.  The water is seeping slowly in the
last five feet.
	 
	The borings are in a clay layer, and hole integrity is not being
adversely affected.  The contractor intends to pump the hole prior to
placing concrete.  There will still be a wet condition, and some of the
water will be displaced by the concrete during placing.
	 
	Are there any special precautions or mix changes that should be
implemented?
	 
	How is the mix design different when placing concrete under water?
(not exactly my problem, but I find the question interesting)
	 
	All input is appreciated,
	 
	Paul Feather

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