] [Thread] [Date] [Author]

["Nick Blackburn" <nblackburn(--nospam--at)fdgoak.com>]

Typically when placing piers in this kind of condition, the contractors
around here (Northern California) pump the shaft full of drill mud to
mitigate caving, water intrusion etc. problems, then place a tremie at the
bottom of the shaft and pump away.  The drill mud floats on the wet concrete
and is collected as it puddles up at the surface. There is, I suppose, a
small amount of mixing of the mud and the concrete.

Water intrusion problems aside, I would consider a drop of more than 6' to
be unacceptable due to the definite possibility of segregation and lack of
consolidation in the concrete, not to mention the problems you noted below.

What logic did the geotech use to determine that an extra three feet was
enough to replace the bell (assuming the bell is now worthless) and deal
with the water problem?  It would seem that the onus is on the geotech to
explain their actions and/or pay for the delay.
Was the water table elevation identified in the report and at what elevation
has it been discovered at?
When you say geotech, was it the actual engineer who signed the report or
was it a field technician?
How long are the piers?
How fast can the concrete supplier get a pump truck and a tremie to the job
site?

Nick Blackburn, PE


-----Original Message-----
From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
Sent: Monday, July 21, 2003 11:23 AM
To: seaint(--nospam--at)seaint.org
Subject: "Water in the Hole"


I've just returned from an interesting little excursion to inspect some
drilled-shaft footings being installed on a light-commercial (one-story)
building project that I designed.

The drilled shafts are there because of the potential for uplift as well as
to support the gravity loads of the building. Uplift comes from the
bodacious expansive soils we deal with here in southeast Texas (and really,
over most of the state).

The shafts are 12" and 18" diameter, with bell diameters of 36" and 54"
respectively. When I arrived they had already poured four of the thirty-odd
12" shafts. They had just informed me prior to my leaving for the site that
they had encountered quite a bit of water in the excavations, but "it's all
right, we talked to the geotechnical engineer and he asked us just to sink
them about three feet deeper."

Of course, the result of that is there was A LOT of water in the holes. The
foundation contractor seemed to think that since he was doing what the
geotech told him to do in increasing the shaft depth, that everything was
hunky-dory.

He was flummoxed when I ordered him to stop placing concrete until we could
get a handle on the amount of water. "The geotech told me just to pump them
and then we could pour," he complained.

Trouble was that the water was seeping in almost as fast as he could pump.
As soon as he withdrew the pump, and got his concrete chute in range (yes, I
said "concrete chute;" around here, they never heard of a "tremie," and they
have no problem dumping the concrete into the excavated hole, even with a 3x
bell, etc. MEMO TO SELF: Add "must use tremie" to Concrete Notes from now
on), there was water from one to three feet deep in the excavation again.

My informing him that his placing concrete in standing water was not
acceptable got the usual
"what-the-h*ll-are-you-talking-about-you-*#%(--nospam--at)-engineer-we-do-it-this-way-all
-the-time" rant.

My concern is, first, that placing 4" slump concrete--or just about any
concrete--into standing water is just inviting the bell to be not worth @*%#
when the soil pressure attempts to lift the foundation off the piers. The
bell is there as much to anchor the foundation as anything else.

Second, that his placing the concrete as he does, from a height, into so
much water, is going to cause caving and voiding of the concrete to a
horrific extent.

Of course with the developer standing there, just wanting them to finish,
I'm pressured to just "let it go," but the developer DOES want to see
"something in writing" about my concerns.

To me, my concerns are valid even without backup data, but I'm going to try
to be dutiful about this. I wonder if anyone here can give me some pointers
to info on the net or elsewhere, about placement of concrete in conditions
similar to these, and the implications thereof.

Thanks.



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