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concrete pier / rock excavation

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[This got longer than I intended, but there is a question at the end]


I've got a client who owns a perfectly good building.  Unfortunately, he
wants to screw it up.

Existing: 
Four story concrete structure plus roof.  Floor system is a "standard" 4-way
flat slab with capitols and drop panels.  Bays are 31'x28'.  At the first
floor level, the columns are 32" octagons.  The concrete columns bear on
nominally reinforced 4' square piers, which bear on hard limestone bedrock.


Future: 
Tear out the first floor slab and construct 2 levels of sloped parking.  We
will have to excavate below the existing first floor 10' to make this work.



Problem:
The bedrock was found to be significantly shallower than was shown on the
original plans.  In the worst location we need to excavate 5' of bedrock
around an existing column.  We'll have a 32" reinforced concrete column
bearing on a 48" mostly unreinforced concrete pier bearing on a totally
unreinforced chunk of limestone.
 
A geotech is involved, and a core was taken immediately adjacent to the
column in question.  The rock is sound to a significant depth.  The geotech
will be present during all excavation work.


Proposed solution:
Unload the upper levels as much as possible (445 kips just with the concrete
weight and misc. dead load).  Saw-cut as close as possible to the existing
concrete pier then excavate the rock outside the cut.  Saw-cut again and
repeat.  We'll "lose" a few inches each cut because of equipment clearances.
Ultimately, we'll have about a 5' tall stone column supporting an enormous
amount of weight.  Wrap the stone column and unreinforced pier section in
confining steel.  Form and pour with self-leveling concrete.  Total
estimated minimum size is around 6' square (I think it'll be slightly
larger).

My concern is with the stone column before the concrete wrap cures.  Does
anyone else see any problems with this?  Are there any other proposed
solutions?  Maybe a fiberglass wrap?


Thanks in advance,

---
Jason W. Kilgore, PE, SE
Leigh & O'Kane, LLC
Kansas City, Missouri




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