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RE: concrete pier / rock excavation

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Jesús,

The general plan now is to saw-cut and excavate one lift, then wrap the
stone column and the unreinforced concrete pier with carbon-fiber and epoxy.
Saw-cut and excavate a second lift, then wrap that portion.  And so forth.
We might cut the rock in an octagonal shape instead of a square shape to
increase the confining pressure.

We're still working on the specifics, and the geotech is being kept in the
loop.

Bolted bars: I assume you would excavate some distance first.  Then drill
and install the first bolt, with bearing plates, and pre-tension.  Drill the
next hole some distance away, say 12", and repeat.  Then do the same thing
in the other direction.  Then drop down to the next level and repeat again.
Finally, excavate another level of rock and repeat again.  Finally, cover
the whole thing with a concrete face.

Micro-piles: how would this work - core-drill a series of piles around the
perimeter and fasten the piles to the existing footing?  Drilling THROUGH
the footing is impractical because of space - the column is 32" wide, the
footing is 48" wide.

The building is not in use during construction, and no construction activity
will be happening in the bays above this column during excavation.

As to the owner/contractor:  The owner wants to simply excavate the rock and
leave the free-standing rock column as-in (no reinforcing).  Composite fiber
reinforcing is expensive, and a bulky reinforced concrete wrap might
eliminate several parking spaces.  The contractor is a direct employee of
the owner, and thinks that anything more than 2x4 walls and 2x10 joists is
"over-engineered".  

The corporation that was formed to finance the construction will be
dissolved within a year of final construction, so the only one left to sue
will be the design team.  Believe me, I'm being *VERY* careful on this one.

---
Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

> -----Original Message-----
> From: Jesus Gomez [mailto:jgomez(--nospam--at)schnabel-eng.com]
> Sent: Friday, May 07, 2004 11:34 AM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: concrete pier / rock excavation
> 
> Jason,
> 
> I see several problems with this. First, the owner has to buy into the
> risk
> of having the rock column fail before it is wrapped. There are ways to
> analyze the vertical capacity of the unconfined rock column but, you have
> to
> ask yourself if this is really worth. If the building is to remain in use
> while doing this work, you should not do this, of course.
> 
> In this type of work, what is typically done is to bolt the rock using
> grouted bars through the rock column below the footing. You have to do
> several rows and several columns of bolts. You want to create a facing to
> attach the bolts to, which can be shotcrete or can be steel plates.
> 
> If you prefer the wrap, you should apply it as you go down in several
> lifts.
> After each excavation lift, you apply the wrap. Then keep going down.
> 
> Another option, which I prefer, is to install vertical or batter
> micropiles
> through the footing before any excavation is made. Then you can excavate
> without worrying about stability and apply a facing to the rock later once
> you finish the excavation or as part of the final touches for the
> basement.
> 
> 	Jesús Gómez, Ph.D., P.E.



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