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RE: Using shoring as structural member

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Greg,

What you are proposing is feasible.  The actual shotcrete is cheap by
comparison.  I have installed similar walls in the past.  I am surprised
that a tie back wall is economical for a 15 foot excavation.  I would
definitely consider a cantilevered wall of some type.  

Another consideration for the elimination of tiebacks might be a
cantilevered wall such as a slurry wall, tangent wall, or a secant pile
wall.  These walls are top installed.  Once they are installed you excavate
without having to install lagging or tiebacks.

Tangent walls are constructed by drilling caissons or auger cast piles at a
given spacing and then installing caissons or auger cast piles in between to
seal off the excavation.  

The secant wall is similar except the first caissons are placed and the
infill caissons actually cut into the prior installed spaced caissons.  The
first caissons are placed with no rebar.

I have done walls as tall as 30 feet with no tiebacks using a tangent wall.

Slurry walls are another possibility or slurried precast walls.  Sometimes
the traditional shores, whalers, lagging, cribbing and tiebacks are more
expensive.

Regards,
Harold Sprague


-----Original Message-----
From: Greg Leyh [mailto:lod(--nospam--at)pacbell.net]
Sent: Thursday, February 18, 1999 9:03 PM
To: seaint(--nospam--at)seaint.org
Subject: Using shoring as structural member


Hi All,

On a 15ft tall basement wall (which is to be shotcreted),
shoring will be required for the raw excavation cut.  Typical
shoring for shotcrete work consists of drilling tiebacks and
placing netting _as_ the excavation proceeds, quite an
expensive proposition.  On formed walls, shoring normally 
consists of sinking I-beam columns, with gravel fill.  These 
in turn support horizontal slats that make up the debris box.
Most of the time, the I-Beams are removed after the wall is set.

Would it be feasible on this shotcrete job to sink standard
I-beam columns for the shoring, but instead of gravel, concrete
is poured down the hole for anchoring, and temporary lateral 
supports hold the I-beams true, until the concrete sets?
The I-Beam debris box could then support the rebar curtain 
and top forming on welded spuds, and the shotcrete could be
blown against the I-beams, spuds and slats.
Having an intimate bond with the concrete, the I-Beams could
now support the tension on the back of the wall, placing the
concrete section almost entirely under compression.

Two possible advantages here are strength and cost.  Tieback work
in the Bay Area is @#! expensive, especially if the excavation
work has to be continually interrupted.  The excavation trim work
must also be rather precise, in order to shoot directly to the cut.
The I-Beam backing would avoid the precise cut requirement, and 
would most likely get rid of the tiebacks as well, since the active
wall thickness is now much greater (14" I-Beam + 12" Concrete).

Perhaps this is a common technique somewhere?  It seems to be 
a cost win over using tiebacks, even considering the backfill.

Any comments would be greatly appreciated.
-- 


-GL
www.lod.org