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Re: CONCRETE: Combining Multiple Construction Tolerances

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How about using soil nail wall?

Martin

----- Reply message -----
From: "Polhemus, William" <wpolhemus(--nospam--at)craworld.com>
To: <seaint-seaosc(--nospam--at)mail-list.com>
Subject: [SEAINT-SEAOSC] CONCRETE: Combining Multiple Construction Tolerances 
 Date: Sat, Jan 31, 2015 6:31 PM

I'd like some opinions, guidance, communal wisdom, and profound insight.

I'd also like a 2015 Metallic Lime-Gold Porsche 911 Carrera Cabriolet...but
 that's not important right now.

I'm struggling with specifying construction tolerances for an underground
pump vault. The walls of the vault will be formed by driving steel sheet
piling to form the rectangular outline, roughly 38 feet by 25 feet, excavating 
 to about 21 feet depth, putting in a 2 foot-thick seal slab, and then forming 
 the walls of the box at the perimeter atop the seal slab. The walls are
nominally 10" minimum thickness (measured from the inside face of the
corrugated sheet pile wall).

I've had enough adventure just defining the tolerance for the sheet pile -
apparently U.S. sheet pile producers and constructors don't like to talk
about tolerances out loud. I can't imagine why not. #RollsEyes

I've finally hit upon the notion of specifying the tolerance from British
Standard EN 12063:1999 Paragraph 8.6.1 (Found naughtily here:
https://www.scribd.com/doc/101105394/EN-12063-1999)<https://www.scribd.com/doc/101105394/EN-12063-1999%29>.
In essence the location of the face of the sheet pile wall may vary no more
 than ±3 inches from the nominal location at the top of the wall, and the
wall may not be out-of-plumb more than 1% H:V. The tolerances "may" be
additive. This seems reasonable, even generous given the circumstances.

Turning to the concrete facing to be cast against the sheet pile wall, I'm
referencing ACI 117-10. Fortuitously, the example given for "use of multiple 
 toleranced items to yield toleranced results" in Section 1.2 of ACI 117
uses a concrete wall as an example, so I'm able to follow that you must look 
 at combining tolerances critically, and it is neither advised nor permitted 
 to combine the tolerances in an absolute sense. Paragraph 1.2.3 states:
"Tolerances are not cumulative. The most restrictive tolerance controls,"
and the Commentary further explains "Accumulations of individual tolerances
on a single item should not be used to increase an established tolerance.
... The separately specified tolerances must remain separate and not
cumulative."

ACI 117 includes Fig. R1.2.3 which explains this graphically, again (and
fortunately for me) using a wall as an example.

For instance, in my application I'm permitted by ACI 117 to vary a maximum
of ±1 inch from the nominal location of the wall at the base of the wall
(paragraph 4.2.1), and the wall may deviate from plumb ±0.3% of the wall
height (paragraph 4.1.1). However, as I understand it, no part of the wall
face may deviate more than the ±1 inch specified in 4.2.1. Within that
2-inch-wide envelope centered on the nominal plane of the wall, the wall
surface may exhibit no more than 0.3% of "waviness" in any 10 feet with
respect to the slope of the nominal plane (paragraph 4.8.2).

All well and good...I think.

But the real trouble begins when I try to accommodate the tolerances of the
 concrete wall with that of the sheet pile.

For instance, with respect to the nominal 10 inch thickness of the wall
measured from the face of the sheet pile, keep in mind the sheet pile may
deviate ±3 inch, meaning that the as-built wall thickness can be as little
 as 7 inches. But ACI 117 allows the wall THICKNESS to deviate from +3/8
inch to -1/4 inch (paragraph 4.5.1). I note that paragraph 4.5.2 addresses
"...walls cast against soil," a similar situation, allowing a thickness
deviation of from +3 inches to -1/2 inch from nominal. This makes a me
comfortable for when the thickness is greater than nominal, but still doesn't 
 give me the warm-and-fuzzies for a thinner wall.

Note too, that the adapted BS spec for sheet pile allows cumulative tolerances, 
 and specifies measurement for the top of the sheet pile, so that on the
off-chance a sheet pile segment starts 3" too close to the face of the wall, 
 and proceeds to slope still further toward the wall at the maximum 1 percent 
 slope from vertical, by the time you get to the base of a 19-foot wall the
 dimension from the sheet pile to the face of the nominal wall is theoretically 
 less than 4-3/4 inches! How can I countenance shaving another 1/2 inch from 
 that?

Heck, how could I get any REBAR in there?

One of the other has got to give, but how to reconcile them? "The more
restrictive tolerance controls" means in effect that I've probably got an
unworkable requirement for driving the sheet pile. I can certainly work in
the concept of a thinner-than-nominal wall for design, but that doesn't solve 
 constructibility. I have to have a box footprint that can be used for this
 vault, after all.

Thoughts?

(N.B. Sorry for making ANYONE think on a Super Bowl weekend. As my interest
 in professional sports has waned over the last few years, it's easy to
forget my timing).

_____________________________________________
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