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RE: Basement wall restrained by wood floor

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I have investigated a number of failures of basement walls.  The predominate
cause has usually been related to the type of soil retained, the detailing
of the wall drainage and the backfill conditions, not the structural
detailing (although I have seen a FEW failures where the top of wall
connection was poorly detailed. I have yet to see one with a diaphragm
deflection or failure problem.  Doesn't mean it can't happen, I just haven't
seen one.).  I have seen both cantilevered designed walls and walls
supported by the 1st floor diaphragm fail when subjected to the loads
imposed by clayey retained soils.  

If a soil has low expansion potential, the usually assumed design loads
appear to be very conservative (i.e., the wall probably never sees the
design loads used).  However, with a material that has a high expansion
characteristic, even with high assumed design loads, poor detailing of the
drainage and backfill conditions can lead to a progressive movement of the
wall as the soil is alternately wet and dry with seasonal moisture changes.
When the wall moves out due to expansion, it doesn't move all the way back
when it dries.  Over the years, the effect is a net movement of the wall in
the direction it is being pushed.

  When expansion potential is very high, consideration should be made to
removal of the entire active wedge behind the wall.  I have seen walls with
conventional drainage (approx. 1' min width of rock behind the wall for most
of the height, wrapped in geotextile fabric with a perforated drain line
leading to a gravity outlet) and normal assumed design loads that have
rotated beyond repair even though they were designed by the usual means
because of expansive conditions.

I guess what I'm trying to say is that we need to listen carefully to our
geotechnical colleagues when we encounter expansive soils to be retained by
a wall that is part of a structure (cantilevered or pinned-pinned).  For
competent, free draining soils, the problems are minimal if good drainage
practices are followed, regardless which way the wall is designed.  One key
detail, that I have found helpful in mitigating moisture changes, that is
often omitted from drainage details is to cap the wall drain with about a
foot of impermeable soil (often the native soil does just fine for this) to
prevent the wall drain from collecting surface waters.  If there is a
sidewalk or other paved area adjacent to the building, this can serve the
purpose as well.

Rather than blanket statements that we shouldn't design walls as
pinned-pinned or we shouldn't design walls as cantilevers, we need to
understand the particular site involved and deal with the issues presented
by that site.  Both types of walls have their place if the important factors
are considered in the design and the detailing AND CONSTRUCTION is carefully
executed.

Another detail that I've seen inappropriately applied is to place a drainage
panel, such as Mirafi, etc., against the back of the wall in lieu of a
conventional drain with clayey backfill placed and compacted against it.
The natural permeability of the soil in place is usually greater than the
permeability of the same clayey soil after it has been compacted.  When
constructed in this manner, the drainage panel has been effectively cut off
from the ground water it is supposed to be draining and might as well not be
installed. The same principle applies to a conventional rock drain but is
usually not a problem because most contractors I have encountered will
simply use the drain rock as backfill.  Thus there is no intervening
impermeable material between the drain and the natural, in-place soil.

Regards,
Bill Cain, SE
Oakland, CA


	-----Original Message-----
	From:	James_F_Fulton(--nospam--at)RohmHaas.Com
[SMTP:James_F_Fulton(--nospam--at)RohmHaas.Com]
	Sent:	Thursday, August 26, 1999 5:15 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Basement wall restrained by wood floor

	In 99.9% of the residential construction I've seen, the basement
wall is 
	supported by a strip footing, usually 20" wide or so. This type of
wall 
	relies on the support at the top provided by the first floor. In one

	direction, the direction of the floor joists, not only the floor
deck but 
	also the joists act to provide the restraint. Pretty stiff and
pretty strong 
	in this direction. In the other direction, parallel to the joists, a
blocking 
	or bridging system has to be used to transfer the transverse shear
at the 
	bottom of the rim joists, that bears on the sill plate that's
anchored bolted 
	into the top of the wall, to the level of the floor deck.  In this
case, the 
	floor deck is indeed providing most of the strength and stiffness it
would 
	seem. But I don't see why it cannot be relied upon do its job. To
design the 
	wall as a cantilever against backfill loads has expensive
implications for 
	residential construction now that a structural footing is needed.