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Retaining wall footing resultant CBC
- To: seaint(--nospam--at)seaint.org
- Subject: Retaining wall footing resultant CBC
- From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>
- Date: Fri, 10 Apr 2009 09:34:17 -0500
I agree with Steve. When I design retaining
walls, I put At-rest pressure on the back face. Remember that the
wall has to move slightly to engage the soil skeleton enough to apply only
Active-level forces. In my mind, the next time it rains, the soil settles
and applies at-rest level force. Then, if the wall can move again, the
soil skeleton is re-mobilized and we're back to active pressure.
The wall can't keep moving, or at least I don't want it
to. Around here we've got a lot of short modular block walls with negative
batter. They weren't built that way, but that's how they are now.
After 15 or 20 years they get taken down and rebuilt by the same landscape
companies that screwed them up the first time.
I also design my footings for resultant in the middle
third. The prospect of permanent uplift force (negative pressure) on the
heel does not appeal to me. Again, with ground water, frost, construction
compaction, and whatever else can happen, any potential gap under the heel will
eventually fill in. Then you don't have the downward force on the heel
that you were counting on (because it can bear now) and your wall has to tip
forward to re-mobilize heel uplift. Same problem, different direction,
As far as the front face fill goes, I will use it in an
At-rest state to resist sliding. My understanding (from Bowles) is
that the movement to mobilize passive resistance is something like an order of
magnitude greater than the movement necessary to mobilize active force. In
other words, you don't want the wall to move enough to engage passive
pressure. You don't want the wall to move at all. So design for
at-rest force all around.
Comments about footings getting big (and walls getting
thick and geez that's a lot of reinforcing) I try to ignore. I have
designed flood walls for the Army Corps of Engineers, where the footing length
is around 1.4 times the wall height. That looks excessive until you look
at the force of saturated fill, the strength of saturated foundation material,
the effects of moving water on your subgrade, the potential for some
Emergency Management bureaucrat to give in to inevitable pressure to install
flashboards when the flood exceeds the design level (it will), and the
value of what the wall is protecting--in my case, on one project, half of
downtown St. Paul.
It takes what it takes. It costs what it
costs. There are things we can cut, and things we can't. Weigh the
marginal cost of the bigger footing against the cost of failure.
Geez I sound old.
Mike Hemstad, P.E., S.E.
Steve Gordin wrote:
As far as I know, active pressure on a cantilevered retaining
develops following some movement (rotation) of that wall under
pressure from the retained soil. If we have active pressure on
retained side, we cannot physically have a simultaneous active
on the toe side, where the movement of the wall occurs toward
the soil, =
i.e., in the opposite direction. =20
The program essentially combines the active and passive
pressures on the =
toe side to resist the active pressure on the heel side. This
does not =
appear physically obvious within the theories/assumptions
Can you please comment on the physical nature behind that
V. Steve Gordin, SE