USACE allows the resultant to be outside the middle third for Unusual Loads or Earthquake Loads on retaining walls (Table 4-1, EM 1110-2-2502). I agree that I would maintain the resultant in the middle third for sustained loads or more critical load cases. I've allowed the resultant outside the middle third for design wind load on a masonry fence - and I told the owner that the wall might deflect (rotate) more under wind load using this design, but it saves on footing width.
I also believe that USACE allows resisting soil pressure to be up to one-half the theoretical passive pressure if small movement is allowed, since the deformation to develop one-half of passive pressure is much smaller than that to develop full passive pressure.
Although designing for active pressure on the resisting side is not logical based on expected wall movement, I thought I once saw a reference that suggested using it for evaluation of wall stresses as a "conservative" assumption for minimum soil pressure - but most engineers just neglect resisting soil pressure for wall design.
Bill Sherman
CH2M HILL / DEN
720-286-2792
From: Michael Hemstad [mailto:mhemstad@mbjeng.com]
Sent: Friday, April 10, 2009 8:34 AM
To: seaint@seaint.org
Subject: Retaining wall footing resultant CBCI 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, same result.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.MBJMinneapolis, MinnesotaSteve Gordin wrote:As far as I know, active pressure on a cantilevered retaining wall =develops following some movement (rotation) of that wall under the =pressure from the retained soil. If we have active pressure on the =retained side, we cannot physically have a simultaneous active pressure =on the toe side, where the movement of the wall occurs toward the soil, =i.e., in the opposite direction. =20The 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 used.=20Can you please comment on the physical nature behind that option?Thank you,V. Steve Gordin, SEIrvine CA