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RE: Retaining Wall with Key

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Bill,
 
I did not have time to read the replies so far, but I too am very curious about this. I recently studied Enercalcs treatment of this and found it only considers active pressure to a depth equal to the bottom of the footing. I was only able to rationalize this in my case by placing the key far enough away from the back of the heel such that the a line intersecting the end of the heel drawn at the angle of repose of the soil did not intersect my key i.e. more towards the toe. Not much to offer beyond that. Good luck.
 

Michel Blangy, P.E.
Design Engineer

-----Original Message-----
From: William.Sherman(--nospam--at)CH2M.com [mailto:William.Sherman(--nospam--at)CH2M.com]
Sent: Monday, July 16, 2007 7:39 AM
To: seaint(--nospam--at)seaint.org
Subject: Retaining Wall with Key

I have been searching for a rational load analysis of a retaining wall with a key into the supporting soil for years, but I have not found a satisfactory solution yet.  Hoping someone on this list can offer something to help resolve this issue.
 
Many references on foundation design indicate that a keyway extended below a retaining wall into the soil will enhance the sliding stability by engaging more passive pressure for resistance.  But they do not explicitly address the soil pressure on the driving side of the keyway.  This is often interpreted to mean that only the passive pressure is considered at the keyway.  What happened to "free-body diagrams"?  What is the basis to neglect loads on the driving side of the keyway?  (Many software programs use this questionable assumption for retaining wall design with keyways.)
 
If significant movement of the wall is permissible and loads are transient, it might be argued that the keyway moves away from the soil and leaves a gap.  But for sustained loads, the gap is likely to fill in over time and re-establish a driving soil pressure on the keyway.  Also, for a centrally located keyway, there is a vertical surcharge force on the soil due to bearing pressure from the wall base that may push the soil against the keyway.
 
Load diagrams in EM 1110-2-2502, Retaining and Flood Walls, by the USACE, do show the driving side soil pressure to be extended to the bottom of the keyway (i.e., a true fee-body diagram with applied loads).  This is the approach I generally use - but it makes the heel on some retaining walls rather long!
 
If minimal movement is desirable and at-rest soil pressures are used on the driving side, it is possible for the at-rest pressures on the keyway to exceed the passive pressures on the keyway (due to higher soil depth on the driving side).  This implies that the keyway makes the wall less stable! 
 
Thus, I have concluded that using a full driving side soil pressure on the keyway may be excessively conservative, but that no driving side soil pressure is overly unconservative.  I'm looking for a rational method that falls somewhere between these two extremes. 
 
 
Bill
William Sherman - CH2M HILL / Denver
Structural Technology Discipline Leader (TeD)
Engineering Design Group - Civil / Federal Engineering (EDG-CFE)
william.sherman(--nospam--at)ch2m.com
720.286.2792