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Re: Existing Property Line Retaining Wall

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Thanks Bob.

Yeah, most soils report I get around here say Unrestrained walls design for 35-55 psf, 65 for restrained, this is for fully drained and no hydrostatic pressure. The Active pressure can increase greatly as you know for sloping backfills. The backfill is level, but there is a slight 2-3% slope. So I chose 45 because it's on the low end and knowing before I did the calc that this wouldn't even be close to the code safety factors.

Yeah, I agree that the general trend of soils reports is to be conservative (low ASBP's and higher Active pressures), but that's why I'm requiring one on this job so everyone is covered.

Thanks again!
-gm

On 2/13/07, Garner, Robert <rgarner(--nospam--at)moffattnichol.com> wrote:

I agree totally with you, Gerard.  The wall movement indicates failure.  The calculations indicate failure.  To ignore failure is folly.  I totally agree on getting a soils evaluation, also.

 

EFP of 45 pcf?  I can't say on this one.  We have a geotech on a job now who recommends 56 pcf.  I think that's a little high, but I must defer to his judgment.

 

Bob Garner, S.E.

 


From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)gmail.com]
Sent: Tuesday, February 13, 2007 2:02 PM
To: seaint(--nospam--at)seaint.org
Subject: Existing Property Line Retaining Wall

 

I was retained to look at an approximately 100 year old unreinforced concrete gravity retaining wall that is along a shared property line of two residences. The wall retains about 9 feet of soil and is about 175 feet long.

40 feet of the wall has rotated 9 degrees into the property on the toe side and the remaining 135 feet has rotated 6-7 degrees. At the worst condition, the top of the wall overhangs the bottom by 15 inches.

There is no soils report available and I'm recommending complete replacement or retrofit via. dead man system for the entire wall. I am requiring that a soils report be generated to establish a design criteria.

Wondering if this seems out of line to any of you? These seem like excessive movements and rotations and only a matter of time before the wall collapses. It's service life is up and the wall doesn't appear to have any drainage behind it, weep holes etc...

I know the wall was initially underdesigned (well, it was probably never designed, just built) for the height of the soil being retained. It's front side was built straight while the stem varies from 15" thick at the top to about 32" 6 feet below (there is one point where I can see the cross-section fully, but only to 6 feet from T.O. Soil). I suspect the wall was originally built entirely on the retained side's property but slid into the toe side property, settled, and rotated. There are large vertical cracks in a couple of spots, but in general, the wall is intact, but simply rotated and moved horizontally.

A quick calc using EFP of 45 psf (low end for bay area soils reports I've seen) and a base wall thickness of 36" wide without hydrostatic build up yields safety factors for sliding and overturning around 0.69 and 0.85 respectively, not close to 1.5 and 2.0 per current codes. With hydrostatic pressure this is obviously going to be worse.

Needless to say, the owners of the two properties are battling over who should pay to fix the wall. I'm not getting into that, but simply stating my professional opinion based on my observations.

I'm confident that my recommendations are valid, but just wondering if any of you have encountered rotations to this degree (say 6-7 degrees) and felt no action was required. The owner on the retained soil side had a structural engineer prepare a drawing addressing only the 40 foot stretch of wall with the 9 degree rotation which is surprising to me.

TIA
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-gm

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-gm