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Re: Existing Property Line Retaining Wall

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Gerard-
Some General Comments:
You have a full scale soil test that has been going on for 100 years. Since it hasn't fallen down yet, the FS is currently >=1 but you don't know by how much and with the leaning, it (the F.S.) is decreasing with time. But your current lower bound for an undrained wall is probably about whatever you figure using a FS=1.0  Using this full size test you can estimate the bearing stress or the fluid pressure (not both as one depends on the other) or you can develop a curve of the relationship to get a feel for how they interact at this location. The most critical issue I've found for Bay Area walls is the drainage condition (you say you don't think there is any) and its effect on progressive movement. Without drainage and with the usual swell potential of many Bay Area soils, you can expect progressive movement of the wall away from the soil seasonally (more in winter, probably little or none in summer). \Yyou can expect the progressive leaning to increase until gravity prevails.
 
As for your recommendation, I think it is right the prudent (and I don't mean over-conservative) one although you don't indicate the consequences of failure (i.e., are there houses, patios, etc. immediately adjacent or is it out in the middle of a field where the occupancy risk is low). What would happen if it just fell over? I assume you wouldn't be concerned unless there is some occupancy  or facilities adjacent.
 
For design of a new wall, you have some good information you can use to check the conservativeness of the geotech's recommendations. It makes for a good educational exercise (and you get a chance to see how much the conservatism is costing your client). You will want to provide good drainage as it tends to mitigate the swelling potential by keeping the water content somewhat more constant instead of the area behind the wall filling up in winter and drying in summer and fall. Fixing 40 feet when you have adjacent areas with almost as much deformation does seem a lttle strange unless the failure consequences are slight.
 
As for who should pay, who has the most to lose? Sometimes a little gentle nudging by discussing the failure consequences will encourage a recalcitrant owner to take more reasonable actions.
 
Good luck with the project. It looks like you are "in the middle" regardless of the fact you are  "simply stating my professional opinion based on my observations", always a little bit of an uncomfortable postion to be in. 
Regards,
Bill Cain, SE
Berkeley CA
 
-----Original Message-----
From: gmse4603(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org
Sent: Tue, 13 Feb 2007 2:50 PM
Subject: Re: Existing Property Line Retaining Wall

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
--
-gm
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-gm

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