retaining wall factor of safety analysis

[wxh3]

I am a homeowner in a subdivision that has gravity retaining walls that are between 3 and 5 years old. (Not a geotech engineer, sorry if that is a problem.) The walls range from a foot high to about fifteen feet in height. The soil type in this area is clay. There were several collapses 3-4 years ago when subdivision was still under construction but we've haven't had any problems in the past three years. We have the original design plan with wall dimensions that have the base being roughly half the wall height and an embedment ranging from six inches to 3.5 feet.

We had a different engineering firm analyze the wall design and they came up with a table of "Factor of Safey" values for sliding, bearing, and overturning. The sliding values computed look horrible, with FOS values ranging from 0.60 for a 2-foot wall to 0.35 for a 13-foot wall.

The main conclusion of the analysis was that the wall design was for a different soil type (friction angle 28-31 deg) than the clays (14 deg) we have. The analysis assumed that clay was used as backfill because a few pictures of collapses appeared to show this. The plan does show a "6-inch min." layer of crushed rock behind the wall that is wrapped in fabric. At the bottom of this layer the plan shows a drainage pipe. It is unknown how well this plan was followed.

My basic question is regarding the analysis FOS values. 99% of our wall length have been standing with no apparent problems for 4 years since they were built. This is despite the analysis that shows a sliding FOS of 0.35-0.45 for them.

Is it realistic to assume that the analysis was overconservative? Shouldn't thousands of feet of walls with a sliding FOS of 0.4 show some evidence of problems after three years? I'm a little unclear what FOS exactly means. I've heard 1.0 FOS means the walls are just sufficient to prevent forces from behind the wall to cause it to fail. What time period does this mean? I assume this also assumes adequate drainage. I would think a FOS less than 0.5 would mean the wall will collapse almost imediately after the backfill is added! It would seem to get a 1.5 FOS with clay backfill you would have to have a wall as thick as it is tall.

Thanks for any answers or advice.

 

[PSlem]

Clay will stand vertically to a height of 2c/gamma. If it doesn't develop tension cracks it's not going to load the wall. But if it does it will fail the wall and sliding is usually the weak mode. At our last meeting an engineer was telling me of a retaining wall that translated ten feet without overturning. Evidently a mud flow. My time share lost almost all its small gravity walls with a hurricane that passed through dumping seveeral inches of rain. They had been there twenty years.

 

[GeoPaveTraffic]

wxh3,

You are correct that a FOS of 1.0 is when the driving forces are exacaly equal to the resistive forces. However, when the computed factor of saftey is less than one, then the analysis is no longer valid, it assumes statics while a FOS<1 indicates movement. Therefore you can't assume that because the computed factor of saftey is a lot less than 1 that the wall should move sooner than if it is a little less than 1.

That said, lets look at a few points that could explain why you are not seeing obvious movement. First the engineering firm that did the reanalysis may have been overly conservative. A friction angle of 14 for clays is not unheard of, but it is low. There are many ways that they could have selected this value, lab tests, experience, correlations, etc. Second PSlem is correct that clays don't always load walls imeadiatly. This may be what you were getting at in part of your question. However, after 3 or 4 years I would have expected most of the clays to have creeped and loaded the walls. Another issue may the weather for the last several years. Dry weather will create less load on the walls than long periods of wet weather.

Now my question is Why is this an issue at this time? Is the builders warranty about to expire? Is the statute of limitations on the orginal engineer about to expire?

 

[wxh3]

This has been an ongoing issue for the past three years since one big collapse of a ~12-foot wall after a torrential rainstorm. Part of the aftermath of that collapse was that an insurance company refused to pay due to an analysis that showed the wall was faulty to begin with. That big collapse did have probably have some contributing factors where a homeowner had added additional dirt above the wall (raising it several feet with a wooden extension) and possibly blocked some drainage.

There is a current lawsuit against the developer, engineering firm, and construction firm. If the lawsuit is won then the money would probably be used to reinforce and rebuild some of the walls. Obviously this mess is having an impact on home sales in the subdivision regardless of whether the walls are really that bad.

 

[DRC1]

I would say you are correct that if the walls had a factor of safety of .35 they would have failed by now. This probably means the model was wrong, but unfortuantely provides little insight into the current conditions. The clay willl have some cohesion (stickness) that will reduce the tendency to slide, which does not appear to have been accouted for in the analysis. I would engage a good geotechnical engineer and get a second opinion and get some rccomendations on how to fix it. You may find what you need to fix the problem is cheaper than messing with lawyers.

 

[wxh3]

So I understand the cohesiveness of clay is another factor that needs to be considered. It appears the analysis used the soil "friction angle" and wall dimensions (and weight) to come up with a FOS. I think the same basic formulas were used for clay and granular fill. Unfortunately I don't have the calculations available to see how they got their values. What I'm hearing here is that one can't necessarily use the same model due to clay's special properties...the model changes.

Does anyone here have experience with a gravity retaining walls and clay backfill? The basic dimesions of our rock gravity walls: H = distance from top of wall to surface. Distance from surface to base underground = H/4. Width of base underground = H/2. The width at top is given as a "1-foot min." Do these dimensions look as unacceptable as what the firm suggested for 14-degree friction angle clay? How would things change if the clay more more typical in friction angle?

Thanks for any rough guess opinions. I agree that we should probably search for another firm to get a 2nd opinion about the walls.

 

[GridSchmid]

Clay as a reinforced material is not generally a good thing. A 14 degree soil would indicate that this is a fat clay. The reinforced should never use cohesion as a design parameter. Once water enters the picture there will be more failures. A 28 degree material (minimum) should have been used. Since this is in the ground and if dismantling the wall is out of the question then I would recommend the following. Sell your house quick. Seriously any additional water will make the situation worse. Make sure that a minimum 8" layer of clay is placed at the top of the wall and extends to the property line. This will minimize infiltration to the reinforced soil mass. If failing continues...as it probably will there is an option to place a propritery system in front of the existing wall. Anchor Wall Systems make a product called Landmark. This has horizontal cores as opposed to vertical cores in the block units. This would allow for a steel channel to be placed every 30" vertical. The beam within the units can then be soil anchored back into the reinforced soil zone to resist sliding, overturning and other failures. This is expensive however. In reality the wall should be removed and replaced correctly. Factors of safety should all be 1.5 with the exception of bearing capacity which should be 2.0 and overstress which can be 1.0.

 

[Mudman57]

In the UK we would generally ignore cohesion for design of retaining walls, except for temporary walls. 14 degrees implies either a clay with a plasticity index over 80% (extremely high, for the UK at least) or a clay with evidence of past movement (polished or striated surfaces within the clay). 14 degrees would result in an enormous wall (as wide as high, as you suggest) and explains how they came up with such a low FoS for the existing walls.

I would want to know how your engineers came up with the 14 degree value. The occurrence of failures in the past may be an indicator that such a low value is in fact appropriate - 14 degrees is well below the level of just being over conservative and must have had some justification. If such conditions were valid then a piled wall would probably have been more appropriate, and the original developer is at fault.