Reinforced Levee Question

[ntschwanz]

Can anyone point me to literature or a case history where the shape of the critical failure surface significantly changed once geotextile reinforcement was added in slope stability models for an embankment on a soft foundation? On a current project the embankment also serves as a levee and the with and without water load conditions result in similar shaped slip surfaces but once reinforcement is added the location and shape of the critical slip surface changes to extend beyond the reinforcement or partly thru the anchorage length. This is somewhat expected but the critical slip surface is significantly different than the original. I would like to find references that discuss the effect of reinforcement on the shape of the slip surface.

Thanks,
Neil

 

[jdonville]

Neil,

The reinforcement is supposed to change the critical slip surface. It only stands to reason that a different surface (failure path) becomes critical if you have reinforced the previous critical surface.

If the reinforced failure path FS isn't much - or any - larger than the unreinforced FS, then you should revisit the reinforcement design.

Jeff

 

[ntschwanz]

Thanks for replying Jeff. I understand that the location of the critical slip surface will change. In this case the location changed significantly and the shape is somewhat unusual (see attached). To try to verify, I did a quick FLAC model and results were very similar. I was wondering if other case histories show this condition as well. Haven't had much luck finding case histories in the literature.

 

[GeoPaveTraffic]

I can't tell much from the posted image, to many layers and lines and no ledgen.

However, I agree with jdonville, the failure surface will move if you add effective reinforcement. Based on what I could make out of the model, you have several soil layers; is it possible that the original critical surface was not a global critical surface but a local one? Just a thought.

As for lititure directly on point, I don't know of any. But common sense is your best giude. As a side note, if the levee is in the US, have you discussed using reinforcement with the COE? I would be surprised if they are going to like the idea.

 

[ntschwanz]

Thanks GeoPave for your thoughts and comments. You and Jeff have confirmed our general thought regarding the slip surface location changing when including reinforcement.

The section is complicated by multiple soil layers that vary laterally in undrained strength and density (this is denoted by all those yellow lines in the soil layers). The "no-reinforcement" slip surface is shallower but is still a global/deep seated surface.

The COE does support using reinforcement and did a lot of initial research and work on reinforced levees over soft foundations here in the U.S. I can do literature searches on our libraries, but I know there has been current work by others. If someone could point me to some references I would appreciate it. Again, thanks Jeff and GeoPave for your input.

 

[jdonville]

ntschwanz,

It seems like the soil reinforcement prevents the failure surface from propagating through the embankment. Is the reinforcement designed to permit you to bear on the "native" soils to construct the embanknment?

I have seen the famous "Bubba" video produced by Tensar where embankment construction in a bayou is facilitated by their product. Your scheme appears similar.

Jeff

 

[fattdad]

It's hard for me to contribute but so much to this thread because my slope stability program remains in DOS and is for circles only - ha. That said, is it possible to look a the location of the unreinforced slip surface and then run the analysis with the reinforcement for a specified slip circle? I can do that with my old-school program and it will show just how much contribution to the overall safety factor the use of the reinforcement provides to the originally-calculated critical surface.

No doubt that the installation of reinforcement will relocate a critical slip surface if you re-do a search.

f-d

¡papá gordo ain’t no madre flaca!

 

[ntschwanz]

Haven't seen Bubba video, do you have a link? This project is actually an existing embankment that will be raised to provide a higher level of protection. It has been in place long enough that the foundation soils have strength gain from consolidation. Agree that the reinforcement prevents the failure surface from propagating through the embankment and the "with reinforcement" critical failure surface will optimize differently and at a higher factor of safety.

It would be an interesting exercise to compare the with and without reinforcement results for the same slip surface, good idea. Will still need to meet design requirements for the most critical surface so will have to report that condition. If you can think of any publications that discuss how the critical slip surface changes after incorporating reinforcement, that would be great for documentation.

Thanks!

 

[msquared48]

I understand from the internet that prairie dogs like Geotextile fabric.

Mike McCann
McCann Engineering

 

[jdonville]

ntschwanz,

Contact Tensar and ask your local rep to bring over the Bubba video for a "Lunch 'n' Learn".

Basically, they compared (using fabric as a reference point) the benefits of using biaxial grid to facilitate construction of a levee in very swampy conditions.

I understand that biaxial Tensar grid is also used to bridge over sloppy mine wastes, too...

Jeff

 

[JML4TCM]

The Bubba video is VERY old and out dated. They compared biaxial geogrid to very light weight and weak geotextiles. Of course a $4/sy grid will work better than a $0.50/sy fabric. It helps if you compare inferior products. Tensar is a good marketing firm.

Current standard of practice is to use a high strength woven polypropylene (or sometimes polyester) geotextiles on soft, "mucky" soils. They are still more economical than the Tensar geogrid and do more (ie, separate, filter, drain and reinforce). You may want to also talk to Mirafi and Huesker. They will give you a more balanced view since they make BOTH geotextiles and geogrids.