Reinforced Soil Slope (RSS) or MSE Wall backfill specifications 1

[Pete1919]

In a typical reinforced soil slope (RSS) or MSE wall, the d50 of the ideal backfill material is a function of aperture size of the geogrid. In other situations, many use granular materials (sand or even silt). Does anyone know if the cohesive soil (CL) can be used as backfill in such structures, and why not. I will appreciate the reference, citations or case studies.

We have only CL soil available at the site or in the vicinity.

 

[GeoPaveTraffic]

You can use cohesive soils, though most grid manufactures do not like it. You and your geotechnical engineer will need to determine the appropriate parameters to use in the design.

Mike Lambert

 

[Pete1919]

Why don't the geogrid manufacturers not like it? Any reason? If I have a good and reliable case study or a good example, I don't care what manufacturers think. I am responsible for the design and its performance.

OK,the soil available is CL (clay with little sand, tr. gravel), has a phi of 29 deg and a cohesion of 200 psf tested at 90 percent compaction and at 1k, 2k, 3k psf normal stress with slow strain rate. The ht of the wall is up 50 feet. I would appreciate any suggestions and input for the practical soil parameters.

I can compact in the field at dry of optimum and therefore increase the phi - make it behave more like sand than clay. Or per one recent Australian paper, use as little as one cm thick sand layer above and below the geogrid (to assure interlocking and resist pull out, which I think I can get from CL also) and use any material I want and design accordingly.

Thanks.

 

[GeoPaveTraffic]

As I said above, you need to work with your geotechnical engineer to determine an appropriate design.

Also, your comment that you can compact dry of optimum and make it behave more like a sand is completely off base. A clay is a clay and will behave as a clay. Your real issue is going to be global stability. Once you make the grids long and strong enough to fix global stability, your wall should be just fine.

Mike Lambert

 

[Pete1919]

(GeoPaveTraffic)
"your comment that you can compact dry of optimum and make it behave more like a sand is completely off base." - Are you sure of this? Please read this peer-reviewed paper "Pullout Behavior of Geogrid in Red Clay and the Prediction of Ultimate Resistance" in EJGE by Feng et al. Also please note I did not say "make it behave more like a sand". What I said was "make it behave more like sand than clay". In other words, phi increases and c decreases as you test the clays dry of optimum. That is what this paper says. Please read! If you don't believe it, have this confirmed yourself in the lab.

Clay is a complex material. Sure it can behave like sand in many respects, for example, if normally consolidated it has no c, the only strength parameter is phi like sand.

Thanks to all. I appreciate your time.

 

[GeoPaveTraffic]

I have not read the paper, but I will add it to the list of one that I would like to get around to reading. As for the quote, I admit that I paraphrased what you said.

As for lab test results, many many studies have shown that long term drained clay strengths are independent of compaction and moisture content and are related to the mineralogy of the clay. The latest work that I have seen is by Dr. Stark of the University of Illinois. His work uses ring shear test data, clay fraction, and liquid limit to develop relationships for fully softened and residual strengths.

If I were designing this wall, I would use fully softened strengths for the clay and something slightly less than fully softened strengths for the interface between the grids and the clay.

Again, the controlling condition is likely to be global stability of your 50 foot or so high slope.

Mike Lambert

 

[Pete1919]

Try telling the client or your contractor that compaction and moisture content do not matter, and get ready to be kicked out of the job. But that is altogether a different issue.

On the other matter which has nothing to do with my original posting, and since you brought it up, there have been many failures/studies of clay slopes in the UK. In despite of the constant moisture content and densities, the slopes failed after many many years of stability. It is due to "dilation" over time....another time dependent parameter.

 

[GeoPaveTraffic]

Agreed the education process can be difficult and time consuming.

Compaction and moisture content make a big difference in undrained and short term strengths, they just have little if any effect on long term strengths.

As for the UK, I expect that several different things are going on. There have been several studies of, I think it was railroad cut slopes in the UK, that failed several years after construction. The clay was overconsolidated and very stiff/hard when the slopes were cut. With time, the induced negative pore pressures dissipated and the slopes started failing. While I will not say that clays never dilate, that would be a very unusual behavior.

A simple way of looking at this is the difference in short term undrained strengths and long term drained strengths. Overconsolidated clay or heavily compacted clay fill has a very high undrained strength. However with time the pore pressures dissipate and the long term drained strength starts to control the behavior. I

Mike Lambert

 

[ATSE]

Pete,
Your general tone does not invite further comments.
But for the benefit of the other readers:
1. Not all publications deserve the same recognition. ASCE papers generally have more qualified authors and get more stringent review.
2. Instead of instructing other engineers to read your favorite paper, consider the very good literature on slope stability available. A very readable and still thorough discussion on slope stability is Soil Strength and Slope Stability by Duncan, Wright, and Brandon. The 2nd edition just came out.
3. Do not disregard material manufacturer's expertise. Many of the larger companies, like Mirafi and Tensar, have some of the best engineers in the industry as employees. You should care what they have to say.
4. Case studies and short term lab testing does not replace engineering mechanics, nor outlier (large variance) behavior.
5. Slope stability and wall design, including MSE walls, is semi-empirical (quoting Duncan). In addition to complex soil mechanics, there is a wide variance of behavior, and commonly accepted failure models do not always fully describe field observations. Humility and caution are in order.

 

[fattdad]

soil compacted dry of the line of optimums will show greater friction angle.
Geogrid walls can be designed using first principals.
The grid dimension does not need to be related to the D50 of the reinforced zone (e.g., consider RECO and their use of metal strips).
I'd never rely on the cohesion for long-term performance.
I'd consider evaluating the fully-softened shear strength of the backfill (i.e., get a bulk sample, hydrate it to its liquid limit and normally consolidate the sample in the direct shear machine - use peak strength for FSS and make a failure envelope).
I'd recognize that the line of optimums is parallel to the ZAV. So, for 95 percent compaction what's the target moisture? Is it 93 to 97, i.e., within 2 percent of the optimum moisture for 100 percent compaction?

I work for the DOT. We'd not allow CH or MH in the reinforced zone. We'd require Type 1 select fill, which I think has less than 15 percent fines. We are also designing for 75 years!

f-d

ípapß gordo ainÆt no madre flaca!

 

[moe333]

I wouldn't use CL, CH, or MH in the reinforced zone....unless significant lateral deformation is acceptable. These materials will creep in the long term, and they will never drain.

 

[Pete1919]

Thanks fattad.
The grid dimension does not need to be related to the D50 of the reinforced zone (e.g., consider RECO and their use of metal strips). - this is quite assuring and makes sense. Koerner, however, provides required d50 and aperture relationship, which I always suspected, only because I have found (and confirmed by Dr Giroud) some fatal mistakes in earlier versions of his book "Designing with Geosynthetics". So I lost confidence in that book.

I hope my tone does not offend ATSE or anybody else. As one can see I do not blindly follow anybody, even if it is Dr Koerner. If I did offend anyone, please know that it was not the intent.

 

[moe333]

Zornberg has free downloads of his papers, many dealing with this topic:

http://www.caee.utexas.edu/prof/zornberg/publications_topic.html

 

[Panars]

It's been done, but not often. MSE walls constructed with clays have often had poor performance, usually due to drainage problems. Government organizations and MSE wall suppliers often have limited control over the skill of the contractor constructing the wall, so they won't take chances with using clay.

In addition to the Zornberg papers given by moe333, I would suggest searching for "native backfill" "marginal backfill" or "marginal soil" along with MSE or RSS.

Geocomp has done some research on it. Their papers are here:

http://www.geocomp.com/files/techni...WallsStateofthePracticeinNorthAmerica_WEB.pdf

http://www.geocomp.com/files/techni...orcedFillSoilforMSERetainingWalls2009_WEB.pdf

Virginia DOT has a paper also.

http://www.virginiadot.org/vtrc/main/online_reports/pdf/05-cr12.pdf

And the Louisiana Transportation Research Center has done some research on it.

https://www.ltrc.lsu.edu/pdf/report_380.pdf

 

[Pete1919]

(Panars, Moe333)
Thanks for the references.
I did recently talk with a major geogrid manufacturer's engineering manager. There are no issues with using CL soil for RSS slopes (ours is 1H:2V). But for the MSE wall, the opinions are shaky and come with a few strings attached.

 

[Doctormo]

Quick comment:

True clay soil and 50 ft. MSE walls are not a good combination unless you like lawyers. Steep slopes have lessor requirements but can experience similar issues. You would have to quantify Atterberg limits and expansive properties of a clay soil to really discuss further. Some sandy glacial till lean clays can be quite strong.

Clay strength properties will change over time so a quick test in a lab may not provide representative long term strength results. Clay soils can also creep over time thus resulting in an unstable structure. Areas with clay soils that have lots of problems routinely design for low phi angles with no cohesion as that is where it ends up after enough time has passed. I have personally seen this happen where tension cracks continue to get wider over time thus permitting water to enter the soil zone and so on. Some clay walls have continued to move for years until such a time as something has to be done or failure occurs.