MSE Wall Sliding Resistance

[blgeo]

I am trying to verify the sliding resistance for an MSE Wall. The geotechnical report provides a friction angle and drained cohesion for the foundation soils. The recommendations from AASHTO (Sect. 11.10.5.3 and 10.6.3.4) and FHWA MSE Wall manual (FHWA-NHI-00-043) seem to indicate that only the frictional component of resistance should be considered. Although both documents do not specifically state that cohesion should be excluded (as far as I can tell).

Can you consider the additional cohesive resistance along the MSE wall base width as well? It does not seem unreasonable to me to neglect the cohesion due to potential construction disturbance along the sliding surface; however Im not certain whether this is common practice.

 

[fattdad]

refer to ncma "segmental retaining walls"

f-d

¡papá gordo ain’t no madre flaca!

 

[blgeo]

For this particular wall, I cannot use the NCMA code since it will be within a highway corridor. I recall that even using that code the cohesion is partially reduced by some factor.

 

[concretemasonry]

Check your highway department. They probably have information on the SRW designs that are accepted since most even have standard design plates for their own use.

The NCMA "code" is really not a code since it has no legal standing, but can be adopted by any authority by reference - same as a building code adoption something like an ASTM standard. If you have a large truck, most reputable SRW licensors (not knock-of producers) would probably be willing to share tests and back-up information on many domestic and international applications and testing programs.

Dick

 

[fattdad]

I'm sure the guidance from NCMA would provide a context to your problem, just like the replies to this thread may.

f-d

¡papá gordo ain’t no madre flaca!

 

[blgeo]

To clarify:

If I calculate the factor of safety in sliding considering only the frictional resistance (W * tan phi), I would estimate a certain factor of safety (FS).

If I calculate the factor of safety considering both frictional resistance and cohesive resistance (W * tan phi + c * B), I would estimate a higher FS.

Is it acceptable to design the wall considering both the contribution of cohesive and frictional resistances. Or does AASHTO not allow this?

 

[JAE]

Maybe I'm not seeing it right, but don't MSE walls use geogrids behind them such that much of the lateral sliding is taken back into the soil via the geogrid? Thus you develop soil friction back throughout the backfill and down into the supporting soil. Maybe I've got this wrong but I thought that was one of the lateral resistant mechanisms of these walls.

 

[concretemasonry]

JAE -

You are exactly correct when it comes to walls over the 4' to 5' height where the transition goes from gravity walls to engineered walls where the SRWs are the veneer that distributes and provides the ties to the various layers of geo-grid. There are many continuous walls single walls of up to 1 mile long with heights ranging from 4' to 40' in the world and the usually associated terrace walls contributing to the global design by an engineer. -I recall one stretch of walls in Spain between Madrid and Vallencia plus a few similar applications in South America and Australia.

Dick

 

[blgeo]

JAE

At this point I am considering that the wall is sufficiently reinforced to act as a single mass. I am attempting to verify external stability of the MSE wall. The lateral earth pressure exerted by the material behind the reinforced zone must be resisted by frictional forces (and potentially cohesive forces) along the base of the wall.

AASHTO does not directly address using cohesion (as far as I can tell). The sections that address sliding talk about selecting a minimum phi (frictional resistance only) based on composition of the reinforced backfill, the foundation material, etc. Im a wondering if it is acceptable to include the cohesive resistance. In cases where cohesion is relatively high and/or the vertical stress applied by the wall mass on the base is relatively low (ie short walls), including the cohesion in the calculation has a dramatic effect on the sliding stability.

In my particular case I need a minimum factor of safety in sliding of 1.5. If I consider frictional resistance only the FS in sliding is 1.4; and considering frictional and cohesive resistance the FS in sliding is 2.0.

 

[Boondog]

"Industry standard" for designing MSE walls is to exclude cohesion. You run the risk of problems during review if your design depends on cohesion to satisfy the minimum FS. I assume you are trying to limit L due to site constraints. In my opinion, you are in a better position to support your design by adding the extra couple of feet to L to get the minimum at c=0 than to argue foundation soil cohesion at the interface.

 

[blgeo]

Thank you Boondog.

My office is actually reviewing a submittal from another firm. We are having this argument with designer on whether or not to use the cohesion.

When I couldn't find a clear answer in AASHTO, I ran a check in MSEW ver. 3.0. Curiously, the additional resistance from the foundation cohesion is considered for sliding in that program.

 

[IDS]

This thread:

http://www.eng-tips.com/viewthread.cfm?qid=80582&page=9

from the slope stability forum discusses the same issue, and explains why C = 0 is appropriate for long term, drained conditions. I agree with others that this is normal industry practice.

Also if sliding is marginal without cohesion, a check of global stability with a slip circle analysis (and long term properties) is also likely to have problems.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Boondog]

MSEW confirmed what you knew, that cohesion in the foundation soils increases sliding resistance. That does not indicate whether or not the analysis is appropriate for you situation. I agree with IDS, a global stability analysis will likely end the discussion. Flip the design into ReSSA and see what you get for the translational analysis.