MSE stability due to slope failure and reinfocement length

[salman85]

i have a problem with manual analyse of MSE stability due to slope failure.
The MSE wall has 11.75 m height with soil reinforcement using geogrid (vertical dist 50 cm) as attached sketch..
analyse using AASHTO LRFD with strength load combination (1.35 EV; 1.5 EH and 1.75 LS) also An abutment stand on the top of MSE wall, it has bearing pressure 153 KN/m2..

my question is, how to modeled the MSE wall when analyse (manually) stability due to slope failure? GWT is 0.5 m beneath toe of MSE..
is it required to assume the soil reinforcement resistance with soil cohesion? how to correlate?

another question is, if looking at the soil reinfocement length analysis, is the uniform reinforcement length (L = 14 m based on external stability) required (not in various length) ?

 

[BigH]

Why do you think you think you have slope failure? What is the foundation material on which your MSE wall is sitting? There are many MSE walls that support abutments . . . but don't have "slope failure".

 

[salman85]

Thanks BigH...i'm sorry, the point is global stability and compound stability..actually, the day before yesterday, i have been able to analyse that...

How about uniform reinforcement length? can i reduced it? because the requirement is less than and the available effective length (preliminary reinforcement length taken from external stability)..

is it any requirement for vertical clearance distance from the highest backfill toward to the nearest reinforcement beneath it..?

 

[RFreund]

Do you mean reduce the lower elevation lengths or the upper?

I don't think I have ever seen the upper grid lengths reduced. They must be there to engage the reinforced soil mass. You can space them according to the maximum spacing allowed but I do not believe you can reduce them.

If the lower grid lengths satisfy internal and external factors of safety I have seen variable lengths used (meaning the grids are shorter at a lower elevation).


EIT

http://www.HowToEngineer.com

 

[salman85]

RFreund :

my first thought the same as you, as per AASHTO LRFD, the uniform reinforcement length is commonly used.

I have another case (calculate by other people), that the lower reinforcement length is longer than upper layer, say the lower layer about 13 meter, the middle layer 10 meter, and the upper layer only 8 meter..how about that?
(my opinion : Based on rankine failure zone, the upper layer must have reinforcement longer than the lower layer)

 

[RFreund]

Interesting...

I wonder if the following is the case:
They do the design of the wall for internal and external stability. The lower and upper grid lengths all extend beyond the failure plane (rankine or coulomb) and meet the requirements for anchorage (pull out). External and Internal Factors of Safety are met. Then they run the global analysis and they need to extend the lower layers to satisfy global stability and their analysis determined that the upper layers did not need to be extended to satisfy global stability. Therefore they figured because internal, external and now global stability are all satisfied then they did not need to reduce the upper layers.
I'm not sure if I'm remembering this correctly but I believe that we have had cases where the lower layers have controlled global stability and needed to be lengthened however we then made all the grid lengths the same and did not use shorter grids at the top. If this is the case, I'm not sure which is right, wrong or other.

EIT

http://www.HowToEngineer.com

 

[soil1999]

Hello Mr salman85, it seems that your issue is already resolved. But if you are familiar with TXDOT, it has great examples on this.

 

[fattdad]

you may want to check the GRS-IBS (geosynthetic reinforced soil - integrated bridge system) publications of the FHWA.

f-d

¡papá gordo ain’t no madre flaca!

 

[salman85]

thanks soil1999, i'll check on Texas department of Transportation

 

[salman85]

thanks fattad, with pleasure i'm gonna check the GRS-IBS..
if you don't mind, could you please give me the link?

 

[RFreund]

GRS-IBS - Guide and Report:
Interesting concept - Facing is non-structural. The grid is closely spaced and designed for 'bin' pressures. Also overturning is not a stability check actually it is not a concern for almost all geosynthetic reinforced walls however it remains in the code due to 'habit'.

http://www.fhwa.dot.gov/publications/research/infrastructure/structures/11027/index.cfm

EIT

http://www.HowToEngineer.com