Design of geogrid reinforced earth wall for highway embankment 1

[bbac]

For the design of geogrid reinforced earth walls, the maximum vertical spacing of geogrids is limited to 800mm in FHWA/AASHTO. There is no such limitations in BS code 8006-1:2010.When the design is carried out for reinforced earth structure with gabion box facing and secondary reinforcement using BS Code, the external, internal and global stability criteria are fullfilled even if the vertical spacing of primary reinforcement is 2m at upper levels.
What is the basis for limiting the maximum vertical spacing to 800mm in AASHTO?

 

[geojosh84]

Is this for a wall or reinforced embankment?

 

[bbac]

This is for wall MSEW.

 

[RFreund]

I can't say for sure, but in my mind the maximum limit is to prevent something "strange" from happening between grid layers. In an MSE wall your are 'assuming' that the geogrid is going to engage the entire soil mass to prevent sliding, pull-out etc. We make some pretty simplifying assumptions regarding the stress distributions of the lateral soil pressure and these maximums (I beleive) are meant to ensure that things behave the way we assume.
If I was to go 6m between grid I would want to be very comfortable in my soil design values (long term conditions and short term).
I know this is easier said than done but, these type of walls are cheap, and very durable. However, they are very difficult to repair. Normally you just tear dwon and replace. I always feel like a little extra grid is insurance against a large rain event or something out of the ordinary.
But I get it, when you have a contractor asking "is it really neccesary" it is nice to have something to fall back on and say "yes, it is".
There might be better answers out there though. Most likely "that is the range of our testing".


EIT

http://www.HowToEngineer.com

 

[oldestguy]

Well stated.