MSE Retaining Wall system design (redi-rock)

[EngineerAll]

I am working on designing a two tier redi-rock retaining wall system. The tiers are 15 feet in height and would require long geogrids. I was wondering if I use a special foundation system say a concrete footing on caissons would that help with sliding and hence reduce the length of geogrid.

 

[GeoPaveTraffic]

Assuming redi-rock is a particular brand of segmental retaining wall block, the short answer is no. These wall work by have a mass of material to resist the lateral forces of the material being retained. While it is technically feasible to resist those forces with a structural foundation system, the cost would be many many many times the cost of the "long" geogrids.

Don't forget to have the global stability of the wall system checked by someone who understands slope stability.

 

[EngineerAll]

Thank you GeoPaveTraffic.

 

[bushel3]

Another importannt thing is the geogrid must be long enough to get outside the active wedge created by the failed soil mass behind the retaining wall. The distance the active wedge reaches is porportional to the height. So the higher the wall the longer the grid, even you found someway to prevent pullout with short grid.

 

[Boonie]

What failure mode is driving your design?(sliding, pullout, global stability). Are you having problems with internal sliding or external? Due to the flexible nature of the segmental systems, rigid foundations are not normally suitable. Modifications to your foundation and retained soil parameters will help with sliding if global stability is driving your design (likey).

 

[Prepakt1]

The last post has it correct. Block wall systems should be on a compacted gravel base. They need to flex.Whenever we have installed walls on a solid concrete or asphalt you need a leveling course and it always seems a lot more trouble to get the walls correct.Anything other than gravel seems to throw the aligmment all off. You would think it would be the other way, a solid base would give you a perfect alignment instead you end up with something out of line and its a major problem to re-align if the base is bedded in concrete. Blocks always settle a bit into concrete and if the setting time from one block to the next is off by a few minutes it will sit differently.

Intrusion Prepakt

http://www.marineconcrete.com

 

[MSEMan]

If you are having problems with sliding something is amiss. For a two tier system this problem sometimes arises when you are taking the full sliding force from the retained embankment and applying only the mass of the lower tier wall to resist. Its a very messy hand calc and rarely makes sense.

FHWA NHI 00-043 gives the following advice for tiered walls:

"For the lower wall consider the upper wall as a surcharge in computing bearing pressures. In lieu of a conventional external sliding stability computation perform a slope stability analysis with failure circles exiting at the base. A minimum factor of safety of 1.5 is generally warranted."

Make sure the person doing the slope stability knows what they are doing. Good luck.