Failure slopes through geogrid

[ivanlocke]

I have a retaining wall 6 concrete blocks high with 5 layers of geogrid inbetween. I am looking at using two strengths of geogrid.

If I use the stronger geogrid for the bottom 3 layers. It will have sufficient strength to counter the failure slope starting at the the bottom of the 6th block. Can I then design the top two layers of geogrid shorter using a failure slope starting at the 3rd block?

Thanks

 

[BigH]

I don't believe that's the way it is done. There are, as you know, several considerations. First is the earth pressure against the wall. The wall facing is basically "just for show" - it holds back the facial soil from falling out. The grids are used to counter the pressures of the fill. You can adjust the strengths of the grids, along with the vertical spacings based on the pressures. The grids need to extend back far enough behind the wall to extend beyond the active movement plane.

Secondly, though, you have the global stability. You have made no comments on the nature of the ground onto which you will place your wall. You need to be congnizant of this too. The design of the present job I am on failed to account for this.

Check out the MESA wall computer program that you can get. It is put out by the Tensar group. You can download it. Also, there is an Italian company with a program too. Some recent texts give good examples of how to design.

Best regards.

 

[PEinc]

Design of these walls by hand calculations is at best a tedious operation. There are a number of sources for software to design geogrid walls. Contact the National Concrete Masonry Association in Virginia. Their web site for software is

http://www.ncma.org/estore/index.htmlt

Or go to

http://www.geocops.com

for other available software.

 

[doresh]

Bigh is correct, Also on the upper part of the reinforced mass the pressure on top of the geogrids is low and you get low resistant to pullout so never shorten your length on top.

You can look for BS8006 as a design guide for such walls, or the FHWA report 82 on the matter.