Placing posts behind geogrid wall

[jdbpe]

I have an allen block wall that is eight feet tall. The wall is supported by geogrid reinforcement placed at 24 inches vertically. We want to place a four foot high fence with fence posts that are two inches in diameter behind the wall after the wall is constructed. It seems to me that this would damage the geogrid and compromise the integrity of system. How else does one place these posts behind wall. During construction? Cutting holes in geogrid? Any thoughts

 

[eric1037]

Allan Block has details for doing this. I don't think it would matter if it was done after construction, if it was planned for in the initial design.

http://www.allanblock.com/Architect/retEngineerframe.htm

 

[ctbailey]

I too will be encountering this problem very shortly. But for guardrail posts.

I intend to include the post locations on the grid cut-sheet.

With the intention that the posts are installed simultaneously with the grid. In my opinon, this will DAMAGE the grid the least.

Think of the alternative: The fence/guardrail crew comes in later, uses their truck mounted auger to "drill" the holes for the guardrail, and what do they do, but rip though 2, maybe 3 layers of grid with the auger.

There is no way they would "dig" the holes, and cut the grid with a razor.

I suppose, a better way would be to specify a jersey barrier or other such gravity barrier.

Huh. Maybe.

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[BigH]

Check out BS1806, I think, for details for a friction slab that could sit at the top of the wall. The posts could be supported within the friction slab. Used for highway projects. I have some details, but . . . need to know how to send.

 

[ctbailey]

BS1806? BigH can you clarify?

thanks

craig

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[MSEMan]

What he meant was BS8006.

You have two options: an independant foundation or an augered one. For an independant foundation you can construct a slab say 250mm thick by 1000mm x 1000mm or so wide and put your fence posts in that. (Size of foundation dictated by the moment on the post of course).

Alternatively just dig the holes through the grid. If the wall is built properly the soil will be stiff enough to allow you to cut through the grid without moving the facing blocks - if not well you will have a problem. The MSE mass is not going to behave any differently just because you have a 250mm wide hole cut through a couple of layers of geogrid.

 

[BigH]

thanks MSEMan - I was trying to put the BS number on it from memory. Came close! Craig - BS 8006 is the British Standards for MSE Walls and, if I remember correctly, reinforced slopes.

 

[DesignCone]

I am just finishing a project right now for a commuter train layover yard. We have a Terrafix MESA wall, 2.5m to 3.0m high X 320m long, that the contractor is currently installing 1.8m chain link fence and then SBGR up against.

The fence posts are 60.3mm dia and are being augured in the granular backfill. The augured hole is 250mm and 917mm deep. We will be going though 2 layers of the geogrid.

The SBGR is being driven and is located such that the post will not hit the longitudinal ribs of the geogrid but will actually "split" through the traverse ribs.

 

[ctbailey]

BigH: Thanks for the clarification. I have to admit, I certainly have a "US-Centric" view on engineering codes and specs.

Craig

___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[BigH]

ctbailey - so do a lot of designers and spec writers which is okay if you are in the States but specifying US grade steel on a job in Laos when everyone knows that they will use Thai or Vietnamese steel grades - well, it does lead to problems. Since I have worked overseas continuously for over 11 years, I am a bit more sensitive to the issue I suppose. The BS8006 is a pretty good book and suggest one compares it with the AASHTO Bridge code section.

ps. I haven't forgotten about forwarding you those pictures - just have to remember when I am home to get it off my home hard drive.

 

[MSEMan]

It is almost beyond the realms of possibility that a few holes cut in the geogrid will "compromise the integrity of the system." You have 2" posts placed say every eight feet - 2% of the linear coverage and assuming grid lengths of say 6' - 5 one hundreths of a percent of the overall coverage area. PLEASE!!!! the excess capacity in the system (after taking account of conservative reduction factors) is probably on the order of 30%.

It is like saying that you can't drill a 1/4" hole in a RC wall because you might go through one rebar.

 

[geociv]

MSEMan has a excellent point. The small holes are insignificant to the overall system. Plus MSE walls generally have a lot of conservatism built into the design. I've even heard of driving piling through the reinforcement (from a well known geosynthetics expert), but have not personally seen it done. If you're really concerned, it would be possible to leave sonotubes (or similar) at the post locations for subsequent installation of the posts.

I wouldn't lose any sleep over it.

 

[BigH]

Ran across this particular site which is relevant to this discussion:

http://www.oregon.gov/ODOT/HWY/GEOENVIRONMENTAL/docs/Guardrail_Posts-Appurtenances.pdf

 

[Boonie]

Drive guard rail posts, don't auger. Insert sleeves for fence posts during the wall construction. This makes things easy for the fence contractor. 4" pvc is adequate.