foundation behind MSE wall

[kgeot]

Hello, I have designed a MSE wall that is 27 feet max in height. Now the owner wants to include a monument sign behind the wall face about 10' at the max height location. The sign weight is about 375 kips. The lateral pressure from a shallow foundation appears to overstress the goegrids. Does anyone know an appropriate analysis method if a deep foundation is used?

 

[Boonie]

MSE walls are not typically designed for lateral loads other than earth pressures. As long as the vertical loads generated by the sign are transferred to a point below the bottom elevation of the reinforced soil zone, it should not impact the design of the wall. For instance,with a pressure relief wall, the building loads are transferred below the bottom of wall elevation using piles and the wall is designed for nothing more than the slab loads. Similar to bridge abutments with deep foundations.

 

[MSEMan]

375 kips? Really? - that's not a monument sign that's a monument- what is it the Statue of Liberty?

It seems over the top to put in a deep foundation just to support a monument sign. Why not treat the shallow foundation just as you would a superimposed bridge abutment bearing shelf and increase the capacity of the grids? (unless of course your wall is already built). You probably want to keep the bearing stress down to about 350 psf which will give you something like a 15' square base (just enough to keep a few feet back from the wall face).

 

[kgeot]

Thanks for the repsonses. (Un)fortunately I'm using large segmental blocks (3'high,8'wide,4'deep) and the heaviest geogrid for which interaction testing has been done (synteen SF110), so there's not an easy way to increase grid strength or number of layers, so the deep foundation looks appealing.

 

[MSEMan]

Are you serious? What kind of block is that and why would anyone use such a chunky piece of concrete just for a facing?

Anyway- not every grid needs to be attached to the facing so you can make the spacing whatever you like. To tension the grid use some u-shaped pins near the face made out of bent rebar.

 

[Boonie]

If you are experiencing a geogrid overstress failure mode and not a connection failure, I agree with MSEMan. You can decrease the grid spacings to as little as 6" without causing a constructability problem. This is normal in geogrid structures over 50' tall. The additional grid will be far cheaper than the deep foundation. You may want to approach the design of this section of the wall as though it had a wrap around facing and negate the connection capacity all together. That facing unit does seem a little bulky for such a short wall application. Good luck.