Lateral soil pressure between basement and soil nail

[shepherd]

We are involved in a building project with a sloped site that has grades in the 1.5:1 range. The preliminary proposal for handling excavation on the up-slope side is to excavate using soil nailing, and then place a basement wall approximately 10 feet in front of the soil nail wall. The soil nail cut would be in the 20 - 30 foot range. It is then proposed to backfill the 10 foot space behind the basement wall which would support 15 - 20 foot of unbalanced fill. The reason for not using the soil nail wall as the basement wall is due to numerous wall offsets.

Is anyone aware of any guidance on reduced lateral soil pressure due to the wall only having soil in the 10' space behind it? Would the only lateral pressure it would see be from a wedge failure that could develop within that 10 foot space?

Thanks

 

[MSEMan]

Do yourself a favor, put an MSE pressure relief wall in front of the soil nail wall. We've recently been involved in similar projects, but the shoring wall was sheets. Should work great in your application and also make the basement wall a very economical design since you'll only need to consider vertical loads.

There's a great new FHWA manual on the SMSE (Shored Mechanically Stabilized Earth) procedure. A good source of advice is the Jim Collin of The Collin Group (Bethesda, MD) who co-authored the FHWA report. Try EarthTec (Leesburg, VA) or Tensar (Atlanta, GA) for practical advice as they have experience in this kind of work. Depending on where you are, you might even be able to find a geotech contractor who could package the whole thing for you.

 

[shepherd]

Thanks for the idea. It just appears uneconomical to construct three separate walls, and I'm sure it would raise some eyebrows with the owner. I believe before we would do that, I would propose putting a coupler on the end of the soil nail and extending into the concrete basement wall for support. Any thoughts on that option?

I'd still like to see what others may think about a reduced pressure. I don't believe in this case we'd need to design for the full pressure?

Thanks

 

[PEinc]

You might see some reduction in earth pressure to the building IF the soil nail wall is a permanent wall with proper corrosion protection, if the theoretical failure plane angling up from the bottom of the new structure intersects the vertical (or near vertical) face of the permanent soil nail wall, or if the backfill material between the new building and the soil nail wall is a much more competent soil than the nailed soil.