P-multipliers for piles in a retaining structure

[abih]

The structural engineer is proposing to reinforcing an existing slope with drilled shafts, which will be 24 inches diameter and 30 inches center-to-center. Basically, 6 inches room is left between shafts for installing tiebacks installed into the body of the retained structure.

What P-multipliers do we need to recommend, considering the spacing is only 1.25(=30/24) times of B?

Recommendations by AASHTO 2000 and US Army 1993 simply do not provide values for spacing below 3B, except uncertain extrapolation is used. Would 0.2 be reasonable? or even 0.1? Thanks.

 

[GeoPaveTraffic]

Where to start...

As far as I'm aware, neither AASHTO or USACE provides recommendations for drilled shafts used to stabilize a landslide, so quit looking for a code solution.

There should be very little vertical load on the drilled shafts, only the vertical component of the tie-back load. So figure the vertical load that will be on the drilled shaft and then look at how the drilled shaft will shead that load. If it is in very small end bearing, figure the overlap in the pressure cones and see if you have an acceptable factor of safety. If the load path is to shead the load in friction, again figure the overlap and see if you are ok.

Other options include installing the tie-backs at a flatter angle so there is less of a vertical load component.

Mike Lambert