Axial Resistance of Tangent Drilled Shaft Walls

[DCA040414]

Can anyone provide a reference to a guide for tangent drilled shaft wall design, specifically the axial resistance calculation. I am currently evaluating the axial resistance of the wall as a shaft group however the group efficiency factors provided by FHWA LRFD Bridge Design Specifications 10.8.3.6 are limited to spacings of 2.5D or more. Are there other methods of determining group efficiency factors for tangent drilled shaft walls?

 

[BridgeSmith]

Shafts in a tangent wall would be a single row. Table 10.8.3.6.3-1 states for shafts at 3D spacing or more, there's no reduction. That would indicate that the diameter of the effective bearing is 1.5D, the area would be PI*(1.5D)^2/4.

Assuming the shafts are spaced at 1D (touching each other) the bearing area for each shaft is just the area of the shaft, plus the areas in front and behind. I would conservatively calculate these areas with a width equal D, and a combined length equal to .5D. so that area is (.5 + PI/4)*D^2

The reduction factor (the way I would calculate it) is [(.5+PI/4)*D^2] / [PI*(1.5D)^2/4]. There's probably a way to simplify that further, but that's all the time I have right now.

 

[EireChch]

You could be conservative and consider it as a strip footing founded at the toe. If that gets you enough capacity then no need to do anything further.

 

[DanielSgeo]

Out of curiousity, why do you have axial loads on a pile wall?

I see the logic of BridgeSmith's approach, but I think the FHWA reduction factors account for end bearing and skin friction. I'm not sure BS's approach is applicable whith skin friction.

I would consider EireChch's approach, and you could take it a step further and calculate skin fricton from the rectrangle inscribed by the tangents.