Preforming Effects on Lateral Capacity of Pile

[cancmm]

I'm currently working on a marine structure which utilizes 24" square prestressed concrete piles on a 1:4 batter angle. Due to the close proximity of a nearby seawall, the upland geotechnical engineer wants us to preform the upper 25' of the embedded pile so as to minimize vibration. Preforming will be done through a simple auger (not jetted). Borings in the area show alternating layers of loose to dense sand with lenses of soft to hard sandstone/limestone.

I guess I have a few questions on this:
1) The structure is primarily subjected to lateral loads. How much of an effect will predrilling have on confinement assuming the diameter of the auguer is equal to the corner-corner distance of the pile? I'm trying to avoid an increase in the effective length and resulting moment in the pile.
2) If confinement is affected, how effective would filling the annulus with sand be in this situation? Would it even be worth trying given the high batter angle? Grouting with concrete would be another but expensive option since this is a water-based operation.
3) Any other ideas or suggestions?

Thanks in advance!

 

[cntw1953]

I don't get it.

Say you have a 100' long pile, an auger will bore the first 25', then the pile will be driven in rest of the way, if this is the case, how?

Deep water drilling and concreting have been done routinely, why not looking into that.

 

[TDAA]

What about augering to a lesser diameter to create a partial displacement situation, which may lead to easier driving therefore reducing the vibration, but not leave the gaps?

 

[cntw1953]

Are the piles installed on land? I thought were in water for "marine" structure. Or the area will be dewatered during pile dribing?

 

[paddingtongreen]

How did you decide to use these particular piles? if it was on the recommendation of you own Geotech Guy, ask him.

I take it these are bearing piles? if so, it should make no difference to capacity but if it is friction from the dispacement, you would lose capacity.

Michael.
Timing has a lot to do with the outcome of a rain dance.