Drilled Piers Axial Compression Criteria 1

[conradlovejoy]

If the soil bearing is high enough, one could encounter a situation where the pier will not fail the soil in bearing before is surpasses the P_allow obtained from treating the pier as a concrete compression member 0.8Φ[0.85A_sf'_c(A_g-A_s)+A_sF_y]. Is column axial failure a factor in drilled pier design or does the surrounding soil somehow make that a moot point?

 

[MotorCity]

Most of the time, column axial failure of a drilled pier is not an issue because although the surrounding soil may not have adequate bearing capacity, it usually has enough lateral capacity to "brace" the pier against buckling failure. The exception to this is if you are drilling through really poor soil, like topsoil.

 

[WARose]

I'm pretty sure I've never encountered a situation like that. Almost always (with a compression only pile) you will wind up with the pile exceeding its allowable skin friction + end bearing capacity before a strength issue arises in the pile (as far as reinforcement goes). An exception might be piles socketed into bedrock.

 

[Retrograde]

Structural strength of the pile can be a limiting factor if it is founded in high-strength rock. I have seen a PDA test result that indicated crushing of the toe of the pile.

 

[hokie66]

Even in high strength rock, bored piles are not realistically designed for that high stress. Who would design a pile for 25 MPa, or for that matter 32 MPa, ultimate bearing? Rock is just not that consistent.

Because we only tend to use a fraction of the concrete column capacity, it is usual to reduce the pile reinforcement from 1%Ag minimum as for columns to 0.5%.