Contribution of concrete in pipe pile 1

[harpoon]

We are installing open-ended pipe piles to bedrock. After installation the piles will be plugged with concrete. Can the contribution of the concrete be taken into account for pile bearing capacity? How can this be estimated.

 

[Focht3]

Well, what are the strata your pipe piles will penetrate before they reach the rock? List them all - with thicknesses and approximate strengths. Have you calculated when a plug should form? Do you plan to auger or jet out the piles? Give us your thoughts on how you might tackle the problem -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[harpoon]

Hi Focht3,

We are preaugering the piles through mine rock fill and firm to soft clay to andesite bedrock. So far I have not taken the contribution of the plug into account since the piles will be preaugered.

The pipe piles will be filled with concrete after installation. I have not taken the contribution of the concrete into account because of the difficulty involved with cleaning out the pipe and ensuring that the concrete will go all the way through. Also I am not sure if the pipe and concrete will act as a unit.

Appreciate your thoughts on this.

 

[thelab]

If you can not be sure that the pipe interior will be cleaned before the pipe is filled, then I would suggest neglecting the contribution of the increase area. However, since they are pre-augered you may have a good chance at cleaning them out. The concrete and steel will exhibit different load-transfer behavior at the tip because of the differences in material modulus of elasticity, but if you use a safety factor of about 2 to 3 at the tip, the strain should be small and the effects not worth considering. In the end, it will be your opinion, but I have used the full end bearing area on a concrete-filled pipe pile a number of times with no known problems (I am knocking on wood)

 

[harpoon]

Hi thelab

Thanks for the comments. IN that case what do you use for design the yield strength of the steel for the entire area?

 

[Focht3]

If you use good quality concrete - and enough of it - then steel/concrete bonding shouldn't be an issue. Since you are predrilling, you should be able to flow some concrete into the annulus around the pile tip. You could also oversize the hole slightly and create a composite pile.

Are you concerned about negative skin friction?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

Even if you pre auger you will have some collapse of the hole. Cleaning out the pipe can be tedious and time consuming unless the piles are reatively short. However, if you dont clean out the pipe, you will have a compressible layer under the concrete, which will prevent the concrete from contributing. Also if the bedrock is below the water table that will greatly complicate the problem (more sloughing, need to tremie conc., etc,)
I would sugest driving the piles with a thicker wall (which since you are driving through rock fill can only help) or drive piles closed end. If you drive closed end you may be able to eliminate augering or at least limit to the upper fill area.