why reduce pile capacity in existimg piles 1

[londonkid]

I have a scheme where we are installing new pile close to existing, it has been suggested to me I should be reducing the capacity of the existing. Why? If we are far enough away and block failure of the pile group is not critical why does it matter? Why is that any different to them being installed at the same time. The existing is actually less capacity than the new piles.?
I see no logic. Maybe if the old were greater capacity keep the load down to the new, I can almost understand the logic. but not this. Any ideas?

 

[SlideRuleEra]

You did not mention if the old pile and the new pile share a new, common pile cap.

If they do, consider that over time the (loaded) old pile have either settled in the soil or foreshortened due to compression within the pile itself - not much, maybe a few hundreds of an inch, but they have settled / foreshortened. No matter what specs are used for the new piling, when they are loaded, they will settle / foreshorten (a little bit) also.

However, when the new common pile cap is constructed the new pile settlement has NOT yet happened. When this settlement does happen the load on the new piles will tend to go lower (as the piles go down in elevation) and the load on the old piles will tend to go higher (since they will not go down in elevation as much or any at all).

By arbitrarily saying that the old piling will be more lightly loaded (for the new design), you are taking into consideration that (in the field) the loads on the old pile will slowly increase as time goes by (hopefully not to exceed their ORIGINAL design capacity).

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[msquared48]

There's a pile of logic to your post Sliderule!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[londonkid]

ok, logic ok. But if the load transfers back to the old pile then once the new pile settles (slightly) then should the old pile settle - load back to new. System in equiqulibrium. Old pile will have more load but below design load.
This is how a newly installed system behaves.

 

[BAretired]

If you are talking about drilled concrete friction piles, I see no reason to reduce the capacity of the existing piles provided you maintain a center to center spacing of 3d where d is the pile diameter.

BA

 

[londonkid]

new will be bored utilising end bearing and friction. I propose to dowel into existing but maintain 3d

 

[BAretired]

In my area, geotechnical engineers do not recommend combining end bearing and friction for straight shafted piles. I don't like to rely on end bearing because loose material can fall into the pile excavation prior to or during concreting.

However you calculate pile load, I see no reason to reduce the capacity of existing piles.

BA