Clear distance between Pile & Caison foundation

[GeotechNath]

I am designing a pile group to support a bridge pier. Just adjoining to the proposed pile location, there is an existing caisson/well foundation. I am searching if there are any guidelines for minumum clear distance required between this pile group and the caison/well foundation. Please post if you know any guidelines / reference.

Pile group consists of 4 piles - bored cast-in-situ piles (in square pattern, each of 1.2m dia). Caisson/well foundation's diameter is 3m.

Thanks in advance

 

[BigHarvey]

From a construction point of view, you have several points to consider :

- possibility of a concrete overbreak in the top meters of the 3 m diameter existing caisson, which might make it difficult to contruct the new piles ( deviation )
- you don't want the ground between the existing caisson and the new pile under construction to collapse in the new pile hole before it's concreted. Murphy's law applies to these soils : when you don't want them to collapse, they do and if you want them to collapse, they don't. If you need to stay close ( less than 1 m ) then think about protecting the new pile excavation with a temporary casing.

From a design point of view it would be better to go deeper than the existing caisson ( provided you know their depth ! )