Minimum Spacings When Piling Adjacent to Existing (Disused) Piles 1

[MWD123]

Hello

We have a site with a large number of existing piles on it, we are planning to construct a new building that will require large number of new piles to support it.

None of the existing piles will be reused, and the geotechnical engineer is happy that they will not be a factor in their design.

The problem we are facing is how to determine the minimum offset between new and existing to allow for construction tolerances. Although the existing pile heads will be surveyed on site, I expect they could have a verticality tolerance of circa 1:75, as will our new piles, the new piles will also have a tolerance of ~75mm on plan. While we could sum all of these tolerances together, that seems unduly conservative (this would only occur if you had the worst case verticality/positional deviation for both piles, and this deviation occurred in the worst possible direction).

Is there a sensible way of summing various piling tolerances to get a more reasonable offset? Usually I would be happy to take a worst case but the site is so tight that this figure will have a significant impact on the foundation design,

If anyone has any literature or case studies they could share that would be much appreciated.

 

[jayrod12]

Whenever I've had to do this, a geotech has told me to provide a minimum of 12" clear between the edge of an old pile and the edge of the new pile.

Are your piles end-bearing or friction? How deep?

 

[MWD123]

We haven't received the pile design yet but I understand it's a combination of both skin friction and end bearing (they're taken down into a layer of medium-dense sands).

Both the existing and proposed piles are circa 20m long.

Currently I'm thinking of a root sum squared approach, this gives us a value close to 12in which makes me think it's fairly sensible.

 

[Smoulder]

If you're worried about getting the piles in is there enough ground left for skin friction?

 

[geotechguy1]

When we design a bridge (or more simply a desk...or even a door) we intuitively know that that the bridge piers or the legs of the desk or the frame of the door, being stronger and stiffer, will attract rather more of the load than the air in the gap.

If we filled the gap under the desk or the door or the bridge with sponges, it would be equally intuitive that the stiffer, stronger bits (the pier and the doorframe and the legs) still attract most of the load.

If we stick a concrete pipe in the ground horizontally for stormwater, we design it knowing that being as it is stiffer and stronger than the surrounding ground it will probably attract more load than just the column of soil above it.

For some reason if we make the problem vertical piles and fill the gaps with soil instead of air or sponges though everyone believes the analogy breaks down (perhaps to pretend none of the load from the soil above a neutral plane is getting transferred into the pile).

Sorry for the long analogy. The old piles still might attract some load even if they are cutoff and disconnected from the structure. You would see it in a well built FE model. Your geotech will want to pretend they don't exist though.

So yes - there's still enough ground left for " friction "...worst case, the load is getting shed into the old pile, and then eventually winds up at the same place as it would if the old piles weren't there (below the neutral plane where all pile loads actually go ) So unless you think the old piles are going to dissolve and leave a void it's OK (are they non-filled steel tubes or h-beams per chance?)

Personally, I'd want all the existing pile heads surveyed (as-builts suck, piling contractors lie, some of the old piles may not even exist or they may not be 20m long) and then at least a low strain pile test to get an idea of the actual lengths. Then I'd want to look at the whole thing in a FEM model.

 

[LittleInch]

If you're that tight for space why not use the old piles if they are the same length?

Or use two of them for one new one?

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