271828,
I'm not the OP, but there are a couple of constructability issues which make three pile caps problematic.
1. The pile caps are normally detailed as more or less triangular, with three directions of reinforcement between the piles. Because everything is not orthogonal and there are different length bars, bar setters don't like them. The excavation is also a bit trickier than a rectangular hole.
2. I have know the setout of the piles to be done incorrectly. the column centreline has to be at the centroid, not the centre in both directions.
I think it depends on the cost of the piles - if your piles are very large and very deep then saving the extra pile would offset the cost of the fiddly rebar fixing and slightly more fiddly formwork needed for a 3 pile cap.
If your piles are small then it won't make much cost difference sticking in an extra pile or two, especially on a small job where mobilisation of the piling rigs is more significant.
Having said that, you could always investigate driving/drilling the pile to a deeper depth to give 50% more capacity and changing the 3 pile cap to a 2 pile cap?
You can detail a 3-pile pile cap as a rectangle with orthogonal reinforcement. This gets rid of the detailing issue and leaves it to a cost comparison for the additional concrete and rebar.
On one project about 15yrs ago we changed a three pile (20" piles with triangle cap from memory) design to a single large pile (48" diameter) and even though our concrete volume more than doubled, we saved $100k on the foundation contract.
Horses for courses. Sometimes, you never know which way is more economical until you get the contractor involved. Dare I say it...this is one advantage of design-build contracts and/or piling done as a performance activity.
The biggest problem I've had with piled foundations is the as-built tolerances. I've had plenty of modified pile caps and additional piles installed due to 4+" out of tolerance installed piles.
