What is the design approach for the spacing of min-piles?

For close spacing of min-piles, it would provide substantial cost savings with the reduction of pile cap size. However, close spacing of piles implies the problem of group effect which tends to reduce the load carrying capacity of each pile member. Notwithstanding this, there is well established rule which govern the minimum spacing of piles, i.e. for friction piles like mini-piles, the centre-to-centre spacing should not be less than the perimeter of the pile.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.