Under the 45o load spread rule, when the horizontal distance between the toe level of adjacent piles is smaller than the vertical difference between the piles, additional load is deemed to be added by the pile at lower founding level. Hence, when the pile at lower founding level is within the zone of 45o spreading from the pile at higher elevation, further load checking is required for the lower pile.
This 45o load spread rule is a common practice of foundation engineers and may not be incorporated in foundation codes. There are some situations where this rule may fail. For instance, when two piles at the same foundation level are situated close to each other, they shall have load effect on each other. However, based on 45o load spread rule, it could not address this concern.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.