Q. Explain Forty-five degree spread rule for driven piles ?

For driven piles the length of piles driven into the ground is generally based on dynamic driving formula such as Hiley's formula. In certain ground (for example chalk) the length of piles driven by adopting pile-driving formulas can be far more than adequate since the soil strength may increase with time. In contrast, in silty soils phenomenon of false set can appear and the piles give a false impression of attaining sufficient bearing resistance from the ground. Henceforth the pile's capacity must be verified and checked later by loading test. 

In case driven piles are founded on different depths below ground, 45-degree spread rule is generally applied to check their load carrying capacities. The base bearing loads of higher driven piles are presumed to spread at 45 degrees from their bases and checking is made if loads from higher piles would influence or get transmitted to deeper ones.