In a dual wheel assembly if P is equal to each wheel load, S is centre to centre distance of dual wheels and d is the distance between walls of wheels, then -

each wheel load acts indepenently upto a depth d.

totsl stress due to the dual wheels at any depth greater than 2 S is equivalent to a single wheel load of magnitude 2 P.

total stress due to the dual wheels at any depth greater than S is equivalent to a single wheel load of magnitude 2P.

total stress due to the dual wheels at any depth greater than 2d is equivalent to a single wheel load of magnitude 2P.