Firstly, I appreciate your reply so much.
Of course, as Engineer (ER), he can invoke the code and it is his decision and risk. Before I raise this question, I check the welding requirement of several state DOT, though some of them require AWS D 1.5 on H pile, I found none of them require AWS D1.5 on pipe pile, instead, they only require AWS D 1.1 on pipe pile. That makes me wondering if AWS D 1.5 really applicable to the said Pipe Pile or not? As Contractor, I think I would have to issue RFI to the ER to ask him make clarification.
Again, I am so grateful to your kind reply and I am looking forward to hear more from you.
Best