For intake and outfall pipe lines project such as that of once-through sea water circulation of large-scale power plant,it is relatively difficult to design and implement the works under water,however there is no specific technical code for this aspect by now.Aiming at questions such as different characters of above or under water works,parameter sensitivity,and reaction modulus of the natural compacted backfill material,comparative studies are carried out and verifications are made by actual pipe tests on site and material tests in laboratory.Key design points and parameters are recommended.