9Marston, Anson, A O Anderson. The Theory of Loads on Pipe in Ditches and Tests of Cement and Clay Drain Tile and Sewer Pipe. Bulletin 31. Ames: Iowa Engineering Experiment Station.
10Marston A. The theory of external loads on closed conduits in the light of the latest experiments [J]. Iwoa Engineering Experiment Station, Bulletin 96, 1930(9):138-- 170.