For Ductile iron pipe, simply increasing the pipe thickness and thus increasing the weight of resistant force may be the solution. Another solution could be to change the backfill specifications to a more structural type backfill involving compacted material around the pipe. As long as a pipe has an average specific weight greater than the fluid it displaces, it will tend to sink.
Eighteen-and twenty-foot laying lengths of push-on joint Ductile iron pipe in the following sizes and classes will not float in water (62.4 lb/ft3) even when they are unlined and entirely empty: 3-, 4-, and 6-inch diameter in all pressure and thickness classes; 8-inch diameter Special Thickness Class 52 and greater; I0-inch diameter Special Thickness Class 54 and greater; and 12-inch diameter Special Thickness Class 56.
Also, Ductile iron ball joint pipe in small sizes, generally 4-inch through 16-inch, will not float in water when empty. Buoyancy data for each type of ball joint pipe can be obtained from the manufacturer. Please refer to Ductile Iron Pipe Research Association’s (DIPRA) Buoyancy Effects on Ductile Iron Pipe for further information and more details.
Alternative materials such as HDPE and PVC have innate buoyancy issues regardless of diameter. They all float and therefore require specific attention (permanent anchoring, such as concrete structures around them) during initial planning and installation.
For more information on the differences and advantages of using Ductile iron pipe over HDPE and PVC, see these helpful blogs by my colleagues:
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