Technical Blog

I sometimes hear the statement that PVC pressure pipe requires “special backfill," while ductile iron does not. This is a mind-set that is not based on engineering principles.

“Backfill Requirements for PVC Pressure and Gravity Pipes” sets the record straight by investigating the concepts of pipe stiffness and pipe deflection. Examples are included using low-quality un-compacted backfill. Even in these extreme conditions, the result is that PVC DR18 and DR14 pressure pipes (the typical PVC municipal pipes) experience in-ground pipe deflections that are less than one-fifth of the allowable deflections.

It is clear that un-compacted native soil provides enough support to keep pipe deflections very low – insistence on costly imported backfill is a waste of an owner’s money.

AWWA Standards – Same for DI and PVC

AWWA installation standards recognize that PVC and ductile iron do not require different backfill. In fact, the AWWA C600 standard (ductile iron) and the AWWA C605 standard (PVC) each include drawings of five trench types. Close inspection of these trench types shows that the two standards have identical requirements.

PVC Gravity Sewer Pipe – A Different Story

It is likely that the “special backfill” mentality arose because of PVC gravity pipe. Gravity pipe is used in high-load conditions:

• Extremely shallow burial (as little as 1 foot) with highway loads
• Very deep burial (in excess of 50 feet) with significant soil loads

These high loads cause backfill material and compaction to become critical components of the engineer’s design.

For details, please click here to read the technical brief.