See the "Urban Myths Die Hard" article for use of PVC gravity sewer applications at depths of 30 feet and more. Many sewer utilities have arbitrarily set depth limits on PVC which are not engineering based. 

This document has been developed by the Uni-Bell PVC Pipe Association for design and installation of PVC fittings where solid-wall PVC pipe is used in non-pressure applications. The guide provides information on the wide range of fittings that are available for various dimension ratios and outside diameters of PVC pipe through 60-inch. Also included is information on installation practices.

Photo demonstrates how flexible PVC can be used in shallow burial applications.