When installed under streets, highways, roads, alleys, driveways and parking lots, conduit is required to be buried below the surface of the finished grade, or concrete or similar cover, by no less than 24 in. under all circumstances.
See 300.5 for installation requirements for conductors rated 600V or less. Also, see Note 1 under Table 300.5 for the definition of: “Cover is the shortest distance in millimeters (inches) measured between a point on the top surface of any direct-buried conductor, cable, conduit or other raceway and the top surface of finished grade, concrete or similar cover. This piece of rigid nonmetallic conduit [not sure if it was Schedule 40 or 80] is driven over every day, and this situation will probably get worse until corrected.
Also, just a reminder that a “warning ribbon” is required according to 300.5(D)(3) if any underground service conductors are not encased in concrete and are buried 450 mm (18 in.) or more below grade. The warning ribbon should be placed in the trench at least 300 mm (12 in.) above the underground installation. The warning ribbon serves to identify the conductors. For conductors rated over 600V, see Section 300.50 for underground installation and warning ribbon requirements.