This is probably a better situation than if the pipe was riding with basically a discrete point load on the the outside of a Reiber bell. However, while AWWA M23 is silent on such application it does say "skids may be required to prevent damage to the pipe and to provide proper long-term support", and also to "properly position the pvc pipe in the casing". You don't mention what you are crossing (i.e. reason for the casing), but something else not mentioned in standards is often, e.g. particularly in jack and bored road crossings etc. there are likely some variable, and perhaps even highly variable load/support conditions due to a different soil envelope around the carrier pipeline where it is directly in the ditch soil away from the roadway, as opposed to the cased line through a very highly compacted (and with select material) roadbed. While you perhaps don't have the following situation as this is a very short casing, this can be aggravated when there is over excavation below the invert where the carrier enters and exits the casing e.g. for deeper jacking or exit "pits". This can be argued at least similar to the situation where a pipeline enters and exoits any other structure. In an arguably more critical service this reality, born of unfortunate experience, is not lost however by gas pipeline regulators in the Code of Federal Regulation where Sec. 193.321 now requires of plastic pipe:
"(b) Plastic pipe that is installed in a vault or any other below grade enclosure must be completely encased in gas-tight metal pipe and fittings that are adequately protected from corrosion.
(c) Plastic pipe must be installed so as to minimize shear or tensile stresses." and
"(f) Plastic pipe that is being encased must be inserted into the casing pipe in a manner that will protect the plastic. The leading end of the plastic must be closed before insertion." [Translation, if a significant enough shear load is applied it is not going to be the very strong steel or ductile iron pipe that shears, and the specific means and methods on how to accomplish the minimization of stresses to the weaker plastic are not supplied.]
Finally, I'm going to hazard a guess you now have (assuming an empty pipe is not now floating in a casing full of groundwater) more than 3" of space between the OD (9.05"?) of the 8" pvc barrel and the (normally larger than nominal) ID of the 12" ductile iron, and this should be understood.