south west doesnt worry about frost expansion stuff... only time we use it is roof runs over 100'. but I get the pun, haha... I think the justification is some of these manufactures buy inspectors boats and Alaskan hunting trips... aside from that's what the manufacture requires...
louisanaapp, the word is vulcanizing. how did I know so many would object... it proves that when someone says "in my 20 years ive never..." well, anyway, draw your own conclusion. I started in san Francisco which had unique rules and ended up here with the same thing...don't know why.
I can tell you the manufacture requires the primer- and it ends up welding the joint...try it and compare. so take your broken joint in the 3' trench. sooner or later you will end up on a job with a 12' concrete foundation with 3 cranes, 5 pump trucks and about 950 concrete trucks in 30 hours or so. hard core? a bit. but not as hard core as the hoppers the cranes are dumping concrete on your pipes from about 40', let alone the pump trucks..(the concrete guys NEED the concrete to fill the voids, aint no gentle about it) and steel guys walking on your pvc in the middle of the frame is another story! in big slabs improperly supported pvc floating to the top is a issue- ive seen joints break just from this...that were not primered!
do what thou wilt... its a little odd because I know you guys have pulled joints apart and the cement alone didn't do a darn thing..."well crap, look at that! All the glue just made a ring around the bell!" :/
a waste of time.... goodness. anyhow, 1" and smaller it is not required...but I do it. but we have inspectors that count the stupid threads on RT fittings!