Thanks guys, all good advice again. I did see the guide in the Caesar manual and it's very good, however I was met with blank looks from the vendor when I requested breaking friction for the slip joint. The closest thing to actual friction values is to assume they are frictionless.

"Tests have shown that this frictional resistance in the joint is unpredictable, varying widely with installation conditions and other factors that are insignificant in other respects. Thus, these joints should be considered as offering no longitudinal restraint for design purposes."

The above is taken from the DIPRA document "Thrust Restraint Design for Ductile Iron Pipe". However I remember reading one of the board member's posts about some underground steam lines groaning very loudly when the joints moved, so a frictionless joint seems unlikely.