RCP pipe joint deflection question

I'm working a 8'sd transmission line where our geotech consultant has determined to have 3.5" inch of differential settlement (immediate and secondary) along 30 foot lengths. I have checked manufacturers like Hanson and Ameron and the brochures indicate somewhere along the lines of 0.5 degrees of allowable rotation between a bell and spigot type of joint. What confuses me is that for a double gasket joint, Ameron advertises the pipes can deflect up to 22 degrees, 30 min. I need to tell the client that for the differential settlements occuring what maximum angular deflections between two pipes this size can handle with what type of joint without loosing it's watertight integrity.