While neither figure represents exactly how soil pressures act on buried pipes (the latter that is a quite complex depiction with many variables), I am aware the second at least loading/force depiction (e.g. from DIPRA) is a simplification that has worked quite well for general thrust restraint purposes for many decades. I am not as familiar with the first i.e. it appears assuming an equal soil pressure (similar to hydrostatic around a point) all around the pipes, but I am curious what exactly is the "ASCE Buried Flexible Steel Pipe textbook" you refer to (what is the full bibliographic reference?) In any reasonable thrust restraint analysis, however, I do not believe you would want to assume that BOTH the loading figures are at play to develop helpful friction in additive fashion at the same time, as that would seem non-conservative and not intended by either reference.
That being said, and while ductile iron pipe does indeed have great columnar strength (as even do even many unrestrained joints thereof, at least when assembled straight and tight to each other, and also laterally well-supported by backfill soil), I agree with bimr that some effective scheme of temporary external buttressing is likely more foolproof/dependable for periodically testing otherwise ordinary unrestrained joint piping. [This may be particularly true if you are not sure of the existing pipe-to-pipe alignment, and how tight together are the existing/unseen buried pipeline joints - e.g. if a pushed on joint has space in the rear, meaning essentially really only water behind say a somewhat not completely inserted, withdrawn or deflected pipe spigot end, one or more existing joints with only water behind same is obviously a less stiff column than if straight and tight against metal, and thrust on the cap could invite accumulated sort of telescoping movement to uncap the new test pipe end or the first buried unrestrained joint back on the new test section?]
All have a good weekend.
