Help on sealing RCP joints

We have been losing massive amounts of dirt over a known RCP drainage pipe. This pipe is 24" ID with typical tongue and groove joints as shown here:
Flow line of pipe is about 9' below grade. Installation is about 40 years old. Pipe is in excellent condition. Problem is definitely related to the joints. Some were not shoved tight in the original installation. Several show signs of a patching concrete smeared at the joints indicating some minor damage back when. No gasket or sealant was used. The leaking problem is associated with 2 large outfield fences, back to back, with posts set over 4' in the ground. I'm confident that these fairly recent post installations have allowed runoff and rainwater to get deeper in the ground than normal due to depth of install and vibration . 47 continuous days of rain doesn't help, though the problem predates our recent monsoons.
here's the job and the question: I have excavated about 50 LF of overburden, exposing about 7 joints (more to go). What would you use to seal these joints?
Intentions at this point: -Scrape, brush, and clean as best we can. -6" wide heavy bed of plastic roof cement with reinforcing cloth imbedded. -12" 22 gauge galvanized strip bound tight to protect the tarred joint during backfill.
Other thoughts: 9" Flashing tape. Either self adhered alone or as a cap over plastic roof cement. $2/LF - 10' /joint
Sonneborn HLM 5000 or similar. $70 / 5 gallon
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