The drip lines are usually poly (black or what ever they are installing now). the pop up sprinkler lines are sch 40 PVC which are white 1/2 inch pipe with Main line feed of 1 inch PVC (at lease in my yard).
I don't know how old you house is or if you had any irrigation work done. If you don't have massive water leak when you turn you irrigation system it was junk pipe that someone left in the ground as stated by wiotte.
I uncover crap all the time when digging in my yard. IMO they take all the junk trash and bury it under your yard from you house left overs after the build. I can explain how all the junk I find when digging?
You would be surprised what mess you find when working irrigation system and crap under ground.
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