as you want to pass your pipe under ground, the important task is to prevent corossion , and protect your pipe external surface . 1" sch 40 pipe with min. spy = 35000 psi will safely do the job for you and you should consider at least two main item for its long service duration.
1- depth...
ASTM STANDARD RELATED BOOK TO STEEL PIPING, TUBING,& FITTINGS WILL BE A GOOD REFERANCE TO YOU .
ASTM A216 : IT HAS THREE GRADE AS WCA,WCB&WCC ,IT COVERS CARBON STEEL CASTINGS FOR VALVES, FLANGES,FITTINGS FOR HIGH TEMPERATURE SERVICE .
GR. WCA GR.WCB GR.WCC...
BOTH 5L GRADE B & A106 GRADE B HAVE THE SAME SMYS = 35000 PSI , BUT A 106 IS RECOMENDED FOR HIGH TEMP , SERVICES & 5L IS ONLY USED FOR NORMAL TEMP. SERVICES
