Underground electrical conduit duct bank set in a trench excavated in undisturbed earth with a minimum three inches of clearance around the ducts. Plastic spacers maintain alignment for multiple duct runs. Six inches of fluidized thermal backfill surrounds the conduits to provide a thermal resistivity value of less than 90 rho. Foil-backed marker tapes are installed sixteen inches maximum on center on each side above the duct bank. The upper trench is backfilled with five-eighths inch crushed rock gravel compacted in eight-inch nominal layers up to the finished grade.
6" FLUIDIZED THERMAL BACKFILL (TO PROVIDE THERMAL RESISTIVITY VALUE OF LESS THAN 90 RHO) FINISHED GRADE 5/8" CRUSHED ROCK GRAVEL BACKFILL COMPACTED IN 8" NOMINAL LAYERS 24" UNDISTURBED EARTH PROVIDE PLASTIC SPACERS FOR MULTIPLE DUCT RUNS 3" MIN 10.76 MIN 3" MIN 3" MIN FOIL-BACKED MARKER TAPES, 16" MAX OC., ONE AT EACH SIDE. 3" MIN
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