Shotrock/crushed stone backfill for reinforced concrete structure

[CivilPipe]

I am designing a 40 foot deep reinforced concrete sewage pumping station. The geotechnical boring hit rock at 5' below the surface. I am thinking of allowing the Contractor to backfill the structure with a mixture of select shot rock less than 12-inches and crushed stone up to about 24 inches below the surface and 24 inches of top soil. Should I require filter fabric on top of the shot rock?

 

[cvg]

I wouldn't recommend open graded rock backfill unless you have a weeping tile / foundation drain. In that case, I would not use large rock but smaller gravel or sand. Otherwise use a good, granular, well graded structural backfill