conflict between stp foundation and existing leaching pools

huangyhg

conflict between stp foundation and existing leaching pools
hi everyone,
during the construction of sewage treatment plant building foundation, about 15 old leaching pools have been found during the excavation.   the leaching pools bottom elevation is about 8 feet deeper than the bottom elevation of the tanks bottom slab.  some of the leaching pools are in the middle of the foot print of the building and will e completely removed but four pools are near the edge of the foot print (about 4'away).
is it acceptable to remove the top section of the four pools and fill it up with compacted fill and do we need field test to verify the proper compaction below the bottom slab of the stp building.
thank you for your help.
before anyone goes on, can you please advise the nature of the soil taht you are wanting to found on?  are you on till, or sand, or clay, or . . .  what is the consistency or relative density of the soils you wish to found on.
the soil at the stp building location is sandy soil (for the top 25' with medium density and the foundation design was based on minimum bearing capacity of 3 tons per square foot.