10.6 Plinth and Backfill specifications:
10.6.1 Gravel backfill specification:
Primary backfill specification:
Primary backfill material should be free-flowing granular material. Compaction should be by
lightweight rollers or vibratory plate compactor until “traffic” depth has been achieved. Compact
gravel evenly to ensure tank is properly supported. Ensure the vibrating machine does not come in
contact with the shell of the tank. Tanks must be installed with primary backfill only within the region
immediately surrounding the tanks. This primary backfill must extend a minimum of 250mm outward
from the tank, and directly beneath the tank.
The following materials are approved as primary
backfill:
Rounded pea gravel:
Minimum particle size 3mm, maximum 18 mm, compacted to a relative density of >70%. Gravel shall be
clean and free flowing, free from large rocks, dirt, sand, roots, organic materials or debris. Upon
screening analysis the backfill material shall have no more than 5% by weight passing 2.36 mm Sieve
Or
Crushed or processed stone:
Minimum particle size 3 mm, maximum 12 mm, compacted to a relative density of
>40%
Dry Gravel density must be at least 1500 kg/m
3
. Material should be washed or screened to remove fine
particles. Upon screening analysis the backfill material shall have no more than 5% by weight passing
2.36 mm sieve
Use of other than specified backfill and bedding materials will void the tank
warranty.
Backfill material shall not be frozen or contain lumps of frozen material at any time during
placement.
