Pea Gravel
Crushed Stone
10.6.2 Concrete backfill specification
Semi dry concrete 25n grade with a ratio of 4.5 aggregate to 1
cement.
Note: Standard concrete mixes should not be used, where sulphates or similar aggressive chemicals are
present in the groundwater.
Lift height (rate of rise):
Determine the lift height (m), or rate of rise (m/h) for the specific concrete type used, to ensure
that a design pressure (P max) of 15kN/m
2
on the tank is not exceeded.
Vibration:
The tank design assumes minimal compaction of the surrounding concrete. Where necessary, this may
be extended to include light internal vibration. Never use deep revibration which will substantially
increase the pressure on the tank, possibly causing failure.
Impact of concrete on discharge:
Under no circumstances should concrete be discharged directly onto the
tank.
10.6.3 Top soil:
Clean native top soil shall not contain rocks larger than 36mm on largest
dimension.
Note:
The use of geo textile barrier fabrics surrounding the primary backfill material is considered good
installation practice. The fabric must be chosen to allow the flow of water in and out of the excavation but
to prevent the movement of fine soil particles into the primary backfill material.
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