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3.4.13 WET GROUND CONDITIONS:
3.4.14 Excavate a hole to appropriate depth allowing at least 300mm for lean mix concrete and hard-core base.
Allow for tank width plus at least 400mm with additional allowance for any necessary shuttering.
3.4.15 De-water the excavation using suitable pumping equipment. Ensure that the pump discharge does not
saturate the ground in the immediate vicinity. In wet ground conditions the installer should ensure that the base
is adequate to support the weight of the tank and its contents. If the base of the excavation is unstable, i.e.
running sand or similar, excavate an additional 250 – 300 mm below concrete levels and fill up with
compacted hard-core. Place a sheet of polythene over the hard-core and up the sides of the excavation
before putting in the concrete cradle.
3.4.16 Lay a bed of concrete (minimum 150mm thick) on top of the polythene at the base of the excavation.
De-watering is to continue until you are satisfied that the concrete has cured.
3.4.17 Lower the tank onto the concrete bed, ensuring that the inlet and outlet are in the correct position.
Ensure the tank is upright and then ballast it with water to a maximum of 500mm deep.
3.4.18 Haunch up the concrete bed at least 450mm all round the base, ensuring that all voids in the concrete
are eliminated and at least 150mm of concrete is left below the tank base.
3.4.19 Backfill to the invert depth with concrete. Ensure that the water level inside the tank is maintained no more
than 250-300mm above concrete backfill level. It is most important that the installer progressively fills the
tank with water to a level above the backfill in order to stabilize pressure on the tank.
3.4.20 Backfill evenly all round the tank, consolidating in layers. The backfilling should start before the base has
hardened and be a single continuous operation so that the tank has a full concrete jacket without joins.
3.4.21 DO NOT use vibrating pokers to consolidate concrete. DO NOT discharge concrete directly on to tank.
Ensure that the concrete is not too wet and that is tamped in around the tank.
3.4.22 See section 4 ‘System Installation’ for connecting all pipe work.
3.4.23 Build up a shell of concrete around neck of tank to 150-200mm thickness before completing the backfill with
a suitable material. Necks must be suitably braced whilst backfilling around this non-ballasted area.
3.4.24 Fit cover and frame. Apply surface finish e.g. turf.
3.4.25 Do not empty tank until the concrete backfill has cured. Tanks may be left filled with water.
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