These problems arise because not enough compaction was carried out when the excavation alongside the underground tank was backfilled. This is an area of about 2.5m in width, which is outside the new tank wall and where the shutters were put up, etc.

For an 8ft tank, this ground will have been excavated to almost 10ft, so there is a lot of scope for the backfill material to settle by a few centimetres – leaving a void under any concrete laid on top. Remember also that shed pillars or a shed side wall may be placed here, so stability is vital. Back filling plays no role in supporting the tank walls – that falls to the reinforcing mesh in the tank walls which must be adequate.

Tank excavations are backfilled with subsoil from the dig, sometimes with gravel or stone available on the farm. Subsoil in particular requires good compaction, more so than gravel or stone.

Most groundwork contractors empty the subsoil in and tamp it down with an excavator bucket. This is okay if each layer of new material is not deep and if the machine is 20t, or heavier. The driver will be mindful that he’s pressing down his bucket within inches of a new and expensive tank wall, so he won’t be banging down too hard.

Daragh King of King Concrete Contractors has raised the bar by purchasing a piece of new equipment to ensure he gets proper compaction when backfilling around a new tank and I watched this machine in action when I went to view the underground tank being constructed in Teagasc Grange recently (see Pages 2 and 3).

Picture one

Daragh had kept this heap of suitable soil close at hand for backfilling around the two new tanks. At the time of digging, the heap was sloped and tamped down to let rain run off. It took a lot of rain but was quite dry when opened up a few weeks ago. Daragh used a one-tracked machine to fill the dumper.

Picture two

The soil is tipped into the excavation and a second digger levels and compacts it using this new, purpose-made roller.

Picture three

The driver rolls the compactor backwards and forwards on the digger arm. He levels and compacts each dumper load of soil as it is tipped in. This ensures that the material is compacted evenly at all depths. It will be good foundation for any concrete apron laid on top.

Picture four

The roller is narrow and this makes it effective at compacting the soil. The spikes are intended to add to the compacting pressure. With care, the digger driver can roll the soil near the tank wall without a fear of hitting the wall.

‘‘You must get all the air out to prevent subsidance later,’’ says Daragh. ‘‘The concrete here could be driven on by a diet feeder and big tractor.’’ Daragh built up block foundation pillars from the bottom of the excavation, outside the tank walls, rising through the backfill, to provide solid foundation for the stanchions of the new shed. He will place a reinforced concrete pad on the block pillars as a ‘landing point’ for the shed stanchions.

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