You have only really have one chance to examine an underground slurry tank – while it’s being built and before it has gone into use.

In this article from the Farmers Journal, Paul Mooney discusses the process involved in installing a large underground slurry tank in Co Limerick.

The tank was built by Croom Concrete and we also supplied the cattle slats and passage slabs.

 

This is a big tank!

It’s 170 feet (52m) long and each of the three adjoining tanks are 16 feet (4.8m) wide and x feet (x m) deep, giving a total capacity of x gallons (x litres). The centre tank has been covered by pre-cast slabs. This tank was built to a full spec that is essentially the same as that of the Department of Agriculture.The external walls are 16 inches (400mm) thick.

It has rebar reinforcing under both internal and external faces. The floor slab is nine inches (225mm) and extends 10 inches (250mm) outside the walls.  Given the width of the tanks,  the whole floor is reinforced and the steel is carried from the floor slab up into the wall.

The farmer requested the precast beams, that we see installed across the width of the tank, as additional support. This would be above the spec laid down by the Department for tanks. When I called, the precast slabs had been laid on the centre tank and the slats were about to be laid on this tank.

 

Compacted Backfilling

The walls of the tanks were nicely backfilled using gravel. Properly compacted backfilling, with subsoil or a stone material, is necessary to cope with the sidewards pressure of slurry when the tanks fill and to thus further support the reinforced tank walls. Backfilling should be built up in layers with each layer compacted before the next is applied. That is because the force of a roller, whacker, etc, declines as the depth of material increases. Proper compaction around a tank excavation is also necessary if a concrete apron is to be laid alongside the tank. Tank walls must be given 28 days to strengthen before backfilling is done. But it should be done before slats are installed as otherwise there is a risk of the side of the excavation collapsing under the crane lorry.

Spine Walls

Here we see one of the two spine walls in this tank and a circulation doorway. Each spine wall has a doorway at each end. The walls are 12 inches (300mm) wide and reinforced on both faces. The walls sit on the floor slab, not on a separate foundation. The reinforcing steel of the floor slab curves up into the spine wall.

The spine walls are 12 inches wide and reinforced on both faces, mainly to safely carry the weight of slats and feed passage slabs.12 inches of width is re- quired to give a six inch seat to the slats and/or slabs on each side of the wall. If the walls didn’t have to carry this weight then a slimmer wall without reinforcing might do – the open doorways at each end mean that the pressure of slurry is equal on both sides of the wall, barring blockage, etc.

 

A precast reinforced beam is used to bridge the doorway and support the slats and passage slabs. Like the wall it is 12 inches wide to give secure bearing to the slats and slabs on top. It is prestressed to carry high weight.This picture shows how slat ribs open outwards to allow dung to fall down into the tank below. This helps keep the top surface of the slats − and therefore cattle − clean. It also reduces the weight of a slat gang. The reinforcing steel in a slat is on the bottom only as the weight will always be pressing down from on top.

Prestressed slabs

Then come the prestressed slabs which make the base for the centre feed passage. Although the slabs are strong enough to support weight, an additional layer of concrete must be poured on top to provide a smooth surface for a tractor to drive on, seal gaps, etc. This will also place the feed passage four or five inches above the pen slats, giving cattle a further reach. Before pouring the concrete, Croom will run reinforcing bars through the lifting hoops of the slabs. This will help knit the whole floor together.The passage slabs are Hollowcored to give a significant reduction in weight. This can also cut down on costs.

There are six steel bars running through this slab, placed under tension when the concrete is poured. Again, this prestressing allows a lighter steel wire to be used to give the same strength but at lower weight.

Slats and slabs should sit evenly on a wall with no rocking. Ideally, not too many spacers should be required to eliminate wobbles. The slat gangs are made in moulds under factory conditions and should always be straight. However, a wall poured
on site will usually have minor unevenness. But the straighter and more even it is the better.

(Mooney, P. (2012). Building a large underground slurry tank. The Farmers Journal, 25th August 2012, p.36 – p.37) – From the farmersjournal.ie