This article from the Farmers Journal features some of the basic issues to consider when building a concrete storage tank.

This is the time of year when farmers start enquiring about slatted concrete tanks, with the idea of having tank, slats and roof in place by next winter. Slatted concrete tanks are a complex piece of construction.

Many farmers want their tank built to the spec of the Department of Agriculture grant schemes and are prepared to pay accordingly. Others build to a somewhat lower spec to suit a budget. Either way, it’s sensible to discuss the specifications of the new tank with the contractor in advance. This article from the Farmers Journal features some of the basic issues to consider.

Water Table:

This springs to mind after the recent wet years. If the water table is significantly higher than the floor of proposed tank, there is risk of the tank floating and being destroyed. If a high water table is suspected it can be checked by digging test holes and examining them after 48 hours. If it is present then the best option is to lower the water table by putting in a pipe drain to a ditch. If no fall is available then the options are building the tank a foot or two higher, putting in a heavier floor and walls to weigh down the tank – or finding another site.

Excavation:

A tank requires a solid base underneath or it is at risk of subsidence and cracks. Space permitting the hole should be dug out a meter beyond the walls on all sides and 150mm (six inches) of hardcore put in and compacted.

Tank Floor:

The floor slabs should be at least 225mm (nine inches) thick and extend beyond the tank walls.

Tank Walls:

Tank walls are being built anywhere from 225mm (nine inches) to 500mm (20 inches) thick. Once the shed pillars are not sitting on the wall then 225mm is acceptable – once reinforcing steel is up to full spec. Some farmers choose to build the walls for such tanks at 300mm (12 inches) but with less steel, reducing overall cost. However, if the pillars of a big shed are to sit directly on the tank walls then the wall needs to be thick and well reinforced. Some farmers go for a 500mm wall with reinforcing. An alternative is that shed pillars sit on piers build outside the tank walls. Farmers should note the Department spec on walls. It requires that tank walls be a minimum of 225mm (nine inches) thick with full reinforcing. If pillars are to sit directly on top then the wall must be at least 400mm with full steel. Alternately, a 300mm (12 inches) thick wall can be used if it incorporates a 600mm x 600mm (two foot by two foot) pier for each pillar. Spine walls should be at a minimum of 300mm thick, reinforced. For ease of agitation they should be centered in the tank.

Safety:

A tank should be one foot narrower than the slats or precast slabs it will carry. This is to allow the minimum six inches bearing (ie seating) at each side for the slat or slab ends. Any narrower bearing would leave risk of a slat or slab end falling into the tank. As every farmer knows agitation points should be outside the building, including for extended tanks, etc. Internal agitation points represent a safety risk going forward. Some farmers put one inside on a “just in case” basis. Covers on agitation points should be galvanized and child proof. Concrete for slurry tanks should be 35N and certified to Department of Agriculture grant standard. Concrete tanks and channels purpose built for silage and effluent should be made with 40N concrete to be able to withstand the acid.

Backfilling:

Backfilling should not be carried out until the walls are at least 28 days old. Concrete achieves strength quicker in warm summer weather, slower in cold winter weather. Subsoil material may be used if it is free of topsoil. Material should be added in layers and compacted.

(Mooney, P. (2013). Agree the details of your tank in advance . The Farmers Journal, 26th January 2012, p.42 – p.43) – From the farmersjournal.ie

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