Farm Waste Structures: Guidance on Construction, Repair and Maintenance

Above-ground circular concrete and rectangular weeping-wall slurry stores

Note No: CGN 001, July 2000
Prepared by ADAS National Building Design Team. Funded by The Department for Environment, Food & Rural Affairs.


Figure 1 Above-ground circular concrete store

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Figure 2 Weeping-wall store

 

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Pre-cast and pre-stressed concrete wall panels are proprietary units manufactured under strict controls. Many manufacturers will only supply units to approved installers.

The floors incorporate the panel foundations and are generally constructed of in-situ reinforced concrete to BS 8110 or to BS 8007 (See References 3 & 4).

Weeping-wall stores must have a perimeter effluent collection channel and an effluent tank. If spurting is likely to be a problem this may be overcome by increasing the width of the channel to equal the net depth of slurry stored or by providing some means of deflecting liquids into the perimeter channel.

Some systems, such as above-ground circular stores made of post-tensioned concrete staves or panels, require special installation techniques. A post tensioning method is illustrated in figure 1. It is advised that only experienced contractors and specialist installers should be engaged to build these systems.

Site Suitability

  • Avoid sites in a flood plain or sites with a high water table especially for below ground level stores.
  • Avoid improperly filled ground or other unsuitable ground conditions such as peat soils or ground liable to subsidence.
  • Locate at least 10 metres from a water-course (See Reference 1).
  • Suitable sites will be on firm and level non-organic sub-strata.

Specifications

Before placing an order for components, or commencing an installation, check that the specification fully conforms to the Control of Pollution Regulations and BS 5502 (See References 1 & 5). The store must be designed to properly withstand all loading conditions. If in doubt seek expert advice. Most pre-cast concrete manufacturers supply detailed specifications of their products. Stores should be designed to the load classifications of BS 5502 part 22 for Class 2 structures. In certain locations Class 1 structures may be required. Such design standards will meet the 20 year durability life requirement.

  • To calculate the size of a store refer to the Water Code (See Reference 2).
  • Wall panels and supports must be strong enough when the store is loaded full to the brim with slurry, irrespective of the freeboard limits.
  • Below ground panels must withstand the external loads defined in Part 50 of BS 5502.
  • Check any construction drawings to see that they conform to the specification.
  • Check the maintenance requirements, especially the method of sealing and maintaining the joints.
  • External maintenance of below ground stores will be very difficult so investigate durability, particularly the joint seals.
  • Check the suitability of collection systems e.g. channels and tanks of weeping-wall stores.
  • Two lockable in-line valves at least one metre apart are required in outlet pipes. (Not applicable to weeping-wall stores.)
  • Check details of the in-situ concrete work particularly the base, the foundations and the method for sealing the joint between base and wall panels.
  • It is recommended that a 200mm thick C35A mix concrete floor slab to BS 8007 cast in one pour should have at least one top layer of A393 steel fabric reinforcement to help reduce cracking. If joints are necessary these should have water-stops to BS 8007 (See Reference 4).

Construction

  • Employ an experienced contractor obtain references and investigate similar projects undertaken.
  • Ensure that the relevant Health and Safety Regulations, including the CDM Regulations are observed (See Reference 6).
  • Ensure that the construction is fully in accordance with the detailed drawings and specification. A suitably qualified independent supervisor should carry out this duty.
  • All concrete work must be built to the correct dimensions and in accordance with the design specification. This includes the mix specifications and the size, type, location and quantities of steel reinforcement. Discrepancies may result in a failure of the structure.
  • Fix prominent permanent notices showing maximum depths (freeboard >300mm) proper operational procedures and safety precautions.

Repair and Maintenance

  • Ask the designer or installer for operating and maintenance instructions.
  • Be aware of all hazards, particularly the presence of toxic gases when entering a partially full tank, and observe all relevant precautions.
  • Inspect tank internally once a year when empty and externally at more frequent intervals. Rectify defective joint seals with specified sealant.
  • Post tension systems and proprietary jointing systems require specialist attention (usually by the installer or manufacturer).

References

  1. The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations 1991 (as amended).
  2. The Code of Good Agricultural Practice for the Protection of Water; MAFF Free publication.
  3. British Standard 8110: Structural use of concrete.
  4. British Standard 8007: Code of Practice for design of concrete structures for retaining aqueous liquids.
  5. British Standard 5502 Part 50: Code of Practice for design construction and use of storage tanks and reception pits for livestock slurry.
  6. Health and Safety Executive: Construction (Design and Management) Regulations.

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