Farm Waste Structures: Guidance on Construction, Repair and Maintenance

Silage clamps and effluent tanks

Note No: CGN 005, July 2000
Prepared by ADAS National Building Design Team. Funded by The Department for Environment, Food & Rural Affairs

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Site Suitability

  • Consult the Environment Agency and Local Planning Authority when selecting a site.
  • Avoid sites in a flood plain or sites with a high water table.
  • Size, site location & suitability carry out survey & investigation (See References 1, 2, 9).
  • Site more than 10 metres from a watercourse with no land drains beneath silo or within 10 metres.
  • Suitable sites will be on firm and level non-organic sub strata.


  • Obtain a professional design, e.g. from chartered civil or structural engineers, that conforms to the Control of Pollution Regulations, the Water Code and BS 5502 (See References 1, 2, 3).
  • Impermeable base e.g. Concrete to BS 8007 with water bars to joints, or Hot Rolled Asphalt if site is suitable.
  • Floor must extend beyond any walls and have an external perimeter effluent collection channel.
  • Effluent collection channels and tanks must conform to the Control of Pollution Regulations (See Reference 1 & 2).
  • Walls and their foundations must be designed to withstand the loading described in BS 5502, Part 22 (See Reference 3).
  • The walls could become overloaded if an effluent drain is not installed at floor level within 500mm of the inside face of the wall.
  • Proprietary wall panels and timber railway sleepers may be used. However professional advice must be obtained on their use and on the support system and foundations appropriate for the site conditions.
  • All silos must be provided with an effluent tank with a capacity equivalent to 3m3 for each 150m3 of silo capacity up to 1500m3 and an additional 1m3 capacity for each 150m3 of silo capacity in excess of 1500m3.
  • Effluent tanks constructed on site using in-situ reinforced concrete, masonry or other structural materials must be impermeable. Take professional advice on an appropriate design.
  • Proprietary silage effluent tanks are common. Tanks and any associated drainage should be installed strictly in accordance with the manufacturers specification.
  • Tanks must have a 20 year durability life without maintenance and require certification to Environment Agency requirements.

GRP effluent tank with load bearing cover slab

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Construction:- Site preparation

  • Remove all vegetation and organic top soil from site.
  • Excavate and trim to formation levels. Protect from weather.
  • Remove soft ground and fill with Department of Transport specification type 1 hardcore to formation level.
  • Compact formation with vibrating roller to achieve a permissible bearing pressure of more than 200kN/m2.
  • Spread in uniform layers and compact as above at least 150mm thickness of Department of Transport type 1 hardcore. Avoid limestone susceptible to acid attack (See Reference 9).
  • Alternatively a minimum of 75mm of C20 concrete to BS 8007 or GEN 2 mix concrete to BS 5328, may be used instead of hardcore (See References 6 & 7).
  • Lay 1000 gauge polythene on formation and lap the joints.

Construction:- Concrete floor slabs

Section through concrete floor slab & joint detail

cgn005_fig3.jpg (18259 bytes)

  • Set rigid formwork to edges of slab and joints.
  • For 150mm deep RC 45 concrete slab and maximum joint spacing at 15m for a 3.5m wide bay, use A252 fabric reinforcement in top with at least 40 mm cover. See BS 8007 for alternative bay size and specification (See Reference 6).
  • Use water bars and sealants to BS 6213. Set and joint water bars before concreting work (See References 5 & 9).
  • Lay reinforcement, lapped one mesh, on spacers.
  • Use ready mixed RC45 concrete; max W/C ratio 0.50, minimum cement content 350 kg/m3 (See References 6 & 7). Only use approved additives.
  • Transport concrete in approved manner.
  • Vibrate concrete using approved mechanical vibrator.
  • Pour slab bay in one continuous operation.
  • Provide uniform and regularly tamped finish to falls.
  • Protect and cure concrete on completion - all to BS 8110 (See Reference 8).
  • Apply joint sealant resistant to effluent attack.
  • Apply protective coating against effluent attack.
  • Do not fully load concrete until it achieves design strength (equivalent to 28 day strength).
  • Concrete specification: see BS5328, BS 8007 and BS8110 (See References 6, 7, 8).
  • Internal and external perimeter effluent drainage channels set into the floor slab require special construction methods. Plastic perforated land drains, laid on the floor near the inside face of the wall, may be used for effluent collection within the clamp. (See Reference 9).

Construction:- Hot Rolled Asphalt (HRA) floor slabs

  • Not all sites are suitable for HRA use.
  • Specifying and laying HRA is a specialised job requiring special equipment (See Reference 9).
  • Special measures will be necessary to ensure water tightness around stanchions and silo wall foundations.
  • Concrete foundations are necessary for walls and other structures.
  • Gas produced from any escaped effluent lying beneath HRA can cause uplift and failure of the HRA.

Example of HRA specification paviour machine application only

  • Top wearing course at least 40mm thick of HRA to BS 594 Part1 Table 3 or 5 (non limestone aggregate), on;
  • Base course at least 110mm thickness of 28mm nominal size dense bitumen Macadam to BS4987 clause 2.2.5, on;
  • Mechanically compacted Department of Transport type 1 non-limestone hardcore, compaction specification and depth to suit ground conditions.

Repair and Maintenance

  • Obtain all information for operation and maintenance from designer or installer.
  • Major structural defects including steel corrosion and concrete decay - refer to expert for advice.
  • HRA can be used to re-surface some forms of defective concrete - refer to specialist (See Reference 9)
  • Minor shallow concrete defects where reinforcement is not affected must be properly cut out prepared and filled with compatible mortar. Epoxy resin mortars and bonding agents are especially suitable.
  • Exposed and corroded reinforcement can cause structural failure - refer to expert for advice.
  • Floor and wall joints must be inspected every year. If defective, properly prepare and repair with effluent resistant sealant
  • Joints with defective concrete may require special attention.
  • Observe precautionary notices; do not use heavier tractors than wall design, failure can occur. Repair impact damage.
  • Do not surcharge or over load walls more than design load criteria.
  • Check and rectify damage and blockages to drains and effluent tank. Be aware of danger and take precautions against toxic gases during inspections.
  • Refer to CIRIA Report 126 for specialist techniques and repair practice (See Reference 9).


  1. The Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations 1991 (as amended) .
  2. The Code of Good Agricultural Practice for Water; MAFF (Free publication).
  3. British Standard 5502: Buildings and structures for agriculture (Parts 20 and 22); British Standards Institution.
  4. Construction (Design and Management) Regulations; Health and Safety Executive .
  5. British Standard 6213: Selection of construction sealants; British Standards Institution.
  6. British Standard 5328: Concrete specifications and procedures; British Standards Institution.
  7. British Standard 8007: Code of Practice for design of concrete structures for retaining aqueous liquids; British Standards Institution.
  8. British Standard 8110: Structural use of concrete; British Standards Institution.
  9. CIRIA Report No 126: Farm waste storage - Guidelines for construction.

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