The theory of silage clamp construction – and everything else – needs to be applied to the real world.  That means real sites on real farms and usually it means that the site isn’t level. There might be some sites where the difference in levels is slight, or you could be building in Holland, but in most real cases you are going to be faced with a slope.

So how do you build a silage clamp on a sloping site? 

This all depends on the level differences you are faced with. Small differences can be easily taken out during the topsoil strip. Incidentally, and not really on topic but why do builders strip off topsoil? It’s not because it’s worth good money to gardeners and landscapers, it’s because soils in the upper layers of the strata contain high levels of organic matter. Whatever the organic matter, be it live plants or invisible humus, it will rot down and decompose over time. If built on, this decomposition will create gaps and voids in the “sub-base” and the construction will settle down into these gaps over time. The settling usually results in stresses that cause cracking in rigid structures like a silage clamp. Subsoils tend to be very low in organic matter so this settlement is minimised. Stripping off the topsoil and building on compacted subsoil is standard good practice for all permanent structures.

So you can take out small level differences during the topsoil strip process by levelling the subsoil, but what do you do on a site with big level differences? Obviously you should try and utilise the site to its best advantage in the schematic design. Generally the material flow should follow the falls of the site wherever possible. For an AD site that means silage at the top, consumption in the middle – AD tanks – and final product – the digestate - at the bottom. For dairy farms the goal is a food product so the MOST important structures in the process are the Dairy and Parlour, so these should (ideally) be at the highest point on the site to avoid potential contamination by a spill or flooding incident. Whatever the overall design, let’s assume you have a couple of prospective clamp sites, neither of which are level, how do we optimise the choice?

Building on “made up” ground

As we will see (or have seen) elsewhere in this series, it is vital that the clamp floor falls towards the drains at the front. You should therefore be using the natural ground levels to assist in this; after all, a day on the digger is much cheaper than lorry loads of stone fill to get the levels correct. But if the ideal site falls the wrong way, can I just build up with some builders rubble? Almost certainly no - it’s not a good idea, and for a couple of good reasons. Firstly the code for good practice (CIRIA report C759F) indicates that building silage clamps on made up ground should be avoided wherever possible. This is because it’s likely that the material will slump and settle over time. However carefully you place and compact the material, it’s going to settle. Specialist ground stabilization contractors can achieve a stable platform, but it is a complex, specialist and expensive process. Secondly, builders rubble cannot be considered a suitable material for the base of a silage clamp construction. Anything that could compact, degrade or disintegrate over time must not find its way under a silage clamp floor; that includes plasterboard, wood or metals.

Digging a clamp into the bank

So the open end needs to be on the low side of the site and you must not build on built up ground. Now you are facing digging out the North Face of the Eiger to position the silage clamp, what the hell are you going to do with all the spoil? In the case of many structures, you would consider a cut and fill solution, where you split the difference in the levels and put the stuff you dig out on the low side to level up the site. But silage clamps are not usual constructions, and the ‘no building on made up ground’ rule still applies.

Can I use the earth or rock face as the walls of the silage clamp?

Historically yes that would have been a potential solution. However the current SSAFO regulations state you must have a floor slab that extends outside the walls and includes an effluent containment drain. By my understanding, that means earth or rock walls are no longer an acceptable solution.

So the spoil needs to go elsewhere, can it at least go behind the walls?

At last some good news, or at least potentially. As long as the walls are designed to be loaded from the outside as well as from the silage, then yes you can – but what about the external drain? Some enforcement officers have misunderstood this matter in the past. With careful design of the external drain and the incorporation of a vertical impermeable membrane, then a clamp built into a bank will comply with the SSAFO regs.

If you would like to discuss designing clamps on sloping sites and how to make them comply with the regulations or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com