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08 Nov 2022

Clamp considerations for successful storage

As featured in Arable Farming Magazine

Clamp considerations for successful storage

by Arable Farming Magazine October 2022 issue


With maize silage volumes down after a hot, dry summer, getting clamping right will be even more important this season.

Do we know as much as we ought to about storing silage for use in anaerobic digesters (AD)? Can we learn from experience overseas and improve silage quality and gas yields in UK AD plants? These are questions occupying the mind of Will Wilson of silage storage expert Ark Agriculture, after more than a decade of building silage clamps across the UK.

We have a phenomenal amount of value going into AD plants, he says, reflecting that this years silage crop is possibly the most valuable silage growers will have made.

You may have drilled this years silage at last years prices, but you will be valuing it at this years prices, he says.

Which makes any losses and loss levels of up to 20% through fermentation and aerobic spoilage are not uncommon even more costly.

Mr Wilson would like to see clamp design move up the priority list when AD plants are being designed and constructed.

Clamp design is not complicated, but it is very easy to get wrong, he says.


You will look at diagrams and it will be a square box drawn with a ruler and it has no detail about the drainage or capacity.

All it will say is there is a four to five metre-high back wall and it has three bays.

So, it will be length times width times height equals capacity, but what we know from experience is a lot more detail needs to be thought about at the early stage to get the design right.

A scenario Mr Wilson often encounters is a lack of capacity.

Clamps went into planning that were maybe 4.5m high, they were too wide, too short and we would end up with a scenario where a clamp to store 10,000 tonnes had to store 20,000t. Initially there may have been plans to build several extra clamps but they never happened.

What then happens is that clamp walls are overloaded, leading to collapse, which increases the risk of a pollution incident.

Choice of clamp base is another area where there is room for improvement, says Mr Wilson.

We see a lot of plants that were built with concrete; it was a cheaper solution at the time.

But we are now seeing after 10-15 years that concrete has not worn as well as asphalt, especially on the expansion joints.

And you are left with fewer options to repair a concrete base than you have with an asphalt base.

These issues should be consid ered on day one of the planning process, along with contingency plans and how Environment Agency requirements will be met.

A contingency plan might be AgBags, a long-term plan might be to have a budget for future extensions to the clamp or satellite clamps, neither of which are quite as good as having capacity on site.

Probably the most logical way of understanding how strong a clamps walls will be is to think about axle load, says Mr Wilson.

This indicates the maximum weight of compacting machinery that can be used on the clamp and how high up the panel can compaction safely occur.

Modern clamp machinery can weigh more than 20t and compacting machinery will have two axles, which means maximum axle weight would be 10t.

So that is something you might want to ask anyone who is building your clamp.

Can you express your design in terms of axle loads? But for those with existing clamps, the first and obvious thing to do is look after it, says Mr Wilson.

People forget about silage clamps and their day-to-day maintenance.

Clamp drains, walls, joints are all important.

You can get big problems with silage clamps that you have not maintained; it is going to cost you a lot of money to sort them out in the long term.

He recommends plant operators pay attention to joints where a concrete base meets an asphalt base.

Those areas are really important to pay attention to because what happens is the joints expand and leachate goes underneath and can travel under the base and undermine the walls.

Filling the clamp is all about reducing the amount of air in the silage and creating a high level of anaerobiosis the anaerobic condition which stops silage breaking down and ensures that the energy that growers have spent so much money trying to bring into the clamp is retained, says Mr Wilson, who has several tips for creating the optimum clamp conditions.

If you are moving material from a field to the AD plant and it is travelling a long distance, you want it to be coming in a regular pattern.

You do not really want to have the first 10 trailers of the day arrive all at once.

I know several AD plants where contractors set their people off staggered times.


The logistical exercise of moving hundreds of thousands of tonnes of silage is a military operation when we start talking about bigger plants.

So one of the golden rules is basically a third of the weight of the grass and a fifth of the weight of the maize that comes into your silage camp per hour should be on the clamps compacting machinery.

So, 100t of maize per hour arriving in the clamp equals 20t minimum weight of compacting machinery on the clamp.

But 100t of maize arriving into a clamp is nothing.

There will be plants that do 100t in 20 minutes.

In which case you need to upscale your compacting machinery or slow your harvesting down, but it is almost impossible to slow down a contractor.

Silage layers should be no thicker than 20cm and compacting machinery should move slowly across the clamp.

The faster you go, the less compaction you achieve.

An emerging trend within the last couple of years has been to either leave clamps uncovered or cover them with a layer of digestate, to reduce labour costs and the amount of plastic waste.

I can see all the reasons why operators do not want to be involved with uncovering and re-covering clamps but the more we look into this the more we understand that the losses are phenomenal, says Mr Wilson, who cites recent research conducted at a plant in Northamp tonshire comparing a digestate covered clamp to one covered with proper sheeting.

Losses were found to be about 40-50% where digestate was being used to cover the clamp.

Which is a phenomenal amount of money, says Mr Wilson.

And there are other implications of not covering or covering with digestate; the loadings on the walls increase massively because your clamp is absorbing a huge amount of rainfall rather than letting it wash off.

Environmentally, you are putting a lot more stress on your leachate collection because you are not collecting rainwater, you are collecting slurry that comes off the digestate that then fills your drains.

Other points to consider include the oxygen transmission rate of silage films, which should be low as possible, and use of sidewall sheets.

Sidewall sheets are still a no-brainer.

I know people do not want to use them, but they do such a good job of protecting the walls and creating tight seals around the edges.

Hopefully, we can develop new products that remove the labour intensity of these solutions.

But at the moment, the best thing to do is try and engineer out the labour requirements, he says.


Looking to the future, he suggests clamp safety is one of the biggest challenges facing AD plant operators.

We have not really addressed the problem that silage camps are probably the most dangerous structures on an AD plant.

You send operators to work at height on unstable surfaces to uncover and re-cover silage clamps and they have to do that in all weathers.

We have to work out how we can help making uncovering silage clamps safer.

Storage costs make up a large proportion of the costs associated with silage and there is room for improvement, says Mr Wilson.

It is all relative to the cost of the value of the silage but how do we reduce the cost? How do we make silage covering simpler? How do we automate more of it? It is a real challenge, especially when we have clamps that arent really designed well to begin with.

And he highlights issues around the environmental impact of silage production for AD, not only in terms of pollution, but the environmental cost of losses too.

If were throwing away 20% of our silage, we have already burned the diesel to do that.

If we could reduce that waste by 5%, we could make a massive difference to the carbon footprint of our feedstock, which in the future may become really valuable, Mr Wilson concludes

Clamp design considerations

• What is the maximum fill height of the finished clamp across the face?

• Is it only designed to be filled level with the tops of the panels or can it be filled higher?

• What is the density of the silage the clamp is designed to hold (t/cu.m)? Grass, maize and wholecrop have different densities, which can also be expressed as dry matter percentages.

• How close can the machinery used on the clamp be driven to the edge of the panel? Some designs mean that compacting machinery cannot operate within four metres of the inside edge of the wall, which is a significant flaw in a clamp that requires even distribution of the weight of compacting machinery.

• What is the maximum weight of the compacting machinery that can be used on the clamp and how high up the panel can compaction safely occur? The best way to understand this is based on axle weight.

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