Is it possible to make silage clamp walls higher?

Running out of silage clamp space is problem that most of us have faced, be it a bumper year or expansion in the enterprise. If you didn’t stick to the old adage “build it twice as big as you want and it will be half as big as you need” when it was constructed, there will come a time when you need to consider more clamp capacity.

What is the best solution for more silage clamp space?

If your clamp is built with fixed walls, it’s unlikely that you can just make it a bit wider without a lot of hassle and cost. Making the clamp a bit longer isn’t easy either as the open front is probably the lowest point so drainage is going to be compromised.  So why not go up? There is plenty of space and it’s not going to spoil the farm layout so increasing the clamp height is the obvious solution. Previously in this series we have looked at overloading a clamp and putting a bit extra silage on top of the pile. It’s a tricky subject and you need to be really careful not to compromise (collapse!) the walls.

What you need to consider

The key issue is how the clamp has been constructed and - more importantly - how it was designed. So we will consider the different ways of building silage clamp walls and assess how, or if, they can be increased in height.

 Earth Banks – might be the oldest and simplest form of clamp wall. The very first silage was made in trenches or pits and hence the term silage pit. And if your clamps are built into a bank in a “pit” then this is possibly the easiest of all clamps to make a bit taller. If you treat the top surface of the earth bank as ground level if you were building a new earth banked silo then you won’t go far wrong. The topsoil needs to be stripped off and then a “shear notch” needs to be formed in the middle of the wall to stop the new earth sliding off. Once this has been formed, the new walls can be built up in compacted layers to the required height. Obviously all this needs to be “engineered” to ensure the angles are correct and that you have suitable soil material, but it’s certainly a “do-able” solution.

And what if the walls are already built up from ground level? Then the same principals need to be applied. You still need the shear notch, you still need enough wall thickness to suit the height and you still need to compact as you go. If the existing wall is too skinny to pile more on top, you will need to re-engineer the base to make the whole lot more substantial but as we will see later – this is one of the easier problems to overcome.

Mass Concrete walls – were a very popular way to build a silage clamp in the 70’s and 80’s and there are generally three types:

-       cast between the columns of a building

-       cast between steel stub posts

-       cast as a freestanding wall

These three wall designs have similarities, and differences to all the other solutions to forming a silage clamp wall

Timber or Concrete sleeper walls – are generally supported by steel columns either in a building or by steel stub posts.

Concrete wall panels and posts – are basically a variation of the sleeper

Free-standing L, T or A shaped concrete – are effectively the same as a cast insitu freestanding in that they rely on the floor to hold them up.  

Prefabricated timber walls – were popular in the 60’s and are again either supported by columns or cantilever off the base slab.

To simplify this we can consider all these different wall types as either supported by steel (or concrete) columns and those acting as a freestanding wall (or cantilever). Freestanding or cantilever walls are generally made at a height and you can’t really increase the wall. Any precast wall is really only ever designed to suit the size it was made so you can’t add to them. You might think of nailing some more timber to the wooden cantilever panels but you really shouldn’t do it. This type of wall was designed for 6t tractors back in the 1960’s so they are past their sell by date now.

The brave might consider shuttering more concrete on top of existing cast walls so we will come back to that one….

Walls built with steel columns are tempting to upgrade, particularly if they are part of a building. It can also be tempting to pop another concrete panel on top of cast insitu walls as a really quick solution. The very brave even consider welding a bit extra column on top stub posts to gain some more height. And whilst these options can seem tempting in varying degrees, none of them can be recommended – not one.

So why can’t I increase the height of the walls?

It all comes down to the design – and some basic physics. If you have all the design information at hand, then you can consider the implications of increasing the height of your walls but for the rest of us we can only assess what’s before us. There is lots of published performance data for steel columns and by just measuring them we can assess their load capacity. It’s also fairly easy to ascertain the performance capacity of prestressed concrete wall panels or sleepers, but for anything else you are just guessing. It’s unlikely the foundation design is documented and you might be thinking that 2 out of 3 isn’t bad. Many of the wall panels have sufficient “spare” capacity to allow you to increase the height, but what about the columns?

Just as a big spanner can crack a nut that a little one won’t shift, so a taller wall can buckle a column that was otherwise fine. Leverage can be your friend and your enemy and it’s certainly no friend to steel columns. Adding to the wall height has more effect than most people expect. For a 15’ bay (4.5m) adding a 3’ (1m) panel on top of a 10’ (3m) wall will more than double the load at the base of the column! And yes, there is a safety factor built into every design, but that’s only around 40% and it’s there for safety. This is also the reason that casting more concrete onto a mass concrete wall is not a good idea. It’s also likely that you have been using up the safety factor in the original design with heavier tractors on the pit. But even if the column and the walls has sufficient capacity, what about the foundations, are you really really sure they are up to twice the loading?

So no – it’s really not a good idea to increase the clamp wall height – not unless you planned to do so when you first built the clamp. 

If you want to discuss options if you’re running out of clamp space or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash @ Jeremy@silageconsultant.co.uk

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