Do you need them, what are the benefits?

The current SSAFO regulations in the UK state that the clamp must have a catchment or containment drain around the outside of the walls. This requirement was detailed in the Control of Pollution regulations of 1991 and has not really changed in the subsequent revisions to the rules. For all leaky clamps constructed in the 70’s and 80’s with sleepers or simple precast walls the external channel was designed to collect the effluent leaking through the wall.

Since then the industry has moved to higher dry matter silage and the design and quality of wall construction has improved enormously. So why do we still need the external drainage channel?

The theory is that this channel is present to catch and safely drain away any accidental leakage of effluent. If all goes to plan then the channel will never collect anything other than rainwater, but the requirement still exists and you can be prosecuted if this is not in place. Even if you are building your clamp into an earth or rock bank the requirement still exists. The drain can be a simple dish gully in the concrete base or a channel formed with rendered blocks or road kerbs. There are no specific volume or flow performance limits in the regulations, but obviously the longer the channel and the more silage there is on the other side of the wall, then the bigger the channel will need to be. If the channel blocks or overflows and a pollution incident occurs, you will still be prosecuted.

All clamp drains need to be connected to a suitable effluent containment tank (subject of another piece in this series) that is also covered by legislation.

 So you MUST have a drain around the outside walls but what else do you need?

Some years ago it was usual to install a drain across the front of the clamp, in line with the front edge of the walls, between the clamp floor and the apron. The purpose of this is to collect any effluent seepage out of the clamp and divert it into the effluent containment tank. This “drain” can be as simple as a difference in floor level falls and as sophisticated as polymer concrete drainage channel as used on fuel station forecourts.

Section drainage channels with heavy duty gratings are great to travel over, robust and effluent resistant, but they are also eye wateringly expensive. The covers need to be removed to clear out and are very likely to become blocked, particularly when high DM maize silage at 5mm chop length is being clamped. As noted elsewhere in this series, sealing of joints in the floor of a clamp is a complex and detailed issue and the preformed channel introduces two more joints between concrete and another material. So these channels are not as popular as they once were.

Insitu box or T channels, covered with heavy duty timber sleepers, answer some of the problems associated with preformed channels, but “Sleeping Policeman” humps and simple gullies are more popular today. The Sleeping Policeman is a robust effluent barrier but driving over it with every load of silage - both in and out of the clamp makes it extremely unpopular with the operators. 

A dish gully valley in the concrete generated by falling the floor of the clamp from back to front and the concrete apron falling back towards the clamp has become the default choice for obvious reasons.

So is that it, is that all the drains I need?

Unfortunately no, you need a drain at the base of the walls too – however counter intuitive that might seem. Surely if you are trying to airtight seal the clamped material, sticking a drain in the floor or in the silage itself is pure madness?! Surely this can only introduce air into the silage with all the problems we know this will cause?

Wrong! These wall drains do a couple of very important jobs. Firstly they allow the effluent to find an escape route out of the clamp and prevent the entire contents of the clamp suddenly sliding out into the yard with all the safety and loss issues this will create. More importantly the drain relieves the hydrostatic pressures that can build up on the walls. The calculation of forces imposed on the walls for silage were laid down in British Standard 5502 many years ago. Designers are bound by the regulations to design a clamp in accordance with these rules, and the rules allow the designer to assume a relief drain is placed and the base of the wall. If you don’t provide the drain, then the wall is almost certainly going to be overloaded leading to either a catastrophic failure or at least a reduced service life.

What type of wall drains are best?

Base of wall drains need to provide a void for the effluent to escape into. For post and plank type walls this can be achieved by creating a gap between the wall & floor to allow the drainage into the external channel. The downside of this can be aggressive effluent attack of the steel columns in a place that’s very difficult to monitor or protect. Base of wall drains can also be pre-formed channels but there are all sorts of reasons why these are not popular. The simplest and most cost effective solution is a plastic perforated drain pipe coil that can be easily installed by just unrolling along the base of the wall and tipping grass onto it, This can be recovered as the silage use consumed and the pipe may be reused in subsequent years.

 If you need further information or help on decided what drains you clamps need, or for any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com