October 2010 - Posts
Excavating drainage trenches carries one very significant hazard, you can’t see what is underneath the ground you are about to disturb. Most of the time, nothing but soil is present, but you have to be sure. Electric cables vary greatly in size and power, but chop through one with a drainage machine and they all go bang.
Unsurprisingly we spend a lot of time locating services, in the office we check online, search CDs, call and write to the utility companies; on site we use a C.A.T. (Cable Avoidance Tool) to scan the area and a little bit of experience in spotting trench lines. The system is effective, although not perfect, and it is amazing how many land owners do not know what is running thorough they land until we tell them. (I think every drainage contract has a story about a farmer telling them they is nothing whatsoever to worry about in this field, only to find a gas trunk main or 33kv electric cable.)
Most of time we uncover old existing clay drains, some are blocked and have collapsed, but an amazing amount of old clay drains still run an incredible quantity of water. I have no idea how old some of these drains are, I know that plastic pipe has been used since the seventies, so I suspect that the ‘youngest’ clay drains are at least 40 years old. Clay drains have been utilised for centuries, although the vast majority have been installed since the Second World War. Regardless of their actually age, they can’t be too many other services which 24 hours a day, every day of the year for such a length of time without ever pausing.
On another subject, all of a sudden it seems like winter is almost here, its crept up on me this year, it’s been relatively dry in the last couple of weeks and we have been working as if it was summer but with the clocks going back this weekend, and the temperature dropping it’s definitely hit me.
How long is a piece of string?
It’s the question which I know is going to be asked, but which I know I don’t have the answer for. I can’t complain about people asking it, after all it is one of if not the most, important question, but very few people understand that is not very easy to answer.
So how much will this cost me then?
The main problem to answering this question is that every scheme is different and more importantly the cost of gravel is different from one place to the next. The permeable fill whether it be gravel or stone is the single biggest cost in land drainage, and it is the transportation of the material which is costly. If a quarry is near you might pay eleven or twelve pounds a ton, perhaps less if the rep wants to get rid of it and if you require enough to drive a bargain. Alternatively if the supply is far away the price might be over eighteen pounds a ton. There are other factors as well, we might have to cross a gas main or if the field is awkwardly shaped requiring numerous short drains and junctions. I prefer to head back to the office and work out a price I’m happy to stick to, but there is an average.
Most of the time we can install a standard trenchless drainage scheme at around nine hundred pounds an acre, but if I quote you a different price, you have been warned.
At present we are working in the Cotswolds on a pipeline site - repairing all the farm drains cut by the pipeline construction. The title maybe a little over the top, but we’re not used to working on the top of a hill!
Anyway the views are a welcome change....
I thought a blog about drainage design might be of interest...
There are a few general rules, or should I say guidelines as every rule can be broken if the alternative is too expensive or unsafe. No doubt I have missed a few out, but its hard to remember everything whilst quickly typing out a post
Firstly, whilst the drains must cover the desired area, the fewer drains and junctions installed the cheaper and therefore the more desirable the scheme. The most cost effective arrangement is often a grid or parallel scheme but herring bone or targeted designs can be used. Obviously the lay of the land plays an important role here - water can’t flow uphill. See the diagram below.
A 100mm diameter main drain is usually able to cope with laterals covering around ten acres. If the area is greater there are two options: spilt the area in two and install two 100mm drains or install a 160mm main drain. Which option will depend, you guessed it, on the price.
- Lateral drains which are usually 60mm in diameter should not be longer than 300 metres in length, although the longer the laterals the easier the design to install.
- On a standard Agricultural design the lateral spacing’s are usually 20 metres apart, tighter spacings are better but add greatly to the expense. If the original spacings prove insufficient more laterals can be added easily
- If the gradient of a drain is steep every endeavour should be made to reduce it, if the gradient of a drain is shallow every endeavour should be made to increase it.
- Laterals should be oriented with the field's contours as much as possible. This way, laterals can "intercept" water as it flows down-slope
- A drainage scheme only moves water from one place to another, it is vital to consider the suitability of an outlet. For example if the outlet is to an overgrown, shallow ditch it is also advisable to clear and if possible deepen the water course.
Like I said there are just guidelines and there are probably some points which other might want to add, after all an old head once said to me, “Draining is the only department in agriculture in which farmers as a whole consider it beneath their dignity to seek or accept advice” www.farmservicesltd.co.uk
- The depth of the drains is dependent upon the outlet but any pipework must be below the depth of common agricultural practices, preferable around a metre.
Of course we’re a family business; everyone is a family business in Agriculture. Okay, its exaggeration, but I doubt if there are many other British Industries were so much gets passed down, father to son. And this is where the arguments normally start.
This clash of generations is exactly how it should be. The old heads, stubborn and reluctant to adapt to change, and young pups over ambitious and with new ideas half of which have failed before. Of course we follow this comfortable pattern and there is enjoyment to be had playing out the stereotype, i like to flip out a couple of ideas almost solely with the intent of raising my father’s eyebrow. But I can report that a momentous event happened the other day, my father accepted that our web site has been a success. The site its self is relatively small and simple, it’s just an on-line brochure, but almost every month it creates a job we probably would not have got without it. When we set it up a couple of years ago, the old man agreed but doubted if it would have much impact. To be honest it’s exceeded my expectations, not that I’m going to admit it that to the old man!
Experience triumphs most of the time, years of not repeating mistakes have generated a successful method, but sometimes the new ideas work too.
Watching a drainage scheme from design, to installation and then to see the improvement is quite satisfying.
Normally the difference is not as extreme as these two pictures, taken from all most the same spot before and after.
This drainage problem was caused by a collapsed drain and was relative easy to fix. It looks more dramatic than it is, but it makes a good set of pictures.
Our yard was broken into on Wednesday night and our small trailer - a standard Ifor Williams - was taken. Very annoying.
It is with depressing regularity that this sort of thing happens. We are, of course, insured but the incident requires a number of phone calls and it is impossible not to spend time in a fruitless search for evidence. Although our office and yard are on a reasonably busy ‘B’ road, both are set back and the nearest farm house is quite far away. We are isolated and that makes us vulnerable. We have tried to make things as difficult as possible, we have alarms, and palisade fencing surrounds the perimeter, but it is impossible to fully protect anything.
One thing that puzzles me, is it really worth the effort?
Our trailer cost around two grand when we brought it new, in 2008, it’s in good nick but it can’t be worth much second hand. In order to steal it, two, perhaps three people lifted two gates from their hinges, knocking off the weld dabbed on to make it prevent such a thing being easy; travelled over a ten acre field, got stuck in the mud and struggled to get free; used a hack saw to cut two supporting 10mm frames on the fencing and pushed back the fence to gain access. It must have been time consuming and hard work in the dark. Even before factoring the risk involved it seems like a hell of a lot of work for little reward.
The problem for us is whether they have seen anything else they fancy?
I have just come back from what might be the wettest field I have known, well I say field, perhaps is a shallow pond, it’s hard to tell. The days of draining fens are thankfully long gone and most of the time a wet field is a muddy field, you walk around and grow on inch or two due to the mud sticky to the bottom of your boots. No such problem here, the ground was soft but the reed grass was thriving and provided a footing, instead the water lapped up against my ankles.
How can land, productive if given the chance, get into such a state? I’m not sure I can answer my own question. The land owner explained that the field had been abandoned by the previous owner, for how long she did not know but guessed at least twenty years. What once had been pasture had been slowly taken over by reeds, ditches had silted up, drainage outlets buried, and water pooling on the surface even through the drought conditions we suffer during spring apparently. So over-grown it proved hard to tell if the field had a fall on it until I used the laser level, but the main hazard or perhaps interest, was the sheer numbers of frogs. Every step I took caused two or three to leap elsewhere, and me to adjust where I was going to place my foot. The frogs clearly loved the field, but so did a Heron who stood patiently, waiting for me to leave so that he could carry on with his frog feast.