Paddock land drainage

[capate]

Firstly pavingexpert, congratulations excellent site, very informative.

I have a 2 ½ acre paddock (agricultural land) which is waterlogged and I am arranging to have a land-drain installed along the top of the field to intercept the ground water and re-direct it to a existing pond and sump (120m). The ground in this area is clay.

I am using a local agricultural contractor who has carried-out work in the area.

Reading your pages I was looking to adopt a collector drain system using a half perforated pipe with stone surround, however, I have been unable to find an agricultural drainage pipe that is only half-perforated (Polypipe Civil Ltd - Field Coil and Land Coil are fully perforated, Hepworths Coil land drain is also fully perforated) the only half-perforated pipes I have been able to find come in 6m lengths and cost *significantly* more.

My question is for agricultural land drainage (not a land drain to a Tescos carpark) is it really necessary to adopt a half-perorated pipe or would a coil (fully perforated pipe) suffice?

My second question is relating to trench width. The convention appears to be to adopt a trench width 300 wider than the pipe. Is this really necessary? I cant see any fluid dynamics reason for this I am guessing the 300 wider requirement is really to facilitate a man standing in the trench whilst laying the pipe. As I am intending to lower the coiled pipe from the surface onto a stone bedding (not standing in the trench) I cant see any particular reason for not using a 300 trench for a 150dia pipe. The narrower trench also results in less stone and less arising to dispose of on site. Any thoughts?

My third question relates to LA Land Drainage Consent - does this really matter?

tia
Steve

[Tony McC]

Half-perforated pipes are a bit of a nonsense, really. It is possible to make a convoluted case for their use, but out in the field, things are fairly simple and the groundwater doesn't worry too much about whether the pipe is half- or full-perforated, and whether the holes are facing up or facing down.

So, you will be fine using fully-perforated.

Trench width is usually calculated for ease of working, but there are some installations where the trench width is part of the attenuation, and so the total volume of pipe + gravel is directly relevant to the efficacy of the system as a whole. However, from the simple description you provided, I'd guess this is a width of convenience rather than design.

LA consent - there's nowt more bolshie than a LA Official who thinks s/he's been ignored. They can make life hell if you get on the wrong side of them - and then there are those who just couldn't care less. The problem is that you don't know which type you have, so it's best to play it safe and get a consent.

[capate]

Thanks for your advice Tony - its appreciated

[TheRobster]

My second question is relating to trench width. The convention appears to be to adopt a trench width 300 wider than the pipe. Is this really necessary? I cant see any fluid dynamics reason for this I am guessing the 300 wider requirement is really to facilitate a man standing in the trench whilst laying the pipe. As I am intending to lower the coiled pipe from the surface onto a stone bedding (not standing in the trench) I cant see any particular reason for not using a 300 trench for a 150dia pipe. The narrower trench also results in less stone and less arising to dispose of on site. Any thoughts?

From the point of view of fluid dynamics, the width of the trench should not make any significant difference. The hydraulic conductivty of most gravel is very high and so should be able to conduct whatever volume of water enters the trench with ease.

There may be another factor, although I aren't sure just how significant this is. The gravel should be surrounded with a permeable geo-membrane in order to keep silt and sediment out of the trench. Membranes have a habit of becoming partially blocked with fine silts over time, reducing the hydraulic conductivity of the membrane. The greater the surface area of the membrane (i.e. the wider the trench that the membrane needs to wrap around) the less effect this will have.

I don't know how significant this will be though, or if it something that you need to worry about. You need to have some gravel though...I wouldn't recommend just using a perforated pipe with a geo-membrane wrapped directly around the pipe. Some older field drainage systems have been built like this and they tend to stop functioning fairly quickly.

[capate]

Thanks for your observations TheRobster.

I do indeed intend to adopt a clean stone surround to the pipe, for the length of the pipe. Also, where the waterlogged ground is at its worse I intend to adopt the clean stone for the full depth of the trench. I also intend to incorporate a geo-textile membrane, but not directly wrapped around the pipe.

Cheers
Steve