Gravel Moles??

[Getnthair]

I want to drain a very flat field (30cm fall in 800m) with little fall to the ditch - perhaps 900mm. The field is quite narrow with the ditch down the long side and is only for grazing. In the past it has been a very god grazing field but I'm slowly losing the use of it - didn't graze it at all in 2012. Dug some test pits and the ground is fairly consistently a loamy clay over a clay subsoil, lots of surface water but little percolation down into the subsoil, some pits showed a little water running in the subsoil - never saw a stone. Adjacent field is very gravelly and rolling so I intend to dig and screen all my gravel from this site - should cost around £5-6/t.

Initial thoughts are to drain across the field to get as much cover and fall as possible on short runs but it will need a lot of close-spaced runs and a lot of gravel.

An idea that the drainage contractor has is to run main drains across the field (every 50-60m) into the ditch and then run gravel moles along the length of the field, every 3-4m, each mole should only have to run 25-30m to a pipe.

I've heard some mixed reports about gravel moling. Is it an Irish system? Will the moles fill up quickly and lose there permeability? Does it work?

My thoughts are well, yes/no, maybe? If it works it could be cheaper - use a lot less pipe. Gravel should hold the mole open and introducing a lot of stone into the soil won't be a bad thing. If it doesn't work I can put a herringbone pattern between the main drains and spend as much as I thought I was going to anyway (no prices yet to work on but imagine none of this will be cheap).

Anybody got any experience of gravel moles - good or bad?

 

[Pan mixer]

I want to drain a very flat field (30cm fall in 800m) with little fall to the ditch - perhaps 900mm. The field is quite narrow with the ditch down the long side and is only for grazing. In the past it has been a very god grazing field but I'm slowly losing the use of it - didn't graze it at all in 2012. Dug some test pits and the ground is fairly consistently a loamy clay over a clay subsoil, lots of surface water but little percolation down into the subsoil, some pits showed a little water running in the subsoil - never saw a stone. Adjacent field is very gravelly and rolling so I intend to dig and screen all my gravel from this site - should cost around £5-6/t.

Initial thoughts are to drain across the field to get as much cover and fall as possible on short runs but it will need a lot of close-spaced runs and a lot of gravel.

An idea that the drainage contractor has is to run main drains across the field (every 50-60m) into the ditch and then run gravel moles along the length of the field, every 3-4m, each mole should only have to run 25-30m to a pipe.

I've heard some mixed reports about gravel moling. Is it an Irish system? Will the moles fill up quickly and lose there permeability? Does it work?

My thoughts are well, yes/no, maybe? If it works it could be cheaper - use a lot less pipe. Gravel should hold the mole open and introducing a lot of stone into the soil won't be a bad thing. If it doesn't work I can put a herringbone pattern between the main drains and spend as much as I thought I was going to anyway (no prices yet to work on but imagine none of this will be cheap).

Anybody got any experience of gravel moles - good or bad?

I have never heard of gravel moles. Why not ordinary ones?

On our clay soil at least the gaps fill up very fast in the gravel backfill if the stone is small, that is why rejects are used which are quite big (can't remember size but certainly much bigger than 20mm for example) I have a hardcore yard which was crushed rubble, we added granite chips courtesy road resurfacing, now it is as waterproof as a concrete slab.

The idea of moles is to be small, therefore the gravel will be small therefore it will block, are you sure you don't mean french drains?

 

[Getnthair]

I have never heard of gravel moles. Why not ordinary ones?

On our clay soil at least the gaps fill up very fast in the gravel backfill if the stone is small, that is why rejects are used which are quite big (can't remember size but certainly much bigger than 20mm for example) I have a hardcore yard which was crushed rubble, we added granite chips courtesy road resurfacing, now it is as waterproof as a concrete slab.

The idea of moles is to be small, therefore the gravel will be small therefore it will block, are you sure you don't mean french drains?

Thanks, but I think gravel mole is the correct term. I think it is a system used in Ireland, the idea being to prolong the life of the mole channel by having a porous infill that also holds the channel open. There would appear to be enough clay to form an ordinary mole channel but as we have high rainfall I'd worry about getting the right conditions to get back onto the ground regularly to redraw moles.

As I understand it, a mole plough is pulled through the ground with a gravel box behind the leg - like a gravel box on a trenchless drainer - gravel fills the mole channel and as far up the leg as you want, width of "leg" fill being dependent on the width of the box used.

Interestingly my drainage contractor wants to use small gravel as he thinks it will block ingress of clay from the sides of the mole channel. Everybody has different ideas but it is my money and I want it to work......

 

[Zetor]

One of these? I like the idea but the small gravel will soon block up in places making wetspots, do you need main drain pipes, can you plough directly from the ditch it would save you some money.

To give you an idea of cost i can supply & plough 80mm pipe for around £1.20/m, no gravel.

 

[Getnthair]

One of these? I like the idea but the small gravel will soon block up in places making wetspots, do you need main drain pipes, can you plough directly from the ditch it would save you some money.
View attachment 45465
To give you an idea of cost i can supply & plough 80mm pipe for around £1.20/m, no gravel.

That looks like it!!!

I can't plough directly from the ditch. Ditch and field are just about at the river and are on a flood plain, field is surrounded by 6' high floodbanks on ditch-side. I am wondering about putting flood valves on the drain outlets.

Thanks for the costing information.

 

[Morph]

Similar land here and conundrum. Looking @Zetor Facebook photos interesting drainage attachment how does it fair, lots of land heave, i presume no stone backfill?

 

[TWF]

http://www.rmbuckets.com/drainage.html .