Land drains on/near agricultural land

[vivienz]

After many months of waiting for the vendor's rubbish solicitors to get their act together, we've finally exchanged on our north Dorset plot and will complete next week.

 

The site is a rural one, and our plot is part agricultural field and part residential.  The local area is heavy clay.  We went to inspect the place at the weekend and met the neighbours who said that there are land drains running all over the place under the properties, but they weren't able to give any specific detail. 

 

Given that we intend to have the existing 1950s pre-fab bungalow demolished and then to re-build, is this something I need to worry about at this stage?  Will the architect or structural engineer take this into account?  Any useful information on this topic would be most welcome.

[PeterW]

Its pretty unlikely that land drains will run under the existing property, but if you are planning on extending its footprint then you may find you will uncover a land drain or two.

 

Unlike leach fields and sewers, as long as the land drain is running away from the property (or you want to build a swimming pool..) then there is little you need to do. If the land drains run across the footprint of the new build and run towards the build, you may want to divert them around the build.

 

What is the water table like as that will define a lot of the requirements for the structural engineer and the foundation design. Any trees in the proximity too..?

 

[ProDave]

I would be more concerned with percolation rates and where you are going to put your leach field, on the assumption you are off mains drainage and are going to be installing a new treatment plant (if you have mains drainage that is obviously not a concern)

 

Clearly it's a wet area, something I have here (though probably for different reasons) But a SE should be able to sort out a foundation system for you.
 

[Jeremy Harris]

We uncovered a few old clay pipe land drains, the type that were just loosely butted sections of clay pipe, with leaky joints.  Our site had been an orchard for a couple of hundred years, with a stable and cart shed at the bottom, and these drains were almost certainly running under those old buildings (built around 1800) to take run-off from further up the side of the valley (where there are a fair few springs) down to the stream that runs along the front of our plot. 

 

In our case, none of them we uncovered were wet, and I suspect more recent developments further up the side of the valley had dealt with the original source of the water.  We did run a perforated drain behind our retaining wall to a soakway, just in case, but I'm reasonably sure the water that used to come down that way now runs to another drain.

 

It's usually easy enough to make sure the area where your foundations are going to go is drained, in our case we have what amounts to a large French drain all around it that connects to our main SUDS-compliant underground storm surge tank, made of Aquacell crates, that takes all our surface water run-off.

[IanR]

Is any part of your proposed foot print (now or in the future if you plan to add garages, outbuildings etc.) within the drainage path of the field and the ditches it drains to. If so then you should consider running land drains around the periphery of the area you wish to protect, and then on to a ditch at the lowest point on your property. The drains need to go at least 500mm deeper than you foundation.

 

My current build is a change of use from Agricultural and there's about 2 acres or so of field that is higher than my foot print, which is in the path to the ditch it drains to. There wasn't much sign of surface water draining across and around the Barns I'm converting, but when we dug the trenches for the land drains I was shocked at how much water was moving through seams in the clay, below the surface.

 

The field itself hasn't had any drainage work done to it in the last 25 years, but was moled at that time, and before that has much older clay land drainage. It all still seems to be pretty effective at draining the land and there's a significant amount of water that drains to the ditch below my Barns after the rain. I'm very glad I'm diverting it around my new foundation.

 

 

[vivienz]

Thanks all, that's very helpful.  I guess that it will be another thing that the SE will need to account for when considering drainage for the property.  I'd really like to have a basement and if this part goes ahead, then certainly that's when any old land drains will come to light.

 

We're building on the residential curtilege rather than any part of the agricultural land - the planning officer said that we would never get permission for a change of use on that, but it fits in nicely with our plans.  We'll be digging plenty of other stuff as we're off-grid for everything apart from electricity; I plan to have a GSHP system installed along with UFH.  Also, there is an old septic tank that will need to be dug out and replaced.

 

Re. trees, there is a wild cherry and overgrown Christmas tree close to the existing property.  Neither of any merit and the PO is happy for these to be removed as part of the redevelopment.

[Jeremy Harris]

I'd have a careful look at the cost comparison between a GSHP and an ASHP.  ASHPs now make no more noise than GSHPs, are only very slightly less efficient if installed properly, cost around 1/3rd the price (installed) and have virtually no ongoing maintenance (whereas just an antifreeze change in a GSHP every few years could set you back several hundred pounds)

 

We were originally going to install a GSHP, but even with me doing most of the labour we would never have come close to saving 1/10th of the additional installation cost from energy savings over the lifetime of the unit (which I assumed to be about 20 years).

 

As an idea of costs, the very cheapest GSHP install for us, including the unit, pipes, antifreeze etc, was around £9k.  Our ASHP cost £1700 and was installed by me in half a day, with around £100 worth of extras.  It's almost completely silent, and we're in a very quite rural area.  The noise of air being drawn in the nearby ventilation intake for the MVHR, plus the faint hum from the wall-mounted PV inverter. make more noise than the ASHP.

[ProDave]

^^^ +1 to the above

 

I looked at a GSHP. Even having my own digger so the ground work would only cost some diesel,  I decided to go for an ASHP.  The cost of all the pipes to bury in the ground and the antifreeze to fill them (which you have to replace every 10 years, where are you going to dispose of the old stuff?) cost more than the actual heat pump.
 

[vivienz]

Hmm.  Very interesting comments re. GSHP v ASHP.  I'm nowhere near starting the costings on things with everything having been in hiatus for so long, but they are very persuasive figures.  Added to the 'good ideas' list.

[dogman]

+1 for an ASHP. Having lived next to a house fitted with one that was facing us (in a rural area) and now living in that house it is hard to tell when it is on. I actually have to open the back door and stick my head out.

 

[Stones]

I looked at retrofitting an ASHP or an GSHP in our last house, heating demand 5000kWh/yr. When our took into account the cost of running the ground loop circulation pump, the GSHP actually had a higher running cost than an ASHP despite having higher CoP. The significantly higher installation cost (vastly inflated because of RHI) was the other nail in the coffin.