Surveys, geology and boreholes

Surveying theodoliteWith a new build the greatest area of uncertainty is underground. Foundations need to keep the house up, and when, like us, you have no access to mains drainage, you need to be sure that the foul drainage is going to work too.

Clearly you can’t just dig the whole plot up just to see what’s down there – far too expensive and anyway it would compromise the foundations if they were built on disturbed ground.

So whatever you do to try and uncover problems under the surface, there’s always the possibility you’re going to be surprised when you come to build. What can you do though is dig a set of trial inspection pits and, of course, set aside a decent contingency in your budget to cover the unexpected. But it still makes sense to have someone carry out a survey of the plot; it just may turn up things that make that perfect plot unviable.

Survey 1

In our current plans to build a house in the Borders, we’ve been here a couple of times. The first time was on a plot near Duns. We had put our bid in subject to a satisfactory ground survey, referring specifically to foundations and drainage.

Our bid was accepted and we called in a local chartered surveyor to take a look. The first report told us that there would be no problem with foundations but the foul drainage would be problematic – the soil was a heavy clay that would quickly become saturated with the output from the soakaway.

We went for a second visit to examine the possibility of putting a discharge pipe out to a local burn. SEPA, the Scottish environment people, were discouraging, and the cost of putting the pipe 50 metres across the adjoining field was not small. We reluctantly backed out of the deal and went to look elsewhere to build.

Survey 2

Fast forward 6 months or so and here we are looking at the plot at Edenmouth. Again we made an offer subject to survey. SEPA had already said they would allow a soakaway but only if we put in a sewage treatment plant, an enhanced sort of septic tank. We were keen to see if we could find a way of reverting to a cheaper solution, and we wanted to check the gravelly soil to make sure we could put some foundations in.

Andy from Firefly Wood arranged for his structural engineer Ewen to visit the site and so one day in February we all turned up. The resident pig hadn’t seen so much excitement before and celebrated by having a nibble at Andy’s camera. We peered into a recently dug service trench and admired the solidity of the walls. Jamie from the farm next door arrived with his JCB and we dug 3 or 4 regulation depth trial pits, and poured water into them. Stop watches out, we counted off the seconds for the water level to sink which it did with dismaying speed.

BGS Edenmouth detail

BGS phone app for identifying underlying rock types
Click for larger image

Ewen then stomped off peering at his GPS phone. It transpired he was using a British Geological Survey app on his phone. He was interested in exactly where one soil type changed to another. He came back muttering under his breath about how it was all gravel.

Chatting with Andy and Ewen when all the dust had settled and the pig had been fended off again, their opinion was the land was eminently suitable for building on. The bad news was because the soil drained so fast, we would still need the treatment plant, but with a cost of £1500 against £750 or so for a standard septic tank, it wasn’t going to break the bank.

Ewen left us to go and work on his report, the design for the drainage, and we waited for SEPA to confirm they would still be happy about our plans. We turned our attention to our plans for ground source heating.

Ground source heating
Closed loop ground source slinky

Ground source – a “slinky” ground loop

A ground source heat pump circulates a mixture of water and antifreeze around a loop of pipe – called a ground loop – maybe as much as 300 metres long buried in the garden. Heat from the ground is absorbed into the fluid and then passes through a heat exchanger into the heat pump. The wetter the ground, the better heat can be transferred to the ground loop.

We were worried that the soil on the plot, while it was really well drained, would be too dry and gravelly to work well with the usual ground source loop.

The alternative would be to drill boreholes down 150 metres, hoping to find water saturated rock which would be ideal. But what lies beneath? This was a different question than Ewen had answered with his solution for footings and drainage. Really the only sure fire way of knowing what’s there would be to drill a trial borehole, but again that would be an expensive trial run.

Enter the British Geological Survey website again.

The geology app Ewen used for the superficial soil information also had data about the underlying bedrock:

Ballagan Formation – Sandstone, Siltstone And Dolomitic Limestone. Sedimentary Bedrock formed approximately 341 to 354 million years ago in the Carboniferous Period. Local environment previously dominated by rivers.

OK, so we have sandstone and limestone down there, but do we have any water?

Boreholes

The BGS website comes up trumps again. Using their borehole scan service you can search and view records made by borehole drillers in years gone by. You have to pay for more recent records but if you’re lucky you will find original handwritten records from historical boreholes near to your location.

We found a borehole record for a site just round the corner near Ednam. A 119m borehole drilled for water at a nearby farm back in 1950 was documented at the time and has recently been scanned for public access. The borehole driller carefully logged all the changes in the rock encountered by the drill as he got deeper and deeper into the ground.

Happily at the bottom of this document they say they found water only 10 feet down (1950 so imperial measures were still around!) and they were able to extract useful amounts of water from it.

So we look like being successful if we drill into similar ground. And although it will be a lot more expensive than laying a ground loop, we could expect 25% more heat from a similar length of pipe. Warm and cosy for nothing! Well, almost nothing … all this still has to fit within our budget. Watch this space.

 

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