Ground test for basement feasibility

[vivienz]

Can anyone give me any pointers as to reputable companies around north Dorset/south Somerset who carry our ground tests?  Or do I need to go further afield, or does it make the slightest bit of difference where they are?

 

We want to have a basement incorporated into our new build and need to find out whether this is possible, particularly before the architect makes too much progress with his design (only just appointed today, so nothing happened so far).  Also, I'm considering GSHP and a borehole, I'm guessing that I'd need some sort of ground test done for the borehole, too, so I may be able to kill two birds with one stone.  Or not.

 

Are there any particular questions I should be asking or warning flags that I should be aware of?

 

 

Edited February 13, 2017 by vivienz
forgot the borehole!

[Declan52]

Any site investigation company will be able to sort you out. They will do a few boreholes confirming what the ground buildup is and from this your structural engineer can design your basement to suit.

Just a word of warning about a gshp they are expensive to install and maintain compared to an ashp so think long and hard before going down that road.

[Jeremy Harris]

As Declan says, you're looking at a capital investment of between double and three times the cost for a GSHP, for an efficiency gain of maybe 10%, if that.  Add in the high cost of things like antifreeze replacement and GSHPs start to look very uneconomical, and the additional cost over an ASHP would never be recovered by the tiny gain in efficiency over the life of the kit, I think. 

 

I was going to do the same as you originally (and we have a borehole for water - there's a lengthy tale about in our blog, here: http://www.mayfly.eu that you may want to have a look at first!)   and then opted for an ASHP at a fraction of the cost of a GSHP, and I've found it performs very well.  It's also a great deal quieter than I expected, and a heck of a lot quieter than the GSHP that was fitted to a holiday place we stayed in.

 

I probably learned more about boreholes than is healthy to know during our adventure.  We're probably not too far from you, either, we're East of Shaftesbury, just over the border into West Wiltshire by a mile or so.  I'd not want to name companies here, but we did get a few borehole people around to quote, and had an "interesting" experience.  Happy to chat offline about our experiences, but perhaps read the borehole and water bits of our blog first to get a flavour of things!

[Triassic]

You can get information local to you by searching the British Geological Survey borehole database http://mapapps.bgs.ac.uk/geologyofbritain/home.html?

[Pocster]

I did my own test holes. Also took the soil to be sampled myself for planning rather than pay a company to come out and charge a fortune.

 

We were fortunate and as expected the top meter soil and then rock; so no shuttering required 

[Ferdinand]

The only test hole I have ever dug I later fell into in the dark when poshed up.

 

Not a good idea.

 

It was not very deep but it was full of water testing the percolation in clay.

 

Unfortunately the percolation in clay had been insufficiently brisk to be of much use in the circumstances.

Edited February 13, 2017 by Ferdinand

[Steptoe]

OK, I've noticed a lot of times people referring to the high cost of antifreeze in GSHP, does nobody run glycol in their ASHPs,? 

Makes for a way more efficient system, and its actually extremely cheap for the small amount you need, generally sub £100 for a typical system, and a heck of a lot cheaper than 'water wetter' , albeit just not quite as efficient as WW. 

That is assuming they are proper ASHP and not remanufactured AC units that a lot of places sell. 

[Jeremy Harris]

4 hours ago, vivienz said:

 Also, I'm considering GSHP and a borehole, I'm guessing that I'd need some sort of ground test done for the borehole, too, so I may be able to kill two birds with one stone. 

 

 

I've just dug out the information I collected when we were looking at adding an extra borehole to run a GSHP; sorry for the delay but I'd archived all the old correspondence and had to dig it out.  We had a hydro-geological survey done and that contains information about both the water borehole and the GSHP borehole that we didn't end up drilling (BTW, I can recommend a very good chap for this sort of survey, and the price was very reasonable too, £250 plus VAT - PM me if you want contact details).

 

We only needed around 4 kW maximum output from the system, and the borehole needed to deliver this would have been 80m deep for the geology where we are.  Very roughly, and assuming that the £1k or so rig mobilisation cost is already paid by the needed ground survey boreholes (not so sure about that in reality, they probably need a much smaller rig) then you're looking at around £80 to £100/metre to drill, line and grout a borehole around our area, so around £6.4k to £8k plus mobilisation cost to just drill the hole, plus around £200 for the down-hole collector and another £250 for the antifreeze.  Worth noting that the antifreeze needs to be accounted for when assessing performance, as both propylene glycol and ethylene glycol mixtures are around 20% worse than plain water (at 20% concentration) when it comes to heat transfer efficiency, as all glycols have both a significantly reduced heat capacity and reduced thermal conductivity when compared with water.   I found that some manufacturers tested and specified their systems based on running pure water, rather than antifreeze mixtures in the primary loop, to enhance, on paper, their performance.  A bit naughty, in my view, but you do need to read the small print carefully to see how they derived the performance data they quote. The same applies to ASHPs, as well, as they run the same antifreeze and inhibitor mixtures, but at very much smaller volume (I think there's about £20 worth of antifreeze in our ASHP system, compared with over £400 worth for an equivalent power output GSAP system).   ASHPs are more critical in terms of making sure the primary antifreeze concentration is high enough, because their internal plate heat exchangers are more likely to experience very cold temperatures when the units are turned off, say when there's a power cut in winter.

 

When it comes to cost, I was quoted £4k supply only for the smallest GSHP I could find, a little 4 kW Kensa unit (good firm, BTW, very helpful, and knowledgeable too).  The total cost of a 4 kW max borehole based GSHP (2013 prices) was around £12500, with me doing the setting up and commissioning. 

 

In the end, I bought a 7 kW max ASHP (a decent one, with inverter control, badged Glowworm, but really made by Carrier, one of the biggest manufacturers of heat pumps around) for £1700 including delivery and installed it in a bit over half a day, plus a day of waiting time for the concrete foundation pad to cure.  Total install cost, including antifreeze and inhibitor, was under £2k.  Not a fair comparison in many ways, as the ASHP was very much more a DIY install than the GSHP could have been.

 

I'll admit to it taking me a fair time to work out how best to set up and run the ASHP, probably a day or more of my time, spent over a couple of weeks,  testing and adjusting settings.  I learned by experiment that ASHPs do not like operating with a flow temperature that's much higher than about 40 deg C, especially in cool, damp, weather (the worst case is around 4 deg C with high humidity) because they start running defrosting cycles.  They do this by reversing the cycle, pumping heat out, like an air con unit, to melt the ice on what is normally the evaporator, but which becomes the condenser when reversed like this.

 

As I didn't want to use the heat pump to produce hot water, just act as a preheating system for a phase change material heat store instant heater (the Sunamp PV) I wasn't bothered about not being able to deliver more than about 40 deg C flow, as our UFH only runs at 24 deg C anyway.  If you do want hot water at, say, 50 deg C, then there are now some very good hybrid heat pump/combi boilers available.  They will run on LPG, if you don't have mains gas, but don't use much gas because the ASHP heats the water to about 40 deg C and then all the combi bit has to do is boost that by around 10 deg C.

 

As before, if you want info on borehole drillers or the hydro-geological survey chap, feel free to PM me and I'll pass the details over.  Given what we went through with our borehole I'd be happier keeping company names off the forum, out of politeness, nothing more.

 

 

 

[vivienz]

Thank you for the detailed information, Jeremy. I've spent a couple of hours this evening reading your blog and I have to say that I'm going right off boreholes.  That aside, my main reasons for leaning towards GSHP rather than ASHP were my perceptions of the former being more efficient and robust and the latter being noisy.  One of the really big things for us moving out to the sticks is for the relative peace and quiet and the thought of a noisy ASHP system was off-putting.

 

I will PM you about a couple of other things, but thanks again, everyone, for the thought provoking comments.

[reddal]

We have 3 boreoles - 2 for GSHP and 1 for spring water.

 

We have our our fair share of issues with the GSHP over the last few years (maybe more than our fair share!). it was an expensive solution and if I was doing it again I would look very carefully at cheaper solutions. Also for a long time the GSHP was very noisy (eventually mostly solved now after much grief) but don't assume they are always quiet...

 

They key thing that persuaded us to go with the borehole / GSHP combination was the ability to do effective passive cooling in summer. This part does work quite well - however for the extra cost you could buy a hell of a lot of active cooling in one form or another - so think about that carefully if its your motivation.

[Jeremy Harris]

14 hours ago, reddal said:

They key thing that persuaded us to go with the borehole / GSHP combination was the ability to do effective passive cooling in summer. This part does work quite well - however for the extra cost you could buy a hell of a lot of active cooling in one form or another - so think about that carefully if its your motivation.

 

As, I think, some here know, we have active cooling, set up by just reversing the ASHP into cooling mode (easy to do on many models that use a 4 way valve defrost system).  In cooling mode the ASHP uses around 400W, and as cooling is only needed in hot weather we're pretty much certain to be generating maybe ten times that much power from the PV system, so effectively the cooling is at no cost.

 

Like you, we find the cooling works pretty well, too.  I have ours set to a flow temperature of 12 deg C in cooling mode, which seems enough to keep the floor surface at around 18 deg C in hot weather.  Enough to cool the house without being cool enough to pose any risk of condensation at all.

[Bitpipe]

We built a passive basement in Berks, with a MBC timber frame passive house on top.

 

The GI for a basement is a bit more complex than what you'd do for strip foundations, DIY is probably out of the question.

 

While GI does not need to be expensive, you want to do it properly once and get everything you need. I found that most firms travel far and wide, they usually rely on a small self propelled rig that they can mobilise on a trailer or in the back of a van.

 

Ideally, your SE should draw up the specification for the GI, not your architect. In reality, the SE designs the basement  - i.e. the steel schedule, concrete spec, waterproofing methodology etc, and needs to get the required info from the GI. BC and any warranty underwriter may want to see the assumptions that the SE used for the design and your LA may have interest in what you find also.

 

SE will primarily be looking for the bearing strength at various points of the basement, water and gas levels, ground build up and any history of made ground, etc. if you have contamination conditions from your LA as part of planning, then you'll also need the requisite contamination reports. To minimise muck away costs, you may need classification of the waste as inert - depends on where it's going and what their requirements are.

 

We required two go-arounds, mostly due to sloppy practice first time on site (I was away, which was unavoidable but a mistake) but also due to some uncertainty in the findings and the vagueness of historical workings in the area. We had a mix of 10m dynamic probes and 5m samples taken, plus ground water and gas monitoring left in place for a few weeks with regular monitoring.

 

Our buildup was classic river bed - clay, gravel and chalk - quite surprising on how much this varied in a 11m2 area, 3.5m deep. SE was concerned about solution features in the underlying chalk so we have a fairly hefty 300mm thick slab with lots of rebar to support the cantilevered walls - we have a suspended timber floor, not a concrete lid on the basement. SE also designed the steel support for that, tying in with the TF design and point loads from above.

 

Whole basement sits on 300mm EPS 200 (the cheapest we could get away with for the required bearing strength) and the walls have 200mm EPS70. We used Sika warrantied waterproof concrete without anything else as the water table was quite low for us (6m). 

 

 

 

[vivienz]

Great, thanks Bitpipe.  I'll get onto finding a suitable SE as soon as possible.  I'll see if the artchitect has any recommendations, but do a bit of my own research too, I think.

 

Love this forum.