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Author Topic: Does GSHP cause subsidence?  (Read 12224 times)
dan_aka_jack
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« on: July 02, 2010, 05:46:04 PM »

I've recently heard a rumour which goes like this:

Quote
in the US, they're now trying to avoid doing ground source heat pumps near buildings, because the heat pumps are drying out the ground and then causing subsidence.

Has anyone else heard that rumour?  Even better, does anyone know of any empirical evidence which either supports or falsifies this rumour?

I'm sceptical.

Here's my thinking (which may be wrong, of course):

Ground source heat pumps, if they have any measurable effect on the ground, should cool the ground (GSHP extracts heat from the ground and moves it to the house).  So what's the mechanism by which GSHP dries the ground?  If anything it should fractionally reduce the rate at which moisture evaporates from the surface, although if the GSHP pipes are installed where they're supposed to be installed (2 meters below surface) then the effect on the surface temp of the ground should be absolutely minuscule.  I suppose it's possible that GSHP might very slightly increase the chance of the ground freezing in winter but that shouldn't affect a well built house (foundations are supposed to go down below the frost line)

Maybe vertical bore holes allow the ground to drain more rapidly or something?  Or perhaps the bore holes make the land less stable?

I tried googling for "ground source heat pumps, drying ground out"  but didn't find anything relevant in the top 10 results.

I've definitely heard that geothermal (drilling a very long way down to access the heat coming from the earth's core) installations can do bad things to the land though.
« Last Edit: July 02, 2010, 05:53:07 PM by dan_aka_jack » Logged

dhaslam
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« Reply #1 on: July 02, 2010, 05:54:43 PM »

In the US they are used for  cooling in summer as well as for heating  so  perhaps they would dry the ground a little in cooling mode.  In comparison to the heat of the sun  on the ground it  would be fairly insignificant.   Some  people just like to complain.
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noelsquibb
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« Reply #2 on: July 02, 2010, 06:25:45 PM »

Digging 2m deep trenches will be the primary source of settlement.

Because its almost impossible to get 100% compaction but over time consolidation ( water moving smaller particles downwards into voids) will give close to 100% depending on particle sizes.

This process can be horizontal as well as vertical, so the whole area could sink over time, where there are close parallel trenches.

So digging 2m deep, parallel and close to a foundation 1m deep, is a guaranteed way to cause foundation movement.

Be surprised if the small variation in temperature as you get close to the pipes, will have any effect.
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Paddling Al
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« Reply #3 on: July 02, 2010, 07:32:15 PM »

I have three barn conversions near me where "Ice Energy" have installed their GSHP?? systems using micro arays.

This is causing heave rather than subsidence.

My memory is that the arrays were a tight packed matrix of pipes no more than 9 sq m.

They don't call themselves ice energy for no reason!!

The barn I have visited has experienced about 50mm of uplift to their patio (over the array as I remember it) though I have not tested the temperature of the ground so it 'could' be from some other cause.
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dan_aka_jack
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« Reply #4 on: July 03, 2010, 08:38:51 AM »

Very interesting.  So, just to be clear: the theory is that the GSHP pipes, combined with a cold winter, have frozen the ground which has caused uplift?  Is the uplift only directly above the GSHP pipes?  Does the uplift threten the main foundations?
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MR GUS
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« Reply #5 on: July 03, 2010, 10:30:17 AM »

I guess these are retro fittings possibly without more than a knowledgeable surveyor assessment?
Or that the client has been informed of proximity problems & made an informed choice to commence?
otherwise this could be seen as dubious & damaging sales practise possibly, the fact that by the fact that 3 sites were mentioned they could possibly be installations by the same area team therefore individual bad practise ..akin to poor double glazing sales & fitters, either way damaging.

more information required really.
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Paddling Al
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« Reply #6 on: July 03, 2010, 10:58:52 AM »

The three barns are a single developmentb on one site.

The uplift is only evident over the array

The matrix was placed in a deep (3m +) excavation immediately adjacent to the barn. The structure is shale so if it is frost heve then I would expect the expansion to be constrained by rhe surrounding undug shale and therefore be most evident upwards. In this area most barns had no actual foundations and the walls were built directly off the shale at whatever depth that was encountered. I have one where the walls start within 200mm of the apparent ground surface.
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« Reply #7 on: July 12, 2010, 09:56:26 AM »

It sounds very optimistic to expect 9m2 to yield enough energy to heat a barn, usually the flux is anywhere from 1-10W/m2 depending on the ground conditions, maybe the designer lost a decimal place or two.

Desperate
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dhaslam
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« Reply #8 on: July 12, 2010, 10:53:10 AM »

From their  website  this video suggests that their ground loops may  not be deep enough but they did use two 100 metre trenches in this example.    If the ground loop is only one metre down it would be severely affected by sub zero weather, quite apart from the  extra heat transfer needed in cold weather.  The ground loop would always need to use a larger area than  building being heated  because the temperature increase  inside needs to be greater than the temperature reducion outside.     

www.iceenergy.co.uk/q3-joy-_107/
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desperate
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« Reply #9 on: July 12, 2010, 12:11:17 PM »

Exactly, 200m2 of trench extracting from an area probably nearer to 300m2 with an energy flux of say 8W/m2.......... bingo you have 2.5Kw to play with, still not a massive input. If you have a leaky old house to heat you will need a huge garden to dig up, and 8W/m2 is near the top end of energy flux from what I read.

Desperate
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GavinA
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« Reply #10 on: October 16, 2010, 05:22:16 PM »

The three barns are a single developmentb on one site.

The uplift is only evident over the array

The matrix was placed in a deep (3m +) excavation immediately adjacent to the barn. The structure is shale so if it is frost heve then I would expect the expansion to be constrained by rhe surrounding undug shale and therefore be most evident upwards. In this area most barns had no actual foundations and the walls were built directly off the shale at whatever depth that was encountered. I have one where the walls start within 200mm of the apparent ground surface.
that sounds like they've doubled or possibly trippled up the ground loop, with 1 or possibly 2 layers on top of each other seperated by 1m of soil. So one loop at 3m depth, another at 2m and another at 1m.

Presuming there's no additional solar input to boost the winter ground temperatures (such as in the chargingtheearth situation), I can easily see this leading to 3-4 metres of frozen ground under the patio in winter, some of which may even be basically permanently frozen, which I'd think could easily lead to a 50mm uplift.
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Brandon
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« Reply #11 on: October 16, 2010, 09:24:31 PM »

We were approached t be expert witness in a case where a lady was seeing just under 300mm of heave in her garden every winter for 3 years!

I forget off hand whom the manufacturer was, but it looked as though the installers were going to get a right royal legal roasting.  We declined the option of standing as witness, as we had plenty on and the whole affair was rather messy, and both camps firmly entrenched, installers being backed by manufacturer (was it worcester?) on their word that the install was as per....

Kensa for whom we install numerous GSHP do say that there shouldn't be any ground array within 5m of the building, as the cooling will effectively lower the frost line, and therefore put foundations at risk.
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rogermunns
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« Reply #12 on: December 25, 2010, 08:20:54 AM »

A bit of a similar situation;

we are slowly installing our system, which is intended to be hybrid collection. We are currently using well water (12.6deg. C yesterday) but we don't yet know how long this will last since we are returning the water to the well.

Next year we will lay ground loops and the intention then is to use them with the well water as well if necessary. We are doing this because our ground is rocky from about 30cm down and also the land slopes 10%. So we are going to scrape down to the rock, install the pipes and then backfill to 1.5metres - i.e. a raised bed. This will take a long time to settle properly although we will power-roll the soil as we put it in.

Having to build a raised bed means keeping the size down (money and ability to do such heavy work) so we hope the system, with the well, will be sufficient.
Also we will make provision for getting solar heated water into the ground. 

Interestingly, over here they place the collector pipes in 'calcaire' which is fine limestone dust. As rainwater gets down there and through general dampness this calcaire 'solidifies' round the pipe.
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dhaslam
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« Reply #13 on: December 25, 2010, 08:51:02 AM »

It might be less expensive to  drill  down into the rock   instead of  having to move all the soil.      Using the well for a while will give a good idea  of how  consistent that  source is.     Having a  heat source  deep into the ground  is always going to be better in extreme weather conditions.       
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rogermunns
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« Reply #14 on: December 27, 2010, 06:16:05 PM »

Simply; too expensive to drill into rock.
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