Geothermal system and sinkholes

When I had my home built 8 years ago, I installed a geothermal heating/ cooling system. It is a vertical system with 3 wells although I am unsure which are supply and which are return wells. I have had no problem with the system itself but several years ago, I noticed a sinkhole about a yard wide in the middle of my lawn. At first, I didn't associate it with the geothermal system, but later I noticed two other sinkholes at about where I believe the other two wells were. In addition, the ground has sunk between the 3 wells and leading up to the house. I have little doubt this is related to the geothermal system but have no idea what is causing this. Is this a common problem with geothermal systems and what can I do about it?
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On Wed, 13 May 2009 11:42:29 -0700 (PDT), bigdog

    Then one where you pull water out is the supply. The own where you put it back is the return
    You're welcome.

    RUN !!!!!
    Can you say 'foundation damage ????
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bigdog wrote:

That Phenomenon is called the *China Syndrome*! Quite common in cracked Nuclear reactors and piss poor installed Geothermal Units. There is a bright side though. Many survived Chernobyl with only slightly massive mutations etc.
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Sounds like your house is about 2-3000sf, if sized for cooling...if your in Northern climes & it was sized for full heating then your 1500-1800sf. The 3 vertical boreholes are probably vertical closed loops & I'd bet the bores drove through an open cave, or a soft stone or peat layer. I'd also bet your contractor didn't use a decent bentonite grout or even a reasonable sludge pumped down the hole (via trembe tube) after the loops were inserted. This grout/sludge insures the holes are sealed from surface water drainage down & water swaps between the water tables a typical vertical loops typically penetrate. Here in MN we're not allowed to use vertical bore wells w/o bentonite grout sealing the loops. I've got a 268' well for domestic & pump & dump geothermal. when installed, I was told we shot through 5 or 6 different water tables. The Grout keeps the rusty iron laden 35' water table from contaminating the various lower water tables. We're also not allowed to return the pumped water into a return well. ECOLAB was fined a big chunk a while back for reinjection wells in it's downtown St.Paul office building. We take fresh water pretty serious here in the land of 10,000 lakes. I'd bet your sinkholes are from the collapse of soft soil or cave layers down below, or surface drainage washing down the tubes & dragging the slurry to the bottom. In either case the install is suspect, but a few yards of surface soil & grass seed should cure the problem. Although it may take a generation or two to fully seal, when it finally stops, be assured your loops will has a good a thermal transfer as is possible.
If your system is keeping your house comfortable, I'd have a couple truckloads of soil spread about & live w/it. If it's not working, call your installer...
Of course I might be mistaken.
goodluck geothermaljones

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wrote:

Thanks for the intelligent reply. The house is about 1760 sq. ft. with a 1200 sq ft basement. The system itself is working fine. The supply and return pipes enter the house about 4-5 feet below grade. Since this home was built about an hour from my previous home, I was not present when the wells were drilled, but having seen the ground after they were covered over, it appeared to me that they dug a trench diagonally from the front of the house to connect to the row of wells about 25 feet from the house. Another trench connected the 3 wells. It is along these two trenches that ground is sinking. I can understand why the ground might be sinking around the head of the wells, but why would it be sinking along these horizontal trenches. At the wells the sink holes are about 2 feet wide and 1 to 2 feet deep. Along the trenches ground has sunk about 6 to 12 inches. Grass already covers the are of the sinking. If it is just a matter of bringing in some dirt to level the ground, I'm OK with that but if this problem is going to persist, I need to find a more permanent sollution. One idea I had is to dig out the trenches again and put a layer of concrete several feet below the turf but if the soil is going to continue to sink beneath it, is that really going to fix the problem.

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I'd buy a few loads of dirt before I started laying concrete. You can get a lot of topsoil & spread it around for the cost of the excavating & concrete work. Concrete may cure the problem, but it may also get in the way of a future fix... though I doubt it. If the system is operating properly & the space is comfortable, it's a "wait & see" thing. I'd monitor the water levels & make sure there aren't any leaks, call the local dirt digger & spread some grass seed. If I have to do it again is 5-6 years, sobeit...
goodluck geothermaljones

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