Our Geothermal Adventure (Chapter 4): A Hole Lotta Sink


It's been three months since our geothermal system was installed.  We've made it through the hottest part of the summer, and proved that a heat pump sized for a Minnesota winter does a bang-up job with air conditioning in the summer.

So far we've discovered only one problem: the sinkhole.

When the contractors buried the plumbing for the loop field, they basically excavated a trench about ten feet wide, twenty feet long, and six feet deep.  That's about 45 cubic yards of material removed.  At the bottom of this pit, they connected the six deep wells to a manifold and a pair of pipes which run under the garage into the utility room.  These pipes circulate the antifreeze solution which transfers heat between the ground and the house.

After all the plumbing was done, the geothermal company just pushed the 45 yards of material back into the hole.  They made no attempt to level the ground, nor did we expect them to.  On the contrary, they made it very clear that they would leave the yard a complete mess and it was our responsibility to fix the landscaping.

A week or so after the geothermal guys left, the landscapers arrived.  They used a bobcat to level and grade the ground and plant grass seed on top.

Now, we had a dry spring and summer and for a while things looked pretty good.  If you've had experience with excavation, though, you can probably see where this is going.

A certain amount of settling is always expected when you dig a hole and refill it.  That's because the granules of dirt, sand, and clay don't just drop back into the same compacted configuration they had been before.  Instead, they're fluffed up a little, and it takes some time to unfluff.  A good soaking rain helps, since the water suspends and lubricates the particles.

This August, we got that rain.  When we got that rain, the ground above the excavation settled.  And collapsed into a big sinkhole.

My best guess is that when they pushed all that material back into the hole, they accidentally left a sizable void in one of the corners of the excavation.  This is easy to do when the dirt is dry and lumpy like it was this past spring.  The void sat there quite happily for a couple months, until we got enough rain to actually soak all the way down to the underground air pocket.

Once the water reached the void, it collapsed and created our sinkhole.

The sinkhole is about a cubic yard in volume, which is to say, big enough to look ugly and alarming, but not big enough to actually be dangerous.  Fortunately it's not in a place visible from outside our yard, so I don't feel like it has to be dealt with this instant to keep the neighborhood looking good.

Right now, I'm thinking that the time to deal with the sinkhole will be in the spring, after we've had a complete freeze-thaw cycle and I can be fairly confident that the excavation is mostly done settling.  I would hate to fill it all in, just to have it sink again.

If I had thought of it at the time, I should have taken the garden hose and run it into the rough-filled pit the geothermal guys left before the landscapers arrived. That would have at least uncovered the void and prevented the dramatic sinkhole, even if the ground would still have settled after being regraded.

Update: A few hours after I wrote this entry, I discovered that I was a little too sanguine about the need to immediately fill in the sinkholes. The sinkholes are trapping runoff which would normally flow downhill and away from the house, and with heavy enough rain some of the water is making it into our basement. Not much, but enough to make me want to go get a couple yards of sand and rough-grade the sinkholes before the next big storm.