Anyone ever experience sink holes?

I received this email today. Below is a second email that we received one year ago while this ground loop was being installed. This is a first for me. Looking for anyone who has experienced something similar.

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Steve,

On the xxx project in xxx, MI the parking lot above the geothermal well field has developed two sink holes.

I have two questions:

1. Could ground water around the wells be drawing the sand around the grout into the below ground water stream?
2. Could a leak in the loop system cause enough water to run out of the system to cause an underground void?

Please let me know what you think or if you have experienced this in the past as the sink holes are continuously growing.

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We have encountered unforeseen drilling conditions at the project site for the xxx At a depth near 225’ we are finding fractured limestone that is caving into the bore hole and will not hold open long enough to set the loop. At a depth near 255’, we are finding a cavernous zone where we lose our ability to return cuttings to the surface as our circulation mud (that lifts the cuttings) disperses into the cavern. The drill bit then quickly is overcome by cuttings and will lock itself into the rock. We have several options to consider, with #1 being the best option for minimizing additional costs to complete the balance of installation:

1. We would like to propose some field design modifications based upon our understanding of the drilling conditions present. The existing design is based upon 9,600’ of total vertical bore depth. This is split between 5 circuits, or 1,920’ per circuit. The field circuits are currently laid out at 6 bores of 320’ each, we propose to change this to 8 bores at an average depth of 240’, giving us the same total vertical bore total as the original design. We would like to present this option to SES for review and consideration.

2. A second option would be to add an additional circuit of 7 bores and add 1 bore to each of the 5 existing, giving us 6 circuits of 7 bores. This would total 42 bores at an average depth of 228’ depth per bore. This option would require a more lateral pipe, larger header and additional trenching.

I am hoping that we could have a phone conversation as soon as you’re available to review and discuss any additional details to consider these solutions.

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Thoughts

The drilling is not causing an underground void. You have encountered an underground void.

And the rule is to back completely off and re-design

 

We have come across something similar in the past.
You may have hit a area of limestone or some other water reactive mineral. As your water and mud mixes with it it can dissolve the formation and cause you lose circulation. You can either drill shallower bores or abandon the site all together.

Another option is double case the hole with a 6" casing or larger to avoid the situation and continue drilling, and have the cuttings come to the surface through the casing. It will get expensive since you will be installing 250' of 6 casing.

I would suggest shorter bores

 

Ground stability is among the reasons a little west of you we tend towards 150' bores.
In our AO glacial activity as well left a wide variety of difficult conditions (near Lakes St. Clair and Erie as well as the Detroit River) around 200', that make the 150 on up policy reliable in almost every circumstance.
Good water tables around here suggests at 150' depth you are in 1-2 or more aquifers which makes performance excellant.

BTW sinkholes are sometimes caused by graders and pavers (poor prep).

 

Not a big deal. We have to redesign the loopfield on the fly on 50% of our drill jobs. MAke sure your flow, pressure drop, pump size etc is still ok.

 

This ground loop has been installed for about a year. At this point I'm looking for suggestions on how to remedy the sinkhole problem. I still haven't seen the site for myself. I'm still waiting for some pictures.

 

Depending on the size of the sink hole your solution will vary but be the same in the end. If it is as big as a volkswagon beetle, you will need to find the actual borehole and grout it shut below the sink. If this proves problematic clean fill sand to seal the cavern, then grout, then compaction.

This is only a big deal if it is your first experiance. Our Maker throws curve balls all the time that the best laid plans and procedures can not account for.
Eric