Ground collapse/sinkhole around well casing

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jbrukardt

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I found a very serious problem at my home the other day, which I have owned about 5 years.

The soil around my well casing has caved in massively. At least 1.5 feet out on all sides of the casing, with maybe a 10 inches of soil as a cap over the hole. Its a massive safety risk, and with enough weight i feel like the whole thing could collapse at any second, or with any weight.

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Videos of the hole, put my cell phone on a tape and lowered it down.


I measured it with a plumb bob, and its 21 feet deep, before stopping at some moist soil.


The details on the well:


Submersible pump well, 60 feet deep, through alternating 10 foot layers of sand and clay per the paperwork, drilled sometime in the 70s.

My first thought was an issue with a leaking pitless adapter eroding the soil when the pipe to the home was under pump pressure. That doesnt appear to be it, I ran the well pump manually (left a hose triggered on) and checked the pitless adapter while the pump was running to see if perhaps it was leaking, and causing erosion. A few drips, but no at-pump-pressure leak. Drips well could have been condensation.

Would a leak way down at the bottom of the well, perhaps a cracked pipe by the submersible pump, cause soil liquifaction and collapse?


My main concerns are as follows:

PVC casing for the well... that seems, not good
Pitless adapter way too shallow (I can reach down and grab it with my hand), I live in maryland, so the frost line is fairly shallow, but still.
Potential column collapse. With the well casing unsupported, thats a lot of weight. Fortunately 2/3s of the casing is still supported, the bottom 40 feet
Danger to persons: I have it cordoned off now, but its a very dangerous situation.


Solutions:

After determining what has caused the erosion:

Fill with bentonite (pricey)
Add a cement cap at the bottom of the 20 feet before i do?
Cheaper option, fill with a aggregate base. Sand? Or gravel?
 
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LLigetfa

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My guess is that surface water going down through the layers of what used to be inpenatrable clay is what is causing the erosion. Hopefully the casing is still sealed at the base and your water supply is not yet in jeopardy. Regardless of price, I think the prudent thing to do is to seal it with bentonite.

I'm sure the well drillers will be along shortly and give good advice.
 

Craigpump

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Call a pump or well guy who has a down hole camera, he can run the camera down the hole and see if the casing is leaking and go from there.

I'd also have the water tested to be sure it is still bacteria free and safe to drink.
 

jbrukardt

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Just ran the calculations for volume, bentonite wont be that terrible i guess, about 1500 bucks for a 21 foot deep by 23 inch wide cylindrical hole
 

jbrukardt

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Call a pump or well guy who has a down hole camera, he can run the camera down the hole and see if the casing is leaking and go from there.

I'd also have the water tested to be sure it is still bacteria free and safe to drink.



I did that myself somewhat, and found no leaks, at least down to 21 feet where the erosion stops. the casing looks dusty and dry, and at the bottom of the eroded pit, its only moist, no standing water.

Now, it could be leaking below the soil line.

And yep, absolutely will get it tested after resolved to make sure i didnt contaminate.
 

Craigpump

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I should have been clearer, run the camera down the inside of the well and look for intrusion into the well itself.

You will want to use Hole Plug, it comes in 3/8 or 3/4" chips. A 50 lbs bag was about $10.00 last time I bought some.
 

jbrukardt

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aha, 10-4. I thought that might be what you meant. I can do that.

Yeah, hole-plug is called "chip bentonite" around here i think, same price per bag. Need 150 bags or so.
 

Justwater

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Back then rotary rigs were just catching on, didn't have grouting laws etc. probably just caved in over time. I wouldn't waste my money on a ton of hole plug. I'd dump a few bags in and fill the rest with fill dirt. jmo
 

LLigetfa

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If I had easy access to river rock or 3/4" stone, I'd be tempted to blend the two. From the purported alternating 10 foot layers of sand and clay, the material could be disappearing in any one of those layers so I would want some sort of seal at each of those clay layers.
 

jbrukardt

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If I had easy access to river rock or 3/4" stone, I'd be tempted to blend the two. From the purported alternating 10 foot layers of sand and clay, the material could be disappearing in any one of those layers so I would want some sort of seal at each of those clay layers.

id considered just filling it with #57 gravel. But this is the home i plan to stay in for a long long time, so i want to ensure i do things right.
 

Smooky

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I would fill the annular space with wet cement grout. Do not fill it with gravel. The gravel will leave space for water and other contamination direct access down the side of your well casing and possibly into your well. Most any well company should be able to grout it or you could grout it yourself. Then you could pour a little concrete slab at the surface.
 

jbrukardt

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I would fill the annular space with wet cement grout. Do not fill it with gravel. The gravel will leave space for water and other contamination direct access down the side of your well casing and possibly into your well. Most any well company should be able to grout it or you could grout it yourself. Then you could pour a little concrete slab at the surface.

understood. Looks like im definitely on the route to a "solid" solution, whether that be hole-plug or a poured grout.
 

Craigpump

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I wouldn't want crushed stone around the pipes, you would be surprised how sharp that can be and the damage it can do to poly pipe. So if you use stone, be sure there is a few inches of either sand or soil around the pipe
 

VAWellDriller

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It's definitely just a poor grout job that has eroded away over time; I see this all the time from certain drillers with poor grouting practices. Cement sounds good, but it can heat up and deform PVC casing if the casing is not cooled, so it's definitely not a homeowner job. As said before, bentonite chips are the easiest and best way for you to do it, or you could call a driller to stop by when they are out doing a grouting job; it wouldn't take long at all to mix and pump in bentonite and would be a lot cheaper than you buying hole plug.
 

LLigetfa

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If using chips, would you also have to mix up a slurry and pour it over the chips to fill the voids? I cannot help but think the voids could cause subsidence. I know any trenches I dig in clay and then put back the same material, if I just fill the trench level, it subsides.
 

jbrukardt

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If using chips, would you also have to mix up a slurry and pour it over the chips to fill the voids? I cannot help but think the voids could cause subsidence. I know any trenches I dig in clay and then put back the same material, if I just fill the trench level, it subsides.

bentonite swells, so that might offset the subsidence? That said, im getting about half a yard extra, so i can refill as it subsides.

Iv'e called 4 well companies here so far to try to find a bentonite source. Three didnt even know what bentonite was, and one quoted me 33 a bag.
 

VAWellDriller

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You're right, the hole plug (bent. chips) will swell up a lot, and make an excellent seal...I can't imagine a drilling company that doesn't know what bentonite is....crazy. Wholesale is about 10/bag here and I'm sure up there too; not sure where in Maryland you are but DSI and Milby Co. are two places that will have it...not sure what the homeowner price is. You get about 5 times more volume for your money if you get someone to mix and pump bentonite, and any reputable driller should have a grout machine that can mix and pump it for you. Actually, the cap of the well may have the name of the company that put the well in, and they should come out and do it for free.
 

jbrukardt

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You're right, the hole plug (bent. chips) will swell up a lot, and make an excellent seal...I can't imagine a drilling company that doesn't know what bentonite is....crazy. Wholesale is about 10/bag here and I'm sure up there too; not sure where in Maryland you are but DSI and Milby Co. are two places that will have it...not sure what the homeowner price is. You get about 5 times more volume for your money if you get someone to mix and pump bentonite, and any reputable driller should have a grout machine that can mix and pump it for you. Actually, the cap of the well may have the name of the company that put the well in, and they should come out and do it for free.

Just called milby right before i got this, got a quote of around 13. Better. Still around 1800 bucks though.

Ill keep calling drilling companies, hopefully can get someone that can mix and pump. like i said, no dice at all so far. Will check the well cap today, see if the company is still around.
 

jbrukardt

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Good news, company that drilled the well is still in business. Will see when they give me a call back if theyre decent. Any particular justification i should use when trying to get them to support their work?
 
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