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working below sea level

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Strictly Surveying

Last Post by bill-c 1 year ago

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john-hamilton

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We had an interesting project yesterday. I had to establish control in a salt mine for installation of a new conveyor. Actually, they were rerouting a conveyor around an area with a weak roof. Surface elevation is about 575 feet (NAVD88), and we were at -1920 feet below that. So about 1340 feet below "sea level". It was a 4 minute ride down in a metal cage, then about 1/4 mile walk. The current mining is taking place about 4 miles further out under one of the great lakes. We do surface leveling (2nd order class I) for subsidence monitoring at this mine, going to do another bluebook level run (about 15 miles) in the fall as required by the state. Interestingly the subsidence at the surface is concentrated in one area and is pretty slow but consistent. We initially did the leveling every year but now only every 5 years. So mining almost 2000 feet down has a mm/year effect at the surface. There is a CORS and water level station above this mine but they appear to be stable

Attachment : image.png

Here is a picture from yesterday

Attachment : image.png

We were given no control to tie into, the project engineer doesn't know of any. They do not have a surveyor on staff, they use an outside survey company, so I'm sure they must have control but we didn't get any. So we scanned the conveyor shown in the background above, and then also at the other end. Their plan is to make a right 90° turn at the point shown, go about 100 feet, then a left 90° down a long passage (450 feet), then a left 90° to tie back in the the existing conveyor (also scanned).

Attachment : image.png

The floor of the mine is basically crushed salt, so we could not set any ground monuments. We set three PK nails in the walls at each end of the long passage, and the idea is that whoever is working down there can resect to get on the coordinate system that we established. There is actually a 65 ppm elevation factor from the surface, but since we have no control I am just making a local coordinate system at elevation -1340.

It was warm (about 78°) and very humid in the mine. I could taste the salt on my lips the rest of the day. Pressure (for the EDM corrections) was 31.42". We had to go through an airlock to get to the work area and my ears popped inside the airlock due to differences in pressure from one side to the other. That was due to the way they maintain air flow in the mine. Good thing is that it is a non-gaseous mine (non explosive) with 40 foot high ceilings. Way different than coal mines I have done work in.

Their production is about 4500 tons per day, so about 1.5 million tons per year. Almost all for road salt

Posted : June 19, 2025 6:24 am

GaryG

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WOW

Posted : June 19, 2025 7:03 am

jhframe

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Interesting project!

I did a tiny bit of work below sea level in the late '70s for a sewer line (108", as I recall) bore under Interstate 5 in the Sacramento Valley. Elevation at the surface was in the plus single digits, and as we leveled down to the bottom of the bore pit it got into the minus single digits. Keeping the level notes straight took a bit of extra concentration.

On that same project there was another bore, a 24" casing for a water line, I think. The contractor had gotten the casing in about 100 feet and was unsure if he was still on line, so my party chief told me to crawl to the far end of the casing to hold a target for line. Even though I was a junior employee at the time, I refused.

Posted : June 19, 2025 7:46 am

squirl

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That's crazy and impressive! This is an example of why I love what we do as surveyors. 😎

T. Nelson - SAM

Posted : June 19, 2025 8:53 am

WA-ID Surveyor

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I don't even like stepping in a ditch, let alone being 2000' feet below ground. Impressive, but not for this guy!!

Posted : June 19, 2025 1:32 pm

Williwaw

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So did you pinch a little salt for some margaritas later?

Just because I'm paranoid, doesn't mean they aren't out to get me.

Posted : June 19, 2025 5:17 pm

Wendell

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Posted by: @williwaw

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So did you pinch a little salt for some margaritas later?

Now I want a margarita.

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Posted : June 19, 2025 11:11 pm

john-hamilton

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It is amazing to me that just this one mine (and there are many others) produce so much salt that is just thrown on the roads and then washes away to the sea.

We have also done subsidence monitoring at another mine nearby that is underneath downtown Cleveland. It also subsides at a small mm/yr rate

Here is a picture of some machinery. Note that they have to disassemble this and take it down in pieces and then reassemble. The tires are about the biggest thing that will fit inside the lift. I have no idea how they get the bucket down there, maybe they remove the cage from the shaft and lower things down by crane

Attachment : image.png

Here is a video about the mine...

Where we were working it was dead quiet, except when crews would drive by in side-by-sides or other equipment. And dark

Posted : June 20, 2025 6:02 am

party-chef

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Thanks for sharing!

I have worked underground a fair amount but never anything close to as deep.

Posted : June 20, 2025 6:32 am

Williwaw

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@john-hamilton There was an article in the WaPo recently about just this issue with all of that salt dumped on roads in winter, washing into and increasing the salinity of many fresh water streams altering the types of plants and organisms that call it home and also impacting fresh water drinking supplies.

https://www.washingtonpost.com/climate-environment/2025/06/04/freshwater-rivers-saltier/

Just because I'm paranoid, doesn't mean they aren't out to get me.

Posted : June 20, 2025 9:45 am