When Abandoned Mines Collapse

[00:01] In December of 2024, a huge sinkhole opened  up on I-80 near Wharton, New Jersey, creating   massive traffic delays as crews worked to figure  out what happened and get it fixed. Since then,  

[00:14] it happened again in February 2025  and then again in March. Each time,   the highway had to be shut down, creating  a nightmare for commuters who had to find   alternate routes. And it’s a nightmare for the  DOT, too, trying to make sure this highway is safe  

[00:29] to drive on despite it literally collapsing  into the earth. From what we know so far,   this is not a natural phenomenon, but one that’s  human-made. It looks like all these issues were   set in motion more than a century ago when the  area had numerous underground iron mines. This  

[00:47] is a really complex issue that causes problems  around the world, and I built a little model   mine in my garage to show you why it’s such a big  deal. I’m Grady and this is Practical Engineering.

[01:12] We’ve been extracting material and minerals  from the earth since way before anyone was   writing things down. It’s probably safe to say  that things started at the surface. You notice   something shiny or differently colored on the  side of a hill or cliff and you take it out.  

[01:28] Over time, we built up knowledge about what  materials were valuable, where they existed,   and how to efficiently extract them from the  earth. But, of course, there’s only so much   earth at the surface. Eventually, you have to  start digging. Maybe you follow a vein of gold,  

[01:44] silver, copper, coal or sulfur down below the  surface. And things start to get more complicated   because now you’re in a hole. And holes are kind  of dangerous. They’re dark, they fill with water,  

[01:56] they can collapse, and they collect dangerous  gases. So, in many cases, even today,   it makes sense to remove the overburden - the  soil and rock above the mineral or material  

[02:08] you’re after. Mining on the surface has a lot  of advantages when it comes to cost and safety. But there are situations where  surface mining isn’t practical.   Removing overburden is expensive, and it  gets more expensive the deeper you go.  

[02:23] It also has environmental impacts like  habitat destruction and pollution of air   and water. So, as technology, safety, and our  understanding of soil and rock mechanics grew,  

[02:35] so did our ability to go straight to the  source and extract minerals underground. One of the major materials that drove the move  to underground mining was coal. It’s usually   found in horizontal formations called seams, that  formed when vast volumes of paleozoic plants were  

[02:51] buried and then crushed and heated over geologic  time. At the start of the Industrial Revolution,   coal quickly became a primary source of  energy for steam engines, steel refining,   and electricity generation. Those  coal seams vary in thickness,  

[03:07] and they vary in depth below the surface too,  so many early coal mines were underground. In the early days of underground mining,  there was not a lot of foresight. Some   might argue that’s still true, but it was a  lot more so a couple hundred years ago. Coal  

[03:22] mining companies weren’t creating detailed  maps of their mines, and even if they did,   there was no central archive to send them  to. And they just weren’t that concerned   about the long-term stability of the mines  once the resources had been extracted. All  

[03:37] that mattered was getting coal out of the  ground. Mining companies came and went,   dissolved or were acquired, and over time, a  lot of information about where mines existed   and their condition was just lost. And even  though many mines were in rural areas, far away  

[03:53] from major population centers, some weren’t,  and some of those rural areas became major   population centers without any knowledge about  what had happened underneath them decades ago.

[04:05] An issue that confounds the problem of  mine subsidence is that in a lot of places,   property ownership is split into two pieces:  surface rights and mineral rights. And those   rights can be owned by different people. So  if you’re a homeowner, you may own the surface  

[04:20] rights to your land, while a company owns the  right to drill or mine under your property.   That doesn’t give them the right to damage  your property, but it does make things more   complicated since you don’t always have a  say in what’s happening beneath the surface.

[04:35] There are myriad ways to build and operate  underground mines, but especially for soft   rock mining, like coal, the predominant method  for decades was called “room and pillar”. This is   exactly what it sounds like. You excavate the ore,  bringing material to the surface. But you leave  

[04:52] columns to support the roof. The size, shape, and  spacing of columns are dictated by the strength of   the material. This is really important because a  mine like this has major fixed costs: exploration,  

[05:05] planning, access, ventilation, and haulage.  It’s important to extract as much as possible,   and every column you leave supporting the roof  is valuable material you can’t recover. So,   there’s often not a lot of margin in  these pillars. They’re as small as  

[05:20] the company thought they could get away  with before they were finished mining. I built a little room and pillar mine in my  garage.  little model is not a rigorous reproduction of  an actual geologic formation. My coal seam is  

[05:37] just made of cardboard, and the bright colors  are just for fun. But, I’m hoping this can help   illustrate the challenges associated with this  type of mine. I’ve got a little rainfall simulator set  up, because water plays a big role in these processes.

[05:52] This first rainfall isn’t  necessarily representative of real life,   since it’s really just compacting the loose sand.  But it does give a nice image of how subsidence   works in general. You can see the surface of the  ground sinking as the sand compacts into place.

[06:07] But you can also see that as the water reaches  the mine, things start to deform. In a real mine,   this is true, too. Stresses in  the surrounding soil and rock   redistribute over time from long-term  movements, relaxation of stresses that  

[06:22] were already built up in the materials before  extraction, and from water. I ran this model for an entire day, turning  the rainfall on and off to simulate a somewhat   natural progression of time in the subsurface.  By the end of the day, the mine hadn’t collapsed,  

[06:39] but it was looking a great deal less stable than  when it started. And that’s one big thing you can   learn from this model - in a lot of cases, these  issues aren’t linearly progressive. They can   happen in fits and starts, like this small leak  in the roof of the mine. You get a little bit of  

[06:55] erosion of soil, but eventually, enough sand  built up that it kind of healed itself, and,   for a while, you can’t see any evidence  of any of it at the surface. The geology   essentially absorbed the sinkhole by  redistributing materials and stresses  

[07:10] so there’s no obvious sign at the surface  that anything wayward is happening below. In the US, there were very few regulations  on mining until the late 19th century,   and even those focused primarily on  safety of the workers. There just wasn’t that  

[07:24] much concern about long-term stability.  So as soon as material was extracted,   mines were abandoned. The already  iffy columns were just left alone,   and no one wasted resources on additional  supports or shoring. They just walked away.

[07:39] One thing that happens when mines are abandoned is  that they flood. Without the need to work inside,   the companies stop pumping out the water. I can  simulate this on my model by just plugging up   the drain. In a real soft rock mine, there can  be minerals like gypsum and limestone that are  

[07:56] soluble in water. Repeated cycles of drying and  wetting can slowly dissolve them away. Water can   also soften certain materials and soils, reducing  their mechanical strength to withstand heavy   loads, just like my cardboard model. And then,  of course, water simply causes erosion. It can  

[08:13] literally carry soil particles with it, again,  causing voids and redistribution of stresses   in the subsurface. This is footage from an old  video I did demonstrating how sinkholes can form. The ways that mine subsidence propagates to the  surface can vary a lot, based on the geology and  

[08:30] depth of the mine. For collapses near the surface,  you often see well-defined sinkholes where the   soil directly above the mine simply falls into the  void. And this is usually a sudden phenomenon. I  

[08:43] flooded and drained my little mine a few times  to demonstrate this. Accidentally flooded my   little town a few times in the process, but  that’s okay. You can see in my model, after   flooding the mine and draining it down, there was  a partial failure in the roof and a pile of sand  

[08:57] toward the back caved in. And on the surface, you  see just a small sinkhole. opened right in the center of a sports complex in  Alton, Illinois. It was quickly determined that  

[09:10] part of an active underground aggregate mine below  the park had collapsed, leading to the sinkhole.   It’s pretty characteristic of these issues.  You don’t know where they’re going to happen,   and you don’t know how the surface soils are  going to react to what’s happening underneath.

[09:24] Subsidence can also look like a generalized and  broader sinking and settling over a large area.   You can see in my model that most of the surface  still looks pretty flat, despite the fact that it  

[09:36] started here and is now down here as the mine  supports have softened and deformed. This can   also be the case when mines are deeper in  the ground. Even if the collapse is sudden,   the subsidence is less dramatic because the  geology can shift and move to redistribute  

[09:53] the stresses. And the subsidence happens more  slowly as the overburden settles into a new   configuration. In all cases, the subsidence can  extend laterally from the mine, so impacted areas   aren’t always directly above. The deeper  the mine, the wider the subsidence can be.

[10:09] I ran my little mine demo for quite a few  cycles of wet and dry just to see how bad   to speed things along. Let’s say this is a  simulation of an earthquake on an abandoned  

[10:23] mine. You can see that by the end  of it, this thing has basically collapsed. And take a look at the surface now. You have some  defined sinkholes for sure. And you also have just   generalized subsidence - sloped and wavy areas  that were once level. And you can imagine the  

[10:40] problems this can cause. Structures can easily be  damaged by differential settlement. Pipes break.   Foundations shift and crack. Even water can drain  differently than before, causing ponding and even  

[10:52] changing the course of rivers and streams for  large areas. And even if there are no structures,   subsidence can ruin high-value farm land,  mess up roads, disrupt habitat, and more. In many cases, the company that caused all the  damage is long gone. Essentially they set a  

[11:08] ticking time bomb deep below the ground with no  one knowing if or when it would go off. There’s   no one to hold accountable for it, and there’s  very little recourse for property owners. Typical   property insurance specifically excludes damage  from mine subsidence. So, in some places where  

[11:24] this is a real threat, government-subsidized  insurance programs have been put in place.   Eight states in the US, those where coal mining  was most extensive, have insurance pools set up.  

[11:36] In a few of those states, it is a requirement  in order to own property. The federal government   in the US also collects a fee from coal  mines that goes into a fund that helps   cover reclamation costs of mines abandoned  before 1977 when the law went into effect.

[11:53] That federal mining act also required modern  mines to use methods to prevent subsidence,   or control its effects, because this isn’t just a  problem with historic abandoned mines. Some modern  

[12:06] underground soft rock mining doesn’t use the room  and pillar method but instead a process called   longwall mining. Like everything in mining, there  are multiple ways to do it. But here’s the basic   method: Hydraulic jacks support the roof of the  mine in a long line. A machine called a shearer  

[12:23] travels along the face of the seam with cutting  drums. The cut coal falls onto a conveyor and is   transported to the surface. The roof supports  move forward into the newly created cavity,  

[12:36] intentionally allowing the roof behind them to  collapse. It’s an incredibly efficient form of   mining, and you get to take the whole seam, rather  than leaving pillars behind to support the roof.   But, obviously, in this method, subsidence  at the surface is practically inevitable.

[12:55] Minimizing the harm that subsidence creates starts  just by predicting its extent and magnitude. And,   problem to solve. Engineers use  a mix of empirical information,  

[13:09] like data from similar past mining operations,  geotechnical data, simplified relationships,   and in some cases detailed numerical modeling  that accounts for geologic and water movement  

[13:22] over time. But you don’t just have to predict  it. You also have to measure it to see if your   predictions were right. So mining companies use  instruments like inclinometers and extensometers   above underground mines to track how they  affect the surface. I have a whole video  

[13:38] about that kind of instrumentation  if you want to learn more after this. The last part of that is reclamation - to repair  or mitigate the damage that’s been done. And this   can vary so much depending on where the mine  is, what’s above it, and how much subsidence  

[13:53] occurs. It can be as simple as filling and  grading land that has subsided all the way to   extensive structural retrofits to buildings  above a mine before extraction even starts.  

[14:05] Sinkholes are often repaired by backfilling  with layers of different-sized materials,   from large at the bottom to small at top. That  creates a filter to keep soil from continuing to   erode downward into the void. Larger  voids can be filled with grout or even  

[14:21] polyurethane foam to stabilize the ground above,  reducing the chance for a future collapse. I know coal - and mining in general - can be a  sensitive topic. Most of us don’t have a lot of  

[14:33] exposure to everything that goes into obtaining  the raw resources that make modern life possible.   And the things we do see and hear are usually  bad things like negative environmental impacts   or subsidence. But I really think the  story of subsidence isn’t just one of  

[14:49] “mining is bad” but really “mining used  to be bad, and now it’s a lot better,   but there are still challenges to overcome.”  I guess that’s the story of so many things   in engineering - addressing the difficulties  we used to just ignore. And this video isn’t  

[15:04] meant to fearmonger. This is a real  issue that causes real damages today,   but it’s also an issue that a lot of  people put a great deal of thought,   effort, and ultimately resources into so that we  can strike a balance between protection against  

[15:20] damage to property and the environment and  obtaining the resources that we all depend on. Mining isn’t exactly construction, but it  is construction adjacent: heavy machinery,  

[15:32] hard work, and lots of consideration of  geology. It’s a fascinating industry that   forms the backbone of of modern society, and  we don’t really get to see much about how it   works. My friend Sam from Wendover Productions  put together this awesome documentary about coal  

[15:49] mining that really gives you a peek behind  the scenes in his “Logistics of X” series.   These videos are so good - just deep dives into  various industries and how they actually work.   I had no idea that so much of the  US coal supply comes from a single  

[16:04] county. And if you want to check it  out, it’s only available on Nebula. You’ve heard me talk about Nebula before. It’s  a streaming service built by and for independent   creators, including a lot of my favorites like  Neo, Wendover Productions, the Coding Train,  

[16:19] and Branch Education. I don’t know about you, but  independently-produced content is most of what I   watch these days. I just like the authenticity  and thoughtfulness of videos that haven’t been   through a writer's room and ten levels of studio  executives. Someone said Nebula’s like Netflix  

[16:35] for people who love trains. And I like that  comparison, not just because I also love trains. Nebula’s totally ad-free, with tons  of excellent channels and lots of   original series and specials like the  Logistics of X. It’s also a great gift,  

[16:49] especially because a yearly membership  is 40% of the link in the description.   My videos go live on Nebula before they come out  on YouTube. If you’re with me that independent   creators are the future of great video, I  hope you’ll consider subscribing. That’s  

[17:04] go.nebula.tv/Practical-Engineering. Thank you  for watching, and let me know what you think!