[00:01] In December of 2024, a huge sinkhole opened&nbsp; up on I-80 near Wharton, New Jersey, creating&nbsp;&nbsp; massive traffic delays as crews worked to figure&nbsp; out what happened and get it fixed. Since then,&nbsp;&nbsp;
[00:14] it happened again in February 2025&nbsp; and then again in March. Each time,&nbsp;&nbsp; the highway had to be shut down, creating&nbsp; a nightmare for commuters who had to find&nbsp;&nbsp; alternate routes. And it’s a nightmare for the&nbsp; DOT, too, trying to make sure this highway is safe&nbsp;&nbsp;
[00:29] to drive on despite it literally collapsing&nbsp; into the earth. From what we know so far,&nbsp;&nbsp; this is not a natural phenomenon, but one that’s&nbsp; human-made. It looks like all these issues were&nbsp;&nbsp; set in motion more than a century ago when the&nbsp; area had numerous underground iron mines. This&nbsp;&nbsp;
[00:47] is a really complex issue that causes problems&nbsp; around the world, and I built a little model&nbsp;&nbsp; mine in my garage to show you why it’s such a big&nbsp; deal. I’m Grady and this is Practical Engineering.
[01:12] We’ve been extracting material and minerals&nbsp; from the earth since way before anyone was&nbsp;&nbsp; writing things down. It’s probably safe to say&nbsp; that things started at the surface. You notice&nbsp;&nbsp; something shiny or differently colored on the&nbsp; side of a hill or cliff and you take it out.&nbsp;&nbsp;
[01:28] Over time, we built up knowledge about what&nbsp; materials were valuable, where they existed,&nbsp;&nbsp; and how to efficiently extract them from the&nbsp; earth. But, of course, there’s only so much&nbsp;&nbsp; earth at the surface. Eventually, you have to&nbsp; start digging. Maybe you follow a vein of gold,&nbsp;&nbsp;
[01:44] silver, copper, coal or sulfur down below the&nbsp; surface. And things start to get more complicated&nbsp;&nbsp; because now you’re in a hole. And holes are kind&nbsp; of dangerous. They’re dark, they fill with water,&nbsp;&nbsp;
[01:56] they can collapse, and they collect dangerous&nbsp; gases. So, in many cases, even today,&nbsp;&nbsp; it makes sense to remove the overburden - the&nbsp; soil and rock above the mineral or material&nbsp;&nbsp;
[02:08] you’re after. Mining on the surface has a lot&nbsp; of advantages when it comes to cost and safety. But there are situations where&nbsp; surface mining isn’t practical.&nbsp;&nbsp; Removing overburden is expensive, and it&nbsp; gets more expensive the deeper you go.&nbsp;&nbsp;
[02:23] It also has environmental impacts like&nbsp; habitat destruction and pollution of air&nbsp;&nbsp; and water. So, as technology, safety, and our&nbsp; understanding of soil and rock mechanics grew,&nbsp;&nbsp;
[02:35] so did our ability to go straight to the&nbsp; source and extract minerals underground. One of the major materials that drove the move&nbsp; to underground mining was coal. It’s usually&nbsp;&nbsp; found in horizontal formations called seams, that&nbsp; formed when vast volumes of paleozoic plants were&nbsp;&nbsp;
[02:51] buried and then crushed and heated over geologic&nbsp; time. At the start of the Industrial Revolution,&nbsp;&nbsp; coal quickly became a primary source of&nbsp; energy for steam engines, steel refining,&nbsp;&nbsp; and electricity generation. Those&nbsp; coal seams vary in thickness,&nbsp;&nbsp;
[03:07] and they vary in depth below the surface too,&nbsp; so many early coal mines were underground. In the early days of underground mining,&nbsp; there was not a lot of foresight. Some&nbsp;&nbsp; might argue that’s still true, but it was a&nbsp; lot more so a couple hundred years ago. Coal&nbsp;&nbsp;
[03:22] mining companies weren’t creating detailed&nbsp; maps of their mines, and even if they did,&nbsp;&nbsp; there was no central archive to send them&nbsp; to. And they just weren’t that concerned&nbsp;&nbsp; about the long-term stability of the mines&nbsp; once the resources had been extracted. All&nbsp;&nbsp;
[03:37] that mattered was getting coal out of the&nbsp; ground. Mining companies came and went,&nbsp;&nbsp; dissolved or were acquired, and over time, a&nbsp; lot of information about where mines existed&nbsp;&nbsp; and their condition was just lost. And even&nbsp; though many mines were in rural areas, far away&nbsp;&nbsp;
[03:53] from major population centers, some weren’t,&nbsp; and some of those rural areas became major&nbsp;&nbsp; population centers without any knowledge about&nbsp; what had happened underneath them decades ago.
[04:05] An issue that confounds the problem of&nbsp; mine subsidence is that in a lot of places,&nbsp;&nbsp; property ownership is split into two pieces:&nbsp; surface rights and mineral rights. And those&nbsp;&nbsp; rights can be owned by different people. So&nbsp; if you’re a homeowner, you may own the surface&nbsp;&nbsp;
[04:20] rights to your land, while a company owns the&nbsp; right to drill or mine under your property.&nbsp;&nbsp; That doesn’t give them the right to damage&nbsp; your property, but it does make things more&nbsp;&nbsp; complicated since you don’t always have a&nbsp; say in what’s happening beneath the surface.
[04:35] There are myriad ways to build and operate&nbsp; underground mines, but especially for soft&nbsp;&nbsp; rock mining, like coal, the predominant method&nbsp; for decades was called “room and pillar”. This is&nbsp;&nbsp; exactly what it sounds like. You excavate the ore,&nbsp; bringing material to the surface. But you leave&nbsp;&nbsp;
[04:52] columns to support the roof. The size, shape, and&nbsp; spacing of columns are dictated by the strength of&nbsp;&nbsp; the material. This is really important because a&nbsp; mine like this has major fixed costs: exploration,&nbsp;&nbsp;
[05:05] planning, access, ventilation, and haulage.&nbsp; It’s important to extract as much as possible,&nbsp;&nbsp; and every column you leave supporting the roof&nbsp; is valuable material you can’t recover. So,&nbsp;&nbsp; there’s often not a lot of margin in&nbsp; these pillars. They’re as small as&nbsp;&nbsp;
[05:20] the company thought they could get away&nbsp; with before they were finished mining. I built a little room and pillar mine in my&nbsp; garage.&nbsp; little model is not a rigorous reproduction of&nbsp; an actual geologic formation. My coal seam is&nbsp;&nbsp;
[05:37] just made of cardboard, and the bright colors&nbsp; are just for fun. But, I’m hoping this can help&nbsp;&nbsp; illustrate the challenges associated with this&nbsp; type of mine. I’ve&nbsp;got a little rainfall simulator set&nbsp; up, because water plays a big role in these processes.
[05:52] This first rainfall isn’t&nbsp; necessarily representative of real life,&nbsp;&nbsp; since it’s really just compacting the loose sand.&nbsp; But it does give a nice image of how subsidence&nbsp;&nbsp; works in general. You can see the surface of the&nbsp; ground sinking as the sand compacts into place.
[06:07] But you can also see that as the water reaches&nbsp; the mine, things start to deform. In a real mine,&nbsp;&nbsp; this is true, too. Stresses in&nbsp; the surrounding soil and rock&nbsp;&nbsp; redistribute over time from long-term&nbsp; movements, relaxation of stresses that&nbsp;&nbsp;
[06:22] were already built up in the materials before&nbsp; extraction, and from water. I ran this model for an entire day, turning&nbsp; the rainfall on and off to simulate a somewhat&nbsp;&nbsp; natural progression of time in the subsurface.&nbsp; By the end of the day, the mine hadn’t collapsed,&nbsp;&nbsp;
[06:39] but it was looking a great deal less stable than&nbsp; when it started. And that’s one big thing you can&nbsp;&nbsp; learn from this model - in a lot of cases, these&nbsp; issues aren’t linearly progressive. They can&nbsp;&nbsp; happen in fits and starts, like this small leak&nbsp; in the roof of the mine. You get a little bit of&nbsp;&nbsp;
[06:55] erosion of soil, but eventually, enough sand&nbsp; built up that it kind of healed itself, and,&nbsp;&nbsp; for a while, you can’t see any evidence&nbsp; of any of it at the surface. The geology&nbsp;&nbsp; essentially absorbed the sinkhole by&nbsp; redistributing materials and stresses&nbsp;&nbsp;
[07:10] so there’s no obvious sign at the surface&nbsp; that anything wayward is happening below. In the US, there were very few regulations&nbsp; on mining until the late 19th century,&nbsp;&nbsp; and even those focused primarily on&nbsp; safety of the workers. There just wasn’t that&nbsp;&nbsp;
[07:24] much concern about long-term stability.&nbsp; So as soon as material was extracted,&nbsp;&nbsp; mines were abandoned. The already&nbsp; iffy columns were just left alone,&nbsp;&nbsp; and no one wasted resources on additional&nbsp; supports or shoring. They just walked away.
[07:39] One thing that happens when mines are abandoned is&nbsp; that they flood. Without the need to work inside,&nbsp;&nbsp; the companies stop pumping out the water. I can&nbsp; simulate this on my model by just plugging up&nbsp;&nbsp; the drain. In a real soft rock mine, there can&nbsp; be minerals like gypsum and limestone that are&nbsp;&nbsp;
[07:56] soluble in water. Repeated cycles of drying and&nbsp; wetting can slowly dissolve them away. Water can&nbsp;&nbsp; also soften certain materials and soils, reducing&nbsp; their mechanical strength to withstand heavy&nbsp;&nbsp; loads, just like my cardboard model. And then,&nbsp; of course, water simply causes erosion. It can&nbsp;&nbsp;
[08:13] literally carry soil particles with it, again,&nbsp; causing voids and redistribution of stresses&nbsp;&nbsp; in the subsurface. This is footage from an old&nbsp; video I did demonstrating how sinkholes can form. The ways that mine subsidence propagates to the&nbsp; surface can vary a lot, based on the geology and&nbsp;&nbsp;
[08:30] depth of the mine. For collapses near the surface,&nbsp; you often see well-defined sinkholes where the&nbsp;&nbsp; soil directly above the mine simply falls into the&nbsp; void. And this is usually a sudden phenomenon. I&nbsp;&nbsp;
[08:43] flooded and drained my little mine a few times&nbsp; to demonstrate this. Accidentally flooded my&nbsp;&nbsp; little town a few times in the process, but&nbsp; that’s okay. You can see in my model, after&nbsp;&nbsp; flooding the mine and draining it down, there was&nbsp; a partial failure in the roof and a pile of sand&nbsp;&nbsp;
[08:57] toward the back caved in. And on the surface, you&nbsp; see just a small sinkhole. opened right in the center of a sports complex in&nbsp; Alton, Illinois. It was quickly determined that&nbsp;&nbsp;
[09:10] part of an active underground aggregate mine below&nbsp; the park had collapsed, leading to the sinkhole.&nbsp;&nbsp; It’s pretty characteristic of these issues.&nbsp; You don’t know where they’re going to happen,&nbsp;&nbsp; and you don’t know how the surface soils are&nbsp; going to react to what’s happening underneath.
[09:24] Subsidence can also look like a generalized and&nbsp; broader sinking and settling over a large area.&nbsp;&nbsp; You can see in my model that most of the surface&nbsp; still looks pretty flat, despite the fact that it&nbsp;&nbsp;
[09:36] started here and is now down here as the mine&nbsp; supports have softened and deformed. This can&nbsp;&nbsp; also be the case when mines are deeper in&nbsp; the ground. Even if the collapse is sudden,&nbsp;&nbsp; the subsidence is less dramatic because the&nbsp; geology can shift and move to redistribute&nbsp;&nbsp;
[09:53] the stresses. And the subsidence happens more&nbsp; slowly as the overburden settles into a new&nbsp;&nbsp; configuration. In all cases, the subsidence can&nbsp; extend laterally from the mine, so impacted areas&nbsp;&nbsp; aren’t always directly above. The deeper&nbsp; the mine, the wider the subsidence can be.
[10:09] I ran my little mine demo for quite a few&nbsp; cycles of wet and dry just to see how bad&nbsp;&nbsp; to speed things along. Let’s say this is a&nbsp; simulation of an earthquake on an abandoned&nbsp;&nbsp;
[10:23] mine. You can see that by the end&nbsp; of it, this thing has basically collapsed. And take a look at the surface now. You have some&nbsp; defined sinkholes for sure. And you also have just&nbsp;&nbsp; generalized subsidence - sloped and wavy areas&nbsp; that were once level. And you can imagine the&nbsp;&nbsp;
[10:40] problems this can cause. Structures can easily be&nbsp; damaged by differential settlement. Pipes break.&nbsp;&nbsp; Foundations shift and crack. Even water can drain&nbsp; differently than before, causing ponding and even&nbsp;&nbsp;
[10:52] changing the course of rivers and streams for&nbsp; large areas. And even if there are no structures,&nbsp;&nbsp; subsidence can ruin high-value farm land,&nbsp; mess up roads, disrupt habitat, and more. In many cases, the company that caused all the&nbsp; damage is long gone. Essentially they set a&nbsp;&nbsp;
[11:08] ticking time bomb deep below the ground with no&nbsp; one knowing if or when it would go off. There’s&nbsp;&nbsp; no one to hold accountable for it, and there’s&nbsp; very little recourse for property owners. Typical&nbsp;&nbsp; property insurance specifically excludes damage&nbsp; from mine subsidence. So, in some places where&nbsp;&nbsp;
[11:24] this is a real threat, government-subsidized&nbsp; insurance programs have been put in place.&nbsp;&nbsp; Eight states in the US, those where coal mining&nbsp; was most extensive, have insurance pools set up.&nbsp;&nbsp;
[11:36] In a few of those states, it is a requirement&nbsp; in order to own property. The federal government&nbsp;&nbsp; in the US also collects a fee from coal&nbsp; mines that goes into a fund that helps&nbsp;&nbsp; cover reclamation costs of mines abandoned&nbsp; before 1977 when the law went into effect.
[11:53] That federal mining act also required modern&nbsp; mines to use methods to prevent subsidence,&nbsp;&nbsp; or control its effects, because this isn’t just a&nbsp; problem with historic abandoned mines. Some modern&nbsp;&nbsp;
[12:06] underground soft rock mining doesn’t use the room&nbsp; and pillar method but instead a process called&nbsp;&nbsp; longwall mining. Like everything in mining, there&nbsp; are multiple ways to do it. But here’s the basic&nbsp;&nbsp; method: Hydraulic jacks support the roof of the&nbsp; mine in a long line. A machine called a shearer&nbsp;&nbsp;
[12:23] travels along the face of the seam with cutting&nbsp; drums. The cut coal falls onto a conveyor and is&nbsp;&nbsp; transported to the surface. The roof supports&nbsp; move forward into the newly created cavity,&nbsp;&nbsp;
[12:36] intentionally allowing the roof behind them to&nbsp; collapse. It’s an incredibly efficient form of&nbsp;&nbsp; mining, and you get to take the whole seam, rather&nbsp; than leaving pillars behind to support the roof.&nbsp;&nbsp; But, obviously, in this method, subsidence&nbsp; at the surface is practically inevitable.
[12:55] Minimizing the harm that subsidence creates starts&nbsp; just by predicting its extent and magnitude. And,&nbsp;&nbsp; problem to solve. Engineers use&nbsp; a mix of empirical information,&nbsp;&nbsp;
[13:09] like data from similar past mining operations,&nbsp; geotechnical data, simplified relationships,&nbsp;&nbsp; and in some cases detailed numerical modeling&nbsp; that accounts for geologic and water movement&nbsp;&nbsp;
[13:22] over time. But you don’t just have to predict&nbsp; it. You also have to measure it to see if your&nbsp;&nbsp; predictions were right. So mining companies use&nbsp; instruments like inclinometers and extensometers&nbsp;&nbsp; above underground mines to track how they&nbsp; affect the surface. I have a whole video&nbsp;&nbsp;
[13:38] about that kind of instrumentation&nbsp; if you want to learn more after this. The last part of that is reclamation - to repair&nbsp; or mitigate the damage that’s been done. And this&nbsp;&nbsp; can vary so much depending on where the mine&nbsp; is, what’s above it, and how much subsidence&nbsp;&nbsp;
[13:53] occurs. It can be as simple as filling and&nbsp; grading land that has subsided all the way to&nbsp;&nbsp; extensive structural retrofits to buildings&nbsp; above a mine before extraction even starts.&nbsp;&nbsp;
[14:05] Sinkholes are often repaired by backfilling&nbsp; with layers of different-sized materials,&nbsp;&nbsp; from large at the bottom to small at top. That&nbsp; creates a filter to keep soil from continuing to&nbsp;&nbsp; erode downward into the void. Larger&nbsp; voids can be filled with grout or even&nbsp;&nbsp;
[14:21] polyurethane foam to stabilize the ground above,&nbsp; reducing the chance for a future collapse. I know coal - and mining in general - can be a&nbsp; sensitive topic. Most of us don’t have a lot of&nbsp;&nbsp;
[14:33] exposure to everything that goes into obtaining&nbsp; the raw resources that make modern life possible.&nbsp;&nbsp; And the things we do see and hear are usually&nbsp; bad things like negative environmental impacts&nbsp;&nbsp; or subsidence. But I really think the&nbsp; story of subsidence isn’t just one of&nbsp;&nbsp;
[14:49] “mining is bad” but really “mining used&nbsp; to be bad, and now it’s a lot better,&nbsp;&nbsp; but there are still challenges to overcome.”&nbsp; I guess that’s the story of so many things&nbsp;&nbsp; in engineering - addressing the difficulties&nbsp; we used to just ignore. And this video isn’t&nbsp;&nbsp;
[15:04] meant to fearmonger. This is a real&nbsp; issue that causes real damages today,&nbsp;&nbsp; but it’s also an issue that a lot of&nbsp; people put a great deal of thought,&nbsp;&nbsp; effort, and ultimately resources into so that we&nbsp; can strike a balance between protection against&nbsp;&nbsp;
[15:20] damage to property and the environment and&nbsp; obtaining the resources that we all depend on. Mining isn’t exactly construction, but it&nbsp; is construction adjacent: heavy machinery,&nbsp;&nbsp;
[15:32] hard work, and lots of consideration of&nbsp; geology. It’s a fascinating industry that&nbsp;&nbsp; forms the backbone of of modern society, and&nbsp; we don’t really get to see much about how it&nbsp;&nbsp; works. My friend Sam from Wendover Productions&nbsp; put together this awesome documentary about coal&nbsp;&nbsp;
[15:49] mining that really gives you a peek behind&nbsp; the scenes in his “Logistics of X” series.&nbsp;&nbsp; These videos are so good - just deep dives into&nbsp; various industries and how they actually work.&nbsp;&nbsp; I had no idea that so much of the&nbsp; US coal supply comes from a single&nbsp;&nbsp;
[16:04] county. And if you want to check it&nbsp; out, it’s only available on Nebula. You’ve heard me talk about Nebula before. It’s&nbsp; a streaming service built by and for independent&nbsp;&nbsp; creators, including a lot of my favorites like&nbsp; Neo, Wendover Productions, the Coding Train,&nbsp;&nbsp;
[16:19] and Branch Education. I don’t know about you, but&nbsp; independently-produced content is most of what I&nbsp;&nbsp; watch these days. I just like the authenticity&nbsp; and thoughtfulness of videos that haven’t been&nbsp;&nbsp; through a writer's room and ten levels of studio&nbsp; executives. Someone said Nebula’s like Netflix&nbsp;&nbsp;
[16:35] for people who love trains. And I like that&nbsp; comparison, not just because I also love trains. Nebula’s totally ad-free, with tons&nbsp; of excellent channels and lots of&nbsp;&nbsp; original series and specials like the&nbsp; Logistics of X. It’s also a great gift,&nbsp;&nbsp;
[16:49] especially because a yearly membership&nbsp; is 40% of the link in the description.&nbsp;&nbsp; My videos go live on Nebula before they come out&nbsp; on YouTube. If you’re with me that independent&nbsp;&nbsp; creators are the future of great video, I&nbsp; hope you’ll consider subscribing. That’s&nbsp;&nbsp;
[17:04] go.nebula.tv/Practical-Engineering. Thank you&nbsp; for watching, and let me know what you think!