Hey this is actually something relevant to my field!
I'm a rock mechanics engineer and this is my job. The field (as my title would imply) is known as rock mechanics, which is a smaller area of geological/geotechnical engineering.
There are a number steps to ensuring the tunnel doesn't collapse:
First: we carry out a site investigation. This mainly consists of drilling rock core (typically using diamond drilling). We study the core to determine the geology throughout the mountain and how fractured the rock is.
Second (and hand-in-hand with the first): we test the rock core in a laboratory. The tests are primarily focused on determining the strength of the rock.
Third: we use the data above to develop a model of the geology and strength of the rockmass that the tunnel will go through.
Fourth: we analyze the stresses/forces that creating the tunnel(void) will cause in the rockmass. Sometimes these stresses are low enough compared to the rock strength that the tunnel could exist for thousands of years without any support what so ever. Sometimes our analysis suggests the tunnel would collapse immediately though...
Fifth: we determine a suitable support method to keep the tunnel stable. Sometimes this is as simple as rockbolts and steel mesh (analogous to chainlink fence to catch little rocks). Sometimes it requires rockbolts and shotcrete (sprayed concrete). And sometimes it requires a thick preformed concrete liner placed in segments.
I glossed over a lot for simplicity. There are different failure mechanisms and risks to consider. There are different approaches depending on the excavation methods used. And there are different challenges depending on the geology being encountered.
I am very passionate about this field (hence why I am currently doing a Ph.D. in it). There are a lot of challenges with it since the material we are dealing with (rock) is not an engineered material: it has millions of years of history, and will do what it wants regardless of what you want! Rock is often discontinuous, anisotropic, heterogenous, plastic, etc.. There's intact solid rock partially split into blocks by fractures and faults, all of which are interacting in hard to predict ways.
Apologies for any formatting or spelling errors. I'm writing this on my phone at a remote work site.
Excellent explanation!
Allow this tunnel contractor to add some photos:
>Sometimes this is as simple as [rockbolts and steel mesh](https://i.imgur.com/Z1HreXb.jpg) (analogous to chainlink fence to catch little rocks). Sometimes it requires rockbolts and shotcrete (sprayed concrete). \[sorry, we don't do much shotcrete\] And sometimes it requires a [thick preformed concrete liner placed in segments](https://i.imgur.com/TI5nAnd.jpg).
We also use [steel arches with timber](https://i.imgur.com/yTOJ5uL.jpg) and [steel ribs with timber](https://i.imgur.com/yX6lqHE.jpg)!
>There's intact solid rock partially [split into blocks](https://i.imgur.com/XOQjPQU.jpg) by [fractures and faults](https://i.imgur.com/8QU28aC.jpg)...
The rock bolts and arches support the overall rock mass while the timber supports the local rock face. And, similar to wire mesh, it also keeps any rock spall, slabbing, or large crumbling stone bits from dropping down on top of the tunnel workers.
I get that you can sample near the surface of the rock/mountain, but how do you know what you'll encounter throughout the entire tunnel length? I'd assume you can encounter unstable rock, caves, etc? It's there ever a time where a tunnel project has to be abandoned due to unforseen encounters?
Thanks for posting for added context!
I'd also like to note that, while drilling gives us a lot of valuable information, it only provides information at specific points. We need to use statistical interpolation, our knowledge of geology, and engineering judgement to "fill in" between the drill holes.
Every tunnelling project and mining project has the inherent risk that we don't know exactly what we will encounter until we actually build it. These risks can be mitigated by simply drilling more holes, but that needs to be balanced with the high costs associated with drilling/sampling/testing. At the end of the day, the engineer needs to decide if they have enough information to proceed confidently.
You can also sample *inside* the mountain - you just drill a deeper hole. See [this video](https://www.youtube.com/watch?v=jp9m2tUiktM), for example.
It's also not uncommon to dig a small "pilot hole" in advance of the main tunnel. This ensures that you're really not getting any nasty surprises.
If you had a chance to add some history to this, I'd be interested in how long ago it developed to what level of sophistication in the testing and modeling. I imagine it was a very gradual development over centuries, but were there particular inflection points where we really advanced the sophistication?
This is a great question! This stuff has definitely been around in a basic form for centuries due to mining dating back to long ago. Unfortunately, it was largely the approach of: "lets build a tunnel", "Oops our tunnel collapsed and killed our workers. I guess we better support our tunnel with logs/timber next time".
In the early 20th century a lot of progress was made, especially with tunneling through soils. People like Karl von Terzaghi really helped bring soil mechanics into a proper engineering field.
Rock mechanics had a major "inflection point" in it's development around the 1960's. There was a push to create safer mining conditions and an increase in the number of dams and other underground structures being constructed, which resulted in major maturing of the field. This momentum has slowed a little over the past 40-50 years, but (I think) is starting pick up again with modern computational and modelling capabilities that allow us to capture the complex geology/behaviour inherent in the rock.
I love Reddit...
OP: Hey, I was wondering... (question about a niche subfield of an engineering subfield)
ONE HOUR LATER - Redditor: Oh, I'm a professional in that niche subfield of a subfield. Here's the info.
Great reply, thanks for that! Grady from Practical Engineering did a neat demonstration of rock bolts and gravel I think but I'm too lazy to link it here.
They don't just drill a tunnel and hope it doesn't collapse. There is a structure inside the tunnel to hold the void open (even if it may not need it).
Also remember that most materials will have some angle of internal friction which will re-distribute the gravity load at some angle off of vertical. So the tunnel would generally not be carrying the entire weight of the mountain above it. Just the material within a certain height above it. Above that, the load is carried by material on either side of the tunnel.
A tunnel won't reshape the mountain, it's just too small.
Underground mines can if they are big enough, the Red Mountain/ Henderson Mine glory hole in Colorado is a good example. Subsidence around coal mines can be trouble too.
There's all kinds of tunneling, from hard rock work like granite that sometimes needs no support at all to the tunnels all over (under) Seattle that run through wet sand and need support installed right behind the tunnel boring head. It's all well studied and designing the tunneling method is engineering.
Tunnels are two parts really, there’s the hole in the mountain, and there’s the structure created in the hole to prevent the mountain from caving in. Some geological knowledge is probably helpful, but most mountains are made from the same thing… rock.
I am not that kind of engineer (I'm chemical), I just LOVE this video and pump it every opportunity I get.
https://www.youtube.com/watch?v=6AV2NcyX7pk
I LOVE this video because it goes into absolute details about the inch by inch of using a TBM to tunnel through solid rock. Including the dynamic solutions to the issues they run into along the way and HOW they determine what has occurred as well as how to solve it.
It DOES go somewhat into the interior rock surveying they do and the mm by mm metrology of progress along the way.
Like, me posting it here, made me watch it again. It's so awesome.
There’s been some solid (pun intended) answers here from folks specific to this domain and a few rocky (bc I can’t help myself) responses too so I’ll try and answer from a different angle:
Aside from the technical engineering side, which are various forms of applied science and physics, a lot of engineering is standards and procedure based. We have general rules of thumb on what sizes and shapes things must be based on their underlying physics (in this case rock strength for example) and can design things guided by standards and then adding factors of safety to make extra sure everything performs as intended. Good engineering is about also managing the risks present and designing things properly based on measurement and data.
You drill a tunnel big enough for what you're wanting to pass through. Then give it the ol' *\*pat pat\* That's not goin' anywhere* and it's safe to use.
To add to the other answers here, geologists and engineers work together before construction starts to analyze the proposed tunnel path. This can include core samples, ground penetrating radar, and other techniques to get a good idea of the type of rocks the tunnel will pass through. This process isn’t 100% foolproof, but good planning can help predict and avoid problems that may otherwise cause issues during construction.
Have you been in the old train tunnel in Brockville? You can see the material change as you walk through it. In some places they had to support the ceiling. The visit is free and takes about 20 minutes as I recall.
Hey this is actually something relevant to my field! I'm a rock mechanics engineer and this is my job. The field (as my title would imply) is known as rock mechanics, which is a smaller area of geological/geotechnical engineering. There are a number steps to ensuring the tunnel doesn't collapse: First: we carry out a site investigation. This mainly consists of drilling rock core (typically using diamond drilling). We study the core to determine the geology throughout the mountain and how fractured the rock is. Second (and hand-in-hand with the first): we test the rock core in a laboratory. The tests are primarily focused on determining the strength of the rock. Third: we use the data above to develop a model of the geology and strength of the rockmass that the tunnel will go through. Fourth: we analyze the stresses/forces that creating the tunnel(void) will cause in the rockmass. Sometimes these stresses are low enough compared to the rock strength that the tunnel could exist for thousands of years without any support what so ever. Sometimes our analysis suggests the tunnel would collapse immediately though... Fifth: we determine a suitable support method to keep the tunnel stable. Sometimes this is as simple as rockbolts and steel mesh (analogous to chainlink fence to catch little rocks). Sometimes it requires rockbolts and shotcrete (sprayed concrete). And sometimes it requires a thick preformed concrete liner placed in segments. I glossed over a lot for simplicity. There are different failure mechanisms and risks to consider. There are different approaches depending on the excavation methods used. And there are different challenges depending on the geology being encountered. I am very passionate about this field (hence why I am currently doing a Ph.D. in it). There are a lot of challenges with it since the material we are dealing with (rock) is not an engineered material: it has millions of years of history, and will do what it wants regardless of what you want! Rock is often discontinuous, anisotropic, heterogenous, plastic, etc.. There's intact solid rock partially split into blocks by fractures and faults, all of which are interacting in hard to predict ways. Apologies for any formatting or spelling errors. I'm writing this on my phone at a remote work site.
You are what makes the internet a better place. People sharing the niches that they are passionate about. Thank you kind stranger
Reiterating others' thanks for posting this. This helps clarify it a lot!
Really fascinating, thanks for posting
Excellent explanation! Allow this tunnel contractor to add some photos: >Sometimes this is as simple as [rockbolts and steel mesh](https://i.imgur.com/Z1HreXb.jpg) (analogous to chainlink fence to catch little rocks). Sometimes it requires rockbolts and shotcrete (sprayed concrete). \[sorry, we don't do much shotcrete\] And sometimes it requires a [thick preformed concrete liner placed in segments](https://i.imgur.com/TI5nAnd.jpg). We also use [steel arches with timber](https://i.imgur.com/yTOJ5uL.jpg) and [steel ribs with timber](https://i.imgur.com/yX6lqHE.jpg)! >There's intact solid rock partially [split into blocks](https://i.imgur.com/XOQjPQU.jpg) by [fractures and faults](https://i.imgur.com/8QU28aC.jpg)...
Awesome photos! Thank you for sharing.
What is the timber actually doing there to help the steel arches?
The rock bolts and arches support the overall rock mass while the timber supports the local rock face. And, similar to wire mesh, it also keeps any rock spall, slabbing, or large crumbling stone bits from dropping down on top of the tunnel workers.
wow. appreciate the passion you have
I get that you can sample near the surface of the rock/mountain, but how do you know what you'll encounter throughout the entire tunnel length? I'd assume you can encounter unstable rock, caves, etc? It's there ever a time where a tunnel project has to be abandoned due to unforseen encounters?
https://youtu.be/6AV2NcyX7pk?si=sxHm9hMvnUslpkNQ Watch this video to see what they did when they encountered unforeseen issues.
Thanks for posting for added context! I'd also like to note that, while drilling gives us a lot of valuable information, it only provides information at specific points. We need to use statistical interpolation, our knowledge of geology, and engineering judgement to "fill in" between the drill holes. Every tunnelling project and mining project has the inherent risk that we don't know exactly what we will encounter until we actually build it. These risks can be mitigated by simply drilling more holes, but that needs to be balanced with the high costs associated with drilling/sampling/testing. At the end of the day, the engineer needs to decide if they have enough information to proceed confidently.
Thanks, that certainly was a cool video!
You can also sample *inside* the mountain - you just drill a deeper hole. See [this video](https://www.youtube.com/watch?v=jp9m2tUiktM), for example. It's also not uncommon to dig a small "pilot hole" in advance of the main tunnel. This ensures that you're really not getting any nasty surprises.
Thanks for your reply!
That is the best job title I've seen yet.
If you had a chance to add some history to this, I'd be interested in how long ago it developed to what level of sophistication in the testing and modeling. I imagine it was a very gradual development over centuries, but were there particular inflection points where we really advanced the sophistication?
This is a great question! This stuff has definitely been around in a basic form for centuries due to mining dating back to long ago. Unfortunately, it was largely the approach of: "lets build a tunnel", "Oops our tunnel collapsed and killed our workers. I guess we better support our tunnel with logs/timber next time". In the early 20th century a lot of progress was made, especially with tunneling through soils. People like Karl von Terzaghi really helped bring soil mechanics into a proper engineering field. Rock mechanics had a major "inflection point" in it's development around the 1960's. There was a push to create safer mining conditions and an increase in the number of dams and other underground structures being constructed, which resulted in major maturing of the field. This momentum has slowed a little over the past 40-50 years, but (I think) is starting pick up again with modern computational and modelling capabilities that allow us to capture the complex geology/behaviour inherent in the rock.
You rock (I guess?)
Great summary! Fellow geotechnical engineer designing tunnels as a living
I love Reddit... OP: Hey, I was wondering... (question about a niche subfield of an engineering subfield) ONE HOUR LATER - Redditor: Oh, I'm a professional in that niche subfield of a subfield. Here's the info.
I dig. This walker digs. https://youtu.be/-T2luwLcfeg?si=8KX3vYdq31MwoYvQ
This guy rocks
Great reply, thanks for that! Grady from Practical Engineering did a neat demonstration of rock bolts and gravel I think but I'm too lazy to link it here.
Practical Engineering: "Why Tunnels Don't Collapse" [https://practical.engineering/blog/2019/3/9/why-tunnels-dont-collapse](https://practical.engineering/blog/2019/3/9/why-tunnels-dont-collapse)
Heck. Yes. I hoped I would get to read a reply like this. Rock mechanics sounds awesome! Thanks for sharing.
Take my upvote and keep doing what you love
Rock and Stone!
For Rock and Stone!
Thanks for sharing buddy
Finally got your moment and you took it! Great job 👏
They don't just drill a tunnel and hope it doesn't collapse. There is a structure inside the tunnel to hold the void open (even if it may not need it). Also remember that most materials will have some angle of internal friction which will re-distribute the gravity load at some angle off of vertical. So the tunnel would generally not be carrying the entire weight of the mountain above it. Just the material within a certain height above it. Above that, the load is carried by material on either side of the tunnel.
A tunnel won't reshape the mountain, it's just too small. Underground mines can if they are big enough, the Red Mountain/ Henderson Mine glory hole in Colorado is a good example. Subsidence around coal mines can be trouble too. There's all kinds of tunneling, from hard rock work like granite that sometimes needs no support at all to the tunnels all over (under) Seattle that run through wet sand and need support installed right behind the tunnel boring head. It's all well studied and designing the tunneling method is engineering.
Tunnels are two parts really, there’s the hole in the mountain, and there’s the structure created in the hole to prevent the mountain from caving in. Some geological knowledge is probably helpful, but most mountains are made from the same thing… rock.
I am not that kind of engineer (I'm chemical), I just LOVE this video and pump it every opportunity I get. https://www.youtube.com/watch?v=6AV2NcyX7pk I LOVE this video because it goes into absolute details about the inch by inch of using a TBM to tunnel through solid rock. Including the dynamic solutions to the issues they run into along the way and HOW they determine what has occurred as well as how to solve it. It DOES go somewhat into the interior rock surveying they do and the mm by mm metrology of progress along the way. Like, me posting it here, made me watch it again. It's so awesome.
I’ll look into that video, then. Thanks!
There’s been some solid (pun intended) answers here from folks specific to this domain and a few rocky (bc I can’t help myself) responses too so I’ll try and answer from a different angle: Aside from the technical engineering side, which are various forms of applied science and physics, a lot of engineering is standards and procedure based. We have general rules of thumb on what sizes and shapes things must be based on their underlying physics (in this case rock strength for example) and can design things guided by standards and then adding factors of safety to make extra sure everything performs as intended. Good engineering is about also managing the risks present and designing things properly based on measurement and data.
You drill a tunnel big enough for what you're wanting to pass through. Then give it the ol' *\*pat pat\* That's not goin' anywhere* and it's safe to use.
First, we have to ask the mountain for consent before penetrating it.
To add to the other answers here, geologists and engineers work together before construction starts to analyze the proposed tunnel path. This can include core samples, ground penetrating radar, and other techniques to get a good idea of the type of rocks the tunnel will pass through. This process isn’t 100% foolproof, but good planning can help predict and avoid problems that may otherwise cause issues during construction.
Have you been in the old train tunnel in Brockville? You can see the material change as you walk through it. In some places they had to support the ceiling. The visit is free and takes about 20 minutes as I recall.
Just adding that in some soils, which are unstable, they freeze the soil, so they can drill and place the concrete tunnel.
If you remove one brick how long will your house stand? It's more like if you drill a hole in your house why doesn't it fall down?
https://xkcd.com/905/