Tunnels play an important role in our constructed environment as passageways for mines, conveyance for utilities, and routes for transportation. But, excavating a tunnel underground in unstable material can lead to some dangerous situations, like the 2010 mining accident in Chile when 33 men were trapped deep in subsurface for more than 2 months.
Rocks are heavy. That may seem self-evident, like many fundamental principles of civil engineering. But when you build things underground, it starts to become a major consideration.
Just like atmospheric pressure is created by the weight of air molecules pressing down on each other, pressure exists in the subsurface of the Earth from the weight of the soil and rock above. This pressure compresses the material in the subsurface more and more the further down you go. Building a horizontal passageway, or a tunnel, through this material, interrupts the flow of these compressive forces. Just like if you remove a column from a building, excavating a tunnel takes away the support from the material above. Where you once had compression throughout the subsurface, now you’ve created a zone of tensile stress, where the material above the tunnel is trying to pull away from itself.
Many materials react differently to tension than they do to compression, and soil and rock are no different. You can imagine soil as a collection of individual particles. The only reason a soil mass has any strength at all is because of the friction between those particles. But friction is a function of the force pressing the particles together. So, if you instead reverse that force and pull the particles apart, the soil loses all its strength. Some soils, like clay, do have a certain amount of natural attraction between the particles, called cohesion, but it’s not enough on its own to resist significant forces.
In other words, you can’t make a rope out of soil – it has no strength against tension. If you build a tunnel in soil, you have to replace the support you removed with some other way to transfer the load of the soil above. This is why many tunnels are lined with materials like steel or concrete, to provide support to the tunnel walls and transfer the stresses in the subsurface around the tunnel. These lining systems add a major cost to the tunnel construction.
Rock, on the other hand, behaves a little bit differently in that it does have some tensile strength. You could make a rope from it. Not that it would be particularly useful, but it’s a good way to imagine the difference between soil and rock mechanics. In fact rock generally has more strength than soil for all types of stress. This additional strength gives rock the ability to transfer forces around a tunnel just like the lining discussed before. But, it’s not as simple as saying tunnels in soil require support and tunnels in rock don’t. Geologists use the term “massive” to describe rock that is uniform without layers or joints. Unfortunately, not all rock is massive. In fact, most geologic units of rock in the subsurface have at least some amount of jointing, or natural breaks. In many cases, the jointing of rock follows specific patterns that can be observed and mapped. But, the problem with joints is that they have no tensile strength, and so no ability to transfer tensile stress. You can see that jointed rock starts to behave more like a soil just with much larger particles. So, even tunnels through rock often require some type of support to prevent collapse.
But, what if there was a way to take advantage of the superior strength of rock without going to the added trouble and expense of lining the tunnel to provide support? Well it turns out there is. Rock bolts are a type of reinforcement for stabilizing rock excavations, usually made from steel bars or bolts. I built this demonstration to show how they work. This is essentially the frame of a table, but the top is completely open. I attached a bottom to the frame to represent temporary shoring of a tunnel roof. Even though our permanent support system doesn’t rely on this, it’s necessary until we get the rock bolts installed.
Like I mentioned before, trying to support a gravel ceiling using rockbolts isn’t the most appropriate use. They do have their limitations. But, this simple construction method dramatically reduces the cost of making tunnels through rock safe from collapse. And public safety is priority number one for civil engineers.