Far From Boring: Meet the Most Interesting Tunnel Boring Machines
http://www.cat-bus.com/2018/01/far-f...ring-machines/
The Giant TBM
- Musk says he can build tunnels cheaper if he just makes them smaller. But in reality, it’s not small TBMs that are the future, but big ones. The cost of a TBM doesn’t get much higher as you increase its diameter. It is therefore cheaper to build one very large tunnel, rather than two smaller ones. So giant, linear tunnel building factories have been constructed, with some reaching up to 17.6m in diameter.
- A large diameter tunnel becomes really interesting for the construction of a metro, if it is large enough to fit not only the tracks, but the station platforms inside it. These large tunnels can also house other necessary infrastructure: siding tracks to park trains at night, ramps so trains can cross over from one level to the other, evacuation paths, power substations — all items requiring space and cost.
The Vertical TBM
- Let’s say you’ve built your metro line using a giant TBM, and installed platforms inside the tunnels — you’ll still need to access those stations from the surface. Traditionally, you would dig some access shafts. This may be slow, labor-intensive, and complicated if there’s ground-water. But in the interesting new world of tunnel technology, there’s a TBM for that: the “vertical shaft sinking machine”.
The Diagonal TBM
- We could now build our subway line deep underground inside giant tunnels, and vertical shafts down to provide elevator accesses. The thing is, if you want a lot of people to access your station, you need escalators, which move many more people per hour. This means we want tunnels to be neither vertical or horizontal, but built at the 30 degree angle of escalators.
- In 1997, Saint-Petersburg opened an extension of its line 5, very deep underground. But one 102m deep station, Admiralteyskaya, wasn’t opened until more than a decade later, because they couldn’t figure out how to build a connection to the surface. For all this time, trains just ran through this ghost station but didn’t stop there, since there was no connection to the surface. The problem was that there are a lot of museums and heritage buildings nearby and the ground is composed of a soft soil.
The Rectangular TBM
- One issue with traditional TBMs is that they build tunnels with a circular cross section. But the internal cross-section of tunnels usually needs to be rectangular; generally we want a flat bottom, walls going straight up, with some relatively constant ceiling height. If we use a circular TBM to build a tunnel, we have a bunch of wasted space on the sides. This can be especially an issue when space is at a premium, or if we want to build tunnels as close as possible to the surface.
- This is especially true for underpasses, which have to be as close to the surface as possible and which also have a relatively small height (as little as 2.2m) but require a good amount of width (4m and more). Traditionally, these would be constructed using cut-and-cover: the road would be dug up, the tunnel placed in, and the road rebuilt on top. This can be a major disruption to the surface roads above.