There are two traditional cross-sections for tunnels. One is the circular section for tunnel boring machines (TBM), and the other is the inverted horseshoe for drill and blast. I am going down a different path. I have classified this as 'new engineering', which means I will have more monitoring, and some flexibility to change as I go along.
Recalling an old drawing from a previous post, the circular tunnel section is too sharp, causing a stress concentration of a factor of two at the crown. I want the stress contours to flow nicer around the opening, like this.
Naturally, I would chop down the sides and bottom to make it more practical for access. I would watch for floor heave. You always wind up with the general dimension of 10 m, just to provide the clearance for everything at the face.
Since this is a fun project, I can throw in all the latest tools. This will probably come in at twice the phony Mel price, but it will last 100 years. Besides, what is the price of a complete disaster?
Now, for the grinders. I'm not going with drill and blast, because this is under a city, and that doesn't seem like very much fun anyway. I'm now off to the internet to look at mining machines.
Ok, I am very impressed with modern longwall mining machines. I will use something that grinds this pattern.
And probably looks something like this. I need some bigger versions.
This generally gives me the most unease, since I'm not really up on these things, but I know what I want.
The roof rock bolt machine is also very modern, using the latest robotics. I will need a fairly dense pattern, with welded mesh and shotcrete. The shale is perfectly happy being confined, but release one direction and it may start to crumble. The idea is to maintain confinement, and I hope this can be done with roof bolter right behind the grinder. I would do convergence measurement and acoustic monitoring to check this out. If not enough, the bolts would have to be tilted towards the face, and the pattern densified.
-to be continued