Breakthrough

[Méo]

Originally Posted by SteamWake

I'm always amazed at how they can dig two tunnels thousands of feet from opposite directions and end up joining right were they are supposed to.

Yeah, I was thinking exactly the same.

...and wondering how they did it.

[Jimbuna]

Satellite precision I'm reckoning.

[Schroeder]

Originally Posted by jimbuna

Satellite precision I'm reckoning.

And how do you receive a satellite signal inside a mountain?
GPS doesn't work like that.
(no, I also don't know how they are doing this)

[krashkart]

The GPS data can probably be relayed. That's my guess anyway.

[Oberon]

Did a bit of Googling, here's what I found:

Tunnel surveyors guide them. Tunnels usually begin from both ends and meet in the middle, necessitating considerable mathematical precision. The route of a tunnel of any type, for example sewer or highway, is designed to run from a known point on the map to another known point on the map and at a specific depth. The design route follows specific route engineering principles which can easily be solved mathematically because they are invariably consistent. When you look at a globe, you see latitude lines running around the earth east to west, located so far above or below the equator. [Think of latitude = altitude] Longitude lines run north to south, polar cap to polar cap. Thus, the earth is divided into imaginary mathematical segments for the sake of navigation. The cake is cut again at the federal government level by federal surveyors who set national monuments on a grid pattern. The cake is further sliced at the state level in which state surveyors set state plane coordinate monuments on a grid pattern. GPS in recent years has taken a lot of work, guesswork, and time out of the process. All geometric route designs can be reduced to triangles in solution, whether curving or linear. The basis of this fact lies in trigonometry and plane geometry. Go back to the grid. Imagine that you drive due south for ten miles, then make a hard right turn, at ninety degrees in angle, and drive due west for ten miles. Convention dictates that north and east are positive. So, your new position after leaving 0,0 is -10,-10 (10 south is minus, 10 west is minus). To avoid negative numbers, massive numbers are added to the equation relative to the size of the nation or state, so your new position might actually be N356,023.56, E544,759.47. These numbers have a specific mathematical relationship to any other point on the grid. To save time, you might have driven from 0,0 to -10,-10 along the shortcut, or hypotenuse of the triangle. You drove S45W 14.14 miles. Mathematically, that's the only bearing and distance you could have driven. This process can be reversed through Trig functions to give the latitude and longitude from the known bearing and distance. The instrument you see surveyors working with measures angles and distances with normally considerable accuracy. No matter which way the surveyor turns, he is navigating by mathematically determined position and guiding the mining operation accordingly. Consider that you may have driven from 0,0 to -10,-10 along an arc running from point to point, like a regular road curve. If you trace a path along the hypotenuse then turn 90 degrees to the arc, the position you immediately strike on the arc now has a specific coordinate (north, south) position. The figure described from point 0,0 along the hypotenuse to the 90 degree turn, to the arc and back again is a right triangle. A lot of points like that make an arc obvious; an infinite number of them describe the arc. Calculators and lap tops make mapping and positioning fast and accurate. In the old west, surveyors/engineers on railroad tunnels used log tables, tablets of paper, and fists full of pencils. It took many hours and the only way to check the math was to do it all again. Three times if you got two different answers, more if you're still in doubt.

"Portaling in" is often the terminology for beginning a tunnel. Suppose for simplicity's sake that a flat rock face is ready to be drilled for the first time, and the tunnel is a perfect circle. The surveyor brings in the line (or direction or bearing or azimuth) that the tunnel should proceed on. He has also brought in the correct elevation to the staging area. He can therefore paint a line straight up and down the face of the tunnel, the centerline, and a perpendicular line directly across the circle through the radius point of the circle, determined by the elevation at that specific position (it's all in the blueprints and math). That horizontal line is referred to as the "spring line." From the centerline along the spring line so far, then up so far, will describe a point on the arc. When one has sufficient points, one paints the circle. This is called painting the face and it is done repeatedly for each new drilling cycle. (Drill the holes, pack the dynamite, explode the rock, muck out the rock, drill the holes...) Next, a tunnel drilling machine is maneuvered into position with one drill bit set on a start point, often center-center. The surveyor determines that the drill steel is precisely on line (the rock face might be askew of the desired line of direction) and either dead level or angled according to the prescribed slope. Once the first hole is drilled out, a long wooden loading pole is inserted into the hole with the end protruding. Now all the miners position their multiple drills to follow the alignment of the guide pole and drilling out the circular pattern. The process is repeated until the tunnel is long enough for the dynamite blasts to have minimal effect (usually about 800 feet), whereupon lasers are mounted on either side of the tunnel to constantly point line and grade. All the tunnel faces following the first are straight ahead and on line, or the surveyors adjust as necessary.

Older tunnel boring machines (moles) had to be guided by lasers and laser targets mounted on the mole fore and aft. Modern moles have automatic guidance systems that are wicked accurate, but basically doing what surveyors have done for countless generations. Greeks and Romans built tunnels.

These are the basics. Tunnels can be very complex. Shafts are another source of challenge.

http://www.answerbag.com/q_view/8061

[Diopos]

Actually, simoultaneous "double tunel" digging is an ancient art!
Link: http://en.wikipedia.org/wiki/Tunnel_of_Eupalinos

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