tunneling

[alinfred]

could someone explain how the engineers of old using there basic surveying equipment could tunnel into mountains from either end and meet in the middle so to speak,what i really would like to know is how this was done when tunneling a long curve

[ndbprr]

First you use surveyors to establsih a tsraight line over the mountain. That establishes where the track will come out and the reference point from which to keep measuring. Then as you dig the tunnel you keep shooting the centerline from a transit to make sure you are staying on course. Even today with laser shooting on long tunnels they can be off by a couple of inchs. Now curved tunnels are a different animal but don't forget that the Egyptians and Greeks established higher mathematics over 2000 years ago. That hasn't changed just the way to do the measuring.

[michaelstevens]

To answer "alinfred's" question;

The railroad's surveyor used a instrument called a "Transit"

The "Transit" consists of telescope (graduated for the horizontal and vertical circles) with planetary axes, mounted on levelled tripod (the modern equivalent is the "Theodolite" or the "Distomat").

The surveyor would calculate and set the required deflection angles for each 10' drilling (for loading with dynamite) both horizontallly and vertically, using reference points as far back as he could see in the tunnel.

[CSSHEGEWISCH]

The first St. Clair Tunnel between Port Huron and Sarnia used this system to keep the tunneling shields in line and they were off by less than an inch when they met.

On the other hand, the tunnelers got careless when one the PATH tunnels was bored between New York and New Jersey and there is an S-curve in the middle of the tunnel because the shields were badly out of line.

[Anonymous]

surveyor's are very good at what they do, blast a hole (abit a very large hole), compare to the base line, blast another hole making sure the base line is followed. when two ends meet they should be within a foot of the baseline, and the deiviances would be fixed at that point. As for curved Tunnerls, that's done the same way large curves fo track are done, with tangents and Degree's off the baseline.

with the advent of tunneling machines it's even easier, as they have gyrolevels built into to them to keep them on target, GPS makes curves easy as the machine can follow the curved baseline almost exactly.

Tunnel planning really isn't any different than planning a route across a large landscape, it's just takes alot more calculating as you go along to make sure you stay on target. Remember, Just like bridges, man has been building tunnels for centuries and has become very good at it.

Jay

[Big_Boy_4005]

For it's length the Chunnel was done very accurately, missing by only a couple of inches in a distance of over 20 miles. Of course this was done with all of the most modern equipment.

The others are correct, the answer is to chart a path, and STAY ON IT!!! This means both horizontally and vertically. Measure twice, cut (blast or dig) once. These aren't high speed operations.[swg]

[alinfred]

thank you all for replying,i have found a web site that shows a transit but not the telescope type ,will keep searching