# CONSTRUCTION

**URL:** <https://boards.straightdope.com/t/construction/4205>\
**Category:** Factual Questions\
**Created:** [October 25, 1999, 8:32pm UTC](https://boards.straightdope.com/t/construction/4205 "1999-10-25T20:32:00Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![BOB](https://avatars.discourse-cdn.com/v4/letter/b/ea666f/32.png) [@BOB](https://boards.straightdope.com/u/BOB)\
**Post date:** [October 25, 1999, 8:32pm UTC](https://boards.straightdope.com/t/construction/4205/1 "1999-10-25T20:32:00Z")

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DEAR CECIL; CONVERGENCE

I¹M SURE YOU¹VE BEEN ASKED THIS QUESTION BEFORE; BUT I MISSED YOUR ANSWER; SO HERE GOES AGAIN.

HOW COME; WHENEVER THEY BUILD A BRIDGE, TUNNEL OR RAILWAY; CONSTRUCTION IS ALWAYS STARTED AT OPPOSITE ENDS USUALLY SIMULTANEOUSLY. THIS METHOD; I WOULD THINK, PRESENTS A LOT OF PROBLEMS IN PRECISELY CONVERGING OR HOOKING UP IN THE MIDDLE; ESPECIALLY UNDER WATER OR THROUGH A MOUNTAIN. (EXAMPLE; THE ³CHUNNEL² OR THE TUNNEL UNDER THE ENGLISH CHANNEL)

HOW DO THEY DO THAT???

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							BUDMATE
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**Author:** ![UncleBeer](https://avatars.discourse-cdn.com/v4/letter/u/977dab/32.png) [@UncleBeer](https://boards.straightdope.com/u/UncleBeer)\
**Post date:** [October 25, 1999, 8:50pm UTC](https://boards.straightdope.com/t/construction/4205/2 "1999-10-25T20:50:00Z")

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Ummm, surveyors?

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“The problem with the world is that everyone is a few drinks behind.” - _Humphrey Bogart_

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**Author:** ![Guy\_Propski](https://avatars.discourse-cdn.com/v4/letter/g/5e9695/32.png) [@Guy\_Propski](https://boards.straightdope.com/u/Guy_Propski)\
**Post date:** [October 25, 1999, 9:28pm UTC](https://boards.straightdope.com/t/construction/4205/3 "1999-10-25T21:28:00Z")

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The reason you start from both ends is simple: you can work twice as fast. This way you have two crews working at once. Accurate surveying and constant checking of progress insures that the two forces don’t miss each other.

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**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\
**Post date:** [October 25, 1999, 9:30pm UTC](https://boards.straightdope.com/t/construction/4205/4 "1999-10-25T21:30:00Z")

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What would be the alternative- start in the middle and work towards the ends? 🙂

As far as bridges go, they’re usually designed as two self-supporting ends that just happen to meet in the middle.  
As to why tunnels are dug in from both ends there are several reasons: First, the longer a tunnel is, the greater difficulty in digging it: ventilation, moving excavated material out, power cables, piping, etc. If you want a tunnel that’s a mile long overall, it’s **much** easier to only have to tunnel half a mile twice than to dig a mile in. Secondly, minor errors in surveying can be accomodated if need be where the sections meet in the middle. But usually the mouths of the tunnel _have_to be exactly where intended. You wouldn’t want to dig through a mountain and come out two yards too far over.

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**Author:** ![Nickrz](https://avatars.discourse-cdn.com/v4/letter/n/bbce88/32.png) [@Nickrz](https://boards.straightdope.com/u/Nickrz)\
**Post date:** [October 25, 1999, 9:58pm UTC](https://boards.straightdope.com/t/construction/4205/5 "1999-10-25T21:58:00Z")

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They use very accurately aligned lasers at either end - to guide the tunnel-boring machinery, and to provide a datum line to measure the deviation from the intended path.  
I think the two chunnel tunnels came out within a millimeter of being dead center.