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The shortest connection between two portals would be a tunnel with constant gradient. Why do long tunnels instead rise from both portals to a culmination point, introducing unnecessary steep gradients?

Some thoughts: If it is for drainage, this could as well be done to one portal only. Shortening of service tunnels cannot be a reason, as they are often not connected at the highest point. Problematic bedrock could as well be circumnavigated laterally.

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    $\begingroup$ You seem to have certain tunnels in mind when you ask this question. Which tunnels are you refering to? With that information, your specific question could be answered. In the general sense, I'm sure that not all tunnels have the grades that you are refering to. $\endgroup$ – hazzey♦ Jul 7 '20 at 20:12
  • $\begingroup$ @hazzey The new Brenner Base Tunnel (55km) was in the news today, but it is also true for Gotthard Base Tunnel (57km), for Lötschberg Tunnel (16.7km) and for Simplontunnel (20km). Ceneri Base Tunnel (15km) however seems to climb to the south with no culmination. $\endgroup$ – Andreas Jul 7 '20 at 20:26
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Laterally circumnavigating problematic bedrock is not always possible or practical for the following reasons:

  • This can increase the length and cost of the tunnel
  • The problematic geologic zone may be too vast laterally
  • Such an alignment may already be minimizing the impact of a problematic zone by targeting the best geological zone available.

As to the tunnel rising to a culmination point from each portal, this is reasonable if the tunnel is developed from both ends simultaneously. Simultaneous development from both ends reduces the overall development time for the tunnel. Developing the tunnel with an upwards gradient is done for drainage purposes. Developing the tunnel with a downwards gradient would require the constant use of pumps, which would increase costs and the toe of the downward active tunnel heading/face would continuously have a pond of water in front of it.

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  • $\begingroup$ Drainage of an unfinished segment is a very plausible argument. If true, I would expect the culmination point to also be the point of final breakthrough. I will check with AlpTransits documentation. $\endgroup$ – Andreas Jul 7 '20 at 21:56

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