kh25
are there any railroad tunnels in the US that do not have portals with arch openings?
Mark Kingsbury
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Chris VanderHeide cv_acr

Tunnel Geology

If the tunnel is blasted through solid rock like the one above, the sides could end up being much straighter, but if the tunnel is build through softer, less stable materials (dirt, clay, loose/unstable rock) and requires lining/reinforcement an arch or tube (a modern tunnel dug by a boring machine will be entirely circular) is a much more stable construction if the tunnel has a stone or concrete portal & lining.

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