by mtuandrew
Alright, so here's my best guess on the tunnels. For our purposes, I'm assuming that each tunnel will be 24' tall outside diameter, to allow 14.5' cars (I don't know the height from rail to ceiling - I'm assuming a 24' bore based on a RR.net conversation that gives the North River Tunnel boring shield's diameter as 23.5'.
Naval Station Norfolk to Hampton: the main navigation channel (Norfolk Harbor Entrance Reach) has a depth of no less than 52.6 feet at mean lower low water (MLLW), but the depth over tunnel is no less than 59 feet. For our purposes, let's say a hypothetical railroad tunnel needs to stay no less than 65 feet under the surface at MLLW while in the main channel. Its path also crosses the Willoughby Channel appx halfway between Sewells Point and Rip Rap, but since that channel is only maintained to 10 feet we can assume the tunnel will pass under it easily.
The main navigation channel spans about 7500 feet, but we only need to maintain a 65' depth for 6000 feet before the channel walls begin to rise. For a hypothetical 2.5% grade and a station whose top is buried (say) 10' under Maryland Ave and Hampton Blvd, that puts us at about 2400' slope. Call it 3000' to be certain, and to include vertical and horizontal curvature. On the eastern side, we have about 4000' between Rip Rap and Sewells Point in which to gain those 55'. On the western, there's only 2000' between the tunnel portal island and mainland, so this tunnel's portal would have to be considerably inland.
That puts us at 4000' + 6000' + 3000' = 13,000', or about a 2.5 mile tunnel between Norfolk Naval Station and downtown Hampton. You might be able to reduce that somewhat if you don't try to use the existing tunnel portal islands, and tunnel under the navigation canal earlier.
Naval Station Norfolk to Hampton: the main navigation channel (Norfolk Harbor Entrance Reach) has a depth of no less than 52.6 feet at mean lower low water (MLLW), but the depth over tunnel is no less than 59 feet. For our purposes, let's say a hypothetical railroad tunnel needs to stay no less than 65 feet under the surface at MLLW while in the main channel. Its path also crosses the Willoughby Channel appx halfway between Sewells Point and Rip Rap, but since that channel is only maintained to 10 feet we can assume the tunnel will pass under it easily.
The main navigation channel spans about 7500 feet, but we only need to maintain a 65' depth for 6000 feet before the channel walls begin to rise. For a hypothetical 2.5% grade and a station whose top is buried (say) 10' under Maryland Ave and Hampton Blvd, that puts us at about 2400' slope. Call it 3000' to be certain, and to include vertical and horizontal curvature. On the eastern side, we have about 4000' between Rip Rap and Sewells Point in which to gain those 55'. On the western, there's only 2000' between the tunnel portal island and mainland, so this tunnel's portal would have to be considerably inland.
That puts us at 4000' + 6000' + 3000' = 13,000', or about a 2.5 mile tunnel between Norfolk Naval Station and downtown Hampton. You might be able to reduce that somewhat if you don't try to use the existing tunnel portal islands, and tunnel under the navigation canal earlier.