Why aren't there more tunnels?

[whc.03grady]

@Jaguars

Disclaimer: This is just background engineering stuff I learned for my job.

There are a few main reasons that there aren’t more tunnels.

The first is the method of construction. A tunneling machine is a specialised machine that costs several times the equipment used on the surface construction. Not only that, but unless your construction contractor evisages a steady supply of tunnel construction in the future, the tunnel machine will need to pay itself off in the one project, where surface excavators can just be moved on to the next project (Or can be sold easily and quickly). They are also slower because there are only two possible points can be done, at the tunnel heads. Every extra day of work costs due to greater labour, plant and finance costs. Drilling and blasting is even slower and more dangerous and the cheapest method, cutting and covering obviously can’t be done under a mountain range.

Second is the unknowns. You can survey a surface route well, but it’s hard to know what sort of rock a long tunnel will encounter, even with geotech investigations. If you end up hitting rock too hard for your tunnel machine then you have a very, very expenive mistake.
Fill from surface cuts can be used on embankments further down the road, but a (long) tunnel will produce a surplus of fill to get rid of. It can be sold but costs money to truck out.
Another thing is that the current road is likely to have been upgraded since times when travellers were on foot. If the route was establised by the native indians and later made into a cart route and then into a highway, the costs and effort have been spread over hundreds of years. To be cost effective, a tunnel has to save a huge amount of time or distance, which is why really long tunnels under the swiss alps can be built.

So why is the railway exempt from all this? Because grade is king, and because at the time that they were built, the costs were different. These days two guys with a digger and a truck can move a huge amount of material cheaply and safely, They can move much more earth/rock before it becomes worthwhile to build a tunnel to save on earthmoving costs. If today’s tunnelling boring machines could have been used in the 1800’s, It would be quite likely that there would be rail tunnels going right under many mountain ranges.

Hopefully that makes things clearer and not worse

Excellent–this pretty much answers it.

[Airman_Doors_USAF]

@Hari_Seldon

Back in the mid 19th century, the Penna RR built a bunch of tunnels through mountains, then discovered there was no way to ventilate them (remember steam locomotives?) and gave up and built things like the famous Horseshoe Curve (Horseshoe Curve (Pennsylvania) - Wikipedia) as an alternative. The tunnels sat empty until the Penna Tpk was built in the 1930s (I believe it was a WPA project originally). Between Philadelphia and Pittsburgh there were originally 7 tunnels. But they were only two lanes through the tunnels, although the turnpike was otherwise four lanes. As traffic increased, they became congested and were mostly abandoned. I haven’t driven that in years; maybe they have all been bypassed. There is at least one tunnel on the NE extension, but it was built as 4 lanes. The fact remains that they are expensive to build, expensive to maintain, and expensive to operate.

4 of the original tunnels are still in use after being twinned. 2 of them are now part of the Abandoned PA Turnpike, a bicycle trail (and really cool, to boot). The other abandoned one is being used by Chip Ganassi as a rolling wind tunnel.