The new tunnel through the Swiss Alps has been in the news recently, and it made me wonder: are tunnels bored through solid rock more or less a permanent asset once the cost of digging them is repayed? Or do they require periodic repair and reinforcement, and would collapse in a few decades or centuries if not maintained?
I can certainly think of a number of 100+ year old rail tunnels in use in the western US that have required extensive repairs over the years. The old Cascade Tunnel which was built in 1900 and abandoned when the new one was finished in 1929 partially collapsed during heavy rains in 2008. The Mullan Tunnel near Helena, MT (built 1883) also recently had a partial roof collapse, but it’s still in use and the short line railroad that uses it just spent a few million fixing it. I don’t know if these collapses are due to lack of maintenance or poor design or acts of god or what, but it certainly seems like you can expect to need to put some money into maintaining or repairing at least a 19th-century style tunnel.
It also seems like, just from personal experience on abandoned tunnels that are part of rail-to-trails paths, the real weak part of old tunnels is the concrete they use to give the tunnel the nice smooth horseshoe shape. Big chunks of it on the ground seems to be more the exception than the rule, whereas tunnels that are just jagged rock usually seem to have less debris on the ground, even though they’re usually older. I don’t know if the concrete is just aesthetic or aerodynamic or if modern concretes might hold up better.
I can’t think of many structures which don’t require some maintenance over the years.
The longevity of tunnels would depend upon many different factors, such as the composition of material the tunnel is carved from, the seismic activity in the area, groundwater or the lack thereof, interior drainage, and the tunnel’s use (water, vehicle traffic, railways, etc.). I would say that each requires at least a minimal amount of maintenance.
Assuming it is in an area of stable geology (no earthquakes, for example), no corners were cut during construction (all bad ground reinforced with sufficient concrete, not just shotcrete and mesh), and it either naturally drains or you keep pumping any water out, it should last indefinitely.
One of the PATH tunnels (between NYC and NJ) dates to 1879 and is still in use today. It’s in the silt (mud) at the bottom of the Hudson River, and it is made of 6 layers of brick with an outer 1/4" iron casing (which I think has mostly disappeared). You can read more about it on my site here.
The OP specified “tunnels bored through solid rock”.
True. My example was of one that is exposed to more severe conditions and still intact.
I imagine that any infrastructural asset like that requires at least some maintenance, but I’m likewise sure that maintenance costs are a tiny fraction of construction costs, at least if they’re kept up with and not allowed to snowball.