3 ms·

> As far as I understand it, and this may be outdated cut and cover is way cheaper than boring. When you ignore all the side effects, sure (especially if you a

> As far as I understand it, and this may be outdated cut and cover is way cheaper than boring. When you ignore all the side effects, sure (especially if you also ignore the risks of blowing the timeline). Trenchless tech is spreading fast even in small-ish (street utility scale) construction. > Maybe new boring machines are faster and cheaper than they used to be. I'm not sure that's even the case, AFAIK TBMs are still custom-built for the job, highly geology-dependent, and require a ton of babysitting. If you hit an unexpected patch of really different geology things can get really dicey.

bluGill 3y ago
> TBMs are still custom-built for the job This is semi-true. They are built for the job, but the design is standard. What is custom is the exact size of the tunnel, and thus all the supports. TBMs generally last a lot longer than the job, so a city could save a lot of money if they planed to keep using their TBM, constantly digging tunnels. Start with a north-south line, then use the same TBM to build an east-west line (or southeast/northwest), and so on. However most cities are unable to find the continuous budget to do that and so when they are ready for the next one the old TBM is no longer worth using, and in any case they no longer have trained crew to run it. > highly geology-dependent, Soft water logged earth needs different machines from hard rock. There are combination machines, but if you can determine which you need you save a lot of money ordering the geology specific machine. Get this wrong and you end up with a TBM and partial tunnel under the city. > require a ton of babysitting. 12 people per shift is the number I've heard (NYC uses double which is one of the smaller reasons things cost so much there). The work is mostly routine though. However the TBMs do break once in a while and they everything stops while you repair them.

bluGill 3y ago

> TBMs are still custom-built for the job This is semi-true. They are built for the job, but the design is standard. What is custom is the exact size of the tunnel, and thus all the supports. TBMs generally last a lot longer than the job, so a city could save a lot of money if they planed to keep using their TBM, constantly digging tunnels. Start with a north-south line, then use the same TBM to build an east-west line (or southeast/northwest), and so on. However most cities are unable to find the continuous budget to do that and so when they are ready for the next one the old TBM is no longer worth using, and in any case they no longer have trained crew to run it. > highly geology-dependent, Soft water logged earth needs different machines from hard rock. There are combination machines, but if you can determine which you need you save a lot of money ordering the geology specific machine. Get this wrong and you end up with a TBM and partial tunnel under the city. > require a ton of babysitting. 12 people per shift is the number I've heard (NYC uses double which is one of the smaller reasons things cost so much there). The work is mostly routine though. However the TBMs do break once in a while and they everything stops while you repair them.