3 ms·

I suspect the cost is not in volume of material removed, but in operation and maintenance of the machine and it's crew. One tunnel instead of two is half as mu

I suspect the cost is not in volume of material removed, but in operation and maintenance of the machine and it's crew. One tunnel instead of two is half as much machine and crew time.

freeopinion 2y ago
Please explain how you reasoned this out. If one 9.5m tunnel requires 70 m³ of removal for every meter of forward progress, and two 5m tunnels requires 40 m³ of removal for every meter of forward progress, won't it take nearly twice as long to dig the single larger tunnel? Won't that require twice the machine hours and twice the crew hours? How does the volume of material removed not directly impact machine and crew costs?
- jcgrillo 2y ago
  Two dump trucks can't pass eachother in a 5m tunnel. That'll put a restrictive upper bound on material removal rate which isn't present in the larger tunnel.
  - Symbiote 2y ago
    Note they use railway trucks to remove spoil (and transport everything else) from a TBM. A narrow gauge temporary track behind the TBM as it advances.
- timschmidt 2y ago
  Seems like larger machines are likely to have larger material removal rates. Dependent on the surface of the digging tool and it's feed rate.
  - Hilift 2y ago
    One of the references in the article mentions using better alloys ("modified tool steel") and 20" discs produces better results and lower overall costs due to time between disc replacement. This could be A8 "chipper knife" steel. https://www.robbinstbm.com/wp-content/uploads/2017/07/Smading_20InchCutters_RETC2017.pdf https://www.robbinstbm.com/wp-content/uploads/2017/07/Smadin... https://www.litechtools.com/products/tbm-disc-cutter/ https://www.litechtools.com/products/tbm-disc-cutter/
    - timschmidt 2y ago
      Makes sense. I was thinking more about the face of the machine itself being larger, likely the size of the tunnel, and would contain many such 20" discs. In other words, given that the material removal rate of each disc is likely fairly similar from machine to machine, the machine with more of them removes more material at once.
      - freeopinion 2y ago
        It seems self-evident that all additional material removed has 0 additional value. In fact it has negative value because it increases the requirements for any support or finish materials. So energizing and consuming 7/4 more discs is just pure waste. It also adds disposal costs. The most obvious argument I can think of to support this approach is if the excess material could be sold at a profit. But that seems dubious.
        timschmidt 2y ago
        > So energizing and consuming 7/4 more discs is just pure waste. That is and assumption and not self evident. There are lots of reasons one might want to use many cutters. It is a common practice in wood and metal machining, for instance[1]. Having additional cutting surfaces reduces the rate of wear, provides redundancy in case of chipping, increases the mean time between required services, allows cutters to run cooler, reduces chatter, improves surface finish, and a host of other reasons. Seems like you could research it. 1: https://www.advancedmachinery.co.uk/content/product/8500_0_z.jpg https://www.advancedmachinery.co.uk/content/product/8500_0_z...
        LargoLasskhyfv 2y ago
        You seem to have focused your 'tunnel vision' on the costs of boring that thing alone. Sort of CAPEX, so to speak. But what about less OPEX later, because of more flexibility, less maintenance burden, when other demands arise or conditions change? If the tunnel is wide enough, two tracks can be built side by side, and another level of two tracks on top of it. That makes room for a total of four (potential) tracks, with the possibility to switch between them not only horizontally, but also vertically via ramps (inside that tunnel profile). It also enables single platform stations for one track on one or two of the two possible levels in such a tunnel. Without any further 'big digs', or blocking the line altogether.
  - freeopinion 2y ago
    Let's say the larger TBM can remove 7/4 the debris in the same amount of time. That means it uses 7/4 more carts or has 7/4 bigger carts and uses 7/4 more energy to power those carts. It places 7/4 more wear on the rails and has a higher probability of increased maintenance costs on those rails, let alone the TBM itself. All of this to make the same forward progress in the same amount of time. Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. You don't get to pretend that larger material removal rates are free. You don't get to pretend that the 7/4 more cutting wheels don't cost more money to replace. The 7/4 more energy to run the TBM isn't free. Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less.
    - timschmidt 2y ago
      > Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less. More expensive than one, potentially less expensive than two. Happens in manufacturing and industry all the time. Transport cost alone could be enough to account for it. > Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. Sounds like you should have a chat with the project lead.

I suspect the cost is not in volume of material removed, but in operation and maintenance of the machine and it's crew. One tunnel instead of two is half as much machine and crew time.

freeopinion 2y ago

Please explain how you reasoned this out. If one 9.5m tunnel requires 70 m³ of removal for every meter of forward progress, and two 5m tunnels requires 40 m³ of removal for every meter of forward progress, won't it take nearly twice as long to dig the single larger tunnel? Won't that require twice the machine hours and twice the crew hours? How does the volume of material removed not directly impact machine and crew costs?

jcgrillo 2y ago
Two dump trucks can't pass eachother in a 5m tunnel. That'll put a restrictive upper bound on material removal rate which isn't present in the larger tunnel.
- Symbiote 2y ago
  Note they use railway trucks to remove spoil (and transport everything else) from a TBM. A narrow gauge temporary track behind the TBM as it advances.
timschmidt 2y ago
Seems like larger machines are likely to have larger material removal rates. Dependent on the surface of the digging tool and it's feed rate.
- Hilift 2y ago
  One of the references in the article mentions using better alloys ("modified tool steel") and 20" discs produces better results and lower overall costs due to time between disc replacement. This could be A8 "chipper knife" steel. https://www.robbinstbm.com/wp-content/uploads/2017/07/Smading_20InchCutters_RETC2017.pdf https://www.robbinstbm.com/wp-content/uploads/2017/07/Smadin... https://www.litechtools.com/products/tbm-disc-cutter/ https://www.litechtools.com/products/tbm-disc-cutter/
  - timschmidt 2y ago
    Makes sense. I was thinking more about the face of the machine itself being larger, likely the size of the tunnel, and would contain many such 20" discs. In other words, given that the material removal rate of each disc is likely fairly similar from machine to machine, the machine with more of them removes more material at once.
    - freeopinion 2y ago
      It seems self-evident that all additional material removed has 0 additional value. In fact it has negative value because it increases the requirements for any support or finish materials. So energizing and consuming 7/4 more discs is just pure waste. It also adds disposal costs. The most obvious argument I can think of to support this approach is if the excess material could be sold at a profit. But that seems dubious.
      - timschmidt 2y ago
        > So energizing and consuming 7/4 more discs is just pure waste. That is and assumption and not self evident. There are lots of reasons one might want to use many cutters. It is a common practice in wood and metal machining, for instance[1]. Having additional cutting surfaces reduces the rate of wear, provides redundancy in case of chipping, increases the mean time between required services, allows cutters to run cooler, reduces chatter, improves surface finish, and a host of other reasons. Seems like you could research it. 1: https://www.advancedmachinery.co.uk/content/product/8500_0_z.jpg https://www.advancedmachinery.co.uk/content/product/8500_0_z...
      - LargoLasskhyfv 2y ago
        You seem to have focused your 'tunnel vision' on the costs of boring that thing alone. Sort of CAPEX, so to speak. But what about less OPEX later, because of more flexibility, less maintenance burden, when other demands arise or conditions change? If the tunnel is wide enough, two tracks can be built side by side, and another level of two tracks on top of it. That makes room for a total of four (potential) tracks, with the possibility to switch between them not only horizontally, but also vertically via ramps (inside that tunnel profile). It also enables single platform stations for one track on one or two of the two possible levels in such a tunnel. Without any further 'big digs', or blocking the line altogether.
- freeopinion 2y ago
  Let's say the larger TBM can remove 7/4 the debris in the same amount of time. That means it uses 7/4 more carts or has 7/4 bigger carts and uses 7/4 more energy to power those carts. It places 7/4 more wear on the rails and has a higher probability of increased maintenance costs on those rails, let alone the TBM itself. All of this to make the same forward progress in the same amount of time. Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. You don't get to pretend that larger material removal rates are free. You don't get to pretend that the 7/4 more cutting wheels don't cost more money to replace. The 7/4 more energy to run the TBM isn't free. Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less.
  - timschmidt 2y ago
    > Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less. More expensive than one, potentially less expensive than two. Happens in manufacturing and industry all the time. Transport cost alone could be enough to account for it. > Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. Sounds like you should have a chat with the project lead.