4 ms·

I find it fascinating that the pressure is so high in their (relatively shallow) drilling area that the people doing the inspection have to go through decompres

I find it fascinating that the pressure is so high in their (relatively shallow) drilling area that the people doing the inspection have to go through decompression chambers. I'm trying to visualize how that could be, but I'm not sure what's going on - is there basically a "perfect seal" in the front of the drill head they don't want to break? And if so, won't the bore holes they are drilling do just that? That's the most interesting part of the article for me.

js2 13y ago
http://www.wsdot.wa.gov/Projects/Viaduct/Contents/Item/Display/1364 http://www.wsdot.wa.gov/Projects/Viaduct/Contents/Item/Displ... Top of the boring machine is 60 ft from the surface, but it's 60 ft high so bottom of it is 120 ft. That's 120/33 = 3.6 atmospheres in the ocean, and my reading of the article above is that it may even be higher here due to groundwater pressure. Anyway, you need decompression at anything over 1 atmosphere given enough time as I recall? Edit: fascinating machine. So yes, it maintains pressure in front of the drill head to resist ground and water pressure. http://www.hitachizosen.co.jp/english/pickup/pickup008.html http://www.hitachizosen.co.jp/english/pickup/pickup008.html http://www.ncus.ntu.edu.sg/Events/Documents/SRMEG%20Seminar%20Tunnel%20Boring%20Machines.pdf http://www.ncus.ntu.edu.sg/Events/Documents/SRMEG%20Seminar%...
ars 13y ago
It seems they are under water! They probably have an airlock behind the machine since I doubt the drill head itself is air tight against the walls. Any bore holes would have to be water proof, or at least pressurized to keep them from filling up.
cincinnatus 13y ago
The water table in Seattle is very high, about 4 feet in that area. So at the point of the blockage they are under 40 feet of water. Every ~33 feet of water is an extra atmosphere of pressure.
- InclinedPlane 13y ago
  The tunnel is not filled with water though. See abduhl's answer for the correct response.
  - msandford 13y ago
    The finished tunnel isn't filled with water because it's lined with concrete. The unfinished tunnel might be filled with water if it wasn't for the pressure that the TBM maintains at the cutting face.
    - avn2109 13y ago
      Not "might." Absent face pressure, the tunnel would definitely be filled with water. This is precisely why they've gone to the trouble of inventing slurry face TBM's and building a recirculating slurry plant on the topside. Hard rock TBM's are much cheaper because they can get away without doing any of that.
abduhl 13y ago
Large TBMs like this generally allow access to what is called the face (the area being excavated) via a double bulkhead system. The ground conditions in Seattle require that the machine be able to control the rate of excavation at all times such that a large sink hole does not form due to uncontrolled loss of ground. This is done by keeping pressure on the face. The pressures required to keep the face stable are a function of the soil properties, lithology, and groundwater. The typical way for someone to access the face is to pressurize ahead of the machine head with gas and have workers enter under pressure (known as a dive or an intervention). Workers generally are limited in how long they can be under pressure and must go through decompression. Edit: real quick calculation on my phone puts them at about 70 psi or 4.5 atm of pressure to do a manned intervention, possibly higher for safety Disclosure: underground tunneling is my profession.
- timr 13y ago
  I've always wondered how they get the kind of seal necessary to maintain that kind of pressure ahead of a tunneling machine. Is the drilled surface smooth enough to provide that kind of seal?
  - abduhl 13y ago
    I typed up a rant about boring vs drilling and the correct terminology but it sounded super nerdy after reading it. I'm not really qualified to answer your question as I'm not as intimate with the machines as I am with the ground/structure side. Most likely the seal is maintained by the machine being too big for the hole being cut or by inflatable packers. The walls of the bore can range from extremely smooth to not smooth at all depending on ground conditions. Sorry.
    - pre-show-mac 13y ago
      You girls are giving away all of our secrets!
    - StavrosK 13y ago
      Are you saying that drilling tunnels is really boring?

js2 13y ago

http://www.wsdot.wa.gov/Projects/Viaduct/Contents/Item/Display/1364 http://www.wsdot.wa.gov/Projects/Viaduct/Contents/Item/Displ... Top of the boring machine is 60 ft from the surface, but it's 60 ft high so bottom of it is 120 ft. That's 120/33 = 3.6 atmospheres in the ocean, and my reading of the article above is that it may even be higher here due to groundwater pressure. Anyway, you need decompression at anything over 1 atmosphere given enough time as I recall? Edit: fascinating machine. So yes, it maintains pressure in front of the drill head to resist ground and water pressure. http://www.hitachizosen.co.jp/english/pickup/pickup008.html http://www.hitachizosen.co.jp/english/pickup/pickup008.html http://www.ncus.ntu.edu.sg/Events/Documents/SRMEG%20Seminar%20Tunnel%20Boring%20Machines.pdf http://www.ncus.ntu.edu.sg/Events/Documents/SRMEG%20Seminar%...

ars 13y ago

It seems they are under water! They probably have an airlock behind the machine since I doubt the drill head itself is air tight against the walls. Any bore holes would have to be water proof, or at least pressurized to keep them from filling up.

cincinnatus 13y ago

The water table in Seattle is very high, about 4 feet in that area. So at the point of the blockage they are under 40 feet of water. Every ~33 feet of water is an extra atmosphere of pressure.

InclinedPlane 13y ago
The tunnel is not filled with water though. See abduhl's answer for the correct response.
- msandford 13y ago
  The finished tunnel isn't filled with water because it's lined with concrete. The unfinished tunnel might be filled with water if it wasn't for the pressure that the TBM maintains at the cutting face.
  - avn2109 13y ago
    Not "might." Absent face pressure, the tunnel would definitely be filled with water. This is precisely why they've gone to the trouble of inventing slurry face TBM's and building a recirculating slurry plant on the topside. Hard rock TBM's are much cheaper because they can get away without doing any of that.

abduhl 13y ago

Large TBMs like this generally allow access to what is called the face (the area being excavated) via a double bulkhead system. The ground conditions in Seattle require that the machine be able to control the rate of excavation at all times such that a large sink hole does not form due to uncontrolled loss of ground. This is done by keeping pressure on the face. The pressures required to keep the face stable are a function of the soil properties, lithology, and groundwater. The typical way for someone to access the face is to pressurize ahead of the machine head with gas and have workers enter under pressure (known as a dive or an intervention). Workers generally are limited in how long they can be under pressure and must go through decompression. Edit: real quick calculation on my phone puts them at about 70 psi or 4.5 atm of pressure to do a manned intervention, possibly higher for safety Disclosure: underground tunneling is my profession.

timr 13y ago
I've always wondered how they get the kind of seal necessary to maintain that kind of pressure ahead of a tunneling machine. Is the drilled surface smooth enough to provide that kind of seal?
- abduhl 13y ago
  I typed up a rant about boring vs drilling and the correct terminology but it sounded super nerdy after reading it. I'm not really qualified to answer your question as I'm not as intimate with the machines as I am with the ground/structure side. Most likely the seal is maintained by the machine being too big for the hole being cut or by inflatable packers. The walls of the bore can range from extremely smooth to not smooth at all depending on ground conditions. Sorry.
  - pre-show-mac 13y ago
    You girls are giving away all of our secrets!
  - StavrosK 13y ago
    Are you saying that drilling tunnels is really boring?