3 ms·

Looks like they’re over digging the trench, then partially backfilling with aggregate. So the sections aren’t sat on sediment, but rather a thick bed of gravel

Looks like they’re over digging the trench, then partially backfilling with aggregate. So the sections aren’t sat on sediment, but rather a thick bed of gravel type material that will distribute the load over a larger space. I would also be interested to know what the tunnels effective weight will be once it’s submersed. Given the sections are hollow, the road and train sections will create a buoyancy effect. Possibly enough to offset the total weight of the concrete, given they need to fill ballast tanks in-order to sink the sections. If that’s the case, they can probably adjust the ballast tanks to keep the sections near to neutrally buoyant, and thus apply little pressure to the sediment below.

earthscienceman 5y ago
This makes sense and, although I didn't include it in my comment, the buoyancy was part of my curiosity. While it's clever to take advantage of the buoyancy created by the air cavity, if they're nearly-buoyant that would also mean that the variability in traffic would create a significantly variable (as a percentage) load. I wonder how this is accounted for. For the record, I don't think I'm "gotcha-ing" the engineers by thinking of these questions. I'm sure there have been answer to these questions since before the project began, it's just that I would like to know the answers. They also must have a way to seal off segments in case of a water breach? Although, that seems awful given there will be human traffic in the tunnel
- mattashii 5y ago
  See the video for answers: After putting in a segment, the trench is backfilled with rocky material (over the segment), so that any buoyancy is negated by the fact that it's held down by a large layer of rocks. This layer presumably also prevents most, if not all, leaks from generating instant floods in the tunnel. Lastly, once completed the project will result in 5 parallel, but mostly independent tunnels (two train tunnels, two highway tunnels, and a service tunnel between the highway tunnels). I presume that the tunnels can be isolated and waterproofed seperately.
mikewarot 5y ago
Concrete is 2.4 times the density of water, according to Google, steel rebar is about 8. The video mentioned that they were plugged (and thus full of air) and were being given additional lift by the barges so they could float and be towed. So it appears they will not be buoyant.
lancebeet 5y ago
I'm not an engineer so I might be missing something, but according to Wikipedia[0]: > The precast concrete tunnel sections will have a rectangular cross-section that is about 40 metres (130 ft) wide and 10 metres (33 ft) high and (also mentioned in the video): > The tunnel will consist of 79 standard elements with a length of 217 metres If it's a perfect cuboid, this should result in a buoyancy corresponding to ~87,000 tonnes. According to the video, each segment weighs (over) 73,000 tonnes, though this presumably doesn't include other stuff that will be added into the tunnel like road surface material, rail tracks etc. [0] https://en.wikipedia.org/wiki/Fehmarn_Belt_Fixed_Link#Tunnel_characteristics https://en.wikipedia.org/wiki/Fehmarn_Belt_Fixed_Link#Tunnel...

earthscienceman 5y ago

This makes sense and, although I didn't include it in my comment, the buoyancy was part of my curiosity. While it's clever to take advantage of the buoyancy created by the air cavity, if they're nearly-buoyant that would also mean that the variability in traffic would create a significantly variable (as a percentage) load. I wonder how this is accounted for. For the record, I don't think I'm "gotcha-ing" the engineers by thinking of these questions. I'm sure there have been answer to these questions since before the project began, it's just that I would like to know the answers. They also must have a way to seal off segments in case of a water breach? Although, that seems awful given there will be human traffic in the tunnel

mattashii 5y ago
See the video for answers: After putting in a segment, the trench is backfilled with rocky material (over the segment), so that any buoyancy is negated by the fact that it's held down by a large layer of rocks. This layer presumably also prevents most, if not all, leaks from generating instant floods in the tunnel. Lastly, once completed the project will result in 5 parallel, but mostly independent tunnels (two train tunnels, two highway tunnels, and a service tunnel between the highway tunnels). I presume that the tunnels can be isolated and waterproofed seperately.

mikewarot 5y ago

Concrete is 2.4 times the density of water, according to Google, steel rebar is about 8. The video mentioned that they were plugged (and thus full of air) and were being given additional lift by the barges so they could float and be towed. So it appears they will not be buoyant.

lancebeet 5y ago

I'm not an engineer so I might be missing something, but according to Wikipedia[0]: > The precast concrete tunnel sections will have a rectangular cross-section that is about 40 metres (130 ft) wide and 10 metres (33 ft) high and (also mentioned in the video): > The tunnel will consist of 79 standard elements with a length of 217 metres If it's a perfect cuboid, this should result in a buoyancy corresponding to ~87,000 tonnes. According to the video, each segment weighs (over) 73,000 tonnes, though this presumably doesn't include other stuff that will be added into the tunnel like road surface material, rail tracks etc. [0] https://en.wikipedia.org/wiki/Fehmarn_Belt_Fixed_Link#Tunnel_characteristics https://en.wikipedia.org/wiki/Fehmarn_Belt_Fixed_Link#Tunnel...