4 ms·

That's not how that works... Any granular material, even sand, will spread the forces laterally. It's impossible for the full weight of the soil above to be app

That's not how that works... Any granular material, even sand, will spread the forces laterally. It's impossible for the full weight of the soil above to be applied to the steel structure. The only reason you need the steel structure at all is to prevent it failing incrementally. Without it, the small amount of unsupported soil immediately above the tunnel can fall, which then leaves a gap for previously supported soil to move into, which can then itself fall, etc. until the whole tunnel comes down. The steel structure therefore only needs to preserve the shape of the soil. Finally, this is steel we're talking about - 3.5mm of steel in compression is still very strong. It's already massively overkill for the tiny amount of weight it actually has to support in this case.

lmilcin 5y ago
> That's not how that works... Any granular material, even sand, will spread the forces laterally. It's impossible for the full weight of the soil above to be applied to the steel structure. Absolutely false. The ground may be plastic on a long time scales (depends on the material) and over time full weight of it can be transferred to the structure. Do following thought experiment: build a bridge out of clay over a scale. The bridge might not suddenly collapse, but it will slowly deform and over time all weight of it will rest on the scale. If you have noticed, the top layers of the tunnel are made of clay. Clay is plastic and will creep over time. It is the same basic principle as buildings falling due to ground giving under foundations.
- Diggsey 5y ago
  Even with a totally plastic material (which is a stretch, since there are certainly granular elements even in clay soil) the material will deform until it fills the gaps around the arch shape. At that point the material itself is forming an arch, and even clay has compressive strength since it has nowhere else to go. While it may increase the load on the steel, it's still a long way from having to support the full weight of the material above.

lmilcin 5y ago

> That's not how that works... Any granular material, even sand, will spread the forces laterally. It's impossible for the full weight of the soil above to be applied to the steel structure. Absolutely false. The ground may be plastic on a long time scales (depends on the material) and over time full weight of it can be transferred to the structure. Do following thought experiment: build a bridge out of clay over a scale. The bridge might not suddenly collapse, but it will slowly deform and over time all weight of it will rest on the scale. If you have noticed, the top layers of the tunnel are made of clay. Clay is plastic and will creep over time. It is the same basic principle as buildings falling due to ground giving under foundations.

Diggsey 5y ago
Even with a totally plastic material (which is a stretch, since there are certainly granular elements even in clay soil) the material will deform until it fills the gaps around the arch shape. At that point the material itself is forming an arch, and even clay has compressive strength since it has nowhere else to go. While it may increase the load on the steel, it's still a long way from having to support the full weight of the material above.