[murbard2]

I don't think the problem is the pressure - holding up one atmosphere is not big deal - I think it's the porosity. It just takes one hole, anywhere, and pscchht you start losing your vaccum. You have to continuously run pumps to get rid of the air that finds its way in, and their efficiency is inversely proportional to the pressure difference.

[Robotbeat]

This is also true of below-water-table tunnels and pipelines of various sort, which are often at much higher pressure and with much greater environmental risk.

[AstralStorm]

It is quite a bit easier to hold back a liquid than a gas. Different minimum pore size and you get hydrophobic chemistry and surface tension on your side.

Especially at such big pipe diameters as required.

You can use cheap materials like modified concrete to patch a hole in an underwater tunnel, not so in a vacuum tube - that needs something much better and more expensive.

[Robotbeat]

Not true. You can use concrete to hold back a vacuum and cheap materials like a thin polymer coating, too. Remember, Hyperloop does not require an ultra hard vacuum.