A vacuum only weighs 1.28 grams less per litre than air. Since there are 1000 litres in a cubic meter, a 3x3x3 meter cube (approximation of a cross section of a unit of the tube) would contain 27000 litres whose air would weigh 76 lbs.
In other words you're talking about sinking 3 meters of tube that would weigh 76 fewer pounds than if there were air. This is probably a small effect compared to the weight of the strong metal casing that would need to surround such a vacuum.
So since we can sink normal air-filled subways quite normally (e.g. the BART in San Francisco) this must be a well-established technology.
Well I was responding to a specific poster who focused on a different issue. Steel tubes can easily withstand a vacuum... a round tube is an extremely strong structure. Sure the joints between pieces would be the harder part, especially with earthquakes and the like forcing flexibility.
In other words you're talking about sinking 3 meters of tube that would weigh 76 fewer pounds than if there were air. This is probably a small effect compared to the weight of the strong metal casing that would need to surround such a vacuum.
So since we can sink normal air-filled subways quite normally (e.g. the BART in San Francisco) this must be a well-established technology.