Underground tunnels for transport systems

In densely populated regions, tunnels hundreds of feet below the earth´s surface may be attractive guideways for high-speed transportation systems. Advantages include stability, isolation from weather variations, ease of positive control of the system, absence of interference from other structures and transportation modes, and potential alternative uses of the tunnels for such functions as storage, fluid transport, and defense purposes. If the system terminals are located near the surface, propulsion and braking requirements of the vehicles can also be greatly simplified. The principal deterrent is construction costs. These are being reduced by mechanized tunnelling mechines now undergoing much development but hard, strong rock still requires slow, expensive drill and blast techniques. Research to rapidly weaken hard rock so mechanical moles can tunnel it economically is being done. Both thermal treatments and the use of water-borne surface active agents are effective weakeners. A 1000-watt continnous-duty CO2laser can reduce the strength of marble or granite by a factor of ten in 3 to 5 seconds exposure time.