tunnel excavation

Drill and blast, or a cut and face machine

The tunnel is lined with pre-cast sections and the machine is sometimes left in a side tunnel.

Yes, it is size dependant for all tunneling methods, and materials dependent. Tunnelers like the one shown can not drill through good granite or other hard rocks effectively, it is all about what the cutting heads can cut through efficiently. Good hard rock drilling and blasting in those cases becomes the option of choice. If the rock is too weathered, soft, and/or unstable, then blasting may not be suitable or efficient (if it is too unstable you end up casing/lining as you progress to support the opened shaft). The size of the boring also is a big deal, smaller diameter borings tend to have more support internally, so cheaper methods of boring may be utilized with out much casing/lining in some cases. A smaller enough hole and short enough distance you can drill it, efficiently even in the hardest rock for relatively low cost.

I used to work for JARVA (now Robins, as I understand it). We made tunneling machines from 6 ft dia up to 30 ft dia.

Soft rock needs high torque on the cutter head and low thrust. Hard rock needs low torque, but high thrust. The largest machine I worked on, 30'-1", had 2400 hp applied to the cutting head @ 4 RPM (IIRC), enough to cut soft rock and 3,000,000 lb thrust for hard rock. This let one machine do both.

For very soft materials, like clay, we built a shield. It supported about a 10 ft section of tunnel while an excavator (or men with shovels) dug the face. Supports were installed right behind the shield.

Unstable rock requires supports as close to the face as possible. We build some machines with a hydraulic arm to pick up a support section and hold it in place while workers installed rock bolts to keep it there.

As you might guess, I was in the machine engineering dept, not in construction, but if you have any specific questions, e-mail me.