China unveils 4.5 m boring-and-blasting tunnel prototype

Deep underground, the machine's most useful move may be a short retreat. Xianglong, a tunnel prototype with a 4.5 m diameter, developed in Wuhan by China Railway Science & Industry Group and Tsinghua University, can pull its cutterhead back behind a protective shield, drill into solid rock, and place explosives before returning to crush the loosened face.

That combination is the new feature. Conventional soft-ground tunnel-boring machines cut through soil, clay, sand, gravel or water-bearing strata while a shield supports the tunnel face. Hard-rock machines use heavy cutters and grip directly to the rock wall. Xianglong is designed for the difficult transition between those conditions, where soft ground can give way to a rock face.

Its hollow-point cutterhead contains drilling equipment and access channels for holes many meters deep. Those holes can be filled with special low-yield, high-velocity explosives and tamped with sand or clay. When detonated, they are intended to create micro-fractures in the rock without throwing out debris or producing a massive concussion; the shield is designed to deflect and absorb the force before the cutterhead sweeps up the pulverized material.

And so what, concretely? A machine that can bore and blast in one system could reduce the need to stop or switch methods when the geology changes. Chinese state media says Xianglong may improve excavation efficiency by 30% over conventional boring machines, and its shield can vary in diameter to fit different concrete lining segments. That could matter to crews building deep tunnels through mixed ground, if the claim holds in operation.

The limit is clear. Xianglong has been built, but its actual performance remains to be seen. The efficiency figure comes from Chinese state media, and no independent operating result is reported here; for now, the machine is a prototype rather than an established tunnel-building platform.