To build a high-speed rail tunnel under the Pearl River Estuary in southern China, engineers turned to a machine that looks like a steel dragon. It has dug as deep as 113 meters below the water, and at its lowest point, the finished tunnel will reach 116 meters.
At that depth, the water pressure is intense, and the ground is far from simple. Along the route, the machine has to pass through 13 layers of rock and soil, five kinds of mixed ground, and six fault zones. At its front, a huge rotating cutterhead crushes whatever is in the way as the machine moves forward.
All that broken material has to go somewhere. Two pipe systems handle it. One sends slurry, a thin, muddy liquid, to the cutterhead to reduce friction. The other carries thick slurry full of broken rock and soil back to the surface. At a processing plant, the solids are separated out, and the cleaned slurry is sent back down for another trip.
Digging is only half the job. Right behind the cutterhead, workers fit precast concrete pieces into place to form the tunnel wall. Each piece is about 2 meters wide, and nine of them make one ring around a tunnel more than 13 meters across. Because the wall goes up just behind the digging, the new tunnel gets support right away, and the work moves faster.
Crush the ground, pump it away, build a ring, move on. Repeated for more than four years, that cycle took the machine over 4 kilometers forward and 113 meters down.