Last week I got a tour of the new tunnels under construction for the Metro Line A (green) extension. One of my students is the division chief, while another is one of the project’s mgrs. The project started last year, and earlier this year the TBM (Tunnel Boring Machine) began operation on one of the sections that links two metro stations.
I’ve wanted to go into the tunnel site for a while, and this month visitors were given the okay. My student gave a call to the tunnel techs, who prepared everything for me and the mgr. We got there around 9 a.m. for a 2hr tour.
The tour began with a 40-meter cage elevator drop to the base of the tunneling operations. This area is an arena-size area, made specifically to put together the 100-meter long TBM. I took pictures throughout the walk.
The silver pipes leading into the two tunnels are ventilation pipes. These carry fresh air to the deepest part of the tunnel, where men cut through the soil and rock. We began a 1-km walk into the tunnel that will some day house the westbound tracks. Along the way, our guide explained the different things we saw: an intake pipe for water, a pipe for pressurized air (pneumatics), a waste pipe, and the electrical cable holding 200 kw of power to light up the tunnel and machinery.
I was surprised to find very little activity in the tunnel. Just the basic needs of the tunneling crews working at the solid walls. The mgr told me this is for safety reasons: no need for any more people than absolutely necessary.
Past the concrete sectionals already in place, we crossed over to a tunnel in which the Austrian Tunnel Method was being used. This uses a squid-armed machine that basically bites its way through soil and rock. In this case, the crew was working its way through a section of solid sandstone, twelve meters high and sixteen wide.
The drill team cuts through 3-ft of rock at each section, and then must shore up the cut with steel lathe and spray-on concrete. The newly cut rock can withstand the pressure for a couple hours before it can collapse, so the workers move expeditiously.
After taking in this team’s work, we backtracked and got into the next tunnel and onto the TBM. Truthfully, I expected some space-age looking machine like something out of the movies or a comic book. In fact this machine resemble a freight train that’s missing its cover.
The TBM rolls on rails, and its auger bit is a mere 8 meters in diameter. Nonetheless, the self-contained machine bores, shores, and extracts earth at the rate of 16 meters per day. It’s a 24-hr operation.
Oddly enough, the tunnel is warm. No need for heating in winter or air-conditioning in summer. The material is extracted by a conveyor belt system. All materials are brought into the tunnel on low-riding, dauchshund-like machines that have steering wheels at either end — there’s no room to turn these puppies around in the tunnel.