A Day in the Tunnel

View of finished tunnel for Line 6 of China's Chongqing Metro

“Groundhog Day,” the movie: Someone said it’s like that. Every day isn’t exactly like every other day, but they do tend to run together so that one looks like another.  We are on a good tunnel job. This one is in Chongqing, China and it’s a subway. There is civilization and a hotel to live in for a change. There are two 6-meter tunnel boring machines making twin tubes for the metro system.

The translator, Nancy, shows up after breakfast and we get in the car for the short drive to the job. The driver stops and I sit on the bumper to change into rubber boots. Nancy picks up her hardhat and we head into the yard where the adit leads into the tunnel. As we start down the decline, the smell of the bored rock and sprayed concrete is strong. I pull up my respirator for a while and we walk down the ramp. It is dark and my cap lamp is a tiny candle in a cavern. The video below shows the entrance to the tunnels:

The decline to the main tunnels is short and we soon come to the bottom. There is a tiny man sitting on a box and leaning against a huge truck tire. He is the pump tender. His job is to make sure the water pumps keep the tunnel from flooding. He is dressed in red coveralls with reflective stripes. He is barely five feet tall.  Nancy says something to him and we continue around the corner. I tell her that I could never have that job because it would make me crazy to sit all the time. She agrees and we walk out into what will soon be a subway station.

There is a large cavern here that was done by drill-and-blast before the tunnel machines arrived. Each tunnel has two tracks raised on a trestle system that runs the entire length of the job. The tunnels are connected at the station, but are about 50 meters apart in the rock. Everything is measured in meters in this part of the world. We go out onto the trestles and step carefully from frame to frame to get to the crossover to the far tunnel. The frames are half a meter apart, but it is a meter or more to the ground if you slip.

There are short walkways that connect the two tunnels at the center of the station. We duck between the ropes to get onto the walkway. The far tunnel has a double-wide expanded metal grating in the station area. It narrows to a single catwalk wide enough for one person as it goes into the bored part of the tunnel.

The night shift workers have collected here to wait for the man-trip out of the tunnel. They are nearly all shirtless as the tunnel is hot and humid and it is too warm to wear much clothing. They are almost all squatted on the grating and smoking. Nancy chats with most of them as she passes and they all chatter back.

 We continue into the tunnel along the narrow walkway occasionally catching a sleeve on the vertical posts of the handrail. It is warm and steamy. See a video of workers on the walkways below:

The tunnel is supported with ring beams every 750mm. There is wire mesh above the rings to hold the rock back and then shot-crete covering the wire between the rings. The trestles for the rails are also on the same spacing and everything is tied together. The opposite wall has knob-and-tube brackets that hold individual cables for the 400 Volt lighting and the phone system. There are tunnel lights on that side wired between one of the 400 Volt cables to ground. It is how they do things here. It works. Below that the high voltage cable snakes along on hooks.  There are water pipes below the cables for bringing cold water in and pushing dirty water out. In the heat, the “cold” water is usually about 80 degrees Fahrenheit.

 The air is mostly clear and the smell of the rock and concrete has gone away now. It is still hazy because the air is so wet, but the need for the air mask is gone and it is cooler without it. After clattering along the grating for several minutes, we reach the back of the machine. The pipe crew is busy adding new plumbing.

The machine is long and we squeeze forward past the machinery to the operator’s cabin.  The cabin is small. The door opens and I make a point of counting heads and waving my hand to blow away the smoke. There are seven people in a space about twice the size of a closet. Eventually, some of them wander out and it is possible to get in to take down the numbers from the previous day’s production and check with the operator for problems.

Today is unusual as this machine has broken through into the next station overnight. The TBM is gripped on the station’s walls with timbers and the crew is busy preparing to install a walking beam structure that will get the machine to the next starting chamber. See a video of the crew performing maintenance on the TBMs in the underground station below:

 After a brief look at the cutterhead and things not normally visible, we start back out to the other tunnel. It is slightly uphill and the tunnel is still a sauna. By the time we get back to the first station, my shirt is dripping. Wiping away sweat only makes room for more. We stop for half a bottle of water and head across the bridge to the near tunnel.

There is a walkway on the right side of this tunnel and most things are a mirror image of the one we just came from. We duck around some surveyors who are looking at the back tunnel. It seems shorter somehow and we are soon at the machine.  This TBM is in maintenance now and there aren’t any big problems to discuss. There are only five people in this cabin, but I make the show of counting heads and waving smoke again. They only smile and shuffle out. 

Nancy chats with the boys out on the deck for a while to find out if anything important has been missed and we head back out to the daylight.  The pump tender has moved up the adit. I pick up his reflective stripes with my cap lamp. He is sitting shirtless in the dark and smiling at the opposite wall. We continue up the ramp and out to the surface. The driver is waiting.