Cut and Cover - Mixed Face Tunnels - Rock Excavation
Class 6 - Session 2

Rebuilding South Ferry is one of the biggest engineering challenges in the USA Today. The plan is to replace the old station and the loop line connecting it to the network. The problem is, they have to build the new station under the loop line. Excavating a tunnel over 1600 feet in length to join up with the number one line to the north. What's more, there's another line, the 4-5, running straight through the site - and it all has to be done without disrupting one of Manhattan's busiest stations.

They've reached a critical stage. They're pushing to clear the remaining rock from the long trench that will house the new station and approach tunnel. It's taking them much longer than expected. It's going to be a battle to hit the 2008 deadline. The problem, they're dealing with one of the hardest rocks on the planet. It's called Manhattan schist and was forged over 450 million years ago by volcanic activity.

rock blasing mats There's only one surefire way of clearing thousands of tons of schist while keeping to the punishing schedule. The blasting team has shifted the rock from both sides of the loop line, but explosions under the loop here could take out the subway. Jeff now has teams drilling away at both sides of the huge block of Manhattan schist lying directly under the South Ferry Station. Further down the trench, Steve is blasting away to get the approach tunnel down to the right depth.

Jeff's got the best tools in the business, but drilling the huge mass of super dense rock is a slow process and time is in short supply. They've got to break through the wall of rock by the end of this week. This is schedule-driven here. We must get under this loop and remove this rock. It's a battle on two fronts. Steve's crew is blasting their way down the trench. Jeff has men drilling day and night to stay on schedule.

They have to defeat the unforgiving Manhattan bedrock. At the other end of the site, master-blaster Steve is making faster progress. But he's got a problem. If you fire explosives in the heart of New York, you can easily injure pedestrians. Steve's crew must kick their heels. All around excavators carry on clearing hundreds of tons of rubble from the trench.

The mechanics have fitted new air filters and Steve's drill is back online. Last hole is packed with explosives and ready to be charged. There's a vital safety operation to complete. The blast zone must be covered by heavy-duty blast mats. These four-ton blankets of steel and rubber should stop rock fragments from firing out of the hole. With every blast, the whole workforce has to retreat. Drilling at the rock face comes to a halt several times a day - interruptions that Jeff can do without. "Little stressful, little stressful working within the time constraints of the transit authority, when we can work can't work."

Up top, the people in the park have to be kept back. "Stay back, please" whether they like it or not. The blast crew must make sure the subway tunnels down below are clear. Too much explosive could damage the tunnel. Even the sound of a blast could cause panic on the train. They have to wait until the trains have passed down the line. The delay makes Steve jumpy. The first explosion could make or break their deadline. There's no stopping the blasting.

Day in, day out, Steve is steadily working his way along the trench, getting it down to track level. But there is one major obstacle to opening up the whole tunnel. The rock base under the loop line, where Jeff and his drilling crews are facing one delay after another. "An exciting time, will have daylight through to an excavation. We'll be able to see the guys. We'll be able to slash into that after open excavation, and we'll be excited."

The rock wall is blocking progress, but further down the trench, the team can start laying the mass of concrete that will cover the walls and floor of the tunnel. The new station will be built here. They're down to the right depth. It's time to lay rebar for the concrete flooring. The hole is now so deep that the crane operator can't see where he's lowering the rebar. It's a dangerous operation. He needs a man to guide him.

Wednesday. 3pm a fleet of concrete mixers is ready to make the first of hundreds of fours stretching forward over the next few months. Suddenly, the whole operation is under threat. The supply of concrete dries up. The tunnel needs to be encased in thousands of tons of concrete. If the cops don't speed up the checks, he's facing months of delays. The first concrete platform is finally laid and left to set overnight.