Excavating from East and West

Mining will occur from both ends of the tunnel. However, the tunnels won’t meet in the middle. With a larger roadheader and a several-month head start, the tunneling crew from the east side will excavate about two-thirds of the tunnel, while the crew from the west side excavates the other third.

Four Sections

The Fourth Bore Tunnel will be excavated in four sections: The eastern topheading and bench, and the western topheading and bench.

Topheading and Bench

First, the upper portion of the tunnel, called the topheading, will be excavated. The topheading’s initial tunnel lining support (shotcrete, lattice girders, and bolts and spiles as needed) will be built as the mining proceeds. By building the top section of the tunnel first, the miners maintain better control of the surrounding rock, thereby increasing safety and allowing for more efficient tunneling.

When the upper portion of the tunnel is excavated and supported, the miners will begin excavating the bench, or the lower portion of the tunnel. In a similar fashion, support will be built as the bench excavation proceeds.