Tunnel Construction Methods
There are two primary methods for building tunnels depending on the conditions where the tunnel is being built.
Tunnel Boring Machine
A Tunnel Boring Machine (TBM) is a giant drill. The TBM slowly drills through the earth, digging tunnels needed for an underground project. Once the TBM is below ground doing its work, it is unlikely that people on the surface will see, hear or feel its operation. Metro continually monitors the progress of the TBM underground to ensure it is operating safely at all times.
The latest technology uses pressurized, closed-face TBMs that maintain pressure in the ground surrounding the tunnel, protect workers inside the TBM, and immediately install concrete tunnel liners that support the tunnel as they proceed in the newly drilled portions of the tunnel. The front section of the TBM is called the shield with a cutter head at the very front. Each cutter head is engineered for the specific ground conditions where the TBM will be operating. A TBM is about the length of a football field at full size and, depending on ground conditions, tunnels at the rate of about 60 feet per day.
Several acres of property are needed to launch the TBM to drill the tunnels. That location is often used to remove the dirt from the tunnels. A smaller property is typically needed to remove the TBM at the tunnel’s endpoint. Between these starting and ending points, TBMs are generally sufficiently deep to avoid any impact to underground utilities.
Exact locations for launching and extracting the TBM, as well as how much space will be needed to do so is determined as part of the environmental process.
Cut and cover tunnel construction generally begins by opening the ground surface to an adequate depth to support or relocate existing utility lines, and to install soldier piles or other earth-retaining structures. The surface opening is then covered with a temporary street decking so traffic and pedestrian movement can continue overhead while excavation proceeds beneath the decking. The temporary excavation will be retained by an approved excavation support system, known as a shoring system. Adjacent building foundations will also be supported as necessary. Once the tunnel is built, the remaining excavated space is backfilled and the street is restored.
This methodology is typically employed when tunnels are quite shallow or other conditions preclude the use of TBMs. Underground stations are also typically built using cut-and-cover, even when the tunnels are built using TBMs. Because construction is not fully below ground, it is likely that people at the surface level will see, hear and feel construction activities.
Safety is Metro’s top priority. Along with evaluating whether a tunnel is appropriate for a project, numerous factors are considered to ensure that a tunnel will be safe for those building, using and living near the project. Underground construction processes continue to improve, as evidenced by recent transportation tunnel and underground sewers projects, and new construction of buildings with deep basements and underground parking structures here in Southern California and worldwide. Metro continually seeks to take advantage of new developments in tunnel engineering and technology. In some cases, our projects have also set new standards for the industry.
All Metro tunnels are evaluated, planned and designed with input from leading experts, including an independent Tunnel Advisory Panel that is comprised of globally recognized geotechnical earthquake engineering and tunnel experts.