The METRO Bus Tunnel, restricted to diesel/electric busses, passes underneath the downtown area of Seattle from King Street Station on the south to the Convention Center at the north end. The 18'-11" (inside diameter) tunnel was constructed using the "two pass" method of construction that incorporates precast concrete segments as the liner for the initial tunnel core, with cast-in-place concrete placed as the final lining of the tunnel.
The segments used in this type of construction are sometimes described as "junk liners" because they perform the function of shoring the tunnel bore, and are not required to make a watertight, final fit.
Approximately 16,000 precast segments were used in the 1¾ mile tunnel, with a typical segment piece measuring 6" x 4'-0" x 8'-10" in size.
