There are a number of successful tunnel projects with sizes similar to the SR 99 tunnel, which is being designed with an exterior diameter of 57.5 feet and length of 1.7 miles. Examples include:
- China' Shanghai Yangtze River Tunnel: Includes two bores, each about 5 miles long with a 50.6-foot diameter.
- Germany's Fourth Elbe River Tunnel: Includes a single bore with a length of about 2 miles and a 46.6 foot diameter.
- Spain's Madrid M30: Includes bores with a length of about 5 miles and a 49.5-foot diameter.
Tunnel boring machines have been developing at a rapid rate with a major increase in diameter, better ground control, and improved reliability. These machines can now safely and efficiently excavate almost and type of soil, rock or groundwater conditions.