Is the tunnel unusually deep?

At approximately 90 feet below ground, station platform and tunnel depth is comparable to several other stations throughout the United States. Locally, it is about the same depth as the Union Square and Chinatown Central Subway Stations in San Francisco. Chinatown Station is approximately 100 feet below ground. Washington Metro, which is the second busiest heavy rail system in the United States, has multiple stations that are much deeper (including ones that are 117 feet, 145 feet and 196 feet deep). 

The Phase II tunnel will be around 45 feet in diameter and needs approximately that much ground cover above it to support itself. Therefore, the tunnel’s base would be approximately 90 feet below ground.