Measured Soil Response to EPB Shield Tunneling

Abstract

The first earth pressure balance (EPB) shield project in the United States, a 3,000 ft (915 m) long, 12.1 ft (3.7 m) diameter, tunnel is described. It is located in San Francisco and passes beneath a heavily trafficked commercial area known as Fisherman's Wharf. The soil profile consists of 20 ft (6.1 m) of rubble fill which overlies a 30 ft (9.1 m) layer of Bay Mud. A colluvium layer underlies the Bay Mud. The tunnel section is located within the Bay Mud, and has a cover of approximately 30 ft (9.1 m); water heads above the crown are typically 15 ft (4.5 m). The EPB shield was designed to deal with these difficult ground conditions without the use of compressed air or dewatering, and to cut through numerous abandoned wooden piles along the route. Surface and sub‐surface ground movements were monitored along the alignment during and after shield passage. The shield was found to initially heave the soil aside slightly as the face passed, but movements thereafter were towards the tunnel. Settlements were typically less than for conventional shields and, the ground water table was unaffected by tunneling.