Prediction and Observation of a Los Angeles Metro Red Line Subway Tunnel’s Response to Adjacent Deep Excavation

The Wilshire Grand redevelopment project in downtown Los Angeles includes demolition of a 16-story hotel and construction of a new 73-story tower. Construction of the basement and foundation of the new tower required excavation up to 93 feet deep (28 m). The south side of the excavation, up to 57 feet deep (17 m), required placement of shoring within about 6 to 10 feet (1.8 to 3.0 m) horizontally away from a 400-foot-long (122 m) section of the closest of the existing Los Angeles Metro Red Line twin subway tunnels adjacent to the site. The tunnels were constructed using a 22-foot-diameter (6.7 m) digger shield, and were initially supported with expanded, precast concrete segments, with final lining consisting of cast-in-place reinforced concrete. The temporary shoring system for the deep excavation consists of soldier piles generally spaced at 8 feet (2.4 m) on center with multiple levels of tieback anchors extending above the subway tunnels. The bottommost level of bracing consisted of rakers supporting walers attached to the soldier beams because installation of tieback anchors at that level would conflict with the tunnels. Numerical analyses were performed to predict tunnel movement caused by the excavation and its effects on the train operations. This paper describes the numerical modeling and comparison of the predicted and observed tunnel responses to the deep excavation and new tower construction.