Design of a Deep Tied-Back Excavation Adjacent to the Los Angeles Metro Red Line Subway

The Wilshire Grand Redevelopment project in downtown Los Angeles includes the demolition of a 16-story hotel built in the early 1950s and the construction of a new 73-story tower that will be the tallest building in the western United States. The construction of the basement and foundation of the new tower required excavation up to 93 feet (28 m) deep. One side of the basement excavation, with an excavation height along that side of up to 57 feet (17 m) required placement of shoring within about 6 feet (1.8 m) to 10 feet (3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. The temporary shoring support system for the Wilshire Grand deep excavation consists of soldier piles spaced generally at 8 feet on center with multiple levels of tieback anchors which extended above the subway tunnels. The bottom-most level of bracing consisted of rakers supporting walers attached to the soldier beams, because a bottom-most level of tie-back anchors could not be installed due to the presence of the subway tunnels. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces (raking struts). Deflection monitoring was also performed on the interior of the Red Line tunnel. The paper presents a description of: (1) deflection and earth loading criteria for design; (2) geometric constraints of shoring design; (3) limitations on tie-back shoring due to limited easement width; (4) comparison of deflection and load from design criteria using results obtained from geotechnical instrumentation; (5) raker preloading methodology; (6) temperature dependence on raker loading; and (7) deflection distribution as a function of distance from shoring wall. Lessons learned regarding preloading of rakers and deflection of shoring and retained earth will be described.