Monitoring of a Los Angeles Metro Red Line Subway Deflection Due to Adjacent Deep Excavation

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 to 10 feet (1.8 to 3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. A monitoring and contingency plan was established for the shoring itself and the subway tunnel. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces. Deflection and vibration monitoring was performed on the interior of the Red Line tunnel utilizing automated total survey machines installed in the tunnel; the monitoring system provided real time data and automated alerts were provided when threshold readings were exceeded. Presented herein is a description of: (1) deflection and earth loading criteria for monitoring; (2) response levels utilized during monitoring; (3) data transmission methodologies; (4) communication protocols; (5) installation of instruments; (6) evaluation of monitoring data; and (7) modifications to monitoring protocols during project.