Observed Performance and Analysis of SEM Cavern Construction in Downtown Los Angeles

AbstractThe Regional Connector Transit Corridor (RCTC) project consists of an 89-m long, 17.7-m wide, and 11-m high span crossover cavern constructed beneath critical infrastructure using the sequential excavation method (SEM) at relatively shallow depth (15 m below ground surface) in downtown Los Angeles. The cavern construction involved a complex three-drift, two-sidewall configuration and was excavated in the Fernando formation, a weak clayey siltstone. This paper presents a comprehensive case study of the cavern excavation and initial support construction. The performance of the urban SEM was assessed and complex ground behavior was examined once per excavation step during the construction process. The overall average surface settlement was 20 mm and the largest single-stage incremental movements occurred in the center bench and invert excavation, accounting for 30%–40% of the total deformation. The subsurface experienced less differential settlement during the side drift excavations due to the ovate excavation profile and the restriction from surrounding infrastructure. The measured center crown vertical displacement varied between 5 and 10 mm and the center invert initial support was recorded to heave 8–12 mm. A systematic procedure was developed to apply a deliberate observational method. The lessons learned support that appropriate real-time analysis improves the quality of regular data review, continuously updating the knowledge of risk and enabling prompt adaptions and optimizations of the design and construction along the SEM tunneling process.