SEM deformation prediction and observation by 3D numerical analysis

This paper summarizes a computational modeling effort carried out during real-time construction of a cavern to predict ground movements and surface settlement. The Regional Connector Transit Corridor (RCTC) tunnel project in Los Angeles required the construction of a 90-m-long, 11-m-wide and 18-m-high crossover cavern. The cavern was constructed by sequential excavation method (SEM) at relatively shallow depth. The SEM cavern was excavated after twin 6.7 m diameter tunnels were excavated via earth pressure balance shield machine through the cavern profile. A three-drift seven-stage excavation configuration was designed and implemented to control ground movements within allowable limits. A 3D numerical model was developed (FLAC(3D)) to simulate the SEM construction process. The model parameters were calibrated during the initial excavation of the left drift. Thereafter, the model predictions of ground movements (convergence and settlement) provided good agreement with field measurements, providing confidence to the construction team as it proceeded.