Analysis of the Tunnel Face Failure during the Excavation and Evaluation of Excavation Method and Sequential on Ciuyah Tunnel, Lebak Regency, Banten Province

The difference in geological and geotechnical conditions during design and implementation are frequent due to the lack of data collection and diverse rock variations. An important step in designing the excavation method, stages, and support system are determining the rock mass’s actual condition. Inaccuracy in evaluating rock mass quality can cause a collapse during tunnel excavation. This study used an example of a collapse case on the inlet side of the Ciuyah tunnel. The classification system used in the design was the JSCE 1975, but collapses occurred at several locations during excavations. An additional assessment of the tunnel stability was carried out by doing surface surveys and evaluations in STA 0+123 using the RMR₈₉, GSI, and JSCE 2016 classification systems. The rock mass in this location consists of tuffaceous claystone and tuffaceous sandstone with very poor quality. The empirical evaluation showed that Daraei’s multigraph could be used to determine the collapse pattern for very poor rock. For the excavation method, stages, and support systems, it is preferable to apply the RMR classification system with consideration for the maximum stand-up time. This research aims to provide information that may be used for other projects with similar geological circumstances.