Ground Collapse in EPB shield TBM site: A case study of railway tunnels in the deltaic region near Nak-Dong River in Korea

This case study reports on the massive ground subsidence that followed the structural deformation of tunnel segments during the excavation of cross-passage of two main tunnels excavated by earth pressure balance shielded tunnel boring machine (EPB shield TBM), constructed in the deltaic region near the Nak-Dong River in Korea. Continuous leakage of groundwater and ensuing progressive soil inflow to the tunnel induced massive ground collapse even though ground treatment was extensively applied to the tunnelling zone. A series of forensic investigations were conducted using compressional wave velocity of sediment cores recovered from the collapsed zone, geostatistical analysis for 3D velocity profile, X-ray computed tomographic imaging of the cores, and the synthesis of reinforced gassy soils. The investigation results and the other relevant case studies confirmed that the existence of gas in soils was a primary cause to form unexpectedly heterogeneous pore structure in treated soils. The omnipresent and fissured pores provided an influx path of groundwater and soil leakage into the tunnel, which was expedited further by high artesian pressure condition and finally resulted in the ground weakening. It triggered the unbalanced stress concentration to the tunnel segment, as validated by the numerical simulation, and caused the structural deformation which finally led to the ground collapse. The findings of this case study highlight the critical importance of in-situ soil characterization before and after soil reinforcement for underground excavation particularly in the deltaic region.