SBT – TBM excavation with a single‐shield machine in unstable and squeezing ground – Excavation and investigation measures

Abstract The continuous excavation in contract section SBT2.1 of the Semmering Base Tunnel posed high demands on advance exploration, prediction, and the selection of countermeasures due to complex geological and geotechnical conditions. Rotary‐percussive drilling was applied to investigate unfavourable ground sections. However, drilling‐related influences and varying spatial orientations of geological units led to significant prediction uncertainties in certain sections, which affected the selection of the planned supplementary and special measures. The special measure SM3 (injection pipe umbrella) proved to be of limited effectiveness due to groundwater inflow into the ground, high requirements regarding prediction accuracy, low flexibility, and considerable technical effort. In contrast, the supplementary measure ZM1 (foam injection) demonstrated high effectiveness and flexibility. It enabled rapid stabilization of collapsed material, filling of voids, and a controlled resumption of excavation even in heavily disturbed ground sections. By providing additional openings in the shield, the injection range and thus the effectiveness of the measure could be further increased. The results show that flexible, non‐stationary support measures offer significant advantages in deep tunnelling under complex geological conditions. Based on these findings, practical “lessons learned” are derived for future TBM projects.