Deformation control for large-section tunnel construction in fractured carbonaceous slate

Excessive deformation has been observed during the construction of large-section (over 100 m²) tunnels in soft rock all over the world. A case study of deformation control for large-section tunnel construction in fractured carbonaceous slate was conducted. Xingzishan tunnel, China, was first excavated using the three-step reserved core soil excavation method (TRCSEM); however, this resulted in serious rockfalls at the tunnel arch and local collapse. To enhance the stability of the tunnel face, three different tunnel support systems with relatively stiff structures were proposed and tested along the tunnel chainage in the same rock condition, but all of them failed. The mechanism of the TRCSEM was revealed by field measurements, highlighting the significance of the stability of the upper bench of the tunnel face. In view of this, the three-step temporary inverted arch method (TTIAM) was employed and used with advance support. With this new method, the upper bench of the tunnel face was reinforced with fibreglass bolts to mitigate the deformation. Compared with the TRCSEM, the TTIAM effectively controlled large deformation at the tunnel face during the excavation in fractured carbonaceous slate with an acceptable construction cost.