Evolution of fractures in the excavation damaged zone of a deeply buried tunnel during TBM construction

Abstract

This paper presents results of in situ measurements of fracture evolution within the excavation damaged zone (EDZ) surrounding the tunnel boring machine (TBM) excavated headrace tunnel No. 3 of Jinping II hydropower station, China. Fractures were measured by borehole images obtained with a digital panoramic borehole camera throughout the processes of tunnel excavation and support installation. Boreholes for the digital camera were pre-drilled perpendicular to the tunnel axis from a pre-excavated branch tunnel, and a series of digital images of the borehole walls were made as the TBM passed. Digital processing revealed the lithology, pre-existing fractures and characters of rock core discing of the surrounding rock mass. Processes of fracture initiation, propagation, and closure were observed, and width, occurrence and zoning of EDZ were measured. The relationship between fracture evolution and TBM construction progress is described in detail. Based on true triaxial tests and theoretical analysis of unloading conditions of deep rock masses, the occurrence of tunnelling induced new fractures at the EDZ is discussed with respect to in situ measurement results. The results provide valuable data for EDZ analysis, future designs and the construction of deeply buried tunnels using TBM.