Deformation Mechanism of Chungliao Tunnel in No. 3 National Highway of Taiwan: Investigation of Microstructure and Mineral Characterization of Chishan Fault via synchrotron radiation techniques

The Chungliao tunnel and Tianliao No. 3 viaduct were seriously deformed because of the fault-creep effect, which are between Chishan Fault and Longchuan Fault in Taiwan. In the past 18 years, the Chishan Fault has caused an average annual rise of 8 cm at the northern end of the Chungliao tunnel, which makes the tunnel highly uplifted, making maintenance work difficult.

In this study, petrographic methods are employed to examine the deformation mechanism of the Chishan Fault. Microscopic observations reveal the following results. Since Chishan-fault zone is influenced by the mud diapir in the southwest, pyrite is found in fractures formed by plate tectonics, and the crystal size and geometry distribution of the pyrite may enhance the fault creeping. In addition, creep, together with fracture development, increases the porosity of the stratum. Based on the synchrotron radiation technique beamline, 3D modeling technology, the fracture development in the fault-zone is more significant than that in the surrounding wall-rocks. A Fourier transform infrared spectroscopy analysis demonstrates that the water content in the specific minerals in the fault is considerably higher than that in the surrounding wall-rocks. These intensify plastic deformations and exacerbate the creep behavior of the stratum.