In situ experiments on width and evolution characteristics of excavation damaged zone in deeply buried tunnels

The seven long tunnels of Jinping II hydropower station are deeply buried. The width and evolution characteristics of excavation damaged zone (EDZ) are the key problem to the design of tunnels excavation and supports. In order to study this problem, several specific experimental tunnels with different overburden and geometric sizes were excavated at this site. Digital borehole camera, sliding micrometer, cross-hole acoustic wave equipment and acoustic emission apparatus were adopted. This paper introduced the comprehensive in situ experimental methods through pre-installed facilities and pre-drilled boreholes. Typical properties of the surrounding rock mass, including cracks, deformation, elastic wave and micro fractures, were measured during the whole process of the tunnel excavation. The width and characteristics of formation and evolution of tunnels EDZ were analyzed under different construction methods involving of TBM and drilling and blasting, the test tunnels were excavated by full-face or two benches. The relationships between EDZ and tunnel geometry sizes, overburden and excavation method were described as well. The results will not only contribute a great deal to the analysis of rock mass behavior in deeply buried rock mass, but also provide direct data for support design and rockburst prediction.