Abstract:

The Dengloushan tunnel in Yunnan province, China, crosses the Xiaojiang fault with strong hydrothermal activity. The present study aims to evaluate the hydrothermal activity pattern at the tunnel site through geological analysis and subsequently evaluate the linear engineering thermal hazard. The regional development pattern of the Xiaojiang fault is analyzed based on the obtained geological data. Furthermore, the distribution pattern of the overall geological structure of the Xiaojiang fault system at the tunnel site and its spatial relation with the hot water distribution on the surface are evaluated. The recharge elevation, heat storage temperature, and circulation depth of hot water in the tunnel area are analyzed using the elevation effect of hydrogen and oxygen isotopes and the silica temperature scale. The overall migration characteristics of hot water in the tunnel area are also determined. Additionally, the thermal borehole data and hydrochemical data at the tunnel site are comprehensively analyzed. Moreover, hot water migration and evolution models for the Xiangbi and Longtanying hot springs near the tunnel entrance and exit, respectively, are determined. Results show that the migration paths of both the hot springs have no intersection with the tunnel axis in space and that the strata of hot water migration are different from those crossed by the tunnel. Thus, the possibility of heat damage owing to hot water at the two places is low. However, the geothermal gradient in the temperature measurement in the borehole shows that although no surface heat is detected in the part of the tunnel through the mountain, a slight risk of thermal hazard still exists at a depth of >450 m.

Key words: tunnel, thermal hazard, Xiaojiang fault, hydrothermal activity characteristics, hydrochemical analysis, hotwatercycle evolution