Initial thermal conditions around an underground research tunnel at shallow depth

The initial thermal conditions around a shallow research tunnel in Korea were investigated based on the data analysis of rock properties, in situ observation, and a computer simulation. The measured thermal conductivity data were used for evaluating different theoretical estimations based on mineral composition, porosity, and saturation. With a consideration of total porosity, which was assumed to be twice of effective porosity, the prediction with geometric mean could be improved. In the case of three-phase system consisted of minerals, water, and air, however, geometric mean overestimated especially in low saturation condition. By using the Chaudhary and Bhandari equation for three-phase system, we could predict the measured thermal conductivity with about 10% error in low saturation condition. With the data of porosity variation and saturation expected at KURT, it was possible to predict the thermal conductivity increases with depth and stabilized at 3.2 W/mK below 50 m. Considering the surface topography around KURT, sequential tunnel excavation, depth dependent rock properties, and seasonal temperature variation of the air, the time and depth dependent thermal conditions around KURT were predicted by a three-dimensional computer simulation using FLAC3D.