An equivalent thermal conductivity model of tunnel surrounding rock mass with fractures based on the series-parallel theory
Chenglin Li, Yiping Liu, Guozhu Zhang, Zhongtao Wang
Abstract
To evaluate the thermal behavior of tunnels, an equivalent thermal conductivity (ETC) model for the fractured rock masses surrounding tunnels was proposed, based on the series-parallel theory, which incorporates the heat transfer characteristics of the surrounding rock. A thermal probe test (TPT) was subsequently conducted at the Dajian Mountain Tunnel (China), and the heat transfer model of the thermal probe was developed and calibrated based on the TPT results. The ETCs of different test areas in the borehole were calculated based on the fracture parameters (dip direction, dip angle, opening, trace length), obtained via a digital borehole photography test. Finally, the new ETC model, the volume-averaged model, and the non-fracture model were compared for accuracy in heat transfer calculation. The results demonstrate that (1) the error of heat transfer calculation error was reduced by 17.2% to 69.2% using the new ETC model. (2) For the ETCs of different thermal response test areas, the minimum and maximum values were 2.15 W/(m K) and 3.7 W/(m K), respectively, with the fracture effect causing up to a 41.2% reduction in thermal conductivity. (3) For the ETC of the borehole, the ETC value was 2.53 W/(m K), with the fracture effect leading to a 31.6% reduction in thermal conductivity. The effect of fractures on the thermal conductivity of rock mass could not be ignored. This ETC model addresses the challenge of determining the ETC for the heterogeneous surrounding rock of tunnels and provides a more accurate representation of the in situ thermal behavior of tunnels.