Study on temperature field and influencing factors of the high geothermal tunnel with extra-long one-end construction ventilation

Abstract

The increasing number of deep-buried high geothermal tunnels with extra-long one-end ventilation distance makes it critical to develop an accurate and efficient computational model to predict the ambient temperature of such tunnels. A numerical model capable of quickly calculating the longitudinal temperature field of the tunnel construction environment has been established by combining the finite difference method and finite element software. The model, which accounts for the initial ground temperature distribution as well as the heat exchanges at the working face, agrees well with the field monitoring results of the Dechang Tunnel. The research results show that the air temperature during tunnel construction varies as follows: stable heating section in the duct → rapid heating section near the working face → slow heating section of the return air → temperature drop section of the return air. The temperature of the characteristic sections (duct outlet, working face, and tunnel maximum temperature section) all show a quadratic function upward with construction length (L). These also show an exponential trend with ventilation volume (v), insulated duct thickness (t1), and tunnel insulation layer thickness (t2). To achieve a temperature change slope greater than 0.5 in the working face of the Dechang tunnel, the recommended criteria include a ventilation volume below 110 m3/s, an insulated duct thickness below 3 cm, and a tunnel insulation layer thickness below 7 cm. The research results can provide a basis for ventilation and heat insulation design of the extra-long high geothermal tunnel.