The cooling effect of high geothermal tunnel construction environment: A case of ice and spray method in an extra-long tunnel

With the tunnel construction is moving towards the direction of "deep, large and long", high geothermal and its heat damage occurs frequently, and pure mechanical ventilation cannot meet the temperature requirements of the tunnel construction environment. In this paper, through on-site monitoring and numerical simulation methods, we conducted an in-depth study on ice block cooling and spray cooling auxiliary methods, revealing their effects on the tunnel temperature and comparing the impact of key parameters such as different shapes and layout of ice blocks, different flow rates and temperatures of spray and different positions of nozzles on the cooling effect. Results show that the fresh airflow will be heated by the surrounding rock, resulting in the worse cooling effect of the tunnel section the further away from the working face. Compared with pure mechanical ventilation, ice blocks, and spray cooling methods cooled the tunnel by 0.5-2.5 degrees C and 2-5 degrees C respectively. For the ice block cooling method, the flaky shape and double-sided arrangement is the fastest cooling design scheme, and the further away from the working face, the faster the cooling speed increases. The role played by the ice blocks is mainly to cool the fresh airflow heated by the surrounding rock and maintain its temperature. For spray cooling method, the tunnel environment can be controlled in the ideal condition by setting the nozzle flow rate and droplet temperature to 0.02 kg/s and 20 degrees C under the conditions of the tunnel cross-section of about 28 m2, fresh airflow of 25 degrees C, and return airflow velocity of about 2.1 m/s.