Detailed thermal environment classification of high geothermal tunnel based on thermal comfort indices

High temperature and high humidity in geothermal tunnels pose significant risks to the well-being of construction workers. This study aims to establish critical environmental thresholds to protect workers in these challenging conditions. Field tests were conducted in a high geothermal tunnel in southwest China, continuous measuring parameters such as dry bulb temperature, relative humidity, wet bulb globe temperature (WBGT), skin temperature, and heart rate. The collected data were analyzed, leading to the development of a nonlinear programming mathematical model. This model enabled a systematic classification of the tunnel's environmental conditions and the calculation of threshold values for various environmental parameters. Furthermore, critical thresholds for heat stress indicators such as the discomfort index (DI), heat index (HI), and relative strain index (RSI) were determined. The study categorized the tunnel's hot and humid environment into five distinct zones: comfortable zone, general comfortable zone, safety zone, heat tolerance zone, and danger zone. The findings indicate that in this geothermal tunnel, the comfort zone thresholds are 26 °C for dry bulb temperature, 30 % for relative humidity, and 20 °C for the WBGT index. The safety zone thresholds are 37 °C, 64 %, and 32 °C, while the danger zone thresholds are 43 °C, 81 %, and 40 °C, respectively. For heat stress indices, the safety thresholds for DI, HI, and RSI are 25.42 °C, 26.31 °C, and 0.28, while the danger thresholds are 28.59 °C, 35.30 °C, and 0.38, respectively. This research provides essential insights for safeguarding the health of construction workers operating in high geothermal tunnel environments.