Effects of initial ground temperature on frost-penetration of a cold-region tunnel: Insights from a laboratory test

The influence of the initial ground temperature ( T r ) on temperature fields and frost-penetration progression remains poorly understood. This study investigates the effects of T r on the formation of frost-penetration tunnels through a laboratory test model. Three cases with T r values ranging from -1 °C to 2 °C, 0 °C to 3 °C, and 1 °C to 4 °C were analyzed. After excavation, the freezing front moved outward into the surrounding rock and extended along the tunnel. In the case of initially unfrozen rock, the average temperature during the coldest month dropped below 0 °C, and the warmest month's average temperature also fell below this threshold for several consecutive years. A lower T r not only quickens the drop in average temperatures for both the coldest and warmest months but also results in a reduction of the rock's average yearly temperature. Despite changes in T r , the annual temperature fluctuation in the rock stays uniform. A reduced T r markedly hastens the rock's freezing process and the development of permafrost. The core principle behind the frost-penetration tunnel is the annual reduction in the rock's average annual temperature, largely influenced by the difference between the average annual air temperature and T r .