Abstract：Auto-temperature-controlled ventilation roadbed is one of positive engineering measures for cooling roadbed. In the engineering measure，an auto-temperature-controlled system is installed at one or two ends of the vent-duct. The system is composed of air temperature responsive unit and control unit，by which the wind door in ventilation duct can be controlled. When the air temperature reaches the designed value，the wind door can be automatically opened or closed. Through the system the effect of the heat flux in summer can be effectively controlled，and natural cold can be effectively used to cool the roadbed to adjust the temperature of permafrost and ensure the long-term stability of railway. In order to verify its engineering efficiency，the system was installed on two sections of ventilation roadbed with different duct diameters in Beiluhe test section in Qinghai—Tibet railway. Through the field tests from April to October，2003，with ground temperature comparison between the ventilation roadbeds with and without the system，the test results showed that，with the application of the system with 30 and 40 cm diameter ducts，the artificial permafrost table were raised by 0.3 and 0.7 m，respectively，and the average temperature under tubes were reduced by 1.2 ℃ and 1.7 ℃，respectively，and the average temperature under roadbed were reduced by 0.5 ℃ and 0.7 ℃，respectively. The time of reaching maximal thawed depth could be advanced for nearly 20 days，and the time for roadbed giving out heat was extended for more than a month. Through thermal calculation，the thermal flux has been decreased nearly half in a month with maximal thermal flux coming into permafrost. Comparison of the cooling effect in auto-temperature-controlled ventilation roadbeds with different duct diameters indicates that the efficiency of duct with 30 cm diameter is slightly higher than that with 40 cm diameter.
