Innovative designs of permafrost roadbed for the Qinghai-Tibet Railway

Under global warming scenarios, the passive method of simply increasing the thermal resistance by raising the embankment height and using insulating materials has been proven ineffective in warm and ice-rich permafrost areas and therefore could not be used in the Qinghai-Tibet Railway engineering. Instead, a proactive "cooled-roadbed" approach was developed and used to lower the ground temperature in order to maintain a perennially frozen subgrade. The concept that local and site-specific factors play an important role in the occurrence and disappearance of permafrost has helped us to devise a number of measures to cool down the roadbed. For example, we adjust and control heat transfer by using different embankment configurations and fill materials. The Qinghai-Tibet Railway project demonstrates that a series of proactive roadbed-cooling methods can be used to lower the temperature of permafrost beneath the embankment and to stabilize the roadbed. These methods include solar radiation control using shading boards, heat convection control using ventilation ducts, thermosyphons, air-cooled embankments, and heat conduction control using "thermal semi-conductor" materials, as well as combinations of above mentioned three control measures. This roadbed-cooling approach provides not only a solution for engineering construction in sensitive permafrost areas but also a countermeasure against possible global warming.