Study On Design And Construction Techniques Of Special Subgrade For Permafrost Areas

The permafrost and its special engineering properties have caused serious problems to the local engineering projects,of which the thermal stability of the roadbed is the most challenging one.In this paper,the design concept based on summarizing the former research results,making full use of the present achievements,considering the engineering practice and controlling the construction quality is adopted.Under the guidance of the concept,many indoor experiments,numerical simulations and field testing works,for purpose of serving engineering design and quality control of roadbed construction,were employed to systematically research special roadbed regulating techniques in permafrost regions.Because of the repeated freeze-thaw cycles,many peculiar roadbed diseases,such as longitudinal cracks,thawing settlement,frost heaving and frost boiling develop in permafrost regions.According to the degree of the sun radiation affect caused by the embankment slopes,the diseases can be classified into those related to low embankment and those related to high embankment.The causes of disease formation are mainly due to the shape of the underlying permafrost morphology,poor environmental conditions and human activities.The study indicates that the reasonable embankment height for keeping the underlying permafrost table be stable,varies with time.Moreover,the definition and mathematical expression of the reasonable embankment height were obtained.Decreasing the slope gradate and building thermal-insulated beam have no obvious effect on increasing the thermal stability of the roadbed.Thus a general designing principle and methods of the roadbed in permafrost regions were suggested. The studies on the mechanism,adaptability,design essentials and construction technology of the special roadbed adjustment show that the thermal insulation material can reduce the amplitude of annual ground temperature variations,resulting delayed thawing of the permafrost.However,it can not change the trend that roadbeds absorb more heat energy than that they release.The thermal-insulated embankment is only applicable in permafrost areas where the ground temperature is low(cold permafrost). The crushed-rock roadbeds can proactively cool down the underlying permafrest and up-lift the permafrost table,which is the reasonable and economic roadbed configuration.The thermal pipes can effectively lower the ground temperature in winter time through phase changing and circling of the media inside them.Furthermore,the cooling effect of the thermal pipes installed in the both sides is better than that in one side of the roadbed,and the effect of the thermal pipes inserted slantingly is better than that inserted vertically.The awning serves as a shelter over roadbed can effectively shade the surface from solar radiation and reduce the heat absorbed by the roadbed.It is available incessantly all the year round.The duct-ventilated and combined roadbeds also have significant effect on cooling down the underlying permafrost.Therefore,the special roadbed adjustments can efficiently avoid the diseases of the roadbed in warm permafrost areas and the roadbed with a wide pavement.At last,some design points for the roadbed adjustment,measures for controlling the construction quality and the necessary notices in the roadbed construction were proposed.