Study On Thermal Disturbance To Permafrost From Engineering Structure In Wudaoliang Area Of Qinghai-Tibet Railway

The safety in operation of railroad in permafrost area depends primarily on the stability of the main constructions, which include subgrade, bridge and culvert. The stability of constructions is determined by the thermal stability of the frozen soil foundation. The thermal disturbance brought by embankment engineering construction in permafrost area mainly includes the dissipation of fillings' heat and the influence from changes of permafrost radiating interface in the subgrade base. As for the thermal stability recovery of permafrost at subgrade base in low temperature permafrost area, the time increases with the height of the subgrade. This recovery process affects the joints in working operations, as well as the stability of subgrade engineering. The problems of thermal disturbance to permafrost caused by constructions of bridges and culvert foundations are much more complicated than subgrade engineering. The problems are not only about the disturbance of constructions to the permafrost, but more importantly, the thermal disturbance and heat dissipation caused by the particularity of types of foundations and working operations are longstanding problems, which directly affect a series of problems including foundation stability and joints in working operations.Based on the regional permafrost geology and characters of thermal stability, this text studies the thermal disturbance on the permafrost in embankment subgrade base from earth embankment height and construction seasons in low temperature permafrost area. According to the observed and computational results, the best construction seasons and construction methods are proposed to ensure the thermal stability of permafrost and embankment stability of high embankment constructions in the low temperature permafrost area.Considering the construction duration of Qinghai-Tibet Railroad and the control and influence of constructions in the Wudaoliang area on the whole line's construction period, the variation of ground temperature field at pile sides after the casting of cement caisson piles was tested, along with the heat of hydration during concrete casting. Numerical simulations about the influence of concrete molding temperature on the refreezing of soil at pile sides were conducted. Therefore, the key technological problems in working operations and construction organization design of local bridge foundation caisson pile constructions were solved. The smooth joints in working operations and the construction duration of controlling constructions were ensured. The stability of bridge foundations was attested by the in-site experiments and the monitoring after construction for three years.In the light of the characters of climate and thermal stability of permafrost in Wudaoling area, innovative improvements were proposed on the types of culvert foundation. The temporal standards during the initial construction period of Qinghai-Tibet Railroad and the past experiences believed that the promotion type of culvert foundation in permafrost area was prefabricated assembly foundation. Based on the construction tools, construction technology and construction capacity at the present state and the climate characters of Wudaoliang area, the idea that cast-in-site concrete monolithic foundation should be used locally is proposed. Through in-site construction test, computational simulation and the characters of joints in working operations, the author found that if local shading and heat insulator with a depth of 6cm were employed, the thermal disturbance on the permafrost at subgrade base from the cast-in-site concrete monolithic foundation would be dissipated within one to two years of freezing and thawing cycle, which would not bring about damage to the permafrost at culvert foundation base and the stability of the foundation itself. Moreover, water creep is rare for culverts with this type of foundation. The absence of joints also reduces frost heaving during operation, which enhances service life and stability.With scientific experiments and theoretical calculations closely related to production practice, the working operations of high embankment constructions, bridge pile and culvert foundations in Wudaoliang low temperature permafrost area were studied, based on the characters and variations of permafrost thermal stability in Wudaoliang low temperature permafrost area. The results have theoretical and practical significance on the constructions in the permafrost areas at Qinghai-Tibet railroad, and they are:The high concrete molding temperature during thawing seasons could not meet the original standards. Through calculations about the influence of concrete heat of hydration on the permafrost thermal disturbance and the mixture ratio experiments of cement addition agents, the acceptable value of molding temperature during thawing seasons can be up to 12℃.The thermal disturbance of permafrost at foundation base can be effectively controlled by the replacement of foundation base of cast-in-site concrete monolithic culvert foundations, along with the laying of insulating materials with a certain depth, which will facilitate the recovery of stable thermal state while maintaining the stability of foundation base. Therefore, with different climate and permafrost thermal stability characters, suitable construction technologies should be employed, and cast-in-site concrete monolithic foundations can be adopted.As for the construction of bridge caisson piles, concrete molding temperature and refreezing of soil at pile sides are the controlling operations. The standards of refreezing of soil at pile sides consist of two concepts, one is the refreezing standards suitable for the joints in the working operations, and another one is the refreezing standards meeting the designed capacity of pile foundations. These two standards are acquired through experiments, thus the capacity of pile foundations can be standardized while the law of soil refreezing at pile sides can be employed. The proceeding procedures are connected, the efficiency of construction is improved and the duration of construction can be scientifically and reasonably arranged.Trough in-site tests, observed data during construction and initial operation periods and numerical simulation results based on the in-site observed data, the bridge and culvert foundation constructions and the initial project results following the above-mentioned construction technologies were proved to be safe and reliable.