Study On The Stability Of Bridge Pile Foundation At Permafrost Regions Under The Locomotive Load

The degeneration of permafrost along the Qinghai-Tibet Railroad (QTR), induced by the global climate warming and human engineering activities has being brought potential risk to the safe operation of QTR. In order to pass the areas withhigh temperature and high ice content safely, the dry bridge, also named by bridge instead of embankment, has been adopted widely along the railroad. The key factor which influences stability of the bridges is the pile foundations locating in the permafrost regions. Moreover, and the key factors influencing the stability of pile foundation are the interaction effect caused by the piles and the permafrost around them under coupling of earth temperature and vibration.In the past several decades, the average temperature has increased by 0.2～0.4℃per year, and annual temperature range decreases year after year on the Qinghai-Tibet Plateau(QTP).Variation in temperature is one of the most important factors that influence the mechanical properties of permafrost and also the one that affec s the bearing capacity of foundation in permafrost areas. Moreover, there are tens of trains running on the railroad every day, the high frequency vibration induced by trains should cause vibration effect on the pile foundations, and influence safe operation of QTR.The Qingshui River Bridge, which is one of the representational bridges along QTR, locating at the high temperature and instable permafrost region, was chosen as the object of study in this thesis. Engineering geology and climatic environment data of the study area are obtained through survey and data collection. The characteristics, attenuation laws and energy distribution of vibration response,, of pile foundation under the locomotive loads,were studied by by strong motion observation tests and temperature tests in-situ Finally, the finite element analysis method was carried out to study the stress and displacement response of the pile foundationsunder dynamic locomotive load. The main conclusions are shown as follows:1. The vibration response of pile foundation is obvious when train passing. There is an evident attenuation effect when vibrating acceleration transfers from bridge deck to pile foundation. Although the vibration response varies in three different directions, in which the up-down is strongest, the amplitudes of attenuation are almost same. The energy of vibration mostly concentrates in the range of 50Hz to 60 Hz, in which the attenuation of energy is the most significant. The energy of vibration is about 10% of the original one when it reaches the pile foundation.2. The field monitoring temperature data were analyzed. It shows thattrain vibration have an influence on temperature field around pile foundations when train passing. However, due to the short effect time in vibration, the maximum value in temperature increases by 0.02℃. 3. The numerical results show that the energy of vibration decreases to 2%of the original one when it transfers from bridge deck to the bottom of pile foundation. The maximum settlement is 0.085mm,which appears at the top of soil around pile foundation. The settlement reduces with increase in distance away from piles, and decreases with depth along piles. The stress concentration appears at the bottom of pile foundation, there are no cracks and ruptures between the pile and the permafrost, and piles and soils around them are in good state of closecontact.