Research On Load Bearing Characteristics Of Underpinning Pile Foundation For Subway To Cross Existing Bridge

Nowadays, the development of domestic cities toward the trend of modernization and large scale. A large number of high-rise buildings have been built, and urban population density increases, which promotes higher demand for commercial, ventilation, transportation and other supporting facilities. Urban limited space is becoming increasingly crowded, and in the following are a series of measures taken in dealing with such cases: underground space is being developed such as construction of underground shopping malls, supermarkets, subways and other infrastructures. Such underground constructions often pass through bases of bridge structures, high-rise buildings and important historical buildings, which requests reinforcement or underpinning for the existing buildings. Application of underpinning technique for buildings increases when original buildings need rebuilding, adding layer and increasing using load, which also promotes the development of underpinning technology towards large-scale and comprehensive direction.Based on domestic and foreign literatures and research achievements in this field, indoor model test of pile foundation underpinning was carried out with combining with Lanzhou Rail Transit Line 1 engineering. For subway crossing existing bridge project, indoor model test, field investigation, and numerical simulation were conducted to carry out comparison research on bridge foundation underpinning. On the premise of cognizing the upper part and lower part of bridge structure system, corresponding calculation and analysis were carried out. Based on the practical engineering, scheme of indoor model test was designed. Based on the engineering geology, hydrogeology, type of bridge upper structure, type of foundation structure, and spatial relation between shield tunnel construction and the existing bridge at the bridge site, design scheme of the bridge pile foundation underpinning method was designed with addition of the number of piles, expansion of caps and other engineering measures. The model test was carried out in three conditions:(1)The original pile foundation was conducted from imposing loads on it to its failure, which tested the bearing capacity of the original pile foundation.(2) Excavation was carried on to the head position of underpinning pile, and pouring of pile cap was conducted after cutting off the original pile underpinning pile. The new system was to be loaded when the maintenance of the platform was completed. The load was two times of the first condition, which tested the bearing performance of the pile foundation.(3)Soil excavation was carried on by opening a hole in one side of model box, which simulates influence of tunnel excavation on bearing performance of underpinning pile. Combined with the three conditions of model, pile foundation settlement, axial force of pile, pile side friction resistance, and load sharing ratio of pile were tested, which provide data of pile foundation settlement, axial force of pile, and pile side friction on conditions of different loading and different depths of excavation. Comparative analysis was conducted on different ranges of test values, and variation law of pile bearing performance was summarized before and after underpinning and when tunnel excavation was in process, and the variation law of pile foundation capacity behavior was summarized before and after underpinning and when tunnel excavation was in process.Based on actual engineering of Lanzhou Yuergou bridge pile underpinning, finite element numerical simulation was conducted by finite element analysis software MIDAS-Civil before and after pile foundation underpinning. Combined with the calculation method of design coefficient of subgrade reaction of pile end and pile side from foreign pile foundation design specifications, axial force diagram, vertical displacement diagram, and stress diagram of cap before and after underpinning were obtained through adding soil spring constraint on pile side and pile side to consider the influence of soil around pile on the bearing capacity of pile foundation. Axial force diagram, vertical displacement diagram, and supporting and stress diagram of cap before and after underpinning were obtained, with which analysis on axial force, vertical displacement, and stress was carried on before and after underpinning and variation law was summarized.Through comparative analysis on different research methods and results of different conditions, useful conclusions and valuable data of pile foundation underpinning were obtained, which provides beneficial reference for research on pile bearing performance before and after pile underpinning and practical engineering application.