Study On The Bearing Characteristics Of A New Type Of Steel Pipe Composite Pile In Collapsible Loess Area

With the rapid advancement of urbanization in China,more and more construction projects had been built in adverse geological conditions in the collapsible loess areas of Northwest China.Due to imperfect foundation treatment and changes in the groundwater environment,there have been a number of building tilting accidents.At the same time,the influence of negative friction resistance caused by loess collapse on the bearing performance of pile foundation can’t be ignored.This paper aims at the application of collapsible loess foundation piles and relies on a correction and reinforcement project for a pile foundation building in Lanzhou,a new type of steel tubular composite pile was invented.There are two advantages that avoid steel tubular corrosion and eliminate collapsibility of loess in pile side in pile forming process.The related research was less,because the new pile was firstly applied for in collapsible loess area and the pile forming technology was special.Therefore,the bear characteristics of the new pile are studied to popularize the application of the new pile in the collapsible loess area and improve the related experimental theories.The main research works were as listed below:First,the research status at home and abroad was explained by loess engineering characteristics,characteristics of steel tubular composite pile and bear characteristics of single pile.Then,some theories were comprehensively analyzed and summarized,which are the load transfer mechanism of foundation pile,the determination method of vertical bear capacity of single pile and the negative friction resistance of pile foundation in collapsible loess area.These provide basic theories for the research of the new steel tubular composite pile.Second,aiming at the characteristics of collapsible loess,the technology for grouting of high-pressure circulation has been proposed innovatively in forming pile.The static load test for 4 piles with 33 m length were designed and carried out.Distribution and development laws of axial force and lateral friction for the new pile were analyzed under different stratum and micro soaking conditions.The influence characteristics of lateral negative friction and bear capacity were revealed under different stratum and micro soaking conditions.Explored a new idea to eliminate negative friction on the pile side during the process of pile formation,and determined the characteristics of the test pile as a friction pile.At the same time,the theoretical calculation of the bearing capacity was carried out according to material failure,instability failure,and considering pile buckling.Comparing the calculation results with the test values of the bearing capacity of the single pile on site,it is clear that the failure mode of the new type of steel pipe composite pile is between the material failure and the instability failure.Finally,the ABAQUS software was used to simulate the bearing behavior of the test piles.The rationality of the models was verified by comparing simulation results with the experimental results.The relationship between the bearing characteristics of the new pile and its influencing factors was further analyzed.With the increase of pile length,diameter of steel pipe,wall thickness of steel pipe,thickness of protective layer of cement mortar,and cement mortar strength,the settlement of pile top gradually decreases,and the Q-s curve tends to be flat.That is,the bearing capacity will gradually increase with the increase of the above factors,but when the pile length exceeds 33 m,this phenomenon is no longer obvious.At the same time,the thickness of the protective layer of cement slurry has little effect on its bearing characteristics compared with other parameters.