Research On Pile-forming And Bearing Behaviors Of Bag-sealed Pile In Karst Area

With the rapid development of infrastructure construction in China,highway,bridge,wharf and other infrastructures are being built in karst area.Bored pile is extensively used in the construction of the above infrastructures because of its good site adaptability and high bearing capacity.However,there are some outstanding problems in the construction of common bored pile in karst foundation with connected karst caves,such as concrete loss,unstable quality of pile-forming and so on,meanwhile,research on pile foundation in karst area mainly focuses on the mechanical characteristics and stability analysis of karst cave roof,however,there are limited studies on the application of new pile foundation in karst area.So a new type of pile foundation that suitable for karst geology is urgent to be researched,which aims to solve the problem of slurry loss of cast-in-place pile in the connected karst cave and improve the bearing capacity of pile foundation on the basis of ensuring the quality of pile.In this article,according to the characteristics of karst geology with connected karst caves,a new type of special-shaped cast-in-place pile named bagsealed pile was proposed.Its pile-forming and bearing characteristics were studied comprehensively in experiment and theory.Firstly,research methods of existing pile foundation in karst area were summarized briefly,and research ideas of bagsealed pile were clarified.Then,nine groups of model experiments were designed and carried out to study feasibility and influencing factors of bag-sealed pile`s pile-forming.The experiment results showed that bag-sealed pile can reduce the loss of slurry during the process of pile-forming and quality of pile-forming was good.At the same time,the influence law of dendritic structure length positively related to pressure and water-cement ratio of grouting fluid was obtained.Based on the theory of large deflection of circular membrane,a calculation formula of the dendritic structure length which can be used for bag-sealed pile type deduction was derived.Compared with the experimental results,the rationality of the calculation model was verified.Secondly,nine groups of model experiments were carried out based on the pile-forming feasibility of bag-sealed pile.Through comparing model bag-sealed pile with model equal diameter pile,the bearing characteristics of the bag-sealed pile were explored.The experiment results show that the ultimate bearing capacity of model bag-sealed pile was 1.5 times that of model ordinary equal diameter pile.Load transfer law of bag-sealed pile was also obviously different.According to the assumed conditions,the article analyzed stress of bag-sealed pile model and put forward the ultimate bearing capacity and settlement calculation model of the bag-sealed pile.Combined with the results of the comparison with model experiment,the analysis showed that the calculated value fit well with the experiment value.In addition,influencing factors of the bearing capacity of bag-sealed pile were further analyzed,the applicable scope of the bag-sealed pile in karst area was discussed and refined.