The Load Transfer Mechanism Of Composite Foundation With CFG Piles And Gravel Piles

The Gravel pile is a reinforcement element of a composite foundation,which is mainly made of gravel,it is formed by vibrating and other methods,because the gravel is a type of discrete materials without bond strength,the gravel pile can be used as good drainage channels.The rigidity of gravel pile is higher than that of the soft soil,and the gravel pile has a certain reinforcement effect when it is used as an vertical reinforcement element.However,with the increasing shortage of land resources,the types of foundations need to be reinforced are more and more diverse,and the requirements for foundation treatment are also higher and higher.For example,for soft and serious liquefaction foundations,only using gravel piles to reinforce can solve the liquefaction problem,but the bearing capacity can not meet the requirements of the superstructure.For these foundations which need reinforcement elements of the rigid piles to solve liquefaction problem,the composite foundation with CFG piles and gravel piles is the most effective treatment method at present.Based on the shear displacement method and considering the effect of group-piles and the bulging deformation of gravel piles,this paper theoretically deduces the calculation formula of load transfer and distribution in the binary composite foundation of CFG piles and gravel piles.The distribution of stress field and displacement field in composite foundation is deduced.In this paper,the three-dimensional simulation of the coupling of discrete element and finite difference is adopted.The solid grid is established by FLAC3 D to simulate the CFG pile,pile cap and soil between piles.The particle model is established by PFC3 D to simulate the cushion and the gravel pile.Through the coupling analysis of PFC and FLAC,the load transfer mechanism of the CFG pile gravel pile composite foundation is deeply studied.Based on the engineering practice,according to the similarity principle,the indoor model test is established to verify the accuracy of the theoretical formula and numerical simulation results.Through the above three aspects of research,the main research results are as follows:(1)Although the stress concentration of the CFG pile occurs in the early bearing stage of composite foundation,the stress ratio of the CFG pile and gravel pile remains between 5 and 10.With the increase of load,the stress growth of gravel pile top slows down,and the stress is more and more concentrated on the top of the CFG pile,which makes up for the lack of gravel piles bearing capacity.(2)When the CFG pile gravel pile composite foundation is loaded,the pile-soil stress ratio of gravel pile will decrease with the increase of upper load.The main reason is that the gravel pile bulges and deforms when it is loaded,which compacts the soil around the pile and improves the load shared by the soil.At the same time,the decrease of stress ratio is very slow,which indicates that the bulging deformation of gravel pile is very limited under the load of the CFG pile,and the compaction effect of soil around the pile is also very limited.(3)PFC3D-FLAC3 D coupling method is used to simulate the load transfer law of the CFG pile gravel pile composite foundation,which can simulate the fluidity of cushion and the swelling characteristics of gravel pile.Therefore,FLAC3 D is used to simulate the continuous medium,PFC3 D is used to simulate the discrete material,and then coupling calculation method can realize the simulation of discontinuous medium in rock and soil.