Research On The Settlement Characteristics Of Composite Foundation With Long-short Piles Under Fill Subgrade

As composite foundation technique has been applied and popularized in civil engineering construction of our country, the concept and theory of the technique has been developed very much. And composite foundation with long-short piles is a new kind of composite foundation technique, currently often applied to reinforce soft soil under rigid basis. Since the significant advantage of this technology in control of settlement, it is being applied to reinforce the soft soil foundation under the fill subgrade of the highway. But now the settlement & deformation theory of composite foundation has the premise that the deformation of the piles and soil is consistent, and is suitable for composite foundation under the rigid basis. But the phenomenon that the pile top pierces into the cushion and the pile bottom pierces into the bearing stratum in composite foundation under the flexible basis is obvious, and makes the deformation of the piles and soil inconsistent. So the theoretical calculation is different from the measured value. In addition, the mechanism and function of the pile group of composite foundation under the flexible base in control of the settlement is different from the single pile's. Therefore, based on the pile-soil interaction theory and the pile group analyzing theory, it has important practical significance to research the settlement characteristics of composite foundation with long-short piles under fill subgrade. And the article has studied the characteristics in the theory analysis and the numerical calculation analysis.(1)The research status of composite foundation with long-short piles is analyzed firstly, the problems in the present research are pointed out, and the design method of composite foundation with long-short piles are discussed.(2)Through the analysis and comparison of traditional settlement calculation methods of composite foundation with long-short piles, their respective characteristics and the applicable scope are found. The Mindlin and Boussinesq Union Solution Law is derived and used to calculate the settlement. And the influencing factors of the settlement calculation are analyzed.(3)The analysis methods of pile-soil interaction of the single pile are introduced, and their respective characteristics and the applicable scope are found. A analysis model, which considers pile-soil-cushion interaction, is brought forward based on the pile-soil interaction theory and the pile group analysis theory. In the model, the soil body is as Mohr-Coulomb Model, the pile as elastic model, the pile-soil interaction simulated by the shear coupling spring and the normal coupling spring, and cushion as the Winkler spring.(4) The model is calculated by FLAC3D, and the calculated results is mostly same as the checking results. So, it has proved that the model is reasonable, and the method that simulating composite foundation with long-short piles under fill subgrade by FLAC3D is feasible.(5) The settlement law of piles and soil in the construction of fill subgrade is analyzed by FLAC3D. The results show that the pile's settlement change linearly with the increase of roadbed load, the characteristics of soil body's settlement is influenced by pile spacing.(6)The influencing factors of post-construction settlement of composite foundation with long-short piles, such as pile spacing, pile length, modulus of cushion and bearing stratum, are analyzed by FLAC3D. The results show that the pie length, the pile spacing, the modulus of cushion and bearing stratum exist the best value for controlling the post-construction settlement.