Study On Mechanism Of Rigid Pile Composite Ground With Cushion

Rigid pile composite ground is used most extensively in composite ground, which is a very active field in geotechnical engineering of China. The strength of rigid pile is high, thus, in order to avoid stress concentration and ensure the co-work of pile and subsoil, there is usually a layer of cushion above pile top. Cushion becomes one of the key techniques. This thesis deals with the mechanism of cushion and part of working properties of rigid pile composite ground with cushion.Firstly, the characteristics of CFG pile, the typical composite ground under vertical load are illustrated to start the analysis of mechanism of cushion. After introduction of several failure models and calculation methods of cushion thickness that are given by former scholars, the possible failure models for cushion are discussed. Moreover, the most suitable failure model is proposed and a calculation table for the convenience of practical calculation is given.Secondly, the concept of negative skin friction resistance and characteristics of distribution are introduced and so are the formation reasons and influencing factors. The Meyerhof theory, which former scholars used to analyze pile foundation and flexible pile composite ground, is introduced for further analysis of the effect of end resistance augment. The test curve of side skin friction resistance of pile is simplified linearly. The distribution of skin friction resistance is controlled by two factors: the maximum value of negative skin friction resistance and depth ratio of neutral plane. Thus, the simplified linear calculation model of rigid pile composite ground is established.Finally, Vesic theory of spherical cavity expansion is applied to calculate the penetration magnitude of pile top into cushion and a new pile-soil stress ratio formula is deduced. Then, according to differential equation of pile displacement and algebraic equation of neutral plane, the depth ratio of neutral planeλis solved by semi-analytical method. In the calculation example, the influence of pile compressive modulus, pile length and compressive modulus of subsoil beneath pile-bottom on the parameterλis analyzed. Moreover, the range ofλis also calculated.