Research On Hole Forming Mechanism And Bearing Character Of The Bottom Hammering Cast-in-place Pile

To meet the requirements of different soil layer,different construction conditions and different forms of buildings,the type and construction technique of piles have rapidly developed in recent decades.Generally speaking,the main pile types include static pressure pile(SPP),the static pressure and vibration pile(SPVP),the hammering pile(HP)and the bored pile(BP),etc.All the pile forming methods have their advantages and disadvantages respectively.For SPP,it owns a series of advantages like low noise,no vibration and no pollution and is widely applied in China.However,when pressed into the hard soil,the static pressure pile is difficult to get the satisfied bearing capacity because of the limited pressing capacity of the pile machine.So it has became a most challenging problem for how to design the pile,which not only can be applied to a variety of soil conditions but also to ensure the construction quality and bearing capacity requirements in pile foundation engineering.The construction technology of bottom hammering cast-in-place pile(BHCP)is proposed under the situation,which has the composite function of pressing and hammering pile into soil.And this method inherits the advantages of static pressure pile.Also it could make up for the shortcomings of the limited pressing force of the pile machine by the bottom hammering method and adapt to a variety of different soil conditions,ensure the quality of the pile body,improve the bearing capacity of single pile,save the amount of material used and reduce construction costs in pile foundation engineering.In this thesis,the field static load tests are done to study the bearing characters,the numerical models of particle flow was built for simulating the pore forming process,the model test was designed to analyze failure mode of internal hammering pile.So the main research contents and results are summaried as follows:1.Based on the comparative test among the internal hammering cast-in-place pile,the ordinary static pressure pile and the long spiral bored pile,the bearing capacity of single pile,the load-settlement relation and the load distribution are systemly analyzed.And the characteristics of Q-S curve,the pile end bearing behaviour and load distribution are studied in details.2.The numerical model of discrete element particle flow for the pile and soil is built to simulate the pore forming process included the static stage and the hammering stage.And the displacement field,the stress field and the porosity are obtained at any time and any location.So the variation law of the stress and displacement near the pile side and pile end are analyzed.Also,the soil particle displacement trajectory at different positions is monitored during the hole forming process to analyze the deformation mechanism of the soil.The end resistance coefficients are calculated by using the small area method based on the the whole process of pile penetration.And the influence of the depth,the diameter of the pile,the diameter of the pressure bar and the nature of the soil properties are analyzed on the end resistance coefficient.3.The soil variation response of different position is analyzed by tracking and monitoring the soil particles of different locations during the process of bottom hammering.And analytic solutions of the displacement,velocity and acceleration were derived by complex damping theory and dynamic equation of porous media based on the hierarchical method,so as to get the attenuation of ground vibration caused by bottom hammering.4.In order to consider the influence of joints and fissures on bearing characters and failure mechanism of the pile end,the model test of the failure mode of the pile end bearing stratum is presented with different jointed inclination.And the failure mode and load transfer path of pile tip under different inclination conditions are obtained.In addition,the results of the model test were compared with the results of numerical analysis,which showed the rationality of the failure mode of the jointed bedrock.5.On the basis of the failure modes of jointed rock at the pile end,the formula for calculating the ultimate bearing capacity of single pile under different jointed inclination was derived.And the ultimate bearing capacity of single pile is calculated and compared with theoretical analysis,which proves the correctness of the theoretical derivation.By analyzing the pore forming mechanism and bearing characteristics of the bottom hammering cast-in-place pile,the results show that the bearing capacity of single pile can be increased by more than 40%compared with the ordinary static pressure piles under the same depth and the same soil condition.It is meaned that this construction method can greatly reduce the number of piles,reduce project cost and shorten construction period in pile foundation engineering.So the research of this paper provides a theoretical guidance for engineering practice,and lays a necessary theoretical basis for its popularization and application.