Bearing Capacity Analysis Of Understorey Incremental Pile Underpinning For Existing Buildings

With the rapid development of urban construction in China,the development and utilization of underground space has also gradually aroused the attention of everyone.Especially with the increase of the number of cars,the defects of the functions of the existing buildings without underground parking lots and other facilities are more and more obvious.Therefore,the development of the basement of the existing buildings is particularly important.When the existing building is added to the floor,it is necessary to use the original pile foundation or add new pile foundation to support the upper building,so that the excavation of the earth beneath the building can be carried out.After pile soil excavation,compared with the original pile foundation,the bearing characteristics of the pile foundation will change obviously,especially the lateral friction resistance of pile.Therefore,it is necessary to analyze the bearing characteristics of the existing building pile foundation under the condition of excavation.This paper consider the upper structure,foundation and foundation work together on a three layers framework structure of pile foundation has carried on the finite element simulation of excavation process,the analysis of the pile soil excavation before and the change of soil parameters under different excavation depth,changed pile spacing,pile length,change condition,such as excavation sequence explores is building pile foundation excavation under the conditions of load-bearing characteristics change law,mainly includes: axial force of pile,pile foundation settlement and pile side friction and the pile soil relative slippage,etc.From the results of ANSYS analysis,(1)with the increase of the excavation depth,the settlement difference of pile foundation and the relative slip amount of pile and soil will increase,and the proportion of the soil load of the building through the pile side is reduced,it also shows that more pile shaft force is passed through the pile end resistance into the soil,and(2)to reduce the excavation of the pile soil.By increasing the number of piles,the length of the pile,the friction coefficient of the pile and soil,the elastic modulus of the pile soil and the selection of the correct excavation order,the settlement of the pile foundation,the settlement of the adjacent pile foundation,the relative slip amount of the pile and soil,the axial force of the pile and so on are reduced,and the load of the building is passed through the pile side resistance.The greater the proportion of soil,the lower the resistance of pile end,which makes the underpinning pile have more safety reserves.(3)increasing the number of piles,adding the bottom plate after excavation,increasing the elastic modulus of the pile end,increasing the length of the pile,increasing the length of the side,and selecting the correct excavation order can obviously improve the settlement difference of the adjacent pile foundation,while the change of the friction coefficient can reduce the settlement of the pile foundation,but it is not obvious for the settlement difference of the adjacent pile foundation.(4)adding the floor to the ground floor can not only reduce the axial force of the pile,reduce the settlement of the pile foundation,but also guarantee the resistance of the pile side,which is beneficial to the safety of the pile foundation.Therefore,the most unfavorable condition of the underground adding layer is the height of the excavation from the earthwork to the design of the bottom plate,and the safety of the floor is closed after the bottom slab has not been poured.