Three-dimensional Numerical Simulation Analysis Of Pile Foundation With Variable Stiffness Under Earthquake

From the new era of 7000 ～ 8000 years ago to the present era of rapid development of science and technology,the pile foundation has experienced a long and rapid development process,from the wooden pile planted in the marsh to the reinforced concrete pile foundation now,it can be seen that the pile foundation is a kind of foundation form which has a long history and is widely used.However,the hidden nature of the pile,its damage is difficult to be found,at the same time,the repair of the pile foundation has become a major problem.Moreover,the earthquake force and wind force are the main factors that cause the horizontal shear force and overturning moment of the foundation of high-rise buildings.Generally speaking,when the earthquake action is the controlling factor,the base horizontal shear force caused by the earthquake is not more than 5% of the total weight of the high-rise building,but the influence and destructive force on the whole building are still considerable.Therefore,it is very important to study the dynamic pile-soil-structure interaction by seismic numerical simulation,which is of great academic significance and practical engineering value.The variable stiffness pile foundation is the result of using the nature of foundation soil and pile group to adjust the "pile-soil supporting stiffness" as the central principle,and finally realize the uneven settlement,the internal force of foundation and the minimization of energy consumption of resources.By using the design method of variable stiffness for pile foundation,not only the better economic effect can be achieved,but also the settlement of foundation and pile cap can be well levelled."Technical Code for building pile foundation"(JGJ 94-2008)added "variable stiffness leveling design" to the pile foundation design.The variable stiffness design scheme plays more and more important role in practical engineering application,but its seismic research is not enough and needs to be further developed.In this paper,four kinds of 3D models are built by ABAQUS software,which can be used to study the behavior of variable stiffness pile foundation under earthquake more directly.On the premise of keeping the materials of pile body as close as possible,a kind of pile layout with equal pile length and three kinds of pile layout with variable pile length are established.On this basis,the internal force of pile body,the settlement of pile cap and the differential settlement of four different schemes under vertical load and seismic load are analyzed The displacement time-history curves of the bottom position,the seventh floor position(where the maximum displacement is)and the top position during the whole process of earthquake are analyzed.The results are as follows:(1)under the vertical load,the settlement curve of the pile cap is consistent,and both sides of the pile cap are large in the middle and small.But different pile arrangement has obvious influence on the differential settlement value.With the increase of the middle stiffness,the differential settlement decreases.When the total amount of the pile is the same,the differential settlement of the variable stiffness scheme is less than that of the equal length scheme.However,under the earthquake action,the differential settlement of the variable stiffness scheme is large,which is disadvantageous to the building safety.(2)when the seismic load is applied,the axial force of the four schemes varies from large to small,while the shear force and bending moment are the largest at the top of the pile,and then decrease along the length of the pile;The horizontal displacement along the pile increases first and then decreases,and the maximum displacement is obtained at the position of 8 ～ 10 m.(3)through the comparison of the internal force analysis of the 11 horn pile with four schemes,the shear force and bending moment at the top of the pile with the same length of the11 horn pile are smaller than those of the other three schemes with variable stiffness,about 18%less.Through the comparison of axial force,it can be found that the maximum value of axial force of No.11 pile is more than 2 times of the other three schemes.(4)the time-history analysis of the special points of the superstructure under the earthquake load shows that the maximum displacement of the superstructure can be reduced obviously by the method of variable stiffness.Combined with the curve chart and table analysis,it can be seen that from scheme 1 to Scheme 4,the vibration displacement of superstructure decreases gradually.It seems that the variable stiffness scheme can reduce the maximum horizontal displacement of the superstructure under the earthquake action.