Research On Mechanical Behavior Of Large Diameter Pile Group Foundation Under Horizontal Load

With the rapid development of large and complex projects such as super high-rise buildings,oil production platforms,wind power generation,and long-span bridges,higher requirements have been put forward for the bearing capacity of foundations,which has led to more and more extensive use of large-diameter pile foundations.For a long time,people’s research on pile foundations has mainly focused on the vertical bearing capacity.However,in order to resist horizontal loads such as earthquake loads,automobile and train braking forces,pile foundations are also required to have good horizontal bearing capacity.At present,the relevant codes and researches on pile foundation design mainly focus on small and medium diameter piles.There are few studies on large diameter(D>2.5m,D is pile diameter)pile foundations.The suitability and applicability of the existing research need further study.Therefore,the main content of this article and related conclusions are as follows:(1)Through a large number of engineering investigations and literature reviews,the current situation of domestic and foreign research on the horizontal bearing characteristics and seismic performance of pile foundations is summarized.Relying on the actual project,the indoor test pile horizontal bearing capacity test is established based on the similarity principle.The purpose of the test is to study the influence of the presence or absence of the pile cap,the number of piles,and the pile spacing on the test pile horizontal bearing capacity.(2)Based on ABAQUS software platform,the solid finite element model of the test pile is established,and the whole process of the test is numerically simulated.Through the comparative analysis of the test data and the finite element calculation results,on the one hand,the sensitivity of the horizontal bearing capacity of the test pile to different parameters is clarified;on the other hand,the correctness of the finite element modeling method is verified,and on this basis,a refined solid finite element for large-diameter monopile and pile group foundations that can consider the nonlinearity of the soil around the pile is established.The basic model lays the foundation for future work;(3)The sensitivity analysis of key design parameters such as the pile diameter,the elastic modulus of the pile concrete,the nature of the soil,and the vertical load is carried out for large-diameter monopile.The results show that increasing the pile diameter to a certain extent and increasing the elastic modulus of the pile has little effect on the horizontal bearing characteristics of a large-diameter monopile;The nature of the soil layer has a significant impact on the horizontal bearing characteristics of a large-diameter monopile,especially when the upper soil has soft soil layer,the pile body will have a larger bending moment peak near the soil interface,even more than the pile body is completely located in soft soil layer;Excessive vertical load leads to obvious P-△ effect,which weakens the horizontal bearing capacity of large-diameter piles.(4)The sensitivity analysis of key design parameters such as the number of piles,pile spacing,and the cross-sectional form of piles is carried out for large-diameter pile groups.The results show that the number of piles,pile spacing,and soil properties have significant effects on the horizontal load-bearing characteristics of large-diameter pile group foundations;the horizontal load-bearing characteristics of pile groups composed of large-diameter solid piles and hollow piles are basically the same.When the wall thickness increases,the horizontal bearing capacity of the pile group foundation increases to a limited extent,and the wall thickness range of 0.1D～0.2D is more appropriate;because the large-diameter single pile has a higher horizontal bearing capacity,the pile group effect is more significant,and there is a big difference in the bearing performance of different positions foundation piles.Generally speaking,the pile group effect increases with the increase of the number of piles,and decreases with the increase of the pile spacing.After the pile spacing reaching 3.5D,the pile group effect is not obvious,and the harder the soil,the smaller the pile group effect;(5)Based on the ABAQUS software,using static-dynamic boundary coupling and viscoelastic boundary modeling methods,a refined solid seismic finite element model of large-diameter pile group foundations that can consider the nonlinearity of the soil around the pile is established.The influence of near-field(including forward directivity and fling-step)and far-field actual ground motion on the seismic response of large-diameter pile group is studied.The results show that under the action of an earthquake load,the bending moment is distributed along the entire pile;under the action of a strong earthquake,most of the soil may enter a plastic state or even be destroyed,which has an important impact on the horizontal bearing characteristics of the large-diameter pile group foundation;The peak ground motion acceleration PGA has a good correspondence with the seismic response of the pile foundation,and can more accurately predict the seismic demand of the pile foundation.