Vertical Bearing Capacity Distribution Of Pile Groups With Defects Under Rigid Caps

In recent years,China’s economic growth has been accelerating,and the scale of engineering buildings has been expanding.As a high-quality foundation form,pile foundation has been paid more and more attention to.In the actual construction of pile foundation engineering,the emergence of defective piles is unavoidable due to technical and environmental factors.Pile defects,to a certain extent,will affect the effective bearing capacity of piles and the settlement of pile foundation,which is a potential threat to the building itself.At present,little research has been done on the actual load distribution of defective pile groups.In engineering practice,the treatment of defective pile groups is usually simple in the way of adding piles,which lacks a more scientific and reasonable theoretical basis.In this paper,the pile foundation with defective piles is systematically studied by theoretical and numerical simulation methods.The main research contents and results are as follows:1.Theoretical analysis:(1)A formula for calculating the bearing capacity distribution of pile group foundation with defect considering stiffness enhancement and stiffness reduction,arbitrary number of piles and pile arrangement is derived.(2)Through a specific example,the concrete application of the algorithm is introduced,and the bearing condition of pile group foundation with single defective pile is studied.The research shows that the load shared by defective pile in pile group foundation decreases when the ultimate bearing capacity decreases.With the gradual weakening of the defect degree of the pile,the force of each pile tends to be the same.When the number of piles is only 2 or 3,the magnitude of load allocated to defective piles is the same as that of normal piles.When the stiffness of defective piles is less than that of normal piles,the settlement value of defective piles increases.In rectangular pile group foundation with four piles,two diagonal piles bear the same load,and the load borne by defective piles increases with the increase of their stiffness.If the stiffness of the defective pile is less than that of the normal pile,the pile foundation inclines toward the location of the defective pile,and the settlement value of the defective pile is the largest.Among the five pile foundations with positive pentagonal arrangement,the load-sharing value of the normal pile far from the defective pile changes slightly,while that of the pile near the defective pile changes more obviously.2.The numerical simulation aspect: Using MIDAS finite element analysissoftware,the normal pile group model is established,and the four pile foundation model with single defect pile is established by modifying the parameters of the model,and the corresponding analysis is made.The results show that:(1)The bearing capacity of defective piles with reduced stiffness is worse than that of normal piles,which can share less load and produce larger settlement at the same time.Influenced by the decrease of the stiffness of defective piles,the load value of defective piles and diagonal piles in defective pile foundations decreases,while the settlement value of the remaining two piles increases,and the cap inclines toward the location of defective piles.(2)Under the same defect degree,the effect of the defect form with reduced stiffness on the inclination of pile cap is greater than that of the defect form with enhanced stiffness.3.By changing the strength and thickness of pile cap concrete,the influence of the change of pile cap stiffness on the bearing capacity of pile group foundation with defective piles is studied.The results show that with the decrease of the cap stiffness,the balance ability of the cap to the pile group’s force and settlement is gradually weakened,and the difference between the settlement of defective pile and residual pile is enlarged.At the same time,the decrease of the rigidity of the cap increases the difference between the maximum and minimum settlement of the cap itself.