The Research On Settlement Behavior Of Super-long Pile Foundation And Long-short-pile Composite Foundation Under Axial Load

The construction of modern cities and the development of construction technology have made the application of super-long piles more and more widespread in civil engineering.Because of the deep penetration of super long pile,its interaction between the pile and soil and the mechanical behavior are quite different from those of ordinary pile foundation.However,the current research on the pile-soil interaction of super-long piles and mutual reinforcing effect between piles is not sufficient.Therefore,improving the deformation trait theory of pile foundations is a pile foundation subject that needs to be solved urgently.The long-short-pile composite foundation can better adapt to the irregularity of the superstructure,and has good economy and operability.Due to the insufficiency of the interaction study between the long pile and short pile,there is still much room for improvement of the settlement analysis of the long-short-pile composite foundation.In summary,the relevant theoretical research on deformation behavior of superlong pile foundation and long-short-pile composite foundation is not perfect and still lags behind the engineering practice.Therefore,based on the full analysis of the development status of pile foundations at home and abroad,this paper carried out the relevant researches on deformation behavior of super-long single pile,super-long pile groups and long-shortpile composite foundation under axial load.The mainly study and principal conclusions may be summarized as follows:Based on the deep strengthening phenomenon of super-long pile lateral friction found in the latest test,a new load transfer model including upper softening and deep strengthening of pile lateral friction is proposed and developed.The proposed softeningstrengthening model is verified by engineering measured data.The results show that the values calculated by the model proposed in this paper are in good consistency with the experimental data.Through the calculation and analysis of two engineering examples,it is found that the bearing capacity of the super-long pile foundation can be effectively improved after considering the strengthening.Within the allowable settlement range of the pile top,the bearing capacity of super-long pile is increased by 20.97%.The softeningstrengthening model proposed in this paper is significantly better than the previous models.Taking the elastic-plastic characteristics of the concrete of pile body into account,the nonlinear iterative calculation method of the super-long single pile under the axial load is improved based on the model proposed in this paper.Comparison of the theoretical calculation values with the experimental value shows that the calculation method put forward in this paper can accurately reflect the actual mechanical behavior of the super-long pile.The skin friction of axially loaded super-long pile increases with the increase of depth.And near the end of pile,the skin friction decreases with the increase of depth which has an obvious descending section.As the load on pile top continues to increase,the skin friction near the pile end increases obviously,showing a gradual expansion pattern.The results are definitely consistent with the characteristics of pile skin friction.By the analysis of the distribution curves of lateral friction resistance along the depth obtained from the static load test of a super-long single pile,it is found that the lateral friction resistance curves under different loads had the one-dimensional similarity.On this basis,the relationship between the lateral friction resistance and the relative depth of the single pile under different loads is derived,and the settlement calculation method for the super-long single pile under different loads is further obtained.Fully considering the influence of the existence of piles themselves on the soil and the mutual reinforcing effect between piles in the group pile foundation,the settlement calculation method of each foundation pile in the pile group is derived by linking the settlement of single pile with that of the pile group foundation.Comparing the analytical results,experimental values and finite element simulations,it is found that the settlement calculation method proposed in this paper is in good agreement with the experimental values and the finite element results,and can better predict the deformation behavior of pile group under similar conditions.Meanwhile,the finite element results show that the settlement of the foundation piles in the super-long pile groups under the axial load varies with the position of the piles.The central pile has the largest settlement under the load,followed by the side piles and the corner piles are the smallest.The load sharing of each foundation pile also varies with the location of the foundation pile.When the same settlement occurs in the foundation piles,the corner piles borne the largest load,followed by the side piles,and the center pile is the smallest.In view of the deficiency of traditional calculation of interaction coefficient which can only considers the existence of two piles and not fully considers the mutual reinforcing effect,an improved calculation method for interaction coefficient is proposed.It not only can fully consider the existence of intermediate piles but also the mutual reinforcing effects among the foundation piles.Moreover,an analytical solution for the settlement of the pile groups which can fully consider the mutual reinforcing effect is derived.By comparing the theoretical solution in this paper with the experimental values,it is shown that the analytical method proposed greatly reduces the settlement error of the traditional calculation model and better matches the working behavior of pile groups in practical engineering,which provides a simple and effective analytical method for calculating the deformation of superlong pile groups in engineering.The calculation formula for the interaction coefficient between long pile and short piles is revised to make up for its deficiency of not fully considering the reinforcing effect between piles based on the interaction coefficient of equal-length piles.Also,the parameter sensitivity analysis of the interaction coefficient is carried out.Based on the virtual soil-pile model at the pile end and regard the shear displacement method as the basic principle,the settlement calculation method of long-short-pile composite foundation is put forward by combining its working characteristics which fully considering the mutual reinforcing effect between piles.The calculation method can well simulate the stress of the soil at the pile end.Through the comparison with the experimental values,it shows that the settlement calculation method proposed in this paper can calculate the settlement of long-short-pile composite foundation more accurately,and the results of the analytical and experimental values are in good agreement.Establishing the calculation model of long-short-pile composite foundation based on the finite element software.The effect of pile arrangement,pile types,the number and length of long piles,pile diameter and soil properties on the settlement behavior of longshort-pile composite foundation are analyzed.The results show that the deformation of long-short-pile composite foundation varies with the different pile arrangement and pile type.When the total number of piles and the load is the same,the settlement of pile foundation decreases with the increase of the number of long piles,the length of long piles and the diameter of piles.The settlement rate also changes from slow to fast.In addition,the smaller diameter of the piles,the faster the settlement rate of long-short-pile composite foundation.Moreover,improving the properties of foundation soil has a significant effect on reducing settlement.