Study On Vertical Bearing Characteristics Of Long And Short Pile Foundation Piles And The Effect Of Combined Foundation Piles In Loess Area

At present,considerable progress has been made in the research on the combined foundation of long and short piles,and most of the research results focus on the synergistic characteristics of long piles and short piles,and the analysis of the influence of a factor in the soil parameters of piles on their bearing deformation characteristics.Based on the geological characteristics of the loess area,this paper first derives the bearing capacity calculation formula of long and short pile foundation piles without wet subsidence based on the load transfer hyperbolic model,and then uses the stress superposition method to derive the calculation formula of pile bearing capacity of long and short pile foundation piles to obtain a bearing capacity calculation method based on the group effect coefficient.Introducing the problem of weakening the bearing capacity of foundation pile by loess collapsibility,considering the effect of collapse time,the calculation formula of negative friction resistance(pull-down load)of foundation pile is derived with reference to consolidation theory,and the calculation method of foundation pile bearing capacity when wet collapse load and pile top load are combined is superimposed with no wet collapse.Combined with the derived calculation formula of long and short pile group effect coefficient,the calculation method of long and short pile group pile bearing capacity when there is wet collapse is obtained.Finally,based on ABAQUS finite element analysis software,the performance,load sharing characteristics and pile effect of long and short pile foundation piles are comprehensively analyzed,and the main work is as follows:(1)Considering that the modulus of the deep holding layer near the long pile end is much higher than that of the upper holding layer when the combined foundation of the long pile length is applied,the long pile foundation pile is considered as an "embedded pile" of the most recent soil layer of the embedded pile end of the pile body,and its bearing capacity is regarded as composed of three parts: pile side resistance,pile end resistance and embedded resistance,while the short pile does not consider the embedded resistance.Based on the hyperbolic model,the differential equation of load transfer is derived,the P-S curve is obtained,and finally the calculation method of pile bearing capacity of short and long pile foundation pile without wet collapse time is obtained;(2)Referring to the consolidation theory,considering the effect of collapse time,the influence of the radial distance between a certain part of the soil and the pile shaft on the settlement amount of the subsidence is introduced,the change law of settlement and time caused by the collapse load of the foundation pile is found,and the analytical solution of the differential equation of load transfer when considering the collapsibility of the soil around the pile is given,and the weakening degree of the bearing capacity of the foundation pile by the collapse load is obtained.The measured data of pile foundation collapsibility test were collected,and the calculation formula of the total pull-down load generated by negative friction resistance was obtained based on the simplified model of negative friction resistance triangle distribution.By superimposing only pile top load and only wet collapse load,the calculation method of pile bearing capacity of foundation piles in loess area when there is wet collapse is obtained.;(3)The stress superposition method was used to virtually extend the long and short pile and short pile foundation piles,and the calculation formula of the weakening coefficient of the bearing capacity of the double pile was obtained first.Then,from the weakening coefficient of single row piles to the long and short pile group foundation of m row n columns,the foundation piles are divided into three types according to the degree of weakening,and the calculation formula of the effect coefficient of long and short piles in m row n columns is obtained.A numerical model is established to verify it,and then a calculation method for the bearing capacity of long and short pile combined foundation based on the pile effect coefficient is obtained;(4)ABAQUS was used to establish 25 pile models with 30 working conditions in 6 examples,and comprehensively considered the effects of pile length,pile spacing,pile number,pile body modulus,inter-end soil modulus and collapsibility on pile settlement and pile shaft force,and obtained the reference values of better pile number ratio,better pile length ratio,better pile spacing,soil modulus between ends and pile body modulus settings in the design of long and short pile foundations.Based on the established model,the influence of pile soil parameters and pile arrangement method on the load sharing ratio of long and short piles and the effect of pile groups was comprehensively analyzed.Finally,taking the numerical model calculation results as the benchmark database,the calculation formula of long and short pile group effect coefficients is obtained by using the segmentation coefficient method.