| With the rapid economic development in the western region of China,a large number of pile foundations have been applied to engineering construction.There is a large area of deep collapsible loess in the western region.After the loess pile foundation is immersed in water,a large amount of collapsible settlement will be generated in the soil around the pile.The relative displacement between pile and soil increases,resulting in objective negative skin friction,which increases the axial force of the pile body,thus causing hidden dangers to the safety of the project.At present,there are a lot of achievements in the study of collapsible loess pile foundation,but most of them are concentrated in single pile,and the study of negative skin friction of collapsible loess pile group is still relatively lacking.In this context,based on the existing pile foundation immersion test and combined with numerical analysis method,this paper explores the distribution law of negative skin friction of single pile in collapsible loess under different working conditions,and analyzes the distribution of negative skin friction of each pile and the change law of pile group effect by establishing pile group model under different working conditions.The specific research contents are as follows :(1)Based on the modulus reduction method,the feasibility of the numerical simulation method is verified by the field test of pile foundation in collapsible loess site,and the variation law of negative friction resistance of single pile under different working conditions is analyzed.The negative skin friction decreases with the increase of the pre-loading on the pile top and the length-diameter ratio of the pile before immersion,and increases with the increase of the elastic modulus of the pile tip soil and the immersion thickness.(2)The neutral point position is usually determined according to the code or relevant engineering experience.In this paper,the linear regression equations of the neutral point depth ratio,the maximum negative skin friction,the maximum negative friction depth ratio and different working conditions of single pile in collapsible loess site are obtained by multiple regression analysis.The specific values of neutral point depth ratio,maximum negative skin friction and depth ratio are calculated to provide reference for practical engineering.(3)For the collapsible loess pile group with cap,the finite element software is used to simulate the negative skin friction of the pile group before and after immersion.The ultimate bearing capacity of the collapsible loess pile group under vertical load,the relative displacement of the pile and soil before and after immersion,the negative friction resistance of the pile side before and after immersion and the axial force of the pile body are analyzed.The results show that under the design load,the positive skin friction is mainly distributed in each pile of the pile group without immersion,and the axial force of the pile body decreases as a whole.The upper load distribution of each pile is quite different.After immersion,the negative skin friction of the pile increases greatly,and the upper load distribution of each pile decreases.(4)Taking collapsible loess pile group as the research object,the numerical analysis of different working conditions is carried out.The results show that with the increase of the number of piles,the average pull-down load of the pile group decreases,the shielding effect increases,and the negative skin friction pile group effect is strengthened.With the increase of pile spacing,the overall average pull-down load of pile group increases,the shielding effect decreases,and the negative skin friction pile group effect weakens.The shielding effect of central pile is greater than that of side pile and corner pile.The shielding difference of central pile is the largest,that of corner pile is the smallest,and that of side pile is between the them.With the increase of pile top load,the decrease of pile group pull-down load increases,and the shielding effect is strengthened.With the increase of immersion thickness around the pile,the pull-down load of each pile of the pile group increases greatly,and the effect coefficient of the negative skin friction pile group increases.Under different working conditions,due to the influence of pile group effect,the negative skin friction and axial force of each pile are different.The pull-down load generated by the external pile is greater than that of the internal pile,but the shielding effect of the external pile is weaker than that of the internal pile.The linear equation of the neutral point position of each pile group is obtained by constructing the multiple regression model,which provides a reference for the determination of the neutral point position in the actual project.(5)Combined with the calculation methods of negative skin friction and negative skin friction pile group effect coefficient,different methods are used to calculate the negative skin friction pile group effect coefficient.By comparing the calculation results with the simulation results,the differences of different calculation methods are analyzed.It is found that the calculation of negative skin friction pile group effect coefficient based on limit equilibrium principle is in good agreement with the simulation results. |
