| The composite soil gene of rigid pile has been widely used in all kinds of buildings and infrastructure construction due to its advantages of large processing depth,high bearing capacity,short construction period and low construction cost.Engineering application,consider the upper structure load differences in different geological conditions and construction area,often by changing the parameters of composite foundation design method to adjust the stiffness of composite foundation and optimization,in order to achieve the effective control of settlement at the same time,to get the most out of soil between piles and pile bearing capacity,reduce the project cost.The most commonly used optimal design method of variable stiffness of composite foundation is to change the pile length,that is,to improve or reduce the stiffness of part of composite foundation by changing the pile length to control the settlement of this area.All kinds of experts and scholars have carried out long-term research and practice on this subject,and the relevant results have also formed norms.In some complicated geological conditions,such as underground rail transit system,the method of adjusting the stiffness of composite foundation by changing the length of pile is no longer applicable.The load sharing ratio between piles and soil between piles can be adjusted by changing the pile diameter and the thickness of cushion.Relative to the length of pile composite foundation,in view of variable diameter plane variable stiffness composite foundation research is relatively small,the existing results are mainly concentrated in the variable stiffness of pile composite foundation-conventional pile interaction research and variable variable stiffness composite foundation pile diameter plane overall seismic performance of the study,involving variable diameter ratio of composite foundation after soil,the pile bearing characteristics and settlement of research needs to be added.By means of model test and numerical simulation,the load sharing ratio of pile and soil and load transfer mechanism of composite foundation with varying pile diameter and varying plane stiffness under normal operating load conditions are studied.The main results are as follows:(1)Under normal operating load conditions,with the decrease of the pile diameter of the variable stiffness pile,the load sharing ratio of the variable stiffness pile in the planar composite foundation with variable stiffness decreases,while the load sharing ratio between the conventional pile and the soil between piles increases.The main reason for this phenomenon is that the diameter of the pile with variable stiffness decreases and the bearing stiffness of the single pile decreases.With the increase of load amount,the load sharing ratio between the variable stiffness pile and the conventional pile in the plane variable stiffness composite foundation increases,while the load sharing ratio between the piles decreases.The main reason for this phenomenon is that with the increase of the upper load,the pile lateral friction resistance and the pile end bearing capacity in the composite foundation gradually play,and the pile bearing stiffness gradually increases.(2)Under normal operating load conditions,the pile-soil stress ratio of the variable stiffness pile and the conventional pile in the plane composite foundation with variable stiffness increases as the diameter of the pile decreases.In the same composite foundation,the pile-soil stress ratio of the variable-stiffness pile is greater than that of the conventional pile,and the amount of overthrust in the cushion increases correspondingly.The lateral friction resistance of the variable stiffness pile decreases while the lateral friction resistance of the conventional pile increases.The lateral friction resistance of conventional piles in the same composite foundation is greater than that of variable stiffness piles,and the neutral point of lateral friction resistance of conventional piles is closer to the pile top than that of variable stiffness piles.The main reason is that the pile lateral friction in the composite foundation is related to the pile lateral normal stress and the pile-soil relative displacement.The stress redistribution of the soil between the piles in the composite foundation reduces the pile lateral normal stress of the variable pile diameter and increases the pile lateral normal stress of the conventional pile.And the relative displacement of the pile soil of the variable stiffness pile increases in its negative friction area,which makes the neutral point move down.(3)The coefficient of bearing capacity is defined as the ratio between the actual bearing capacity of the pile and the soil between the piles in the composite foundation and the characteristic value of the bearing capacity of the single pile.Under normal operating conditions,the diameter of the variable stiffness piles in the plane composite foundation with variable stiffness decreased,and the coefficient of bearing capacity of the conventional piles,the variable stiffness piles and the soil in the same composite foundation increased.The main reason is that under the same load condition,as the diameter of the pile with varying stiffness decreases,the load sharing ratio of the pile body decreases,while the load sharing ratio of the conventional pile and soil increases.Meanwhile,the characteristic value of the bearing capacity of the single pile of the pile with varying stiffness decreases with the diameter of the pile itself.So in view of the change diameter plane stiffness of the composite foundation in foundation treatment engineering,within the scope of permissible settlement option with smaller diameter pile composite pile diameter plane of composite foundation with variable stiffness,not only can reduce the engineering cost and improve the economic benefit,but also makes the single pile composite foundation bearing capacity between pile and soil into full play,so as to realize the goal of foundation treatment design optimization. |
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