| The interaction between the rigid long-short-pies composite foundation is more complicated than that in equal length pile composite foundation,and the specific performance and reasons are also different.In the traditional design concept which based on the bearing capacity,the existing specifications comprehensively describe the performance and interaction of different pile types and soils through the concept of performance coefficient of bearing capacity.In most cases,the determination of this coefficient is still based on experience,and the coefficients of each component are independent from each other,which cannot well reflect the working mechanism of the interaction between pile and soil.This thesis is based on the numerical simulation,reduced scale model test and the theoretical analysis about rigid long-short-piles composite foundation,and the force transfer mechanism under vertical load has been studied.In particular,the load sharing and deformation stiffness of pile-soil units are discussed,the existing design and calculation method of long-short-piles composite foundation based on equal settlement criterion is improved.The major research works and summary of this study are detailed as below:(1)The cushion of long-short-pies composite foundation was analyzed based on the discrete element-finite difference coupling method.Based on the bearing characteristics of soil,it is considered that the thickness of the effective cushion should be higher than the height of the blocking zone under the maximum allowable settlement condition of the upper structure,and the rationality of calculating the cushion model with Winkler elastic foundation beam model is verified.(2)The distribution model of pile lateral friction in composite foundation and the interaction between pile and soil are considered comprehensively.The calculation model of composite foundation with long-short-pies considering pile-soil interaction is established.To meet the load-bearing characteristics premise,as far as possible to reduce settlement,make full use of the stiffness of piles and soil,The influence of different parameters on the long-short-pies composite foundation was calculated and analyzed.From the perspective of interaction,the mechanism of the influence is explained,and it is concluded that:When the pile length of long pile or short pile increases,the overall vertical compression stiffness can be effectively increased,while the pile length of long pile increases or the pile length of short pile decreases.As the pile end distance increases,the interaction between the pile ends decreases,resulting in the increase of the stiffness of the pile body elements of short pile and long pile.The increase of cushion thickness is equivalent to the increase of a layer of spring whose stiffness is equal to the stiffness of the cushion,resulting in the decrease of the overall vertical compression stiffness.The pile body and the soil above it are analyzed as a whole,the increase of cushion thickness reduces the unit stiffness of the whole.The load sharing of pile body and stiffness of pile body element are reduced and the load sharing of soil body and stiffness of soil body element are increased.The increase of the stiffness of the bearing capacity of the pile end is equivalent to the increase of the spring stiffness of the pile end by a fixed value,resulting in the increase of the overall compressive stiffness.According to the analysis of pile element,the increase of long pile element stiffness is smaller than that of short pile with the same stiffness increment,resulting in the load sharing of long pile is smaller with the increase of the stiffness of the bearing capacity of the pile end.However,when the contact stress increases to the turning point,the increase of the bearing stratum of the pile end results in the increase of the penetration on the pile,and the overall stiffness decreases gradually with the increase of the bearing stratum of the pile end.Similarly,the stiffness of the pile-soil element and pile-soil stress ratio of long-short-pile are reversed after the turning point.The increase of the elastic modulus of the pile improves the overall stiffness and the element stiffness of the pile by reducing the compression of the pile,but the increase is nonlinear.With the increase of the elastic modulus of the pile,the main control factor of the pile settlement gradually inclines to the settlement of the pile end,and the increase of the element stiffness of the pile gradually decreases.Moreover,the elastic modulus mainly affects the compression of the pile,and the increasing effect of the element stiffness of the long pile is more obvious than that of the short pile;The increase of pile diameter can improve the element stiffness and overall stiffness of pile by increasing the bearing area of pile end and pile side.However,because the area of pile end is in direct proportion to the second power of pile diameter and pile side is in direct proportion to first power of pile diameter,the increase of pile diameter increases the load sharing of long pile and short pile,but the stress ratio of pile-soil and the mobilization coefficient of long pile decrease with the increase of pile diameter.(3)Based on the finite difference numerical simulation and test results,the bearing characteristics and stiffness evolution of the whole composite foundation and pile-soil element under different types of composite foundation are systematically analyzed.Among them,the bearing characteristics are analyzed by the load sharing of each component of the same contact stress,and the stiffness is described by the load sharing of each component under the same basement settlement condition.From single pile composite foundation to four piles composite foundation,the whole part of composite foundation is affected by the increase of loading plate size andthe interaction of pile caused by the increase pile numbers.It weakens the integral stiffness.The bearing characteristics and stiffness performance of piles vary with the change of foundation types,which are mainly affected by the soil interaction.From single pile foundation to single pile composite foundation,the soil changes from passive stress to active stress,which weakens the stiffness of pile element.From single pile composite foundation to four piles composite foundation,the increase of pile numbers leads to the increase of pile-pile interaction effect,and the increase of soil stress area leads to the increase of soil settlement under the same stress,which makes the pile subject to additional settlement.Under the influence of pile and soil,the element stiffness of four piles composite foundation is smaller than that of single pile composite foundation.The change of soil element stiffness is mainly affected by the size of the foundation and the load-sharing effect of the pile.When compared natural foundation and single pile composite foundation,the pile shares more load than soil in composite foundation.However,under the condition of the same foundation settlement,the relationship between the soil load sharing and the stiffness of the pile unit is analyzed by the stiffness distribution principle,and it is considered that the stiffness of the load sharing unit of the soil is dominated by the stiffness of the pile.When the stiffness of the pile unit is small,the shared load of the soil is large.The slope of the secant and the load sharing curve of the soil is regarded as the stiffness of the soil element.the smaller the stiffness of the pile element,the greater the stiffness of the soil element.When the soil unit transitioned from a single-pile composite foundation to a four-pile composite foundation,the pile replacement rate did not change,and the soil was mainly affected by the size effect.Under the same stress condition,the larger the soil area size,the greater the soil settlement,so the stiffness of the soil element in the four-pile composite foundation is smaller than that of the single-pile composite foundation.(4)From single pile composite foundation to four pile composite foundation,the stiffness correction coefficient of each unit of pile and soil is different.According to the test results,the stiffness correction coefficient of short pile elements is about 0.6?0.7,the stiffness correction coefficient of long pile elements is about 0.85?0.9,and the stiffness correction coefficient of soil elements is 0.7?0.8.The composite pile group effect coefficient is about 0.7?0.8,and the change of the overall stiffness of the composite foundation is greatly affected by the soil unit.(5)The stiffness correction coefficient is introduced,and an improved formula for calculating the bearing capacity of rigid long and short pile composite foundations based on the equal settlement criterion is given,and the reliability of the formula is verified with engineering examples. |
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