| In recent years,a large scale of infrastructure has been built in collapsible loess area,and special engineering properties of loess has not been fully understood,which results in a large number of engineering failures and accidents.Meanwhile,many projects are too conservative to exaggerate the influence of loess collapsibility,which results in investment waste of engineering construction.There exists a contradiction between theory and practice.Therefore,the essential problems such as collapsibility of collapsible loess and negative friction of deep foundation need to be deeply researched on basis of loess engineering and loess mechanics,which could reveal the inner mechanism of loess engineering problems and provide a theoretical basis for the design and construction of scientific and reasonable engineering.As a kind of strongly-structured soil,stress would be released inevitably in the process of sampling,transportation,and preparation,which results in the disturbance of the intact loess.Due to various geographical and spatial positions of natural loess,the dispersion of test results at the aspects of collapsibility,dry density,water content and shear strength are very large,which is difficult to be compared and analyzed directly.Remolded loess is always applied as filling into the model test of interaction between loess and structure,and the mismatch of similar materials in model test results in relatively large experimental errors.Therefore,research on preparation technology and principle of artificially prepared loess has been conducted in this paper,and optimum mixing ratio and control standard of collapsible loess with different collapsibility degree are obtained,which guarantees the repeatability and reliability of the test results.In view of wide application of the pile foundation and diaphragm wall foundation in collapsible loess area that possess broad application prospects,experimental study of underground continuous wall and pile foundation immersion were conducted with the artificial preparation of collapsible loess foundation soil as filling material.Analysis on load transfer mechanism and distribution of negative friction characteristics of deep foundation in the collapsible loess foundation were conducted,which verifies correctness and generalization of artificially prepared collapsible loess material and technology.Calculation formula of vertical ultimate bearing capacity of deep foundation is put forward and improved by considering negative friction.In the following are main research contents and innovations:(1)Quartz powder,sand,bentonite,gypsum and industrial salt were chosen to prepare artificial collapsible loess with the air fall method.Physical and mechanical properties between artificially prepared collapsible loess and natural loess were analyzed,and a set of preparation technology and method for artificially prepared collapsible loess was put forward.Orthogonal tests and single factor analysis tests were carried out to investigate the influence degree and regularity of various factors on collapsibility by selecting clay mineral content,compaction degree,initial water content and soaking pressure as the influencing factors.Research shows that the application of the air free fall method in artificial collapsible loess preparation can simulate deposition process and formation conditions of the aeolian loess.Artificially prepared collapsible loess sample possesses similar large pore and porosity of natural loess,which has a high coherence with nature collapsible loess in the view of physical and mechanical indexes.After soaking,artificially prepared collapsible loess produces larger collapsible deformation,which can be used for similar material of model test of interaction between collapsible loess and structure.(2)Scanning electron microscope(SEM)test of artificially prepared collapsible loess was carried out,and the influence of initial moisture content,compaction degree and consolidation pressure on the microstructure of soil was qualitatively analyzed.By extracting the anisotropy,equivalent diameter,oblate,filling ratio and area ratio of the microstructure parameters,relationship between micro and macro parameters such as cohesion,internal friction angle and coefficient of collapsibility is established from microscopic perspective to explore the influencing factors and mechanism of loess collapsibility.Research shows that there exists significant correlation between area ratio,anisotropy,and shear strength index.Area ratio,equivalent diameter and anisotropy are positively correlated with cohesion and internal friction angle,and oblate degree,filling ratio and angle is negatively related to cohesion and internal friction.The influence of equivalent diameter and filling ratio on collapsibility coefficient is significant.(3)Model tests of ultimate bearing capacity and bearing capacity under water immersion condition of underground diaphragm wall were carried out with artificial prepared collapsible loess as the foundation.Combined with the numerical simulation results,the variation law of the axial force,wall side resistance,load sharing ratio,neutral point depth and negative friction distribution of the diaphragm wall were analyzed.Research shows that the vertical bearing capacity of diaphragm wall consists of lateral friction,lateral friction,wall resistance and soil reaction force,and the four are not synchronous.The underground continuous wall is end bearing friction type foundation.After soaking,the depth ratio of neutral point is between 0.64 and 0.73,which is close to the measured result of pile foundation neutral point.Because the core soil is not affected by soaking,the side friction of the inner wall and the counterforce of the cap soil can effectively reduce the additional settlement of the diaphragm wall foundation.(4)Based on model test of ultimate bearing capacity and bearing capacity under water immersion condition of super long single pile and pile group foundation,change law of ultimate bearing capacity,load transfer mechanism,neutral point depth and negative friction resistance of super long single pile and group pile foundation are analyzed.The study shows that: with the increase of soaking time,the relationship between the collapsibility and time mainly includes initial gentle section,immersion steep drop section,medium gentle section and gentle section after water cut.The negative frictional resistance develops from top to bottom,and the depth of neutral point moves down gradually.The neutral position is closer to the recommended value of technical specification for building pile foundation.In the steep drop period of water immersion for collapsible deformation of soil layer,downward load acting on the pile increases rapidly,which leads to the increase of axial load and pile end load and gradual increase of the settlement of pile foundation.Collapse of soil layer presents a synchronous trend with the negative friction of pile side and the occurrence and development of pile foundation settlement.(5)Based on site pile immersion test data and model test results of underground continuous wall and pile foundation water,the parabola distribution method is used to analyze the distribution of negative friction,and the parabola method is proposed to calculate the pull down load caused by collapse.The parabolic method takes into account the actual distribution of the negative friction along the pile body,and the downward load is obtained by integration.The maximum value of negative friction is about half of the depth of the neutral point,and the maximum value of negative friction is 1.5 times of the average value of negative friction.The difference between the drop load calculated by parabola method and the measured result is within 10%,and the former method is more reasonable and reliable,which can effectively reduce the waste of a large number of engineering investments caused by excessive conservative calculation of the pull down load. |
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