Laboratory Model Test Study On Vertical Bearing Characteristics Of Mini-long Pile Groups In Loess Area

The pile foundation plays an important role in infrastructure construction in China,it has been rapidly developed and applied,especially in high-rise buildings and large Bridges.Due to the special geological conditions and improper design and construction of pile foundations in the large-thickness loess region of the west,building tilting accidents are endless.Therefore,a kind of miniature steel pipe pile with small diameter,long pile length and high strength that can be used to rectify the deviation of buildings and reinforce the foundation with pile foundation is gradually recognized by the engineering.At present,there has been a comprehensive understanding and development of the vertical bearing capacity of the single pile of miniature steel pipe pile,the theoretical research on the vertical load of this kind of miniature long pile group is not consistent with the practice.Based on this,the article mainly does the following work:First of all,based on the understanding of the research status of single pile s,group pile-bearing cap system at home and abroad,the load transfer law of single piles-soil system under ordinary loads on vertical pile foundations,the w orking shape of group piles,and single piles are described.The calculation theory and settlemen t calculation method of the ultimate bearing capacity single pile and group pile and the settlement calculation method of common pile foundation under vertical load are expounded.In view of the buckling stability of long piles and super-long pile foundations,the judgment criteria,analysis methods and influencing factors of pile foundation buckling stability are summarized,and the energy method for buckling stability of pile foundation is especially introduced.Secondly,in view of the bearing capacity of long miniature piles under vertical load in the western loess region with large thickness,the model tests of single pile and group pile-cap system were designed and carried out.The basic physical and mechanical properties of the loess used in the model test were measured by geotechnical test.Through the data acquisition of pile strain sensor,pile end pressure sensor and pile top displacement sensor,the vertical bearing capacity and settlement characteristics of single pile under three conditions of l ength-diameter ratio and four pile group pile-cap system under three conditions of pile spacing are studied and analyzed.The results show that the load-settlement curves of single pile showed a steep decline trend and the load-settlement curves of pile group showed a slow change.The distribution law of axial force of single pile and pile group is basically the same along the pile depth.With the increase of depth,the axial force of pile body decreases from the top of pile to the end of pile.The lateral friction resistance of single pile increases first and then decreases along with the pile depth,but the lateral friction resistance of test pile group does not continue to decrease at the pile end position,but suddenly increases,resulting in the strengthening effect of pile end resistance.Meanwhile,the pile group effect coefficient increases with the increase of pile spacing,especially when the pile spacing is 4~6 times the pile diameter,the pile group effect is obviously improved.Finally,based on the basis of indoor model test,using geotechnical finite element software Midas GTS/NX,via the geotechnical test results and selecting reasonable parameters for the model,the relative literatures introduce Moore-coulomb soil,pile combined with beam element and element to simulate the pile soil interaction.Finite element simulation was carried out for single pile with three pile diameters of 20 mm,30mm and 40 mm,and for group pile-bearing cap system with five pile spacing of 2.5D,3D,4D,5D and 6D.The analysis shows that the load-settlement curve of single pile and pile group has no obvious inflection point.The pile caps of the 2.5D and 3D pile spacing models were asymmetrically inclined under the conditions of small pile diameters in the pile-cap system.When the pile spacing increased,the deformation characteristics of each base pile in the pile group were basically consistent.