Experimental Study On Bearing Capacity Of Long Spiral Bottom Expanded Pile Under Lateral Cyclic Load In Sandy Soil

The long spiral pile with enlarged bottom is a new type of pile formed by improving the making technology and construction equipment of spiral pile and setting an enlarged head at the end of spiral pile.In recent years,the research on lateral bearing capacity of single pile by most scholars still lags behind the development of engineering practice.In this paper,the long spiral bottom-expanded pile under low cyclic lateral load is taken as the research object.Firstly,the foundation reaction coefficient of the model pile under lateral load is determined through the test of the lateral foundation coefficient of the soil in the model groove site,and the influence ability of the soil layer on the bearing capacity of the pile is studied.Second,by two groups of low level cyclic loading indoor model test,the long screw expanding bottom pile bending moment,deflection and stress change rule and the load-bearing characteristics,with the same size and the straight pile and expanding bottom pile comparison research,at the same time to compare three kinds of pile type under lateral cyclic loading hysteresis curve,analysis of long screw expansion at the bottom of the pile with cyclic number change under different load levels of energy dissipation characteristics;In addition,the influence law of different pile parameters on lateral bearing capacity was studied by numerical simulation,and the response law of long spiral bottom-expanding pile was analyzed to determine the more reasonable pile design parameters.Finally,the bearing capacity of single pile under lateral cyclic load of long spiral bottom-expanding pile is studied in combination with the morphological characteristics of pile body thread and expansion head,and on this basis,the calculation and solution of lateral bearing capacity of long spiral bottom-expanding pile are explored.The main conclusions can be drawn as follows:（1）To carry out experimental study on the lateral bedding coefficient,the results show that the effects of different pile type on soil resistance was not significant,the proportion coefficient of ground resistance coefficient m value as a whole with the similar power function attenuation trend displacement,with the increase of lateral displacement within2mm displacement change of m value is bigger,increases as the lateral displacement,different pile type m value will flatten out,eventually tends gradually 8MN/m⁴.（2）Through the indoor model test of low cyclic lateral load,the results show that the position of maximum bending moment and the overall bending moment distribution of the long spiral bottom-expanded pile are different from those of the bottom-expanded pile and straight pile due to the existence of pile screw and expansion head.As the compressive soil changes from elastic deformation to plastic deformation,the bending moment and stress distribution of the long spiral enlarged bottom pile will appear asymmetry in the process of forward and reverse loading.According to the pile deformation,it is found that the deflection gradually decreases along the depth,and the deformation trend of the long spiral bottom-expanding pile under positive and negative loading is basically symmetrical.The deformation of the test pile presents a parabolic change on the whole,which is more stable than other pile types.The influence of load change on pile stress and bending moment distribution is more obvious than that of cycle number.Under lateral cyclic load,the peak load of the long spiral bottom-expanded pile with the same section size can be increased by about twice as much as that of the extended bottom pile and straight pile.During the whole loading process,the serious disturbance range of the soil around the long spiral bottom-expanding pile is 2-4 times the pile diameter,and the maximum influence range of the soil is near the top surface of the pile.As the pile depth increases,the influence range gradually decreases and presents a conical shape.（3）The hysteresis curve obtained,based on the indoor model test of long spiral of pile under cyclic loading energy dissipation characteristics were analyzed,and the results show that the long spiral kneading expanding bottom pile load early hysteresis loops shrink effect is obvious,difference of energy consumption characteristics of the straight pile and expanding bottom pile,loading to late hysteresis curves tend to be full,knead shrinkage phenomenon is improved;Compared with other pile types,the shape and variation of the hysteretic loop of long spiral enlarged bottom pile are more stable.When the load is small,the number of cycles can significantly reduce the energy dissipation characteristics,while when the load is increased,the effect of the number of cycles on the energy dissipation characteristics is not significant.（4）According to the indoor model test and numerical simulation analysis,the lateral bearing capacity of the long spiral bottom-expanded pile can be up to 25%when the pile diameter is increased.When the length of long spiral bottom-expanding pile increases from0.8m to 1.2m,the maximum reduction of lateral displacement can reach 42.8%.However,when the pile length reaches a certain depth,the effect of reducing lateral displacement by increasing the pile length becomes weak.Therefore,simply increasing the pile length cannot continuously improve the pile bearing capacity.With the increase of the diameter of the expanding head,the bearing capacity of the model pile will increase slightly,but the advantage is not obvious.When the soil modulus or pile modulus increases,the lateral displacement generated by pile decreases,and the maximum displacement of pile top decreases by 13%～15%.On the whole,when the pile length is 1.2m and the pile diameter is60mm,the bearing capacity is improved significantly.The soil compression modulus is30₍（6）,the elastic modulus of the pile body is 6.5₍（6）,the distance to diameter ratio is 1.0,and the blade width is 10mm.（5）Based on the basic assumptions of Winkel foundation model,the general solution of the differential equation was obtained.Considering the bearing characteristics of the long spiral enlarged bottom pile,combining with the parameters of the pile thread and the shape characteristics of the enlarged head,the lateral bearing capacity of the long spiral enlarged bottom pile was determined by calculating the parameters of the pile deflection and bending moment according to the boundary conditions.It is verified by the case that the measured value is in good agreement with the theoretical calculation value of lateral bearing capacity of the long spiral enlarged bottom pile,and the hypothesis proposed in the calculation of lateral bearing capacity of the long spiral enlarged bottom pile is reasonable to a certain extent.The general solution of theoretical calculation given in this paper can provide reference for the design of this kind of pile.