Study On Vertical Bearing Capacity Of Large-Diameter Cast-in-place Pile In Q₄ Loess Area Of Lanzhou

Large diameter cast-in-place piles have many advantages,such as high bearing capacity,small deformation,wide application range,relatively simple operation,and more environmentally friendly.With the continuous development of many construction projects,large diameter cast-in-place pile foundation,as the main form of high-rise buildings and bridges,its application amount occupies a high proportion.But so far,due to the complexity of the pile-soil system,there is no more accurate method to calculate the bearing capacity of the general pile.For large diameter cast-in-place piles,the current theory has been unable to meet the needs of practical engineering.Therefore,it is urgent to study all aspects of large diameter cast-in-place piles.In this paper,based on the relevant experimental data at home and abroad,combined with a specific engineering project,we conducted a field test in a Q₄ loess area in Lanzhou City.There are three test piles,and one of them is selected for analysis.The test pile is given with diameter-800 mm,the length-26 m.The test pile only carried out the bearing capacity test.Every 1m symmetrical buried a pair of steel stress meter in the top 10 m of the test pile,and every 2m symmetrical buried a pair of steel stress meter in the remaining 10 m of the test pile.The static load test method is used to study the vertical bearing characteristics of large diameter cast-in-place piles.The study includes the analysis of the ultimate bearing capacity of large diameter cast-in-place pile under vertical load,the distribution of load along the pile,the developing cause of friction resistance and base resistance,and the analysis of load-settlement curve.The following is the main content of this article:?1?According to the results of the experiment,we got the load-settlement curve and the settlement-time log curve of the large diameter cast-in-place pile,and the relationship between the load and the settlement is analyzed in depth.And finally three major deformation stages of the pile foundation are analyzed,they are the elastic deformation stage,the plastic deformation stage and the elastic-plastic deformation stage.At the same time,according to the discriminant method of the P-s curve,we obtained the vertical ultimate bearing capacity and the bearing capacity characteristic value of the test pile.?2?The analysis of the axial force diagram shows that the axial force decreases nonlinearly along the pile direction.And as the load level of the pile increases,the axial force curve becomes more and more parallel.Especially when loaded into the last two stages,the two axial force curves are almost translational.which indicates that the friction of the pile is very close to the limit at this time.In the process of load transfer,the pile end resistance has always occupied a small percentage of the load on pile top?about 10%?.In other word,duringthe course of the work,the load on pile top is mainly borne by the side friction resistance of the pile,so the test pile is obviously a friction pile.?3?On the whole,the side friction resistance of the pile first increases,and then decreases,and finally increases along the pile direction under the load on pile top.As the load level increases,the side friction resistance also increases and gradually approaches a stable value.At the depth of 6m and 20 m,two maximal values of the side friction resistance of the pile were presented.Negative friction was generated at about 14 m,and the neutral point of the test pile was in 13 m and 15 m.?4?According to the experimental results and the calculation of the correlation theory,the pile tip resistance-pile end displacement curve is drawn.And then the load transfer function in exponential form is obtained by analyzing the relationship between pile tip resistance and pile end displacement.?5?From a general view,the friction at different depth of the pile increase to a stable value as the relative displacement between pile and soil increases.The developing degree of side resistance is well corresponding to the relative displacement between pile and soil.And the developing degree of side resistance is also corresponding to the properties of the soil layer.?6?The elastic-plastic deformation of the pile tip soil and the rebound situation of pile shaft and tip of the pile are analyzed,and we finally obtained the elastic settlement and permanent settlement of the test pile.