Vertical Bearing Capacity Of Rigid Composite Pile In Loess Region Characteristics And Engineering Application Research

In recent years,with the rapid development of Chinese social economy and the acceleration of urbanization,in order to solve the problems of more people and less land in urban infrastructure construction,a large number of middle and high-rise buildings began to spring up like bamboo shoots,which put forward higher requirements for building foundation bearing capacity.The rigid composite pile is a new type of cement-soil flexible pile formed by deep mixing and high pressure rotary jet,and the rigid pile is inserted in the center before the initial setting.When the pile tip is stressed,it can not only take advantage of the high lateral friction resistance of the flexible pile,but also take advantage of the high bearing capacity of the rigid pile,so it has been widely concerned by the engineering and academic circles.This paper will focus on the practical engineering application of the rigid composite pile under the geological condition of loess,through the field test pile to obtain the loading displacement curve trend of the dynamic composite pile,derive the theoretical formula to study the dynamic deformation law of the dynamic composite pile under axial load,and carry out the finite element simulation of the static load test process on the site.The simulation results are compared with the field test and theoretical calculation results.The error is relatively small,which can provide some reference for similar projects.The details are as follows:(1)According to the concepts of lateral friction resistance adjustment coefficient and end resistance adjustment coefficient of composite section of pile in the calculation formula of vertical bearing capacity of single pile of rigid composite pile,using finite element numerical simulation,it is concluded that the value in loess area is related to the core pile diameter of rigid composite pile.According to the general section structure of rigid composite pile,the recommended value is between 1.5 and 1.8.The recommended value is 2.0,and the reduction coefficient of natural foundation soil is 1.0.Combined with the geological prospecting data and pile test parameters in this paper,the bearing capacity obtained by substituting the suggested value into the standard calculation formula is in good agreement with the ultimate bearing capacity obtained from the field static load test results,indicating that the parameter values are relatively reasonable.(2)The mechanical analysis model of single pile of rigid composite pile was established,the basic differential equation of internal force and deformation of rigid composite pile was derived,and the theoretical analytical solutions of pile tip settlement,axial force and lateral friction of internal and external core pile were solved by using the basic principle of shear displacement method,and the analytical solutions were verified by the results of pile test in practical engineering.The results show that the deviation between the theoretical calculation results and the field test data is small,and the trend of Q～S curve is basically the same.The theoretical analytical formula has good rationality and reliability.Secondly,the effects of the ratio of core to pile,the ratio of pile length and the elastic modulus of soil-cement pile on the mechanical characteristics of the stiffened composite pile are analyzed.The results show that the pile head displacement of the stiffened composite pile decreases with the increase of the ratio of core to pile and the pile length.When the core pile ratio is between 0 and 0.4,the pile tip displacement decreases rapidly with the increase of the core pile ratio,and when n is larger than 0.4,the decreasing trend becomes slow.When the pile length is between 10D and 20D,the settlement value of pile top changes the most.The improvement of bearing capacity of rigid composite pile is mainly due to the improvement of pile lateral friction resistance.When the core pile ratio is between 0.2 and 0.8,the lateral friction resistance between pile and soil interface is proportional to the core pile ratio,while the lateral friction resistance between inner and outer core interface and core pile ratio is inversely proportional.(3)The finite element software PLAXIS was used to simulate the field static load test,and the simulation results,field pile test results and theoretical calculation results were compared.The results showed that the overall change trend of the simulation results was consistent with the field test results and theoretical calculation values,and the error was relatively small,the maximum error was 20%.Secondly,the residual stress of pile body,the co-working characteristics of internal and external core piles and the stress distribution of pile body are analyzed.The results show that when the pile body is at the maximum load,the difference of settlement value between the core pile and the core pile is only 2mm,which can be ignored,that is,the collaborative load between the core pile and the core pile can be guaranteed.Under the vertical load of the rigid composite pile,the phenomenon of "stress concentration" is obvious on the top of both the inner core pile and the cement soil pile.The stress of the inner core pile and the cement soil pile decreases continuously along the direction of the pile.The core pile decreases almost linearly after the maximum stress value,and the pile tip decreases sharply.However,the stress value of soil-cement pile decreases first with the increase of depth,and then the stress value remains almost unchanged along the pile body.The increase value is small,and then increases suddenly at the pile tip.Most of the load on pile top is carried by rigid core pile,while the load of cement soil outside pile is small,accounting for only 6.12%-10.25%.(4)Verify the theoretical calculation results and numerical simulation results based on the case of a foundation treatment project in Lanzhou New Area.The experimental results show that the Q～S curve of the rigid composite pile under the geological condition of loess presents a slow deformation trend,without obvious steep drop section,and the tail of the s～lgt curve does not bend down significantly over time,showing a gentle regular arrangement.The ultimate bearing capacity of the rigid composite pile in the loess region is controlled by the deformation condition,and when the load is at the designed ultimate load,The stiffness composite pile is still in the elastoplastic deformation stage,the pile bearing capacity is still in a certain surplus state,and the pile quality can be guaranteed.Secondly,the effect of the foundation treatment of the project is compared with that of bored pile.It is found that the use of the foundation treatment of the composite pile can significantly reduce the cost of the project and the construction period.