Research And Application Of Reinforcement Mechanism Of Rotary Jet Grouting Method For Existing Pile Foundation Of Bridge

With the rapid development of economy and society,some old bridges have been unable to withstand the increasing traffic flow,and need to be reconstructed or reinforced.Post grouting technology is widely used to reinforce the existing pile foundation of bridges.Aiming at the problem that the slurry of traditional post grouting technology is uncontrollable and the effective utilization rate is low,this paper uses the jet grouting method to reinforce the existing pile foundation of the bridge.After reinforcement,the friction characteristics of the pile-reinforcement-soil interface have a great influence on the bearing characteristics of the existing pile foundation,and the depth（confining pressure）is the key factor affecting the friction characteristics of the pile-reinforcement-soil interface.Therefore,it is necessary to carry out in-depth research on the friction characteristics of pile-reinforcement-soil interface at different depths and the bearing characteristics of existing pile foundations after reinforcement.The shear tests of pile-solid-soil interface under different confining pressures were carried out to explore the friction characteristics of pile-reinforcement-soil interface under different confining pressures.Based on the shear test results,the load transfer model of pile-reinforcement-soil interface at different depths was established and the calculation formula of bearing capacity of existing pile foundation after reinforcement was established.FLAC^3D finite difference software was used to explore the influence of different reinforcement length,depth and width on the bearing capacity of pile foundation.The reliability of the bearing capacity calculation formula and numerical simulation results of the pile foundation was verified by the field test of the jet grouting method,and the optimal grouting reinforcement scheme was determined.It provides a theoretical basis for the reinforcement of the existing pile foundation of the bridge by the jet grouting method,and forms a set of analysis theory and design method for the bearing characteristics of the pile foundation of the jet grouting method,which has certain guiding significance for the reinforcement of the existing pile foundation of the bridge in Dezhou（Shandong-Hebei boundary）.The main conclusions are as follows:（1）Aiming at the problem that most shear test devices do not consider the confining pressure,a pile-reinforcement-soil interface shear test device with controllable confining pressure was developed.The device consists of a reaction device system,a constant pressure loading system,a data acquisition and control system,etc.It can perform shear tests on multiple interfaces,keep the interface contact area unchanged during the shear process,and avoid the influence of contact area changes on the test results during the shear process.（2）A series of pile-reinforcement-soil interface shear tests under different confining pressures were carried out.The test data show that the ultimate side friction resistance of pile-reinforcement-soil interface,the residual side friction resistance of pile-soil interface and reinforcement-soil interface increase with the increase of confining pressure（the depth of pile into soil）.This trend gradually slows down with the increase of confining pressure,and finally tends to be stable.The residual side friction resistance of pile-reinforcement interface increases linearly with the increase of confining pressure.The pile-solid interface is affected by the bite force,and the interface failure mode is brittle failure.The shear process has experienced elastic stage,brittle failure stage and shear slip stage.The pile-soil interface and the reinforcement-soil interface show a trend of rapid growth,slow growth and softening during the shear process.（3）Based on the shear test results of pile-reinforcement-soil interface,the load transfer model of pile-reinforcement-soil interface at different depths is established,and the calculation method of bearing capacity of pile foundation after reinforcement is established.The reliability of the calculation formula of bearing capacity of pile foundation after reinforcement is verified by comparing the theoretical value with the static load test results of pile foundation.According to the construction process of jet grouting method and theoretical analysis,the calculation formulas of various construction parameters in the construction process of jet grouting method are improved.（4）The finite difference software FLAC^3D combined with orthogonal test was used to study the influence of different reinforcement depth,width and length of jet grouting method on the ultimate bearing capacity of pile foundation.The range analysis showed that the reinforcement length had the most significant effect on the ultimate bearing capacity of pile foundation,and the optimal reinforcement scheme of different increase range of ultimate bearing capacity of each pile foundation was determined by analytic hierarchy process,which provided some guidance for strengthening the existing pile foundation of bridges in Dezhou（Shandong-Hebei border）area.（5）The field test results of S2 and S4 test piles are compared with the theoretical calculation results and numerical simulation results.The error between the theoretical value and the field test results is 2.1%and 5.4%,respectively.The error between the numerical simulation results and the field test results is 4.1%and 8.6%,which are within the allowable reasonable error of the project.By comparing the optimal scheme of numerical simulation with the on-site construction scheme,the superiority of the on-site construction scheme is reflected.