Experimental Study On Bearing Behavior Of Distributed Post-grouted Bored Piles

Bored cast-in-place pile is widely used because of its high bearing capacity,strong adaptability and simple construction.However,the defects such as pile end sediment and pile side mud skin brought by its construction technology greatly affect the exertion of pile foundation bearing capacity and the control of pile foundation settlement and deformation.The post-grouting technology is generally regarded as an effective means to improve the above problems and increase the bearing capacity of pile foundation steadily because of its significant improvement effect,concise process and convenient construction.At present,the application of post-grouting technology has developed from medium and small diameter piles to large diameter and deep and long piles,and the corresponding type of post-grouting has also developed from simple pile end(side)post-grouting to combined post-grouting and distributed post-grouting.However,the engineering practice of distributed post-grouting is ahead of the theory,and there is still a lack of systematic research on the reinforcement mechanism of distributed post-grouting process and the load-bearing mechanism and load transfer law of distributed post-grouted piles.In this paper,the load-bearing characteristics of large-diameter distributed post-grouted piles and their advantages will be studied in depth by means of model tests,theoretical verification,field in-situ tests and numerical simulations,etc.The main work and results are as follows.(1)A set of model test device which can accurately control the grouting amount is designed and manufactured by using single chip microcomputer and electromagnetic valve.Through the model test simulating field distributed grouting and 18 groups of large-scale contact surface direct shear tests,the relationship between shear stress and shear displacement of post grouting pile-soil contact interface under different working conditions is studied,the improvement effect of different grouting parameters on pile-soil contact interface is analyzed,and the shear mechanics model of distributed post-grouted pile cement soil sand interface is established based on hyperbolic function model.(2)Based on the direct shear test results of a large number of contact surfaces,the analysis shows that post-grouting effectively improves the ultimate shear strength and initial shear stiffness of the contact interface,and the shear strength enhancement coefficient is between1.16 and 2.91.The relationship between the normal stress and ultimate shear stress of the pilesoil contact surface under different grouting amounts still follows the Mohr-Coulomb criterion.Increasing the grouting amount can significantly improve the equivalent cohesion of the contact surface,which is higher than that under the same conditions without grouting 5.8 to 18.8 times higher,while the equivalent internal friction angle barely changes.Under the same conditions,the shear strength of the contact interface increases significantly with the increase of the normal stress.The effect of increasing the shear strength of the contact interface by increasing the amount of grouting gradually weakens under the condition of large normal stress.(3)By carrying out the visualization model test of slurry diffusion of distributed postgrouting and conventional side post-grouting,the forming mechanism of the two grouting methods of the slurry consolidation was proved,and the digital image cross-correlation technology was used to analyze the results.The diffusion law of the grout and the displacement and deformation of the soil around the pile during the grouting process reveal the advantages of distributed post-grouting in improving the bearing capacity of the pile foundation from macro and micro perspectives.Based on the time-varying characteristics of the viscosity of the powerlaw type slurry,the classical slurry diffusion theory formula is modified,and the modified diffusion formula is used to verify the rationality and reliability of the experiment.(4)Based on the in-situ static load test results of two post-grouted test piles,the influence of different grouting types on the bearing properties of the pile foundation was studied,and the advantages of distributed post-grouting in improving the bearing performance of the pile foundation were compared and analyzed.After grouting,the ultimate bearing capacity of the distributed post-grouting pile is increased by 52.2%,which is better than that of the combined post-grouted pile under similar conditions.The resistance of distributed post-grouted piles and combined post-grouted piles has been significantly improved,and the problems of pile end sediment has been significantly improved.After grouting,the strength and stiffness of the soil on the side of the pile are significantly improved,which further improves the degree of frictional resistance on the side of the pile.Under the condition of similar soil layer and depth,the lateral resistance enhancement coefficient of distributed post-grouted piles is slightly larger than that of combined post-grouted piles,and generally the lateral resistance enhancement coefficient of test piles increases gradually with the increase of depth.(5)Practice shows that distributed grouting makes the pile side grout more evenly distributed and covered better because of its small spacing,multi-section,high density,and multi-depth grouting.When the geological conditions and grouting conditions are similar,the effect of distributed post grouting on improving the bearing characteristics of the pile foundation is more significant.