Experimental Study On Grouting Reinforcement Mechanism Of Bored-pile Post Grouting Technology

The post-grouting technique has been proven to be an economical and effective method to improve bored-piles’bearing capacity.However,there are not advanced theories to guide the application of post-grouting technique in practical engineering projects,and the research on the mechanism of post-grouting reinforcement still needs to be further investigated.Therefore,a research on the reinforcement mechanism of post-grouting technique has been performed,which based on the pile-grouting project of Yueqing Bay Bridge in Zhejiang Province.The main work and research results are as follows:（1）Field tests were conducted to investigate the reinforcement mechanism of tip-grouting.The ultimate bearing capacity,tip resistance-displacement response,grout migration and the diffusion mechanism of cement grout were discussed.The results indicate that tip-grouting can increase the ultimate bearing capacity of bored pile by 38.03%to61.87%,and the grouting effect in gravel sand and fully weathered tuff is better than that in clay soil.Tip-grouting can improve the mechanical properties of the soil beneath the pile tip,the properties of the pile-soil contact surface and the lateral friction resistance of the pile can be improved by grout migration.Cement slurry has different diffusion mechanisms in coarse grained soil and clay soil,and the failure mode of pile end soil after grouting is also different.（2）Model tests were conducted to investigate the reinforcement mechanism of shaft-grouting.The vertical bearing capacity,horizontal bearing capacity,slurry-soil interaction mechanism and bearing mechanism of model pile were discussed.The results indicate that shaft-grouting can improve model piles’vertical and horizontal bearing capacity significantly,and the synergistic bearing capacity of slurry stone and model pile is one of the key factors which affect the bearing capacity of shaft-grouting pile.（3）The erosion effect of seawater on slurry stone was introduced according to the engineering background of Yueqing Bay Bridge.The erosion effect of seawater on the strength degradation of slurry stone was studied by means of micro-penetration test（MCPT）,and the chemical principle and microstructure characterization of the slurry stone were studied by means of XRD analysis and SEM scanning.The results indicate that the corrosive ions in seawater such as SO₄^2-and Mg²⁺would react with the cement hydrate in slurry stone,and the reaction products would destroy its microstructure.The erosion rate is related to the cement content,erosion time and ion concentration of seawater.（4）Collecting the data of post-grouting construction parameters of bored piles in Yueqing Bay Bridge Project,the relationships between key parameters of post-grouting construction and external construction conditions such as grouting soil layer and grouting method were investigated by means of SPSS 22.Combined with the slurry diffusion mechanism,the causes of these relationships were discussed.The results indicate that the grouting pressure is affected by grouting depth,grouting soil type,grouting method and the age of pile.The grouting volume is directly related to grouting pressure:the grouting pressure of straight-pipe grouting often fails to reach the value of the design termination pressure,resulting in a relatively large grouting volume,in contrast,the grouting pressure of U-shape grouting is usually higher and the quality of grouting can be guaranteed without exceeding the amount of grouting.