Study On Optimal Design Of Post-grouting Cast-in-situ Piles Based On Pile Body Compression Deformation Control

Post-grouting at the pile end can effectively improve the physical and mechanical properties of the soil at the end of the pile and the soil around the pile by treating the sludge at the end of the pile and the soil around the pile.It can effectively control the settlement of the pile top and increase the bearing capacity of the pile foundation.In recent years,in order to further improve the vertical bearing capacity of single piles,cast-in-place piles are developing in the direction of super-long piles.The maximum pile length of currently-constructed bored piles has exceeded 150 m.The research shows that the settlement of the pile top of the grouting pile after the super-long pile is mainly based on the compression of the pile body.Therefore,the influence of the pile compression on the bearing capacity must be considered in the design process of the post-grouting pile.Based on the pile foundation project of the Outer Ring Project of Longhu Financial Center in Zhengzhou City,this paper analyzes the load bearing behavior,pile end resistance and pile side of two post-grouted cast-in-place piles with different pile lengths through the vertical load test of the same diameter and different pile length,the performance of the resistance,the relationship between the pile-soil relative displacement and the resistance of the pile side,etc..Besides,compared with the analysis results of the finite element simulation software,proposed an optimal design idea of the post-grouting cast-in-place pile based on the control of the pile compression.The main conclusions are as follows:(1)The relative displacement of the pile-soil is affected by the amount of compression of the pile body and the settlement of the pile end.In the elastoplastic working stage,the lateral resistance of the pile on the pile body is proportional to the relative displacement of the pile-soil;After the displacement limit value is exceeded,the softening of the pile side resistance will occur and the pile side resistance will decrease.(2)When the other conditions are the same,increasing the pile diameter can increase the vertical stiffness of the pile body and effectively control the pilecompression.The smaller the length-diameter ratio of the pile,the more significant the control effect on the pile compression.When the conditions of the bearing layer allow,reducing the pile length can effectively reduce the compression of the pile body and improve the bearing capacity of the single pile.(3)The post-grouting cast-in-place pile end resistance with large pile length has hysteresis,reducing the pile length and increasing the vertical stiffness of the pile body will help to transfer the pile body load to the bottom of the pile,making the pile end resistance and a certain range of pile side resistance above play more effectively.Alleviate the adverse effects caused by the pile end resistance and the pile side resistance being played asynchronously due to the excessive pile length.(4)A method for optimal design of bearing capacity based on pile compression control is proposed: Reduce pile compression by reducing pile length or increasing pile diameter.On the one hand,the relative pile-soil relative displacement on the pile body is controlled to prevent the softening of the pile side resistance.On the other hand,the load on the top of the pile is effectively transmitted to the lower pile body,and the side resistance and the end resistance of the lower pile are fully exerted.And the rationality of the method is verified through analysis and calculation of actual engineering.