The shaft friction can be improved greatly by base grouting

and the increment is the major part of ultimate bearing capacity increment of the overlength large diameter pile. There is a weak layer between a pile and the soil around it

so the grout always flows upward along the weak layer

squeezing the soil to a larger diameter and improving the shaft friction. According to the results of the self-balanced static load testing method

the SPT test and the CT scanning test of a project with overlength large diameter piles

the strength of soils in a large range above pile- bases was improved and the shaft friction was increased 12.39 %-52.87 %

the ratios of shaft friction in the increment of ultimate bearing capacities was 56 %-88 %.