Abstract: 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 %.

Key words: base grouting, overlength large diameter pile, shaft friction, the self-balanced static load testing method