Settlement Ratio of Pile Groups in Sandy Soils from Field Load Tests

This note studies settlement ratio, $R_s$ , of pile groups in sandy soils, defined as the ratio of the settlement of a pile group to that of a single pile at the same average load per pile. 31 cases of field pile-group load tests and the corresponding field single-pile load tests were collected for this study. More than one-half of the cases consist of 3-diameter spaced, 9-pile groups. Based on the field test data, statistical analyses of $R_s$ at different load levels were conducted for pile groups with cap-ground contact (PGCs) and pile groups with freestanding caps (PGFs), respectively. The mean of $R_s$ decreases with the load level for both PGCs and PGFs, whereas the coefficient of variation of $R_s$ increases with the load level. The influence of cap-ground contact on $R_s$ does not appear to be significant based on a comparison of the mean $R_s$ values of these PGCs and PGFs. In addition, a comparative study on $R_s$ and group resistance ratio $R_r$ , which is defined as the ratio of the average resistance of a pile in a group to that of a single pile at the same settlement, was conducted to clarify possible misunderstanding between $R_s$ and pile group efficiency factor $\eta$ for driven pile groups in sandy soils. The value of $R_s$ compares settlement at the working load and is often larger than unity. The value of $\eta$ compares failure loads, which occur at different settlements for pile groups and their respective single piles. $\eta$ is usually larger than unity due to soil densification and additional contributions from the cap-ground contact for PGCs.