Development of Downdrag on Piles and Pile Groups in Consolidating Soil

Development of pile settlement (downdrag) of piles constructed in consolidating soil may lead to serious pile foundation design problems. The investigation of downdrag has attracted far less attention than the study of dragload over the years. In this paper, several series of two-dimensional axisymmetric and three-dimensional numerical parametric analyses were conducted to study the behavior of single piles and piles in $3\times 3$ and $5\times 5$ pile groups in consolidating soil. Both elastic no-slip and elasto-plastic slip at the pile–soil interface were considered. For a single pile, the downdrag computed from the no-slip elastic analysis and from the analytical elastic solution was about 8–14 times larger than that computed from the elasto-plastic slip analysis. The softer the consolidating clay, the greater the difference between the no-slip elastic and the elasto-plastic slip analyses. For the $5\times 5$ pile group at 2.5 diameter spacing, the maximum downdrag of the center, inner, and corner piles was, respectively, 63, 68, and 79% of the maximum downdrag of the single pile. The reduction of downdrag inside the pile group is attributed to the shielding effects on the inner piles by the outer piles. The relative reduction in downdrag $\left(W_r\right)$ in the $5\times 5$ pile group increases with an increase in the relative bearing stiffness ratio $(E_b/E_c)\text{,}$ depending on the pile location in the group. Compared with the relative reduction in dragload $\left(P_r\right)\text{,}$ $W_r$ at the corner pile is less affected by the group interaction for a given surcharge load. This suggests that the use of sacrificing piles outside the pile group will be more effective on $P_r$ than on $W_r\text{.}$ Based on the three cases studied, the larger the number of piles in a group, the greater the shielding effects on $W_r\text{.}$ Relatively speaking, $W_r$ is more sensitive to the total number of piles than to the pile spacing within a pile group.