Development of Negative Skin Friction and Dragloads on Piles and Pile Groups in Soft Clay

Abstract

One of the challenging problems that faces geotechnical engineers in pile design in soft soils is the determination of negative skin friction (NSF) value. Negative skin friction causes additional compressive force on the pile, which decreases the pile carrying capacity. In this paper, a 3D finite element (FE) model is presented to model a field case study for instrumented driven pile in soft Bangkok clay subject to NSF. Assigning short and long term material sets to the soft and medium stiff clay layers was necessary to provide well short and long term settlement predictions. The developed 3D FE model was used to investigate the behavior of pile groups in soft soils for surcharge and embankment loading. Piles were spaced at three times the pile diameter for all investigated cases. For surcharge loading, the maximum group effect occurred for the central pile, while the minimum group effect was observed for the corner pile. Smaller dragloads were observed to develop on piles in pile groups due to the “shielding effect” on the inner piles by the outer piles. The larger the number of piles in a group, the greater the shielding effects on the reduction of dragloads. This reduction suggests using sacrificial piles around pile groups in consolidating soils to obtain more economical designs. The depth to the neutral point was larger for the central piles compared to the corner piles. For embankment loading, the maximum group effect was noted for the central pile. The width of embankment increased as the number of pile rows increased for constant spacing between piles. Increasing the number of pile rows was not associated with further reduction in the dragloads on the central piles, which indicated that the shielding effect was counteracted by the effect of increasing the extent of loading.