Development of Neutral Plane on a Pile in a Consolidating Ground

Abstract

When a pile is installed in a layer of soft and compressible soil, the pile would be subjected to skin friction as the consequence of the relative settlements between the soil and the pile. Negative skin friction (NSF) is mobilized when soil settles more than the pile while positive skin friction (PSF) occurs vice versa. The location where NSF changes to PSF is called the neutral plane which is also the position where the relative movement is zero. This paper presents the results of a series of axis‐symmetric coupled‐consolidation finite element modeling of a wish‐in‐place pile embedded in a consolidating ground. Geometric non‐linearity technique is adopted and the pile‐soil interface is assumed to follow a bi‐linear Coulomb type frictional behavior. Attempts have been made to study the effects of the soil’s compressibility, pile head loading and pile end‐bearing type on the development of skin friction and neutral plane with time. It is found that NSF is fully mobilized near the ground surface. In the case of floating piles, the neutral plane locates at about 65% of the pile embedment length in long term regardless the soil’s compressibility. The plane moves upward when a pile head loading is applied. The amount of dragload can be substantial compared to the pile head loading. 90% of the neutral plane’s final position is reached when only 50% average degree of consolidation is complete, regardless the soil’s compressibility and the magnitude of pile head load. For fixed‐end piles, neutral plane always locates at the pile toe in long term. However, the position is not reached instantly when surcharge is placed in the case of loaded piles.