Abstract:
Negative skin friction (NSF) is a common problem for a pile foundation in soft soil arises
when soil settles more than the pile. Negative skin friction induces an additional compressive
axial force (Drag load) and an additional settlement (Down drag). This would lead to
structural and serviceability problems. The drag load depends on the location of the neutral
plane(NP) and the distribution of negative skin friction throughout the pile. Negative skin
friction is influenced by many factors. This research is based on the investigation of the
mobilization of negative skin friction with the change of soil layer thicknesses, for a soil
profile of soft soil overlying a stiff residual soil (TOP) & soil profile of soft soil sandwiched
between stiff residual soil layers (SANDWICHED). A two-dimensional axisymmetric model
is built using the finite element program, PLAXIS 2D. Pile was modelled as a volume
element and a wished-in-place pile. The verification of the model was conducted using a
known case by Indraratna et al., (1992) which was conducted for soft Bangkok clay. The
position of the neutral plane and the magnitude of drag load is analyzed with the change of
soft soil layer thickness for different pile diameters, with and without an axial load on the
pile.
The position of NP varies within 0.85T -1T (where T is the soft soil layer thickness) with the
change of soil layer thickness in both soil profiles when an axial load is applied on the pile.
When the pile is not loaded NSF mobilizes through the residual soil layer as well. i.e. the NP
lies at the residual soil layer. There is not a significant variation in the position of NP for a
particular layer thickness with the change in the diameter of the pile. NP and the distribution
of NSF do not have a significant effect on the change in the magnitude of the axial load
applied, if the axial load ≠ 0. The model results have a deviation in a range of 10-48% with
the values obtained from the guidelines proposed in CIDA for the soil profile of soft soil
overlying a stiff residual soil (TOP) and the deviation is less than 20% for the & soil profile
of soft soil sandwiched between stiff residual soil layers (SANDWICHED).
The settlement of the pile is smaller when a load is not applied on the pile. Hence, the
settlement of soil is significant. When a load is applied on the pile both the settlement and
axial shortening of the pile are significant compared to the settlement of the residual soil
layer. Thus, the NP occurs within the soft soil layer.
Bituminous coatings can be used to reduce the mobilization of NSF. Since both the NP and
drag load can be reasonably obtained from the FEM model, the effectiveness of bituminous
coating can also be assessed using FEM. It can be stated that the FEM analysis can be used to
predict the variation of NSF with reasonable accuracy.
