When the fill starts consolidating under its own overburden pressure, it develops a drag on the surface of the pile. This drag on the surface of the pile is called 'negative friction'. Negative friction may develop if the fill material is loose cohesionless soil. Negative friction can also occur when fill is placed over peat or a soft clay stratum as shown in Fig. 15.32c. The superimposed loading on such compressible stratum causes heavy settlement of the fill with consequent drag on piles.
Negative friction may develop by lowering the ground water which increases the effective stress causing consolidation of the soil with resultant settlement and friction forces being developed on the pile.
Negative friction must be allowed when considering the factor of safety on the ultimate carrying capacity of a pile. The factor of safety, Fs, where negative friction is likely to occur may be written as
Figure 15.32 Negative friction on piles
Computation of Negative Friction on a Single Pile
The magnitude of negative friction Fn for a single pile in a fill may be taken as (Fig. 15.32(a)).
(a) For cohesive soils
(b) For cohesionless soils
Negative Friction on Pile Groups
When a group of piles passes through a compressible fill, the negative friction, Fn , on the group may be found by any of the following methods [Fig. 15.32b].
Equation (15.82) gives the negative friction forces of the group as equal to the sum of the
friction forces of all the single piles.
Eq. (15.83) assumes the possibility of block shear failure along the perimeter of the group which includes the volume of the soil γLnA8 enclosed in the group. The maximum value obtained from Eqs (15.82) or (15.83) should be used in the design.
When the fill is underlain by a compressible stratum as shown in Fig. 15.32(c), the total negative friction may be found as follows: