Negative skin friction

Ground that settles more than the pile drags it down: the friction on the shaft acts downward and adds a load, the drag load $Q_n$. Units, symbols and assumptions are set out in Method and assumptions.

The ground that settles

The ground settles around the piles after they go in, under a new fill such as an approach embankment or with a lowered water table (AASHTO 3.11.8). It is one zone of the ground, set in Soil by the depth it settles down to, the bottom of the deepest stratum that settles: everything above that depth goes down with it. It has to reach below the pile’s head, and it stops at the top of a rock, which doesn’t settle.

The neutral plane

The neutral plane is where the pile and the soil move together, found by force equilibrium as AASHTO puts it (C10.7.2.3.1): “the depth where the nominal permanent load plus drag load equals the nominal side resistance plus tip resistance”. Above it the side acts down, below it up:

$$Q_d + R_s(z_n) = R_p + R_s(L) - R_s(z_n)$$

with $Q_d$ the dead load on the most loaded pile, $R_s(z)$ the pile’s nominal side resistance from the head down to $z$, the same as the capacity takes, and $R_p$ its nominal tip. FHWA GEC 12 (§7.3.6.1, Step 4) also takes the tip half and not at all mobilized; those neutral planes are given beside it.

The drag

The drag $Q_n$ is the side above the neutral plane, $R_s(z_n)$, and the pile’s largest load is there, $Q_d + Q_n$. Where the ground settles, the drag is a load on the pile (AASHTO’s DR, 3.11.8); where it doesn’t, the neutral plane only places a group’s equivalent footing.

References

• AASHTO (2024). LRFD Bridge Design Specifications, 10th ed. Articles 3.11.8, 10.7.1.6.2 and C10.7.2.3.1.
• Hannigan, P. J., et al. (2016). Design and Construction of Driven Pile Foundations, Vol. I, FHWA-NHI-16-009 (GEC 12). Federal Highway Administration. Sections 7.3.5.7 and 7.3.6.1.