Pile settlement · Rischio

How Rischio calculates the settlement of a drilled shaft and of a pile group: the shaft's load-transfer curves and the group's equivalent footing.

The settlement comes without a check: Rischio gives the value, and the limit is the designer’s. Units, symbols and assumptions are set out in Method and assumptions.

One drilled shaft

A single drilled shaft in soil settles by AASHTO’s normalized load-transfer curves (10.8.2.2.2), O’Neill and Reese’s trend lines for the side and the base, in sand and in clay, as settlement over the diameter. Each piece of the shaft mobilizes part of its side resistance as it moves, and the base part of its tip resistance. Rischio finds the movement at which the head carries the load, and from it the share of side and base and the load down the shaft, which the studio draws as the axial load.

AASHTO settles driven piles as a group (10.7.2.3), so a single driven pile has no settlement of its own.

A group

The group settles as an equivalent footing (AASHTO 10.7.2.3; 10.8.2.2.4 for drilled shafts): its plan the group’s outline, at two thirds of the piles’ penetration into the bearing layer, or at the neutral plane where that is deeper. The bearing layer is the toe’s stratum and the strata of its same soil right above it. The nominal dead load spreads below the footing 1 horizontal to 2 vertical:

\Delta\sigma'_i = \frac{Q_g}{(B_g + z_i)(L_g + z_i)}

at the middle of each stratum’s part below the footing, $z_i$ below it. Each clay consolidates by its $C_c$ , or by $C_s$ and $C_c$ when it is overconsolidated, on its $\sigma'_o$ at the middle, as a footing’s clay does, and with its $c_v$ it also comes with its time, as a footing’s clay in time. Each sand settles by Hough’s method from the SPT (AASHTO 10.7.2.3.2 and 10.6.2.4.2b). The piles’ elastic compression, with the drag where the ground settles around them, is added (10.7.2.5).

References

AASHTO (2024). LRFD Bridge Design Specifications, 10th ed. Articles 10.7.2.3, 10.7.2.5 and 10.8.2.2.
O’Neill, M. W., & Reese, L. C. (1999). Drilled Shafts: Construction Procedures and Design Methods, FHWA-IF-99-025. Federal Highway Administration. The normalized load-transfer curves.
Brown, D. A., Turner, J. P., Castelli, R. J., & Loehr, E. J. (2018). Drilled Shafts: Construction Procedures and Design Methods, FHWA-NHI-18-024 (GEC 10). Federal Highway Administration.
Hannigan, P. J., et al. (2016). Design and Construction of Driven Pile Foundations, Vol. I, FHWA-NHI-16-009 (GEC 12). Federal Highway Administration. The group’s equivalent footing.