Downdrag (negative skin friction)
FHWA GEC 12 neutral-plane analysis with layer-specific t-z/q-z transfer, pile self-weight, explicit consolidation or measured settlement, load combinations, and convergence evidence.
When the ground around a pile settles — a new embankment consolidating soft clay, a lowered water table, liquefaction reconsolidation — the soil drags the pile down instead of holding it up. The downdrag tool analyses this with the neutral-plane (unified) method of Fellenius, as adopted by FHWA GEC-12: a t-z load-transfer solve driven by an imposed free-field settlement profile.
How the neutral-plane method works
Wherever the soil settles more than the pile, the interface friction acts downward on the pile (negative skin friction); wherever the pile settles more, the shaft resists. The depth where the two settlements are equal is the neutral plane — there is no relative movement there, and the internal axial force peaks:
Above the neutral plane the force builds as the negative friction accumulates; below it the shaft and the toe shed the force back to the ground. The tool solves the coupled problem — the neutral plane, the force profile N(z), and the pile settlement profile — by driving the axial t-z load-transfer model with the imposed soil settlement.
Dragload is a permanent structural action
Head load
The head load is the sustained (dead) compression at the pile head. It shifts the neutral plane down and adds directly to the maximum force. Leave it at zero to see the pure dragload the ground imposes on an unloaded pile.
The settlement profile
The free field can be supplied as a measured/prescribed linear profile or calculated with explicit one-dimensional e-log(p') consolidation inputs. The prescribed profile uses:
The ground-surface settlement (downward positive). Even 10–20 mm fully mobilizes negative skin friction — downdrag is triggered by remarkably little movement.
The depth where the free-field settlement dies out — typically the bottom of the consolidating layer. Settlement varies linearly from s₀ at the surface to zero here, and is zero below.
The consolidation option requires each compressible layer's Cc, Cr, initial void ratio, preconsolidation stress, sustained effective-stress increase, and degree of consolidation. Initial effective stress comes from the declared layered profile and groundwater. Pile movement at a depth is the accumulated compression of material below it.
The pile, soil profile and groundwater inputs are the same as the axial tool's — the static capacity run supplies the interface friction f(z) and toe resistance the load-transfer solve mobilizes. Each layer gets its own Reese–O'Neill t-z curve; the bearing stratum gets a separate q-z curve. Section-dependent axial area and effective pile self-weight are retained.
Reading the results
- Neutral plane — where the axial-force curve peaks and the pile and soil settlement curves cross (the “Soil − pile” panel crosses zero there).
- Dragload — the accumulated negative friction above the neutral plane; max axial force also includes effective pile self-weight above that depth.
- Head settlement — the serviceability consequence: the pile settles with the ground at the neutral plane plus elastic shortening.
- Shaft resistance below NP — the geotechnical resource left to carry the maximum force (with the toe).
Fail-closed applicability