Structural Engineering & Statics
where the load goes
Concept

Downdrag — where it appears

The downward friction a settling soil applies to a pile that is settling less than the ground around it. It adds to the applied load rather than resisting it, so the largest axial force in the pile is below the ground surface, at the depth where the relative movement reverses.

Named by 5 essays across one field — each of them below, with the objects they name alongside it.

The worst force in a pile is not at the top of it. A 0.6 m pile 24 m long through ground that is settling, carrying 800 kN at its head. Above the neutral plane the soil moves down past the shaft and the friction acts downward, so the axial force grows with depth; below it the friction acts upward in the ordinary way and the force falls again to the 300 kN the base takes. The maximum is 1282.87 kN at 13.78 m — 1.60 times the load applied, and it is at a depth where nothing is applied, nothing is connected and nothing can be inspected. A pile section chosen for the head load is under-sized by that factor over the middle third of its length.

The ground that hangs on instead of holding up

A pile is driven through fill that has not finished settling. The fill goes down past the shaft, the friction along that length turns round, and the pile is now carrying the soil rather than the other way about. The worst force is not at the head and not at the toe, and nothing at that depth can be seen.

internal-forces · Downdrag Most of a long pile is doing nothing. Head deflection against embedded length, both measured in the pile's own characteristic length T = 14.67 m. A pile shorter than about two T is a lever with nothing holding its foot and deflects several times as much; past four T the curve is flat to within a per cent, because the ground below that depth is never asked for anything. A lateral check is a check on the top four T of a pile, however deep it goes for its axial load — and adding length to fix a lateral deflection is the one remedy that does not work.

The pile that is too short to bend

A laterally loaded pile is long or short in its own characteristic length, and the two are different structures. The slender pile bends and the ground below four characteristic lengths never hears about it; the monopile is two lengths deep, rotates about a point, and every tabulated coefficient written for the first case is wrong for the second.

internal-forces · Lateral pile The worst force in a pile is not at the top of it. A 0.75 m pile 30 m long through ground that is settling, carrying 1200 kN at its head. Above the neutral plane the soil moves down past the shaft and the friction acts downward, so the axial force grows with depth; below it the friction acts upward in the ordinary way and the force falls again to the 600 kN the base takes. The maximum is 2331.39 kN at 18.86 m — 1.94 times the load applied, and it is at a depth where nothing is applied, nothing is connected and nothing can be inspected. A pile section chosen for the head load is under-sized by that factor over the middle third of its length.

The coating that takes the resistance with it

The cure for downdrag is to make the pile slippery, and it works — a bitumen slip layer takes the drag on this pile from 1,131 kN to 34. It also removes the shaft friction that was holding the pile up, in the same proportion and over the same length, and past a certain smoothness there is no neutral plane to find because there is no equilibrium.

internal-forces · Downdrag Where the pile and the ground settle alike. For a pile 24 m long and 600 mm across, carrying 800 kN, in ground whose top 18 m settles — by 100 mm at the surface, less with depth — over a toe that resists by moving: on the left, how far the ground (dashed) and the pile (solid) settle down the pile's length; on the right, the force in the pile. Above 15.9 m the ground settles more than the pile and hangs on it; below, the pile settles more and the ground holds it up. The force peaks there, at 1,407 kN against the 800 kN applied, and the toe, settling 10.6 mm, mobilises 623 kN; the head settles 13.6 mm.

The plane where the pile and the ground agree

Downdrag is usually found by equilibrium: the ground hangs on the pile above a neutral plane, holds it up below, and the plane is where those balance the load and the toe's resistance. But the toe's resistance is not a number the designer can supply. It is whatever the toe's own settlement mobilises, and the toe settles because the ground drags the pile down. Solved by compatibility instead — the plane where the pile and the ground settle by the same amount — the drag, the neutral plane, the toe's force and the pile's own settlement all turn out to grow with how far the ground goes, and each kilonewton put on the head costs the pile three quarters of one, not half.

internal-forces · Downdrag Heave puts the pile in tension where settlement put it in compression. For a 24 m pile 600 mm across carrying 200 kN, in clay whose top 18 m moves, by 100 mm at the surface: the axial force down the pile, compression positive, when the ground heaves (solid) and when it settles as far (dashed). Settling, the pile's compression peaks at 910 kN at 17.2 m and its toe carries 324 kN. Heaving, it is in tension below 8.9 m, peaking at 520 kN at 17.3 m, and its toe carries nothing: lifted, it parts from the soil beneath, so everything holding the pile down comes from the shaft below the plane.

The pile the ground pulls apart

Clay that swells after its trees are felled rises past the piles in it, and the same compatibility that put a pile into compression in settling ground puts it into tension in heaving ground, at nearly the same depth. Nearly everything else changes. A lifted toe parts from the soil and holds nothing, so the shaft alone anchors the pile; the head load that added to the downdrag now subtracts from the tension; and the tension peaks 17 metres down, far below the short cage a lightly loaded pile is given. A house's piles in swelling clay are tension members nobody reinforced.

internal-forces · Downdrag

Named alongside it

The objects these essays reach for when they reach for this one.

Load pathNeutral planeCompatibilityPileSettlementEffective stressEquilibriumFrictionPile groupShaft frictionAxial forceBending moment

All concepts