Negative skin friction is shear stress acting downward on the shaft of a pile, developed when the surrounding soil settles more than the pile does. Instead of resisting the load on the pile, the soil in that zone hangs on it and adds to it. The accumulated force is called drag load, and the additional settlement it produces is called downdrag.<\/p>\n
The mechanism is a relative displacement problem, not a soil type problem. Shaft friction acts in whichever direction opposes the relative movement between pile and soil. Where the pile moves down relative to the soil, friction acts upward and supports the pile. Where the soil moves down relative to the pile, the same friction acts downward and loads it.<\/p>\n
Soil settles more than the pile \u2192 shaft friction reverses \u2192 force accumulates down the shaft \u2192 maximum at the neutral plane<\/p>\n
The condition is that the soil around the upper part of the pile continues to settle after the pile has taken up its own displacement under load.<\/p>\n
| Situation<\/th>\n | Why it produces downdrag<\/th>\n<\/tr>\n<\/thead>\n | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Recently placed fill over compressible ground<\/td>\n | The fill loads the underlying soil, which consolidates for years<\/td>\n<\/tr>\n | ||||||||||||||||||
| Normally consolidated or soft clay<\/td>\n | Continues to consolidate under its own weight and any added load<\/td>\n<\/tr>\n | ||||||||||||||||||
| Groundwater lowering<\/td>\n | Increases effective stress throughout the profile, causing widespread settlement<\/td>\n<\/tr>\n | ||||||||||||||||||
| Surcharge or embankment adjacent to the piles<\/td>\n | Loads the surrounding ground independently of the structure<\/td>\n<\/tr>\n | ||||||||||||||||||
| Piles installed through a layer still consolidating<\/td>\n | Consolidation was incomplete before the pile was installed<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Equally important is when it does not occur. In stiff overconsolidated clay, settlement under existing stress conditions is small, and the relative movement required to mobilise negative friction does not develop. Calculating drag load in such ground adds a load that will not materialise.<\/p>\n The mechanism also requires time. It develops as consolidation proceeds, over months or years, which means it is a long-term drained problem rather than a condition present at the end of construction.<\/p>\n What Is the Neutral Plane?<\/h2>\nAlong the upper part of the pile, soil settles more than the pile and friction acts downward. Along the lower part, the pile settles more than the soil and friction acts upward in the conventional sense. Between them is a depth at which pile and soil move together, no relative displacement occurs, and shaft friction is zero.<\/p>\n That depth is the neutral plane, and it is the defining feature of the problem.<\/p>\n Above the neutral plane the soil loads the pile. Below it, the soil supports the pile. The neutral plane is where the two exchange.<\/strong><\/p>\n Two quantities follow from its position.<\/p>\n The axial force in the pile increases from the structural load at the head, accumulating negative friction with depth, and reaches its maximum at the neutral plane:<\/p>\n Qmax = Q + Pn<\/strong><\/p>\n where Q is the load applied at the pile head and Pn the accumulated drag load. Below the neutral plane the force decreases again as positive shaft resistance is developed.<\/p>\n The settlement of the pile equals the settlement of the soil at the neutral plane, because that is the depth at which the two move together. This is the practical significance of the concept: locating the neutral plane gives the pile settlement directly.<\/p>\n Its position follows from equilibrium. It is the depth at which the applied load plus the drag load accumulated above equals the positive shaft resistance below plus the end bearing.<\/p>\n That relationship makes its position dependent on the load and on the pile geometry:<\/p>\n The maximum axial force therefore occurs at a depth that changes with the load case, which is why the position is calculated rather than assumed.<\/p>\n This is the question on which practice has moved, and it deserves a direct answer rather than a formula.<\/p>\n The traditional treatment adds the drag load to the structural load and checks the sum against the pile capacity. That is conservative and simple, and it remains the approach embedded in much of design practice.<\/p>\n The difficulty is that the two loads do not act simultaneously in the way the addition implies. Drag load is generated by relative displacement. If the pile settles more \u2014 as it would if it approached its ultimate capacity \u2014 the relative displacement reduces, the negative friction reverses to positive, and the drag load diminishes. The mechanism that creates drag load is the same mechanism that removes it when the pile begins to fail.<\/p>\n The essential question is not whether the pile can carry the additional force. It is where the neutral plane sits, and how much the structure settles as a result.<\/strong><\/p>\n This does not make the capacity check meaningless. The structural section of the pile must carry the maximum axial force Qmax, and that is a genuine ultimate limit state for the pile as a structural member. What changes is the emphasis: the geotechnical consequence of downdrag is settlement, and the structural consequence is section capacity, and treating it purely as a reduction in geotechnical capacity conflates the two.<\/p>\n The unit negative friction is calculated in the same way as positive shaft friction, using effective stress parameters because the mechanism is a long-term drained one:<\/p>\n fn = \u03b2 \u00b7 \u03c3\u2032v<\/strong><\/p>\n integrated over the shaft area from the ground surface to the neutral plane. The result is Pn, the drag load.<\/p>\n Two refinements matter in practice.<\/p>\n Mobilisation.<\/strong> Negative friction, like positive friction, requires relative displacement to develop fully. A commonly used threshold is of the order of 10 mm. Where the calculated long-term settlement of the soil in the negative zone is less than that, only a proportion of the drag load develops, and applying the full value overstates the effect. The proportion is taken as the ratio of the calculated settlement to the threshold.<\/p>\n Adequacy.<\/strong> Where the load on a pile already exceeds its resistance, the concept does not apply: the pile is failing, relative displacement is dominated by pile movement rather than soil movement, and no neutral plane exists. The correct response is to address the capacity deficiency, not to calculate drag load.<\/p>\n Once the drag load is established for each pile in a group, the settlement it produces is calculated by treating the accumulated forces as an equivalent load acting over the length of pile below the neutral plane.<\/p>\n The drag loads from the individual piles are converted into a distributed load and applied over that reduced length. Stress increments below it are calculated either by Mindlin\u2013Geddes, or by Boussinesq with an equivalent composite modulus in the zone occupied by the columns. Settlement follows from the same drained elastic procedure used in ordinary settlement analysis<\/a>, using long-term drained parameters because the mechanism is itself long-term.<\/p>\n The result is added to the settlement calculated from the structural load alone.<\/p>\n The last row is not a failure to act. Where the calculated settlement at the neutral plane is within what the structure accepts, designing the pile section for the maximum axial force and accepting the settlement is a rational outcome rather than an omission.<\/p>\n |