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Chapter 8 — Deep Foundations: Lateral Loads, Groups & Settlement

8.1 Laterally loaded piles

Piles resist lateral loads and moments from wind, seismic action, earth pressure, water, and eccentric vertical loads. A laterally loaded pile behaves as a beam on an elastic foundation, with the soil providing a stiffness that varies with depth. The key parameters are:

  • the pile's flexural stiffness \(EI\);
  • the soil's modulus of subgrade reaction (or \(p\)–\(y\) curves);
  • the fixity at the pile head (free or fixed).

Short, stiff piles fail by soil failure; long, slender piles fail by structural yielding. The characteristic length distinguishes the two.

8.2 Pile groups

Piles are usually used in groups. Group behaviour differs from single piles:

  • Group efficiency — the capacity of a group is not simply the sum of single-pile capacities; overlapping stress zones can reduce efficiency, especially in clay.
  • Block failure — a closely spaced group may fail as a single block.
  • Settlement amplification — group settlement exceeds single-pile settlement because the stress influence zones overlap and extend deeper.

Design checks both single-pile and group (block) capacity and settlement.

8.3 Negative skin friction (downdrag)

Negative skin friction occurs when the soil around a pile settles more than the pile — for example, a pile driven through a consolidating fill or a soft clay into a firm stratum. The settling soil drags down on the shaft, adding load rather than resisting it. The result is downdrag, which must be added to the design load and can be substantial. Mitigation includes bitumen coating or a sleeve on the upper shaft.

8.4 Settlement of piles

Pile settlement has two parts:

  • Elastic shortening of the pile under load; and
  • Settlement of the soil at the base and along the shaft, from the load transferred to it.

Settlement is estimated from elastic theory, load-transfer (\(t\)–\(z\)) analyses, or empirical correlations, and is confirmed by the load test. Group settlement is larger and extends deeper than single-pile settlement.

8.5 Uplift and tension

Piles may be required to resist uplift (tension) from wind, seismic, or hydrostatic forces. Uplift capacity comes from shaft friction and, for some types, from an enlarged base or anchor. The available shaft resistance in tension is generally lower than in compression.

8.6 Key takeaways

  • Laterally loaded piles act as beams on elastic foundations; short piles fail in the soil, long piles yield structurally.
  • Group capacity ≠ sum of single piles — check block failure and settlement amplification.
  • Negative skin friction (downdrag) adds load where the soil settles more than the pile; mitigate with coatings or sleeves.
  • Pile settlement = elastic shortening + soil settlement; groups settle more.
  • Uplift capacity relies on shaft friction, generally lower than in compression.