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6.5 Load transfer mechanisms

Piles are often classified according to the mechanism of transferring foundation loads to the subsoil. This classification does not refer to the properties of the pile itself (geometry, material, construction method) but rather to the subsoil and to the loading conditions:

  • Friction piles in relatively homogeneous soil transfer vertical loads in the subsoil mainly via the development of skin friction along their surface (Figure 6.12a).
  • End-bearing piles, which toe rests on a relatively stiff formation, compared to the above soft soil, transfer the vertical load mainly to the lower stiff formation (Figure 6.12b).
Figure (a) on the left shows a pile installed in homogeneous soil. The pile is subjected to an axial compressive force at its head. This force is resisted by friction stresses developing along the length of the pile, and (relatively low) normal stresses developing at the pile toe. Figure (b) on the right shows a pile installed in soft soil, underlaid by stiff soil or rock. The pile toe is embedded in the stiff soil layer. The pile is subjected to an axial compressive force at its head. This force is resisted by friction stresses developing along the length of the pile, and (relatively high) normal stresses developing at the pile toe.
Figure 6.12. (a) Friction pile, and (b) End-bearing pile.

In practice, all piles act both as friction and end-bearing, and transfer loads via both skin (shaft) friction and end bearing resistance. The predominant mode of load transfer characterises them as friction or end-bearing piles: If the skin friction resistance Qsf is greater than about 80% of the end bearing resistance Qb, the pile is referred to as a friction pile, otherwise it is referred to as an end bearing pile. A special case is that of piles subjected to negative skin friction at their upper part, as a result of settlement of surficial soil layers. In that case the surficial soil layers impose a compressive load on the pile Qnf, rather than contributing to its resistance (Figure 6.13a).

On the other hand, laterally loaded piles subjected to lateral forces and bending moments resist the applied loads though bending, and the development of passive earth pressures at the upper part of the pile (Figure 6.13b).

Figure (a) on the left presents a pile installed in layered soil. The top soil layer is very soft soil, the middle soil layer is stiffer soil, and the bottom soil layer where the pile toe is embedded is stiff soil or rock. The pile is subjected to an axial compressive force Q at its head, and a surcharge is applied on the soil surface near the pile. The following forces are developing on the pile. A downwards friction force along the top part of the pile shaft Qnf, an upwards friction force along the bottom part of the pile shaft Qsf, and a normal force at the pile toe Qb. Figure (b) on the right presents a pile in homogeneous soil. The pile is subjected to a horizontal force at its head. The horizontal force results in bending of the pile near its head, and to a reaction pressure developing from soil, which acts along the top part of the pile's shaft.
Figure 6.13. (a) Pile subjected to negative skin friction, and (b) Laterally-loaded pile.

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Fundamentals of foundation engineering and their applications Copyright © 2025 by University of Newcastle & G. Kouretzis is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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