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6.1 Introduction to Part 6

There are situations where shallow foundations are proven inefficient or uneconomical for transferring loads from the structure to the subsoil. For example, bearing capacity and/or settlement calculations do not satisfy the relevant acceptance criteria, and any ground improvement methods (preloading, replacement of surficial soil with foundation improvement layer etc.) are either not effective, or too expensive. Pile foundations, which embedment depth is large compared to the width of the foundation, are designed to safely transfer the load from the structure through unsuitable subsoil layers to deeper, suitable bearing strata. In the following, some typical situations when deep foundations are used are compendiously described.

Case 1:

The surficial soil layers may feature low shear strength, or may be too compressible to carry the design vertical compressive load from the structure while satisfying shallow foundation bearing capacity and settlement requirements. Deep foundations are used to transfer the load to a stiff formation encountered at a “reasonable” depth, acting as end-bearing piles (Figure 6.1a). In the absence of a stiff formation at “reasonable” depth, which of course depends on the type of the structure, the load is transferred to the soft soil formation via soil resistance developing along the shaft (friction pile, Figure 6.1b).

Figure (a) on the left shows a sketch of a pile loaded with a vertical force. The pile is driven through soft soil, and the pile's toe is embedded in stiff soil/rock. Figurer (b) on the right shows a sketch of a pile loaded with a vertical force. The pile is drive though uniform soft soil, and the pile's toe is too embedded in soft soil.
Figure 6.1. Typical pile cases: (a) End-bearing pile, and (b) Friction pile subjected to compressive vertical load.

Case 2:

Shallow foundations cannot efficiently transfer high lateral or uplift loads, and bending moments. On the other hand, pile foundations can resist uplift loads through shaft resistance (Figure 6.2a). Lateral loads and bending moments can be transferred by single piles through bending (Figure 6.2b), or by pile groups of vertical or sometimes battered piles, which lateral and bending stiffness is combined when connected with a pile cap (Figure 6.2c). Piles, and especially pile groups, are often used for the foundation of structures transferring significant lateral loads to their foundation, such as tall retaining walls supporting slopes or backfills (Figure 6.2d), highway signs, electricity poles, wind turbines etc.

Figure (a) on the left shows a pile in uniform soil subjected to a tensile force. Figure (b) next to it shows a pile in uniform soil subjected to a vertical compressive force, a bending moment and a horizontal force at its head. Figure (c) next to it shows a pile group consisting of two vertical piles and two batter piles. The piles are connected with a cap at their head. A column is founded on the pile cap and a vertical compressive force, a bending moment and a horizontal force are applied on the column. Figure (d) on the right shows a pile group consisting of two vertical piles connected with a cap at their head. A wall is founded on the pile cap. The wall is supporting a slope, and is loaded by a horizontal pressure.
Figure 6.2. Typical pile cases: (a) Tension pile, (b) Laterally loaded single pile, (c) Pile group subjected to a combination of loads, and (d) Tall retaining wall subjected to high lateral earth pressure.

Case 3:

Scour around footings could result in loss of bearing resistance at shallow depths, and the foundation of a bridge pier must extend below the potential scour zone. Guidelines for bridges require that the design of the foundation should not consider the bearing resistance and lateral support above the level of expected scour. In that case, a pile foundation will transfer the loads from the bridge superstructure below the scour zone (Figure 6.3a).

Case 4:

Seismic liquefaction during a strong earthquake may result in sudden loss of the shear strength of loose saturated sands, and flow of the material in areas of non-horizontal ground surface (e.g., riverbanks). Liquefied sands offer significantly reduced support, thus loads must be transferred in deeper, competent layers (Figure 6.3b).

Figure (a) on the left shows a pile group consisting of 4 vertical piles connected with a pile cap. The pile group is supporting a column. The top part of the piles is driven through the scour zone, and piles are embedded in competent soil below that. Figure (b) on the right shows a pile group consisting of 4 vertical piles connected with a pile cap. The pile group is supporting a column. The top part of the piles is driven through liquefiable sand, and piles are embedded in competent soil below that. The pile's toes are embedded in rock-type material.
Figure 6.3. Typical pile cases: (a) Pile group transferring loads below the scour zone, and (b) Foundation in liquefiable soil.

Case 5:

In urban areas, deep foundations may be necessary for supporting structures adjacent to locations where future excavations are planned (Figure 6.4a), to avoid tilt and the requirement for future underpinning.

Case 6:

Pile foundations can be used in areas of expansive soils to resist undesirable seasonal movements. Loads, including uplift and downdrag are transferred to a deeper formation, which is not affected by moisture changes (Figure 6.4b).

Figure (a) on the left shows a pile group consisting of three piles connected with a pile cap. The pile group is close to a deep excavation, which is supported by a retaining wall and multiple anchors. Figure (b) on the right shows a pile subjected to a compressive force at its head. The pile is driven into two soil layers, with the top soil layer being swelling soil, and the bottom soil layer being non-swelling soil.
Figure 6.4. Typical pile cases: (a) Foundation of structures in areas of future excavations, and (b) Foundation of structures in expansive soils.

Apart from foundation of structures in the cases described above, piles are also used in a variety of geotechnical engineering applications, such as stabilisation of slopes or active landslide areas, as retaining elements for excavations etc. (Figure 6.5). The same (or similar) analysis methods, design guidelines and construction techniques apply.

 

Figure (a) on the left shows a slope in soil, and two potential sliding surfaces. Piles are used to stabilise the slope. The length of the piles is significantly larger than the depth of the potential sliding surfaces. Figure (b) on the right shows piles supporting a trench excavated in soil. Prestressed anchors are used for additional support.
Figure 6.5. Other uses of piles: (a) slope stabilisation, (b) support of deep excavations.

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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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