As Civil, Structural and Geotechnical Engineers, we are all too aware of the importance of skin friction in the design of foundations for load-bearing structures. Skin friction can be a major factor in the stability of a foundation, and an understanding of how to design for it is essential for any engineer. In this blog, we’ll explore the basics of skin friction of piles, looking at how they are best modelled in design, as well as some common problems and solutions.

Skin friction is created by the interaction between the soil and the surface of the pile. This interaction can be modelled using a variety of methods, but the most common is the Coulomb model. This model assumes that the soil behaves like a viscous fluid, and that the skin friction is a function of the normal force between the soil and the pile (Fn), the coefficient of friction (μ), and the area of contact between the soil and the pile (A).

The Coulomb model is a good starting point for understanding skin friction, but it has some limitations. One of the most important is that it does not account for the effect of shear stress on the soil. This can be a major problem in practice, as shear stress can cause the soil to fail and the foundation to collapse.

A common solution to this problem is to use a more sophisticated model that takes into account the effect of shear stress on the soil. One such model is the Extended Coulomb model, which is widely used in practice. This model is similar to the Coulomb model, but it includes a term for the shear stress (τ) on the soil. This term is essential for modelling the behaviour of the soil correctly

The skin friction of the soil can be divided into two main types.

- Positive Skin Friction
- Negative Skin Friction

### Positive Skin Fiction

It is one of the important components of pile design. Both the end bearing, and positive skin friction take the load applied on the pile. There are many methods to calculate the positive skin friction of piles.

Impact of the soil overburden, SPT value, etc. are considered in evaluation skin friction of pile foundations.

### Negative Skin Friction on pile foundations

The negative skin friction is not a useful force to be considered in the pile designs. It reduces the pile capacity and increate the load applied on the pile.

When there are compressive soils, skin friction on the piles will be developed. One such soil type is the peat. It develops downward force on the pile reducing it capacity.