The ultimate unit skin friction (qs) is limited to approximately 10 MPa or 1450 psi.
This limit is reached because the adhesion factor, which relates the skin friction to the soil or rock strength, converges to a value of around 0.10 when the unconfined compressive strength of the surrounding material reaches extremely high values, such as 100 MPa. This phenomenon is particularly relevant in weak rocks and intermediate geomaterials (IGMs).
In simpler terms, as the strength of the surrounding ground increases dramatically, the skin friction between a foundation element (like a pile) and the ground does not increase proportionally. Instead, the relationship flattens out, and the skin friction approaches its upper limit. Beyond this point, even a further increase in soil or rock strength will not significantly increase the skin friction that can be mobilized.
