In the case of friction piles, the load is acting downwards while frictional force to resist that load acts upward. That frictional force is called skin friction.
However, in cases when pile and a part of the soil in which the pile is there is settling. And if this settlement is more than the pile settlement, it will also apply downward frictional drag on the pile surface and the force arising on the pile surface due to negative frictional drag is called negative skin friction.
In such a case when loading is applied, there will be two forces acting downward: acting load and negative skin friction. Therefore part of the load-bearing capacity of due skin friction pile will be utilized to overcome negative skin friction and the rest would be available to resist the acting load.
Therefore, the load-bearing capacity of the pile is reduced. There are several reasons for soil settling more than the pile:
- If the settling layer is very compressible and when a little bit of loading on this soil is applied, it will settle more than the pile. Hence, negative skin friction will develop.
- If the settling layer is a recently filled soil, then over time its consolidation will take place. And because of this over time consolidation, negative skin friction will develop.
- A third reason is the effective stress increases.
As we know that the consolidation is a function of effective stress. Therefore, when effective stress increases then settlement will be more. As a result, negative skin friction will be developed. Change in affective stress happens when the water table rises or goes down.
The process of calculation of negative skin friction is similar to skin friction as shown earlier. However, it acts in a downward direction.