Skin Friction of Soil

Skin Friction of Soil

Skin friction of soil is considered in pile designs. Mainly, there are two types of skin frictions. Negative and positive skin friction are considered in pile designs.

Positive skin friction is calculated based on the type of the ground condition. Depending on the parameters of the soil, skin fiction is calculated. Generally, geotechnical parameters are given by the soil investigation report(geotechnical investigation report). Depending on the parameters given in the report we can calculated the positive skin friction. Some gotechnical investigation reports provides the skin friction values. There are different layer thickness for different types of soils. This has to be considered when the friction is calculated.

When the unit skin friction is given depending on type of soil, skin friction can be calculated by multiplying the skin friction per unit area and surface area of the pile that touch the soil. This is very easy method when compered with the evaluation method with geotechical parameters.

There are many methods that can be used to calculated skin friction of soil. Depending on the available geotechnical parameters and with the available methods, skin friction can be calculated.

When there is weak soil which could be settle with the overburden pressure, negative skin friction would be act on the piles. For example, when there is peat, there could be negative skin friction on the piles. Specially, when the standard penetration value (SPT) is less than five, it is said that there would be negative skin friction on the pile.

Negative skin friction affect to the geotechnical capacity of piles while exerting additional axial force on the piles. When there are different types of layer that can create both positive and negative skin friction on piles, skin friction is calculated by reducing negative skin friction from the positive skin friction.

In addition, when the pile are driven in very weak soil, pile structural capacity checks should be done in addition to the geotechnical capacity checks. Piles will deform freely in a weak soil when compared with a good soil. Therefore buckling effect shall be considered for the structural design. Further, end condition of the piles must be considered when the effective lengths are calculated.