In the weak soil zone pile moves up with respect to surrounding soil. It is same as tension uplift,i.e. surrounding soil moves down with respect to pile. This is known as negative skin friction as shown in figure. It is also known as downdrag phenomenon and occurs when the soil layer surrounding the pile settles more than the pile. Negative skin friction develops when the soft or loose soil strata are located anywhere in between and the pile shaft is subjected to compressive load. Also negative skin friction develops due to increase to effective stresses.
F.S.= (Ultimate Pile capacity – Negative Skin Friction force )/ Working Load.
The net effect is to reduce the F.S. |
– 
)/ 
= 
, for cohesive soils. 
is the depth of compressible layer, C is cohesion. 
= 0.5 P 
, for cohesionless soil. 
