Negative Skin Friction
Negative skin friction is an effect that arises as a result of the settlement of soil around the pile. A soil deforming around the pile tends to pull the pile down thus reducing its bearing capacity. In extreme cases this effect may eliminate the influence of shaft friction. The pile is then supported only by elastic subsoil below the pile base.
The negative skin friction Fs,nk,rep is given by:
where: | Op | - | pile periphery |
n | - | number of layers in the negative friction zone | |
hi | - | depth of ith layer | |
K0,i,rep | - | representative value of the coefficient of earth pressure at rest | |
δi,rep | - | friction between soil and pile at ith layer | |
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φi,rep | - | representative value of the angle of internal friction at ithlayer | |
σv,i-1,rep | - | horizontal stress in soil at i-1 layer | |
σv,1,rep | - | horizontal stress in soil at ith layer | |
pi,a,rep | - | surcharge at ith layer | |
Δσi,v,w,rep | - | change of vertical stress σv at ith layer | |
the following relation holds: | | ||
If a slip surface is defined then the value of negative skin friction Fs,nk,rep is provided by:
where: | Op | - | pile periphery |
hi | - | depth of ith layer | |
ci,rep | - | representative cohesion of slip surface | |
- for bitumen 10*103 N/m2 | |||
- for bentonite 20*103 N/m2 | |||
- for synthetic material 50*103 N/m2 |
The value of representative cohesion along a slip surface can also be introduced directly by the user.