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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

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.

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