Geotechnical Engineering

DHARM

676 GEOTECHNICAL ENGINEERING

Negative skin friction may also occur by the lowering of ground water which increases

the effective stress inducing consolidation and consequent settlement of the soil surrounding

the pile.

It is necessary to subtract negative skin friction force from the total load that the pile

can support. In such a case the factor of safety will be modified as follows:

Factor of safety = Ultimate pile load capacity

Working load Negative skin friction force+

Sometimes this may also be written as

Factor of safety =

Ultimate pile load capacity Negative skin friction force

Working load

−

Values of negative skin force are computed in just the same way as positive skin friction.

For cohesive soils:

Qnf = P. Dn. c ...(Eq. 16.39)

where Qnf = negative skin friction force on the pile,

P = perimeter of the pile section,

Dn = depth of compressible layer settling in relation to the pile

and c = unit cohesion of soil layer which is setting.

For cohesionless soils:

Qnf =

1

2

PDn^2. γ K tan δ ...(Eq. 16.40)

where γ = unit weight of soil in the compressible zone,

K = earth pressure coefficient (Ka < K < Kp), and

δ = angle of wall friction (φ/2 < δ < φ)

Sometimes negative skin friction may develop even in the zone of the fill, if the fill itself

is settling under its self-weight.

When a large magnitude of negative skin friction force is anticipated, a protective sleeve

or coating may be provided for the section that is embedded in the settling soil. Skin friction is

thus eliminated for this section of the pile and a down drag is prevented. Negative skin force

may be computed even for pile groups.

16.5.6Factor of Safety

Where load tests are not performed, it is usual practice to apply a factor of safety of two to

determine the design load.

Piles subject to uplift develop resistance to pull-out only by skin friction. Point-bearing

resistance does not apply, but the weight of the pile may be included in the uplift capacity.

Generally, a larger factor of safety is employed for uplift than for conventional downward

loading. The strength of pile to pile-cap connection becomes critical in the case of uplift forces,

since tensile force at this location negates any pull-out resistance of the pile.

Qap =

()QWsf + p

η

...(Eq. 16.41)

where Wp is the weight of the pile and factor of safety η is to be not less than 2.