Soil foundations - Geotechnical Investigation and Design Tables - page 274

are therefore a convenient terminology to describe the dominant load bearing
component of the pile.

•

The % shared between these two load carrying element depends on the pile
movement and the relative stiffness of the soil layers and pile.

Table 21.13 Pile loads and displacements required to mobilise loads.

Load carrying element Symbols Required displacements

Shaft Q s Ultimate shaft load 0.5 to 2% of pile diameter =

(Skin friction in sands and adhesion in clays) - typically 5 mm to 10 mm

Base Q b Ultimate base load 5% to 10% of pile diameter

- typically 25 mm to 50 mm

=

Total Ultimate load (Q ult ) Q s Q b Base displacement governs = +

•

Choice of the Factor of Safety should be made based on the different response of
pile and base. Maximum capacity of shaft is reached before the base.

•

If the foundation is constructed with drilling fluids and there is uncertainty on the
base conditions, then design is based on no or reduced load carrying capacity on
the base.

•

If the movement required to mobilise the base is unacceptable then no base bearing
capacity is used.

•

The shaft would carry most of the working load in a pile in uniform clay, while
for a pile in a uniform granular material the greater portion of the load would be
carried by the base.

21.14 Pile shaft capacity
•

The pile shaft capacity varies from sands and clays.

•

Driven piles provide densification of the sands during installation while bored
piles loosen the sands.

•

The surface of bored piles provides a rougher pile surface/soil interface ( δ ), but

this effect is overridden by the loosening/installation (k s ) factor.

Table 21.14 Shaft resistance for uniform soils (values adapted from Poulos, 1980).

Soil type Relationship Values

Bored Driven

Clay Shaft adhesion C a C u 0.45 (Non fissured) 1.0 (Soft to firm)

= α α = α =

α =

0.3(Fissured)

α =

0.75 (Stiff to very stiff)

C a 100 kPa maximum 0.25(Very stiff to hard) = α =

Sands Skin friction f s k s tan δσ v Not recommended (Loose) k s tan 0.3 (Loose) = δ =

k s Earth pressure k s tan 0.1 (Medium k s tan 0.5 (Medium = δ = δ =

coefficient dense) dense)

δ =

Angle of friction between k s tan

δ =

0.2 (Dense) k s tan

δ =

0.8 (Dense)

pile surface and soil k s tan 0.3 (Very dense) k s tan 1.2 (Very dense)

δ = δ =

σ v Vertical effective stress

=