Geotechnical Engineering

DHARM

PILE FOUNDATIONS 675

For H-piles,
fs = N ...(Eq. 16.38)
(kN/m^2 )
According to De Beer, point resistance of bored piles in granular soils is less than that of
driven piles, almost about one-third. This may be because of lack of compaction at the base and
the disturbance of soil at the base during boring.

Piles in cohesive soils

Relationship between unit cohesion, c and the static cone penetration resistance, qc, is as follows:

q

c

ccq

18 15

<< for qc < 20 kg/cm^2 (2000 kN/m^2 )

(for normally consolidated clays)
qc/22 < c < qc/ 26 for qc > 25 kg/cm^2 (2500 kN/m^2 )
(for over consolidated clays)
If qc is not known directly, the N-value may be obtained from the correlations between
qc and N, and it may be used for determining c.
Once the c-value is known, Eqs. 16.12 and 16.15 may be used to obtain qb and fs through
c and ca.

16.5.5Negative Skin Friction

‘Negative skin friction’ or ‘down drag’ is a phenomenon which occurs when a soil layer

surrounding a portion of the pile shaft settles more than the pile. This condition can develop

where a soft or loose soil stratum located anywhere above the pile tip is subjected to new

compressive loading. If a soft or loose layer settles after the pile has been installed, the

skin-friction-adhesion developing in this zone is in the direction of the soil movement, pulling

the pile downward, as shown in Fig. 16.11. Extra loading is thus imposed on the pile.

Qup

Fill

Firm soil

Soft loose soil stratum

De

Positive skin friction

(helps carry load)

Qeb

Negative skin friction

Fig. 16.11 Negative skin friction on a pile