Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 241

′

enough in soils having c/p ratios greater than three, Equation (4.15) should be

applied with some engineering judgment for high c/p

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values. The same engi-
neering judgment should be applied for deep-penetrating piles in soils with
high undrained shear strength, c, where the computed shaft frictions, f,using
Equation 4.14 above, are generally higher than previously specified in API
RP2A. In the case of very long piles, some reduction in pile capacity occurs,
because the shaft friction may reduce to some lesser residual value on continued
displacement.
For piles end bearing in cohesive soils, the unit end bearing, q (in kPa), may
be computed by:

q = 9c (4.16)

It is obvious that in open-driven piles the shaft friction, f, acts on both the
inside and outside of the pile. The total resistance is the sum of the external
shaft friction, the end bearing on the pile wall annulus and the total internal
shaft friction or the end bearing of the plug, whichever is less.
Ifthepipepileisconsideredtobeplugged,thebearingpressuremaybe
assumed to act over the whole cross-section of the pile. For unplugged piles,
the bearing pressure will be calculated on the pile wall annulus only. Whether
a pile is considered plugged or unplugged may be based on static calculations.
For example, a pile could be driven in an unplugged condition but act plugged
under static loading.
In some cases, piles are driven in undersized drilled holes, piles are jetted in
place or (in some minor projects) the piles are drilled and grouted in place. In
these situations, the soil disturbance resulting from installation will affect the
shaft friction values. In general, f should not exceed values for driven piles;
however, in some cases for drilled and grouted piles in overconsolidated
clay, f may exceed these values.
In determining f for drilled and grouted piles, the strength of the soil-grout
interface, including potential effects of drilling mud, should be considered. As
discussed by Kraft and Lyons (1974) , a further investigation and check should
be made of the allowable bond stress between the pile steel and the grout.
The shaft friction values, f, in the cohesive layers should be as given in
Equation (4.14) . End-bearing values for piles tipped in cohesive layers with
adjacent weaker layers may be as given in Equation (4.16) , assuming that the
pile achieves penetration of two to three pile diameters or more into the layer
in question and the tip is approximately three pile diameters above the bottom
of the layer, to avoid punch through.
Some modification in the end-bearing resistance may be necessary if these
distances are not achieved.

Shaft Friction and End Bearing in Cohesionless Soils
A simple method for assessing pile capacity in cohesionless soils will be dis-
cussed. There are reliable methods for predicting pile capacity that are based