Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 279

4.9.2 Unit Shaft Resistance and Unit End Bearing for
Uncemented Materials
In cohesive soil, the unit skin friction has been assessed based on the method
proposed by Semple and Gemeinhardt (1981) . This method was developed
from back-analysis of pile installations in normally consolidated to heavily
overconsolidated clays from many areas.
The unit shaft resistance component of SRD is derived using the API RP2A
(1984) procedure for static pile capacity, modified by a pile capacity factor F p ,
which is a function of the overconsolidation ratio (OCR), as follows:

0 : 3

F p = 0 : 5 ð OCR Þ (4.52)

The unit end-bearing component of SRD is taken as 9S u .
The OCR is defined as the ratio of the maximum past effective consolidation
stress and the present effective overburden stress. OCR is a function of the
undrained shear strength ratio (S u /p

), which is equal to 0.22 in normally con-
solidated clay with a shear stress angle equal to 26 o . In general, OCR should be
obtained by CPT.
In cohesionless (granular) soil, shaft resistance and end-bearing components
of SRD can be derived using the API (1984) procedure for static pile capacity
together with the soil parameters specified by Stevens et al. (1982) for the par-
ticular soil type. A limiting skin friction of 15 kPa has been taken for the cal-
careous and carbonate sand layers.

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4.9.3 Upper- and Lower-Bound SRD
Based on Stevens et al. (1982) , four cases have been assessed to obtain upper-
and lower-bound SRD values in uncemented and weakly cemented soil layers:

1. SRD for lower-bound, coring pile is the outside shaft friction in addition to
the inside shaft friction as half the outside. End bearing is unit end bearing
multiplied by steel annulus area.
2. SRD for upper-bound, coring pile is similar to the previous case but with the
full shaft friction on the inside of the pile.
3. SRD for lower-bound, plugged pile is the outside shaft friction in addition to
the end bearing multiplied by full cross-section area.
4. SRD for upper-bound, plugged pile for cohesionless soil layers is with out-
side shaft friction from case 3 increased by 30% and end bearing from case 3
increased by 50%. For cohesive layers, outside shaft friction is as in case 3,
and end bearing from case 3 is increased by 67%.

For the sandstone and limestone layers, plugged conditions are unrealistic,
and only the coring cases are considered.