Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 261

ThenewCPT-basedmethodsforassessingpilecapacityinsandarepre-
ferred to the previous method. However, more experience is required with all
these new methods before any single one can be recommended for routine
design instead of the previously presented method. API stated clearly that the
new CPT-based methods should be used only by qualified engineers who are
experienced in interpreting CPT data and who understand the limitations and
reliability of the CPT-based methods.
The assumption is made that friction and end-bearing components are
uncoupled. Hence, for all methods, the ultimate bearing capacity in compression
(Q d ) and tensile capacity (Q t ) of plugged open-ended piles are determined by:
Q t = Q f + Q p = P o Z f c,z dz + A q q p

(4.29)

P o Z f t,z dz

Q f = (4.30)

As the friction component, Q f , is calculated by numerical integration, so the
results are sensitive to the depth increment used, especially in the case of CPT-
based methods. The depth increments for CPT-based methods should be in the
order of 1% of the pile length (or smaller) and, in any case, the depth increment
should not exceed 250 mm.
There are four recommended CPT-based methods mentioned in API RP2A:

1. Simplified ICP-05
2. Offshore UWA-05 ( Lehane et al., 2005a,b )
3. Fugro-05 ( Lehane et al., 2005a ; Kolk et al., 2005 )
4. NGI-05 ( Lehane et al., 2005a ; Clausen et al., 2005 )

The first method is a simplified version of the design method recommended
by Jardine et al. (2005) , whereas the second is a simplified version of the UWA-
05 method applicable to offshore pipe piles. The other three methods are sum-
marized by Lehane et al. (2005a) . It is important to avoid calculating friction
and end-bearing components from different methods.
The unit skin friction formulae for open-ended steel pipe piles for the first three
recommended CPT-based methods (Simplified ICP-05, Offshore UWA-05 and
Fugro-05) can all be considered as special cases of the general formula:

A r max

h i − c h i (4.31)

q cz σ v

P a

L − z L − z 1

v ,1

d min

f z = u : ,v ½ tan δ cv

D D

where f z is the unit skin friction, soil constant-volume interface friction

angle; L is the pile length underneath the seabed, A r =1

δ cv is pile -

(D i /D) 2 ; D i is the pile

−

inner diameter (D i = D 2t); z is the depth under the seabed; q cz is the CPT tip

resistance at depth z; D is the outer diameter; t is the wall thickness and P a is
the atmospheric pressure equal to 100 kPa.

−