Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 260

around the piles begins to consolidate, so the pile capacity increases with time.
This process is usually referred to as set-up. The rate of excess pore pressure
dissipation is a function of the coefficient of radial consolidation, pile radius,
plug characteristics and soil layering.
In the most popular case, where the driven pipe piles supporting a structure
have design loads applied to the piles shortly after installation, the time-
consolidation characteristics should be considered in pile design. Note that
in traditional fixed offshore structure installation, the time between pile instal-
lation and the platform 3 months, but if the

commissioning and start-up come early, this information should be transferred
to the engineering office, as the expected increase in capacity with time is an
important design variable that can affect the safety of the foundation system
during the early stages of the consolidation process.
The behavior of piles subjected to significant axial loads in highly plastic,
normally consolidated clays was studied using a large number of model pile
tests and some full-scale pile load tests.
From the study of pore-pressure dissipation and load test data at different
times after pile driving, empirical correlations were obtained between the
degree of consolidation, degree of plugging and pile shaft shear-transfer
capacity. This study revealed that there is no significant change in capacity
with time for closed-ended steel piles in heavily overconsolidated clay. This
is contrary to tests on 0.273 m (10.75 inch) diameter closed-ended steel piles
in overconsolidated Beaumont clay, where considerable and rapid set-up in 4
days was found, so the pile capacity at the end of installation was never fully
recovered.
So it is very important to highlight that the axial capacity of the pile with
time is under research and development and there is no solid formula or equa-
tion to follow. The focus should be on research done on the specific site loca-
tion and on the previous history of the location.

' s becoming totally loaded is 1 -

4.6.4 Pile Capacity Calculation Methods
API RP2A (2007) presents new methods for calculating pile capacity based on
the cone penetration test (CPT).
As outlined previously, a simple method for assessing pile capacity in
cohesionless soils is the recommended method in previous editions of API
RP2A-WSD. There are reliable CPT-based methods for predicting pile capa-
city. These methods are all based on direct correlations of pile unit friction
and end-bearing data with cone tip resistance (q c ) values from CPT. These
CPT-based methods cover a wider range of cohesionless soils, are considered
fundamentally better, and have shown statistically closer predictions of pile
load test results.