Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 253

Laterally Loaded Piles Reaction
The pile foundation in a fixed offshore structure should be designed to carry
lateral loads, whether static or cyclic. Additionally, the designer should consider
overload cases in which the design lateral loads on the platform foundation are
increased by an appropriate safety factor.
The designer should be sure that the overall structural foundation system
will not fail under overloads. Note that the lateral resistance of the soil near
the surface is significant to pile design and the effects of scour and soil distur-
bance on this resistance during pile installation should be considered. Under lat-
eral loading, clay soils behave as a plastic material, which makes it necessary to
relate pile-soil deformation to soil resistance. Therefore, lateral soil resistance
deflection (p-y) curves should be obtained using stress-strain data from labora-
tory soil samples and should be one of the deliverables in the geotechnical
report. The ordinate for these curves is soil resistance, p, and the abscissa is
soil deflection, y. By iterative procedures, a compatible set of load-deflection
values for the pile-soil system can be developed.
Matlock (1970) performed a comprehensive study of the design of laterally
loaded piles in soft clay, and Reese and Cox (1975) performed a study of lat-
erally loaded piles in stiff clay.
In the absence of more definitive criteria, Figure 4.11 and Table 4.14 may be
used for constructing ultimate lateral-bearing-capacity curves and p-y curves.
It is noted that these p-y curves are recommended for estimating pile bend-
ing moment, displacement and rotation profiles for various (static or cyclic)
loads. Different criteria may be applicable for fatigue analysis of a pile that
has previously been subjected to loads larger than those used in the fatigue
analysis and that resulted in “ gapping ” around the top of the pile.

Lateral Bearing Capacity for Soft Clay
According to API RP2A (2007), for static lateral loads the ultimate unit lateral
bearing capacity of soft clay p u has been found to vary between 8c and 12c,
except at shallow depths, where failure occurs in a different mode due to mini-
mum overburden pressure. Cyclic loads cause deterioration of lateral bearing
capacity below that for static loads.
In the absence of more definitive criteria, the following is recommended: p u
increases from 3c to 9c as X increases from 0 to X R according to:
p u = 3c + γ X + J ð cX/D Þ (4.19)

and

p u = 9c for X ≥ X R (4.20)

For a condition of constant strength with depth, Equations (4.19) and (4.20)
are solved simultaneously to have the following equation:

X R = 6D =½ðγ D/c Þ + J (4.21)