Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 266

Application of CPT
When using CPT measurement as the basis for the previous methods to calculate
the unit skin friction and end bearing for the pile, some precautions should be
taken, the peak unit skin friction in compression and tension at a given depth,
f cz and f tz , are not unique and are both dependent on pile geometry. In general,
the axial load and deformation response are affected by the pile penetration
depth, the pile diameter and its wall thickness. Note that increased pile penetra-
tion will decrease these ultimate values at a given depth.
In the case of doing the test to obtain the q-z data for axial load-deformation
response, the end-bearing Q p is assumed to be fully mobilized at a pile tip-
displacement value of 0.1D.
Soil types such as carbonate sands, micaceous sands, glauconitic sands, vol-
canic sands, silts and clayey sands have unusually weak structures with compres-
sible grains. These types require special consideration in situ and in the
laboratory tests for selection of an appropriate design method and design para-
meters according to Thompson and Jardine (1998) and Kolk (2000) for pile
design in carbonate sand, and according to Jardine et al. (2005) .
When using CPT in cohesionless soil, such as gravels, when particle sizes
are in excess of 10% of the CPT cone diameter, one possible approach could
be to use the lower-bound q c profile. In this case, one can estimate the end-
bearing capacity profile from the adjacent sand layers.
In the case of using CPT in weaker clay layers near the pile tip, it is recom-
mended to obtain q c data averaged between 1.5D above the pile tip and 1.5D
below the pile tip level, provided q c does not vary significantly. The UWA
method should be used, if significant q c variations occur, to compute q c,av .
A thin clay layer, less than around 0.1D thick, is a problem especially when
CPT data are discontinuous vertically or not all pile locations have been inves-
tigated. From a practical point of view, the offshore piles usually develop only a
small percentage of q p under extreme loading conditions. So, the finite element
method can be used in calculating the pile capacity and settlement of a pile tip
on sand containing weaker layers. It also may be considered in assessing the
axial pile response under such conditions.
It is recommended that the end-bearing component be reduced in the case of
the pile tip being within a zone up to

± 3D from such layers. When q c data aver-

aging is also applied to this 3D zone, the combined effects may be unduly

conservative and such results should be critically reviewed. This rule also
applies to large pile diameters (D

±

> 2 m).

4.6.5 Pile Capacity under Cyclic Loadings
Environmental loadings are developed by winds, waves, currents, earthquakes
and ice floes. These loadings are consider the source of cyclic loading, as these