Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 220

so-called standard penetration resistance of the soil. The penetration resistance
is influenced by the stress conditions at the depth of the test. Based on settle-
ment observations of footings, Peck et al. (1974) proposed the following rela-
tionship for correction of confinement pressure. The measured N-value is to be
multiplied by a correction factor C N to obtain a reference value, N 1 , correspond-
ing to an effective overburden stress of 1 t/ft 2 (approximately 107 kPa)

⋅

N 1 = N C N (4.1)

where C N is a stress correction factor or standard penetration test and p ′ is the

effective vertical overburden pressure (see Equation 4.2 ).

⋅

C N = 0 : 77 log10 ð 20/p ′Þ (4.2)

Preliminary knowledge of the geologic conditions in an offshore area will
aid in selection of exploration tools best suited to the job. However, at the
present time, sub-bottom profiles are the main source of information about
soils and rock that may be encountered.
In many areas of offshore oilfield activity, previous investigations have been
made. Although information may be for a location miles away from the desired
location, it can provide insight into foundation conditions. Furthermore, general
information about the character of materials within the probable depth of a soil
boring may sometimes be obtained from well records.
Geophysical exploration precedes drilling activity in offshore areas. The
depth of a geologic structure important from a production standpoint is gener-
ally well below the zone of interest for foundations; records from seismic
exploration are probably of very limited value for foundation purposes. How-
ever, shallow-penetration surveys are frequently available and do provide useful
data on soil stratigraphy.
Many in-situ testing devices have been developed and are in use to deter-
mine soil properties and conditions in the ground. Among them are the cone
penetrometer, commonly known as the

cone, the pressure meter and
the vane shear device. None of these provides a sample for other tests; if sam-
ples are required, sampling must be done between the in-situ tests or in a com-
panion boring.
The Dutch cone was developed principally to define granular soil strata that
would serve to support point-bearing piles. The cone is modified to measure
both side friction and point-bearing resistance. Experienced operators claim
to be able to identify soil types by the ratios of these resistances. Modern
cone equipment has been used successfully only to shallow penetrations at
sea. Remotely operated sea floor equipment presently being developed in
Europe may have a substantially greater depth capability.
The pressure meter is designed to determine soil behavior by using an
expanding pressure cell to measure load-deformation characteristics. The pres-
sure cell is either driven or pushed into undisturbed soil to begin testing or is

“ Dutch ”