surveys should be compared with lithologic information obtained from conventionally drilled
and sampled boreholes at the site to verify the CPT results.
Conventional CPT tools record bearing pressure on the conical penetrometer tip as a
function of depth. Penetrometer tools equipped with a calibrated friction sleeve attachment
allow for the interpretation of subsurface lithologic changes on a continuous vertical scale
based on cone and friction resistance criteria (Sangerlat, 1972; Schmertmann, 1978).
Measured CPT values also are used to estimate relative formation density and bearing
capacity variations as a function of depth. CPT surveys are applicable to many sites where
the subsurface formations are uncemented and unlithified, free from impenetrable obstructions
such as rock ledges, hardpans, caliche layers, or boulders, and where cone advancement can
be achieved through the formation with minimal stress to the testing equipment. Dependent
upon the site geology, a standard CPT survey can be used as a reconnaissance tool to provide
preliminary site data for planning, or the surveys can be integrated into a broader
investigation program to provide supplemental data between widely spaced drill holes or other
data measurements. At sites where the technique is applicable, CPT surveys can provide a
continuous vertical profile of subsurface stratigraphy and indicate formation permeability.
Cone penetrometer devices are used in off shore and land based applications. The
equipment is highly portable and can be adapted to a variety of specialized applications.
Instruments are commonly truck mounted with equipment to manipulate the probes and rods
and to record and interpret the survey results. Other versions of the tools can be adapted to
drill rods for use with a drilling rig. In addition to conventional surveys that measure the
mechanical response of the formation to the CPT probe, specialized probes have been
developed that can provide measurements of in situ pore pressure, formation resistivity,
formation thermal response to penetration, and seismic source detection. Further probe
specialization can provide measurement of soil moisture by nuclear methods, in situ
pressuremeter measurements, formation fluid and gas sampling, and soil sampling.
Application of CPT is limited to sites where mechanical penetration of the subsurface
formation can be achieved through the zone of interest. In some cases, the penetrometer used
in combination with a drilling rig can allow the CPT survey to progress through difficult
subsurface zones by penetrating these zones ahead of the survey. The continuous survey is
interrupted at these points and no data are collected.
Geophysical Techniques
Geophysical surveys, including surface and borehole methods, are conventionally
applied to site investigations as a means to obtain subsurface information over broad lateral
and vertical extents of the investigated area. The applicability of a particular method or tool
to a site is contingent on the purpose of the survey and the scope of the site investigation.
Integration of one or more geophysical techniques into an overall site investigation plan can
maximize the amount of information obtained for the site and can potentially allow extension
November 1992
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