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.

4.2.5

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

4 21