Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 43

Indirect methods provide a measure of material properties; examples are geo-
physical methods and the use of the cone penetrometer, which through correla-
tions allow an estimation of material type.

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Specific Categories

Geophysical methods provide indirect data ( Section 2.3.2) .

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Reconnaissance methods provide direct and indirect data ( Section 2.3.3).

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Continuous cone penetrometer (CPT) ( Section 2.3.4 ).

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Test and core borings provide direct data ( Section 2.3.5).

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Remote borehole sensing and logging provide direct and indirect data (Section
2.3.6) .

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Method Selection
Basic Factors
Selection is based on consideration of the study objectives and phase ( Section 1.3), the size
of the study area, project type and design elements, geologic conditions, surface condi-
tions and accessibility, and the limitations of budget and time.
The various methods in terms of their applicability to general geologic conditions are
listed in Table 2.1 .

Key Methods
Geophysical methods , particularly seismic refraction surveys, provide the quickest and often
the most economical method of obtaining general information over large land areas, or in
areas with difficult access, such as mountainous regions or large water bodies. They are
particularly useful in investigating shallow rock conditions.
Test pits and trenches are rapid and economical reconnaissance methods for obtaining
information on shallow soil, groundwater conditions, and depth and rippability of rock,
and for investigating landfills of miscellaneous materials.
Test borings are necessary in almost all investigations for the procurement of soil and
rock samples below depths reachable by test pits.
Other methods can be generally considered to provide information supplemental to that
obtained by key methods.

2.3.2 Geophysical methods

The more common geophysical methods are summarized in Table 2.8.

Seismic Methods: General
Theoretical Basis (see also Section 11.2.2)
Elastic waves , initiated by some energy source, travel through geologic media at character-
istic velocities and are refracted and reflected by material changes or travel directly
through the material, finally arriving at the surface where they are detected and recorded
by instruments (Figure 2.20). There are several types of elastic waves.
Compression or primary (P) waves are body waves that may propagate along the surface
and into the subsurface, returning to the surface by reflection and refraction, or that may
travel through the materials as direct waves. P waves have the highest velocities V p and
arrive first at the recording instrument.
Shear (S) waves are also body waves propagating and traveling in a manner similar to P
waves. S waves travel at velocities V s , from approximately 0.58 V p for well-consolidated