Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 65

Cavity Exploration in Soluble Rock ( Section 10.4.4)
The bulk density of limestones is about 160 pcf (2.6 g/cm 3 ), and that of soils generally
ranges from 100 to 125 pcf (1.6 to 2.0 g/cm 3 ). In karst regions, a gravity-low anomaly may
indicate an empty cavity, a cavity filled with low-density material, or a change in soil or
groundwater conditions. Microgravimetric instruments have been developed in recent
years which permit a precision of 0.01 mgal (10 gal) or better (Greenfield, 1979), equiva-

lent to a change in soil thickness of about 10 in. (24 cm) for a density contrast between soil
and rock of 62.4 pcf (1.0 g/cm 3 ). A detected anomaly is then explored with test borings.

µ

Infrared Thermography
An infrared scanner, which can detect very small variations in temperature, is mounted on
a sled that can be towed by a person or a vehicle. Multicolor images are displayed and
recorded. Cooler areas are depicted in green or blue and the warmer areas in orange or red.
In engineering studies, infrared scanners are most useful in detecting water leaks in sewers
or water lines. In limestone areas, such leaks are often precursors to sinkhole development.

2.3.3 Reconnaissance Methods

General
Reconnaissance methods of exploration are divided into two general groups as follows:

Large excavations allow close examination of geologic materials and include test
pits, test trenches, and large-diameter holes, which can be made relatively rap-
idly and cheaply. Adits and tunnels, although costly to excavate, are valuable for
investigating rock mass conditions.

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Hand tools and soundings provide low-cost and rapid means of performing prelim-
inary explorations. Samples are recovered with the hand auger and 1in. retractable
plug sampler, and probings with bars provide indications of penetration resistance
to shallow depths. The cone penetrometer test (CPT) and the standard penetration
test (SPT) are used also as reconnaissance methods for preliminary explorations.

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Methods
The various methods are summarized in Table 2.12, including:

Test pit or trench excavation

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Large-diameter holes

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Adits and tunnels

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Bar soundings

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Hand auger or posthole digger

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One-inch retractable plug sampler

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Continuous penetrometer test (see Sections 2.34 and 3.4.5)

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Standard penetration test (see Section 3.4.5)

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2.3.4 Continuous CPT(ASTM D5778)

History
In 1932, the Dutch developed a simple device to measure continuously soil properties in
situ . At the end of steel rods was a penetrometer consisting of a cone tip. Over the years,
the rods were advanced by driving them with a hammer (dynamic force), applying dead