ASTM D4403-84(1994)
(Practice)Standard Practice for Extensometers Used in Rock
Standard Practice for Extensometers Used in Rock
General Information
Standards Content (Sample)
Designation: D 4403 – 84 (Reapproved 1994)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for
1
Extensometers Used in Rock
This standard is issued under the fixed designation D 4403; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope quired. For in-situ rock testing, instrument sensitivity better
than 0.0012 in. (0.02 mm) is necessary for proper interpreta-
1.1 This practice covers the description, application, selec-
tion.
tion, installation, data collecting, and data reduction of the
3.1.2 Most field measurements related to construction in
various types of extensometers used in the field of rock
rock do not require the precision of in-situ testing. Precision in
mechanics.
the range of 0.001 to 0.01 in. (0.025 to 0.25 mm) is typically
1.2 Limitations of each type of extensometer system are
required and is readily obtainable by several instruments.
covered in Section 3.
3.1.3 As the physical size of an underground structure or
1.3 This standard does not purport to address all of the
slope increases, the need for highly precise measurements
safety concerns, if any, associated with its use. It is the
diminishes. A precision of 0.01 to 0.04 in. (0.25 to 1.0 mm) is
responsibility of the user of this standard to establish appro-
often sufficient. This range of precision is applicable to
priate safety and health practices and determine the applica-
underground construction in soil or weak rock. In most hard
bility of regulatory limitations prior to use.
rock applications, however, an instrument sensitivity on the
2. Significance and Use order of 0.001 in. (0.025 mm) is preferred.
3.1.4 The least precision is required for very large excava-
2.1 Extensometers are widely used in the field of engineer-
tions, such as open pit mines and large moving landslides. In
ing and include most devices used to measure displacements,
such cases, the deformations are large before failure and, thus,
separation, settlements, convergence, and the like.
relatively coarse precision is required, on the order of 1 % of
2.2 For tunnel instrumentation, extensometers are generally
the range where the range may be 3 ft. (1 m) or more.
used to measure roof and sidewall movements and to locate the
3.1.5 For long-term monitoring, displacements are typically
tension arch zone surrounding the tunnel opening.
smaller than those that occur during construction. Therefore,
2.3 Extensometers are also used extensively as safety moni-
greater precision may be required for the long-term measure-
toring devices in tunnels, in underground cavities, on poten-
ments.
tially unstable slopes, and in monitoring the performance of
3.2 Extensometers:
rock support systems.
3.2.1 Rod Extensometers—A large variety of rod extensom-
2.4 An extensometer should be selected on the basis of its
eters are manufactured. They range from simple single-point
intended use, the preciseness of the measurement required, the
units to complicated multipoint systems with electrical readout.
anticipated range of deformation, and the details accompany-
The single-point extensometer is generally used to detect
ing installation. No single instrument is suitable for all appli-
support system failures. The rod can also serve as a safety
cations.
warning device in hazardous areas. Generally, the rod exten-
3. Apparatus
someter is read with a depth-measuring instrument such as a
dial gage or depth micrometer, however, various electrical
3.1 General—Experience and engineering judgment are
transducers such as LVDTs (linear variable differential trans-
required to match the proper type of extensometer systems to
formers), linear potentiometers, and microswitches have been
the nature of investigation for a given project.
used where remote or continuous readings are required (as
3.1.1 In applications for construction in rock, precise mea-
shown in Fig. 1). Another type of readout recently developed is
surements will usually allow the identification of significant,
a noncontact removable sonic probe digital readout system
possibly dangerous, trends in rock movement; however, pre-
which is interchangeable with the depth micrometer type.
cise measurement is much less important than the overall
Multipoint rod extensometers have up to eight measuring
pattern of movement. Where measurements are used to deter-
points. Reduced rod diameters are required for multipoint
mine rock properties (such as in plate-jack tests), accurate
instruments and have been used effectively to depths of at least
measurements involving a high degree of precision are re-
150 ft (45 m). The rod acts as a rigid member and must react
in both tension and
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