ASTM D4403-84(2000)e1
(Practice)Standard Practice for Extensometers Used in Rock
Standard Practice for Extensometers Used in Rock
SCOPE
1.1 This practice covers the description, application, selection, installation, data collecting, and data reduction of the various types of extensometers used in the field of rock mechanics.
1.2 Limitations of each type of extensometer system are covered in Section 3.
1.3 The values stated in inch-pound units are to be regarded as the standard. The SI values given in parentheses are provided for information purposes only.
1.4 The text of this standard references notes and footnotes which provide explanatory material. These notes and footnotes (excluding those in tables and figures) shall not be considered as requirements of the standard.
1.5 This practice offers a set of instructions for performing one or more specific operations. This document cannot replace education or experience and should be used in conjunction with professional judgement. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word "Standard" in the title of this document means only that the document has been approved through the ASTM consensus process.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
e1
Designation: D 4403 – 84 (Reapproved 2000)
Standard Practice for
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.
e NOTE—Editorial changes were made throughout in December 2000.
1. Scope * tially unstable slopes, and in monitoring the performance of
rock support systems.
1.1 This practice covers the description, application, selec-
2.4 An extensometer should be selected on the basis of its
tion, installation, data collecting, and data reduction of the
intended use, the preciseness of the measurement required, the
various types of extensometers used in the field of rock
anticipated range of deformation, and the details accompany-
mechanics.
ing installation. No single instrument is suitable for all appli-
1.2 Limitations of each type of extensometer system are
cations.
covered in Section 3.
1.3 The values stated in inch-pound units are to be regarded
3. Apparatus
as the standard. The SI values given in parentheses are
3.1 General—Experience and engineering judgment are
provided for information purposes only.
required to match the proper type of extensometer systems to
1.4 The text of this standard references notes and footnotes
the nature of investigation for a given project.
which provide explanatory material. These notes and footnotes
3.1.1 In applications for construction in rock, precise mea-
(excluding those in tables and figures) shall not be considered
surements will usually allow the identification of significant,
as requirements of the standard.
possibly dangerous, trends in rock movement; however, pre-
1.5 This practice offers a set of instructions for performing
cise measurement is much less important than the overall
one or more specific operations. This document cannot replace
pattern of movement. Where measurements are used to deter-
education or experience and should be used in conjunction
mine rock properties (such as in plate-jack tests), accurate
with professional judgement. Not all aspects of this guide may
measurements involving a high degree of precision are re-
be applicable in all circumstances. This ASTM standard is not
quired. For in-situ rock testing, instrument sensitivity better
intended to represent or replace the standard of care by which
than 0.0012 in. (0.02 mm) is necessary for proper interpreta-
the adequacy of a given professional service must be judged,
tion.
nor should this document be applied without consideration of
3.1.2 Most field measurements related to construction in
a project’s many unique aspects. The word “Standard” in the
rock do not require the precision of in-situ testing. Precision in
title of this document means only that the document has been
the range of 0.001 to 0.01 in. (0.025 to 0.25 mm) is typically
approved through the ASTM consensus process.
required and is readily obtainable by several instruments.
1.6 This standard does not purport to address all of the
3.1.3 As the physical size of an underground structure or
safety concerns, if any, associated with its use. It is the
slope increases, the need for highly precise measurements
responsibility of the user of this standard to establish appro-
diminishes. A precision of 0.01 to 0.04 in. (0.25 to 1.0 mm) is
priate safety and health practices and determine the applica-
often sufficient. This range of precision is applicable to
bility of regulatory limitations prior to use.
underground construction in soil or weak rock. In most hard
2. Significance and Use rock applications, however, an instrument sensitivity on the
order of 0.001 in. (0.025 mm) is preferred.
2.1 Extensometers are widely used in the field of engineer-
3.1.4 The least precision is required for very large excava-
ing and include most devices used to measure displacements,
tions, such as open pit mines and large moving landslides. In
separation, settlements, convergence, and the like.
such cases, the deformations are large before failure and, thus,
2.2 For tunnel instrumentation, extensometers are generally
relatively coarse precision is required, on the order of 1 % of
used to measure roof and sidewall movements and to locate the
the range where the range may be 3 ft. (1 m) or more.
tension arch zone surrounding the tunnel opening.
3.1.5 For long-term monitoring, displacements are typically
2.3 Extensometers are also used extensively as safety moni-
smaller than those that occur during construction. Therefore,
toring devices in tunnels, in underground cavities, on poten-
greater precision may be required for the long-term measure-
ments.
This practice is under the jurisdiction of ASTM Committee D18 on Soil and
3.2 Extensometers:
Rock and is the direct responsibility of Subcommittee D18.12 on Rock Mechanics.
Current edition approved Aug. 31, 1984. Published November 1984.
*A Summary of Changes section appears at the end of this standard.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 4403
3.2.1 Rod Extensometers—A large variety of rod extensom-
eters are manufactured. They range from simple single-point
units to complicated multipoint systems with electrical readout.
The single-point extensometer is generally used to detect
support system failures. The rod can also serve as a safety
warning device in hazardous areas. Generally, the rod exten-
someter is read with a depth-measuring instrument such as a
dial gage or depth micrometer, however, various electrical
transducers such as LVDTs (linear variable differential trans-
formers), linear potentiometers, and microswitches have been
used where remote or continuous readings are required (as
shown in Fig. 1). Another type of readout recently developed is
a noncontact removable sonic probe digital readout system
which is interchangeable with the depth micrometer type.
Multipoint rod extensometers have up to eight measuring
points. Reduced rod diameters are required for multipoint
instruments and have been used effectively to depths of at least
150 ft (45 m). The rod acts as a rigid member and must react
in both tension and compression. When used in deep applica-
tions, friction caused by drill hole misalignment and rod
interference can cause erroneous readings.
3.2.2 Bar Extensometers—Bar extensometers are generally
used to measure diametric changes in tunnels. Most bar
extensometers consist of spring-loaded, telescopic tubes that
have fixed adjustment points to cover a range of several feet.
The fixed points are generally spaced at 1 to 4-in. (25 to
100-mm) increments. A dial gage is used to measure the FIG. 2 Bar Extensometer
displacements between the anchor points in the rock (as shown
distances, such as found in large tunnels or powerhouse
in Fig. 2). If the device is not constructed from invar steel,
openings. Tape extensometers consist of a steel tape (prefer-
ambient temperature should be recorded and the necessary
ably invar steel), a tensioning device to maintain constant
corrections applied to the results. Bar extensometers are
tension, and a readout head. Lengths of tape may be pulled out
primarily used for safety monitoring devices in mines and
from the tape spool according to the need. The readout may be
tunnels.
a dial gage or a vernier, and the tensioning mechanism may be
3.2.3 Tape Extensometers—Such devices are designed to be
a spring-loading device or a dead-weight (as shown in Fig. 3
used in much the same manner as bar extensometers, however,
and Fig. 4). The tape and readout head are fastened, or
tape extensometers allow the user to measure much greater
stretched in tension, between the points to be measured.
Accuracies of 0.010 to 0.002 in. (0.25 to 0.05 mm) can be
expected, depending on the length of the tape and the ability to
tension the tape to the same value on subsequent readings, and
provided that temperature corrections are made when neces-
sary.
3.2.4 Joint Meters—Normally, joint meters consist of an
extensometer fixed across the exposed surface of a joint (as
demonstrated in Fig. 5), and are used to measure displacements
along or across joints. The joint movements to be measured
may be the opening or closing of the joint or slippage along the
joint. Rod-type extensometers are generally used as joint
meters with both ends fixed across the joint. Preset limit
switches are often mounted on the joint meter to serve as a
warning device in problem areas such as slopes and founda-
tions.
3.2.5 Wire Extensometers—Such devices utilize a thin stain-
less steel wire to connect the reference point and the measuring
point of the instrument (as shown in Fig. 6). This allows a
greater number of measuring points to be placed in a single
drill hole. The wire or wires are tensioned by springs or
weights. The wire is extended over a roller shiv and connected
FIG. 1 Rod Extensometer to a hanging weight. Wire extensometers tensioned by springs
D 4403
FIG. 3 Tape Extensometer with Vernier Readout and Deadweight
FIG. 4 Tape Extensometer with Dial Gage and Tension Spring
have the advantage of variable spring tension caused by anchor
movements. This error must be accounted for when reducing
the data. Wire-tensioned extensometers have been used to
measure large displacements at drill hole depths up to approxi-
mately 500 ft (150 m). The instruments used for deep mea-
surements generally require much heavier wire and greater
spring tensions. Although wire extensometers are often used in
open drill holes for short-term measurements, in areas of poor
ground or unstable holes it is necessary to run a protective
sleeve or tube over the measuring wires between the anchors.
3.3 Anchor Systems:
3.3.1 Groutable Anchors—These were one of the first
anchoring systems used to secure wire extensometer measuring
points in the drill hole. Groutable anchors are also used for rod
type extensometers. Initially PVC (poly(vinyl chloride)) pipes
clamped between the anchor points were employed to isolate
the measuring wires from the grout column (as shown in Fig.
7), however, this arrangement was unreliable at depths greater
than 25 ft (7.5 m) because the hydrostatic head pressure of the
grout column often collapsed the PVC tubing. To counteract
this condition, oil-filled PVC tubes were tried. The use of oil
enabled this method to be used to depths of over 50 ft (15 m).
As an alternative to this system, liquid-tight flexible steel
conduit is used to replace the PVC pipe. This alternative
system seems to work well and can be used in most applica-
tions. Resin anchors fall in this category and are very success- FIG. 5 Joint Meters
ful.
3.3.2 Wedge-Type Anchors—These consist of a mechanical applications in hard rock. Fig. 8 shows the two basic types of
anchor that has been widely used for short-term anchoring wedge anchors: (1) the self-locking spring-loaded anchor, and
D 4403
practical or possible due to the instrument location or area
conditions.
3.4.2 Electrical Transducers—For remote or continuous
readings, electrical transducers are used rather than dial gages.
LVDTs are often used because of their accuracy, small size, and
availability. LVDTs require electrical readout equipment con-
sisting of an a-c regulated voltage source and an accurate
voltmeter, such as a digital voltmeter or bridge circuit. The use
of linear potentiometers or strain gages is often desirable
because of the simplicity of the circuitry involved. The
disadvantage of using linear potentiometers is their inherently
poor linearity and resolution.
3.4.3 When very accurate measurements are dictated by
certain excavations, for example, the determination of the
tension arch zone around a tunnel opening, extensometers
which can be calibrated in the field after installation shall be
used. In all cases, the accuracy of extensometers, either
determined through calibration or estimation, should be given
in addition to the sensitivity of the transducers. The strain-
gaged cantilever extensometer (shown in Fig. 10) has been
used successfully for many years. The strain-gaged cantilever
operates on the principles of the linear strain produced across
a given area of a spring material when flexed. This type of
extensometer readout is normally used when rock movements
FIG. 6 Wire Extensometers
of 0.5 in. (12.5 mm) or less are expected. Strain gages produce
a linear change in resistance of 1 to 3% of their initial
resistance, over their total measurement range. Because of this
(2) the mechanical-locking anchor. Self-locking anchors, when
small change in resistance, it is absolutely necessary to provide
used in areas subject to shock load vibrations caused by
extremely good electrical connections and cable insulation
blasting or other construction disturbances, may tend to slip in
when using this type of transducer. Standard strain-gage
the drill holes or become more deeply-seated, causing the
readout equipment can be used with this type of extensometer,
center wedge to move. Another disadvantage of the wedge
however, care must be taken to protect this equipment from the
anchor is that no protection is offered, if using wires, to the
hostile environments found in most field applications. Vibrat-
measuring wires in the drill hole against damage that might be
ing wire and sonic readouts are also reliable and are becoming
caused by water or loose rock.
more common than strain-gage readouts. Provision should
3.3.3 Hydraulic Anchors—These anchors have proven to be
always be made for mechanical readout capability.
successful in most types of rock and soil conditions. Fig. 9
4. Procedure
shows the two basic types of hydraulic anchors manufactured
for use with extensometer systems: (1) the uncoiling Bourdon
4.1 Preparatory Investigations:
tube anchor, and (2) the hydraulic piston of grappling hook
4.1.1 Select the location, orientation, length, and number of
anchor, which is limited to soft rock and soils. Both anchors
anchors for each extensometer on the basis of a thorough
have the disadvantage of being rather costly. The Bourdon tube
review of both the construction and geotechnical features of the
anchor works well in most rock and soil conditions and the
project. Among the items to be considered are: direction and
complete anchor system can be fabricated before installing it in
magnitude of anticipated rock movements, location and nature
the drill hole. There have been other specialized anchor
of other instruments to be installed, and the procedures and
systems developed, however, these systems have proven to be
timing of construction activities before, during, and after
too costly and unsuccesful for most applications.
in
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