ASTM D4403-84(2005)
(Practice)Standard Practice for Extensometers Used in Rock
Standard Practice for Extensometers Used in Rock
SIGNIFICANCE AND USE
Extensometers are widely used in the field of engineering and include most devices used to measure displacements, separation, settlements, convergence, and the like.
For tunnel instrumentation, extensometers are generally used to measure roof and sidewall movements and to locate the tension arch zone surrounding the tunnel opening.
Extensometers are also used extensively as safety monitoring devices in tunnels, in underground cavities, on potentially unstable slopes, and in monitoring the performance of rock support systems.
An extensometer should be selected on the basis of its intended use, the preciseness of the measurement required, the anticipated range of deformation, and the details accompanying installation. No single instrument is suitable for all applications.
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 .
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 judgment. 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.
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.
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Designation: D4403 − 84 (Reapproved 2005)
StandardPractice for
Extensometers Used in Rock
This standard is issued under the fixed designation D4403; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 2.2 For tunnel instrumentation, extensometers are generally
used to measure roof and sidewall movements and to locate the
1.1 This practice covers the description, application,
tension arch zone surrounding the tunnel opening.
selection, installation, data collecting, and data reduction of the
various types of extensometers used in the field of rock
2.3 Extensometers are also used extensively as safety moni-
mechanics. toring devices in tunnels, in underground cavities, on poten-
tially unstable slopes, and in monitoring the performance of
1.2 Limitations of each type of extensometer system are
rock support systems.
covered in Section 3.
2.4 An extensometer should be selected on the basis of its
1.3 The values stated in inch-pound units are to be regarded
intended use, the preciseness of the measurement required, the
as the standard. The SI values given in parentheses are
anticipated range of deformation, and the details accompany-
provided for information purposes only.
ing installation. No single instrument is suitable for all appli-
1.4 The text of this standard references notes and footnotes
cations.
which provide explanatory material. These notes and footnotes
(excluding those in tables and figures) shall not be considered
3. Apparatus
as requirements of the standard.
3.1 General—Experience and engineering judgment are re-
1.5 This practice offers a set of instructions for performing
quired to match the proper type of extensometer systems to the
one or more specific operations. This document cannot replace
nature of investigation for a given project.
education or experience and should be used in conjunction
3.1.1 In applications for construction in rock, precise mea-
with professional judgement. Not all aspects of this guide may
surements will usually allow the identification of significant,
be applicable in all circumstances. This ASTM standard is not
possibly dangerous, trends in rock movement; however, pre-
intended to represent or replace the standard of care by which
cise measurement is much less important than the overall
the adequacy of a given professional service must be judged,
pattern of movement. Where measurements are used to deter-
nor should this document be applied without consideration of
mine rock properties (such as in plate-jack tests), accurate
a project’s many unique aspects. The word “Standard” in the
measurements involving a high degree of precision are re-
title of this document means only that the document has been
quired. For in-situ rock testing, instrument sensitivity better
approved through the ASTM consensus process.
than 0.0012 in. (0.02 mm) is necessary for proper interpreta-
1.6 This standard does not purport to address all of the
tion.
safety concerns, if any, associated with its use. It is the
3.1.2 Most field measurements related to construction in
responsibility of the user of this standard to establish appro-
rock do not require the precision of in-situ testing. Precision in
priate safety and health practices and determine the applica-
the range of 0.001 to 0.01 in. (0.025 to 0.25 mm) is typically
bility of regulatory limitations prior to use.
required and is readily obtainable by several instruments.
3.1.3 As the physical size of an underground structure or
2. Significance and Use
slope increases, the need for highly precise measurements
diminishes. A precision of 0.01 to 0.04 in. (0.25 to 1.0 mm) is
2.1 Extensometers are widely used in the field of engineer-
ing and include most devices used to measure displacements, often sufficient. This range of precision is applicable to
underground construction in soil or weak rock. In most hard
separation, settlements, convergence, and the like.
rock applications, however, an instrument sensitivity on the
order of 0.001 in. (0.025 mm) is preferred.
3.1.4 The least precision is required for very large
This practice is under the jurisdiction of ASTM Committee D18 on Soil and
Rock and is the direct responsibility of Subcommittee D18.23 on Field Instrumen-
excavations, such as open pit mines and large moving land-
tation.
slides. In such cases, the deformations are large before failure
Current edition approved May 1, 2005. Published June 2005. Originally
and, thus, relatively coarse precision is required, on the order
approved in 1984. Last previous edition approved in 2000 as D4403–84(2005).
DOI: 10.1520/D4403-84R05. of 1 % of the range where the range may be 3 ft. (1 m) or more.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4403 − 84 (Reapproved 2005)
3.1.5 For long-term monitoring, displacements are typically
smaller than those that occur during construction. Therefore,
greater precision may be required for the long-term measure-
ments.
3.2 Extensometers:
3.2.1 Rod Extensometers—Alarge variety of rod extensom-
eters are manufactured. They range from simple single-point
unitstocomplicatedmultipointsystemswithelectricalreadout.
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.
FIG. 2 Bar Extensometer
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
corrections applied to the results. Bar extensometers are
have fixed adjustment points to cover a range of several feet.
primarily used for safety monitoring devices in mines and
The fixed points are generally spaced at 1 to 4-in. (25 to
tunnels.
100-mm) increments. A dial gage is used to measure the
3.2.3 Tape Extensometers—Such devices are designed to be
displacements between the anchor points in the rock (as shown
used in much the same manner as bar extensometers, however,
in Fig. 2). If the device is not constructed from invar steel,
tape extensometers allow the user to measure much greater
ambient temperature should be recorded and the necessary
distances, such as found in large tunnels or powerhouse
openings. Tape extensometers consist of a steel tape (prefer-
ably invar steel), a tensioning device to maintain constant
tension, and a readout head. Lengths of tape may be pulled out
from the tape spool according to the need. The readout may be
a dial gage or a vernier, and the tensioning mechanism may be
a spring-loading device or a dead-weight (as shown in Fig. 3
and Fig. 4). The tape and readout head are fastened, or
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
demonstratedinFig.5),andareusedtomeasuredisplacements
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-
FIG. 1 Rod Extensometer tions.
D4403 − 84 (Reapproved 2005)
FIG. 3 Tape Extensometer with Vernier Readout and Deadweight
FIG. 4 Tape Extensometer with Dial Gage and Tension Spring
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
to a hanging weight. Wire extensometers tensioned by springs
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 an-
choring 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 FIG. 5 Joint Meters
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).
D4403 − 84 (Reapproved 2005)
sured with conventional measuring devices such as dial gages,
LVDTs, strain gages, and the like.
3.4.1 Depth-Measuring Instruments—A dialgage,ora
depth micrometer are the simplest and most commonly used
mechanical measuring instruments. Used in conjunction with
extensometers,theyprovidethecheapestandsurestmethodsof
making accurate measurements. When using the dial gage or
depth micrometer, the operator is required to take readings at
the instrument head, however, local readings may not be
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.
LVDTsareoftenusedbecauseoftheiraccuracy,smallsize,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
FIG. 6 Wire 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
As an alternative to this system, liquid-tight flexible steel
in addition to the sensitivity of the transducers. The strain-
conduit is used to replace the PVC pipe. This alternative
gaged cantilever extensometer (shown in Fig. 10) has been
system seems to work well and can be used in most applica-
used successfully for many years. The strain-gaged cantilever
tions. Resin anchors fall in this category and are very success-
operates on the principles of the linear strain produced across
ful.
a given area of a spring material when flexed. This type of
3.3.2 Wedge-Type Anchors—These consist of a mechanical
extensometer readout is normally used when rock movements
anchor that has been widely used for short-term anchoring
of 0.5 in. (12.5 mm) or less are expected. Strain gages produce
applications in hard rock. Fig. 8 shows the two basic types of
a linear change in resistance of 1 to 3% of their initial
wedge anchors: (1) the self-locking spring-loaded anchor, and
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
shows the two basic types of hydraulic anchors manufactured
4. Procedure
for use with extensometer systems: (1) the uncoiling Bourdon
tube anchor, and (2) the hydraulic piston of grappling hook
4.1 Preparatory Investigations :
anchor, which is limited to soft rock and soils. Both anchors
4.1.1 Select the lo
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