ASTM D4403-20

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Designation: D4403 − 12 D4403 − 20
Standard Practice 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*
1.1 This practice covers the description, application, selection, installation, data collecting, and data reduction of the various
types of contact type extensometers used in the field of rock mechanics. Laser or other non-contact extensometers are not covered
here.
1.2 Limitations of each type of extensometer system are covered in Section 5.
1.3 The values stated in inch-pound units are to be regarded as the standard. The SI values given in parentheses are mathematical
conversions to SI units that are provided for information purposes only.only and are not considered standard. Add if appropriate,
“Reporting of test results in units other than inch-pound shall not be regarded as nonconformance with this standard.
1.4 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice
D6026.
1.4.1 The procedures used to specify how data are collected/recorded or calculated in this standard are regarded as the industry
standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not
consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives;
and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations.
It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.
1.5 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.
1.6 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.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.
1.7 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D6026 Practice for Using Significant Digits in Geotechnical Data
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 Instrumentation.
Current edition approved Aug. 15, 2012Jan. 1, 2020. Published November 2012March 2020. Originally approved in 1984. Last previous edition approved in 20052012
as D4403–84(2005).D4403–12. DOI: 10.1520/D4403-12.10.1520/D4403-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*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
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3. Terminology
3.1 Definitions—Terms not defined below may appear in Terminology D653.
3.2 Definitions of terms specific to this standard are included in Section 5.
4. Significance and Use
4.1 Extensometers are widely used in the field of engineering and include most devices used to measure displacements,
separation, settlements, convergence, and the like.
4.2 For tunnel instrumentation, extensometers are generally used to measure roof and sidewall movements and to locate the
tension arch zone surrounding the tunnel opening.
4.3 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.
4.4 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 the installation. No single instrument is suitable for all
applications.
4.5 In applications for construction in rock, precise measurements will usually allow the identification of significant, possibly
dangerous, trends in rock movement; however, precise measurement is much less important than the overall pattern of movement.
4.6 Data collection of extensometers can be simple or low tech, such as manual readings at the instrument location, or complex
or high tech where there are electronic readings taken at the site and either downloaded at the instrument locations or transmitted
to a data collection and analysis center.
4.7 It is important to realize the pros and cons and costs between each type of extensometers. In the case of manual readings,
not as much data may be collected, important data may be missed and the person taking the readings may be put in harm’s way
and may not be able to safely continue collecting data just when the data is needed the most or becomes more important. Whereas,
with electronic data collection as the system becomes more sophisticated, the data collected can be done more safely, provide
important data that might be missed, and may allow for real-time data analyses that are timelier and more accurate.
4.8 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 adjusted in the field after installation shall be used. In all cases, the
accuracy of extensometers, either determined through calibration, should be given in addition to the sensitivity of the transducers.
NOTE 1—Notwithstanding the statements on precision and bias contained in this test method, the precision of this test method is dependent on the
competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are
generally considered capable of competent and objective testing. Users of these test methods are cautioned that compliance with Practice D3740 does
not in itself ensure reliable results. Reliable testing depends on many factors; Practice D3740 provides a means of evaluating some of those factors.
5. Apparatus
5.1 General—Experience Experience, safety considerations, costs, and engineering judgment are required to match the proper
type of extensometer systems to the nature of the investigation for a given project.
5.1.1 In applications for construction in rock, precise measurements will usually allow the identification of significant, possibly
dangerous, trends in rock movement; however, precise measurement is much less important than the overall pattern of movement.
Where measurements are used to determine rock properties (such as in plate-jack tests), accurate measurements involving a high
degree of precision are required. For in-situ rock testing, instrument sensitivity better than 0.0012 in. (0.02 mm) is necessary for
proper interpretation.
5.1.1 Most field measurements related to construction in rock do not require the precision of in-situ testing. Precision in the
range of 0.001 to 0.01 in. (0.025 to 0.25 mm) is typically required and is readily obtainable by several instruments.The required
precision of measurements necessary will vary with the application, as well as the capability of the measurement device. Examples
of precision requirements are as follows:
5.1.1.1 Precision levels better than 0.0012 in. (0.02 mm) for measurements used to determine rock properties using in-situ rock
testing (such as plate-jack tests),
5.1.1.2 Precision levels in the range of 0.001 to 0.01 in. (0.025 to 0.25 mm) for measurements in underground tunnels and
general construction in rock,
5.1.1.3 Precision levels in the range of 0.01 to 0.04 in. (0.25 to 1 mm) for larger underground openings or rock slopes, and weak
rock or soil conditions,
5.1.1.4 Precision levels in the range of 1% of the expected range of movement for very large excavations, such as open pit mines
and large moving landslides.
5.1.3 As the physical size of an underground structure or 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 often sufficient. This range of precision is applicable to underground construction
in soil or weak rock. In most hard rock applications, however, an instrument sensitivity on the order of 0.001 in. (0.025 mm) is
preferred.
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5.1.4 The least precision is required for very large excavations, such as open pit mines and large moving landslides. In such
cases, the deformations are large before failure and, thus, relatively coarse precision is required, on the order of 1 % of the range
where the range may be 3 ft. (1 m) or more.
5.1.2 For long-term monitoring, Greater precision is also required for long-term monitoring applications, where displacements
are typically smaller than those that occur during construction. Therefore, greater precision may be required for the long-term
measurements.
5.2 Extensometers:
5.2.1 Rod Extensometers—A large variety of rod extensometers are manufactured. They range from simple single-point units
to complicated multipoint systems with electrical readout. multi-position systems using either a manual, an electronic, or
combination of manual and electronic measurement devices. 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 extensometer 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 transformers), linear potentiometers, and microswitches vibrating wire displacement transducers have
been used where remote remote, threshold or continuous readings are required (as shown in Fig. 1). Another type of readout
recently developedor transducer is a noncontact removable sonic probe digital readout system, which is interchangeable with the
depth micrometer type. Multipoint rod extensometers Depending on the diameter of the hole, multi-position rod extensometers can
have up to eight measuring points. Reduced rod diameters are required for multipointmulti-position instruments and have
reportedly been used effectively to depths of at least 150350 ft (45(107 m). The rod acts as a rigid member and must react in both
tension and compression. When used in deep applications, friction caused by drill hole misalignment and rod interference can cause
erroneous readings. Rods are usually constructed of some type of stainless steel or fiberglass. To reduce temperature effects,
something that is typically an issue or concern, rods can be constructed of invar steel, graphite, or composite carbon fiber.
5.2.2 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)100mm) increments. A dial gage is used to measure the displacements
between the anchor points in the rock (as shown in Fig. 2). If the device is not constructed from invar steel, ambient temperature
should be recorded, and the necessary corrections applied to the results. Bar extensometers are primarily used for safety monitoring
devices in mines and tunnels.
5.2.3 Tape Extensometers—Such devices are designed to be used in much the same manner as bar extensometers,extensometers;
however, tape extensometers allow the user to measure much greater distances, such as found in large tunnels or powerhouse
openings. Tape extensometers consist of a steel tape (preferably 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
gage, a vernier, or another suitable gage. 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 necessary.
FIG. 1 Rod ExtensometerExamples of Rod Extensometer, Single Point (top) with a Manual Readout and (bottom) Electronic Readout
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FIG. 2 Bar Extensometer
5.2.4 Joint or Crack Meters—Normally, joint meters consistOne type of a joint or crack meters consists of an extensometer fixed
across thean exposed rock surface ofcontaining a joint (as demonstrated in Fig. 5), and are used to measure displacements along
or across joints. The example shown here shows a dial gage, but it could be an electronic or vibrating wire LVDT type transducer
that measures the displacement as well. 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 or crack meters with both ends fixed across the joint. on
opposite sides of the joint and situated such that deformations normal or parallel to the joint surface trace is measured. Preset limit
switches are often mounted on the joint meter to serve as a warning device in problem areas such as slopes and foundations.
5.2.5 Wire Extensometers—Such devices utilize a thin stainless 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 approximately 500 ft (150 m). The instruments used for deep measurements 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.
5.3 Anchor Systems:
5.3.1 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 or
the heat of hydration of the cement in the grout often collapsed the PVC tubing. tubing when the rated capacity was exceeded.
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. m) but is no longer
recommended because of environmental issues. This can be avoided if PVC schedule 80 is used or if ABS plastic is used instead
of PVC, or the temperature of the grout is controlled using ice or cold water in the grout mix. Oil filed tubes. This alternative
system seems to work well and can be used in most applications. Resin anchors fall in this category and are very
successful.successful, as well.
5.3.2 Wedge-Type Wedge- or Expansion-Type Anchors—These consist of a mechanical anchor that has been widely used for
short-term anchoring applications in hard rock. There are different types of wedge and expansion anchors; Fig. 8 shows the two
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FIG. 3 Tape Extensometer with Vernier Readout and Deadweight
FIG. 4 Side and Top View of Tape Extensometer with Dial Gage and Tension Spring
FIG. 5 Joint MetersExtensometer Set Up as Joint or Crack Meter
to Measure Dilation or Shear
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FIG. 6 Wire Extensometers
basic types of wedge anchors: (1) the self-locking spring-loaded anchor, and (2) the mechanical-locking anchor. Self-locking
anchors, when used in areas subject to shock load vibrations caused by blasting or other construction disturbances, may tend to
slip in the drill holes or become more deeply-seated, causing the center wedge to move. Another disadvantage of the wedge anchor
is that no protection is offered, if using wires, to the measuring wires in the drill hole against damage that might be caused by water
or loose rock. The expansion anchor is retrievable, and it is discussed in 5.3.4.
5.3.3 Hydraulic Anchors—These anchors have proven to be successful in most types of rock and soil conditions. Fig. 9 shows
the two basic types of hydraulic anchors manufactured for use with extensometer systems: (1) the uncoiling Bourdon tube anchor,
and (2) the hydraulic piston of grappling hook anchor, which is limited to soft rock and soils. Both anchors have the disadvantage
of being rather costly. more costly than other types of anchors and require activation through hydraulic lines routed to the head
of the hole. The Bourdon tube anchor works well in most rock and soil conditions, and the complete anchor system can be
fabricated before installing it in the drill hole. There have been other specialized anchor systems developed,developed; however,
these systems have proven to be too costly and unsuccesfulunsuccessful for most applications.
5.3.4 Retrievable Anchors (see Fig. 10)—These can be a mechanical or hydraulic/pneumatic anchor.
5.3.4.1 The special design of the mechanical anchor allows complete system retrievability. The mechanical anchor discussed
here consists of a cylindrical body and three contacting shoes spaced at 120° angle that isis in a collapse configuration that is
smaller than the borehole diameter. Using the installation tool and rods, the anchor is placed in the drill hole to the depth required.
The anchor is spring-loaded and is actuated from the collar of the hole, and the shoes make immediate contact with the borehole
walls. The central screw is then turned to exert a radial force from the shoes to the drill hole wall. The anchoring capacity is very
high, and the contacting shoes are designed to adjust to small borehole deformations while still exerting the anchoring force. This
process is reversed to then remove the anchors one at a time, starting from closest anchor to the borehole collar.
5.3.4.2 For the retrievable hydraulic anchor the string of sensors is assembled (with variable lengths of connecting rods to enable
positioning of the anchors at the required depths), inserted into the pipe or borehole, and then locked in position by pneumatically
or hydraulically actuating the various anchors, which remain fully expanded throughout the monitoring period. When monitoring
has been completed, the pressure is released, which retracts the anchor pistons and allows removal of the string for further use.
5.3.5 Snap Ring Anchors (see Fig. 11)—This type of anchor is quickly and easily installed in boreholes in hard or competent
rock. Anchors are pushed to the required depth on the end of setting rods, and then a cord is pulled to remove the locking pin, which
allows two retaining rings on each anchor to snap outward and grip the borehole. Up to eight anchors can be installed at various
depths in a 3-in. (76 mm) diameter borehole. Particularly useful in upward-directed boreholes. Anchors can only be used with rigid
rods and are not likely affected by blasting.
5.3.6 Packer Anchor—This type of extensometer anchor is a solution for where it is important that the surrounding strata
between anchors are not affected by grout. The borehole between each anchor is therefore left open or filled with compressible
material resulting in the anchors being insensitive to shear displacements. This not only ensures no interference but also reduces
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FIG. 7 Grouted Anchor System
FIG. 8 Wedge Anchors
the amount of grout needed in comparison to grout able anchors. Packer anchors can be used in rock or soils. The anchor employs
a geotextile bladder that is inflated with grout. The geotextile allows water to go thru but retain all solids. Once the grout sets, it
forms an effective and stable anchor.
NOTE 2—There have been other specialized anchor systems developed; however, these systems have proven to be too costly and unsuccessful for most
applications.
5.4 Extensometer Transducers—These extensometers convert displacements occurring in in-situ in situ materials between two
anchored points to mechanical movements that can be measured with conventional measuring devices such as dial gages, LVDTs,
strain gages, and the like.
5.4.1 Depth-Measuring Instruments—A dial gage, or a depth micrometer are the simplest and most commonly used mechanical
measuring instruments. Used in conjunction with extensometers, they provide the cheapest and surest methods of making accurate
measurements. When using the dial gage or depth micrometer, the operator is required to take readings at the instrument head,head;
however, local readings may not be practical or possi
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