ASTM D4403-84(1994)

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
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 compression. When used in deep applica-
This practice is under the jurisdiction of ASTM Committee D-18 on Soil and
tions, friction caused by drill hole misalignment and rod
Rock and is the direct responsibility of Subcommittee D18.12 on Rock Mechanics.
Current edition approved Aug. 31, 1984. Published November 1984.
D 4403
FIG. 1 Rod Extensometer
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
FIG. 2 Bar Extensometer
have fixed adjustment points to cover a range of several feet.
The fixed points are generally spaced at 1 to 4-in. (25 to meters with both ends fixed across the joint. Preset limit
100-mm) increments. A dial gage is used to measure the switches are often mounted on the joint meter to serve as a
displacements between the anchor points in the rock (as shown warning device in problem areas such as slopes and founda-
tions.
in Fig. 2). If the device is not constructed from invar steel,
ambient temperature should be recorded and the necessary 3.2.5 Wire Extensometers—Such devices utilize a thin stain-
corrections applied to the results. Bar extensometers are less steel wire to connect the reference point and the measuring
primarily used for safety monitoring devices in mines and point of the instrument (as shown in Fig. 6). This allows a
tunnels. greater number of measuring points to be placed in a single
3.2.3 Tape Extensometers—Such devices are designed to be drill hole. The wire or wires are tensioned by springs or
used in much the same manner as bar extensometers, however, weights. The wire is extended over a roller shiv and connected
tape extensometers allow the user to measure much greater to a hanging weight. Wire extensometers tensioned by springs
distances, such as found in large tunnels or powerhouse have the advantage of variable spring tension caused by anchor
openings. Tape extensometers consist of a steel tape (prefer- movements. This error must be accounted for when reducing
ably invar steel), a tensioning device to maintain constant the data. Wire-tensioned extensometers have been used to
tension, and a readout head. Lengths of tape may be pulled out measure large displacements at drill hole depths up to approxi-
from the tape spool according to the need. The readout may be mately 500 ft (150 m). The instruments used for deep mea-
a dial gage or a vernier, and the tensioning mechanism may be surements generally require much heavier wire and greater
a spring-loading device or a dead-weight (as shown in Fig. 3 spring tensions. Although wire extensometers are often used in
and Fig. 4). The tape and readout head are fastened, or open drill holes for short-term measurements, in areas of poor
stretched in tension, between the points to be measured. ground or unstable holes it is necessary to run a protective
Accuracies of 0.010 to 0.002 in. (0.25 to 0.05 mm) can be sleeve or tube over the measuring wires between the anchors.
expected, depending on the length of the tape and the ability to 3.3 Anchor Systems:
tension the tape to the same value on subsequent readings, and 3.3.1 Groutable Anchors—These were one of the first
provided that temperature corrections are made when neces- anchoring systems used to secure wire extensometer measuring
sary. points in the drill hole. Groutable anchors are also used for rod
3.2.4 Joint Meters—Normally, joint meters consist of an type extensometers. Initially PVC (poly(vinyl chloride)) pipes
extensometer fixed across the exposed surface of a joint (as clamped between the anchor points were employed to isolate
demonstrated in Fig. 5), and are used to measure displacements the measuring wires from the grout column (as shown in Fig.
along or across joints. The joint movements to be measured 7), however, this arrangement was unreliable at depths greater
may be the opening or closing of the joint or slippage along the than 25 ft (7.5 m) because the hydrostatic head pressure of the
joint. Rod-type extensometers are generally used as joint grout column often collapsed the PVC tubing. To counteract
D 4403
FIG. 3 Tape Extensometer with Vernier Readout and Deadweight
this condition, oil-filled PVC tubes were tried. The use of oil the drill hole. There have been other specialized anchor
enabled this method to be used to depths of over 50 ft (15 m). systems developed, however, these systems have proven to be
As an alternative to this system, liquid-tight flexible steel too costly and unsuccesful for most applications.
conduit is used to replace the PVC pipe. This alternative 3.4 Extensometer Transducers—These extensometers con-
system seems to work well and can be used in most applica- vert displacements occurring in in-situ materials between two
tions. Resin anchors fall in this category and are very success- anchored points to mechanical movements that can be mea-
ful. sured with conventional measuring devices such as dial gages,
3.3.2 Wedge-Type Anchors—These consist of a mechanical LVDTs, strain gages, and the like.
anchor that has been widely used for short-term anchoring 3.4.1 Depth-Measuring Instruments—A dial gage, or a
applications in hard rock. Fig. 8 shows the two basic types of depth micrometer are the simplest and most commonly used
wedge anchors: (1) the self-locking spring-loaded anchor, and mechanical measuring instruments. Used in conjunction with
(2) the mechanical-locking anchor. Self-locking anchors, when extensometers, they provide the cheapest and surest methods of
used in areas subject to shock load vibrations caused by making accurate measurements. When using the dial gage or
blasting or other construction disturbances, may tend to slip in depth micrometer, the operator is required to take readings at
the drill holes or become more deeply-seated, causing the the instrument head, however, local readings may not be
center wedge to move. Another disadvantage of the wedge practical or possible due to the instrument location or area
anchor is that no protection is offered, if using wires, to the conditions.
measuring wires in the drill hole against damage that might be 3.4.2 Electrical Transducers—For remote or continuous
caused by water or loose rock. readings, electrical transducers are used rather than dial gages.
3.3.3 Hydraulic Anchors—These anchors have proven to be LVDTs are often used because of their accuracy, small size, and
successful in most types of rock and soil conditions. Fig. 9 availability. LVDTs require electrical readout equipment con-
shows the two basic types of hydraulic anchors manufactured sisting of an a-c regulated voltage source and an accurate
for use with extensometer systems: (1) the uncoiling Bourdon voltmeter, such as a digital voltmeter or bridge circuit. The use
tube anchor, and (2) the hydraulic piston of grappling hook of linear potentiometers or strain gages is often desirable
anchor, which is limited to soft rock and soils. Both anchors because of the simplicity of the circuitry involved. The
have the disadvantage of being rather costly. The Bourdon tube disadvantage of using linear potentiometers is their inherently
anchor works well in most rock and soil conditions and the poor linearity and resolution.
complete anchor system can be fabricated before installing it in 3.4.3 When very accurate measurements are dictated by
FIG. 4 Tape Extensometer with Dial Gage and Tension Spring
D 4403
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
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
small change in resistance, it is absolutely necessary to provide
extremely good electrical connections and cable insulation
when using this type of transducer. Standard strain-gage
readout equipment can be used with this type of extensometer,
however, care must be taken to protect this equipment from the
hostile environments found in most field applications. Vibrat-
ing wire and sonic readouts are also reliable and are becoming
more common than strain-gage readouts. Provision should
always be made for mechanical readout capability.
4. Procedure
4.1 Preparatory Investigations:
4.1.1 Select the location, orientation, length, and number of
anchors for each extensometer on the basis of a thorough
review of both the construction and geotechnical features of the
project. Among the items to be considered are: direction and
magnitude of anticipated rock movements, location and nature
FIG. 5 Joint Meters
of other instruments to be installed, and the procedures and
timing of construction activities before, during, and after
certain excavations, for example, the determination of the installation of the instrument. If the instrument is installed
where rock bolts are used for support, the deepest extensometer
tension arch zone around a tunnel opening, extensometers
which can be calibrated in the field after installation shall be anchor shall be located beyond the end of the rock bolt. The
used. In all cases, the accuracy of extensometers, either length of the extensometer shall depend upon the anticipated
determined through calibration or estimation, should be given depth of rock influenced by excavation, expressed for example
in addition to the sensitivity of the transducers. The strain- in terms of tunnel diameter or slope height. As a general rule,
the deepest anchor (reference point for all subsequent anchors)
shall be plac
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