1、Designation: D 4403 84 (Reapproved 2005)Standard Practice forExtensometers Used in Rock1This standard is issued under the fixed designation D 4403; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number
2、 in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This practice covers the description, application, selec-tion, installation, data collecting, and data reduction of thevarious types of ext
3、ensometers used in the field of rockmechanics.1.2 Limitations of each type of extensometer system arecovered in Section 3.1.3 The values stated in inch-pound units are to be regardedas the standard. The SI values given in parentheses areprovided for information purposes only.1.4 The text of this sta
4、ndard references notes and footnoteswhich provide explanatory material. These notes and footnotes(excluding those in tables and figures) shall not be consideredas requirements of the standard.1.5 This practice offers a set of instructions for performingone or more specific operations. This document
5、cannot replaceeducation or experience and should be used in conjunctionwith professional judgement. Not all aspects of this guide maybe applicable in all circumstances. This ASTM standard is notintended to represent or replace the standard of care by whichthe adequacy of a given professional service
6、 must be judged,nor should this document be applied without consideration ofa projects many unique aspects. The word “Standard” in thetitle of this document means only that the document has beenapproved through the ASTM consensus process.1.6 This standard does not purport to address all of thesafety
7、 concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Significance and Use2.1 Extensometers are widely used in the field of engi
8、neer-ing and include most devices used to measure displacements,separation, settlements, convergence, and the like.2.2 For tunnel instrumentation, extensometers are generallyused to measure roof and sidewall movements and to locate thetension arch zone surrounding the tunnel opening.2.3 Extensometer
9、s are also used extensively as safety moni-toring devices in tunnels, in underground cavities, on poten-tially unstable slopes, and in monitoring the performance ofrock support systems.2.4 An extensometer should be selected on the basis of itsintended use, the preciseness of the measurement required
10、, theanticipated range of deformation, and the details accompany-ing installation. No single instrument is suitable for all appli-cations.3. Apparatus3.1 GeneralExperience and engineering judgment arerequired to match the proper type of extensometer systems tothe nature of investigation for a given
11、project.3.1.1 In applications for construction in rock, precise mea-surements will usually allow the identification of significant,possibly dangerous, trends in rock movement; however, pre-cise measurement is much less important than the overallpattern of movement. Where measurements are used to det
12、er-mine rock properties (such as in plate-jack tests), accuratemeasurements involving a high degree of precision are re-quired. For in-situ rock testing, instrument sensitivity betterthan 0.0012 in. (0.02 mm) is necessary for proper interpreta-tion.3.1.2 Most field measurements related to constructi
13、on inrock do not require the precision of in-situ testing. Precision inthe range of 0.001 to 0.01 in. (0.025 to 0.25 mm) is typicallyrequired and is readily obtainable by several instruments.3.1.3 As the physical size of an underground structure orslope increases, the need for highly precise measure
14、mentsdiminishes. A precision of 0.01 to 0.04 in. (0.25 to 1.0 mm) isoften sufficient. This range of precision is applicable tounderground construction in soil or weak rock. In most hardrock applications, however, an instrument sensitivity on theorder of 0.001 in. (0.025 mm) is preferred.3.1.4 The le
15、ast precision is required for very large excava-tions, such as open pit mines and large moving landslides. Insuch cases, the deformations are large before failure and, thus,relatively coarse precision is required, on the order of 1 % ofthe range where the range may be 3 ft. (1 m) or more.3.1.5 For l
16、ong-term monitoring, displacements are typicallysmaller than those that occur during construction. Therefore,greater precision may be required for the long-term measure-ments.1This practice is under the jurisdiction of ASTM Committee D18 on Soil andRock and is the direct responsibility of Subcommitt
17、ee D18.23 on Field Instrumen-tation.Current edition approved May 1, 2005. Published June 2005. Originallyapproved in 1984. Last previous edition approved in 2000 as D 440384(2005).1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive,
18、PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2 Extensometers:3.2.1 Rod ExtensometersA large variety of rod extensom-eters are manufactured. They range from simple single-pointunits to complicated multipoint systems with electrical readout.The single-point extensometer is generally
19、used to detectsupport system failures. The rod can also serve as a safetywarning device in hazardous areas. Generally, the rod exten-someter is read with a depth-measuring instrument such as adial gage or depth micrometer, however, various electricaltransducers such as LVDTs (linear variable differe
20、ntial trans-formers), linear potentiometers, and microswitches have beenused where remote or continuous readings are required (asshown in Fig. 1).Another type of readout recently developed isa noncontact removable sonic probe digital readout systemwhich is interchangeable with the depth micrometer t
21、ype.Multipoint rod extensometers have up to eight measuringpoints. Reduced rod diameters are required for multipointinstruments and have been used effectively to depths of at least150 ft (45 m). The rod acts as a rigid member and must reactin both tension and compression. When used in deep applica-t
22、ions, friction caused by drill hole misalignment and rodinterference can cause erroneous readings.3.2.2 Bar ExtensometersBar extensometers are generallyused to measure diametric changes in tunnels. Most barextensometers consist of spring-loaded, telescopic tubes thathave fixed adjustment points to c
23、over a range of several feet.The fixed points are generally spaced at 1 to 4-in. (25 to100-mm) increments. A dial gage is used to measure thedisplacements between the anchor points in the rock (as shownin Fig. 2). If the device is not constructed from invar steel,ambient temperature should be record
24、ed and the necessarycorrections applied to the results. Bar extensometers areprimarily used for safety monitoring devices in mines andtunnels.3.2.3 Tape ExtensometersSuch devices are designed to beused in much the same manner as bar extensometers, however,tape extensometers allow the user to measure
25、 much greaterdistances, such as found in large tunnels or powerhouseopenings. Tape extensometers consist of a steel tape (prefer-ably invar steel), a tensioning device to maintain constanttension, and a readout head. Lengths of tape may be pulled outfrom the tape spool according to the need. The rea
26、dout may bea dial gage or a vernier, and the tensioning mechanism may bea spring-loading device or a dead-weight (as shown in Fig. 3and Fig. 4). The tape and readout head are fastened, orstretched in tension, between the points to be measured.Accuracies of 0.010 to 0.002 in. (0.25 to 0.05 mm) can be
27、expected, depending on the length of the tape and the ability totension the tape to the same value on subsequent readings, andprovided that temperature corrections are made when neces-sary.3.2.4 Joint MetersNormally, joint meters consist of anextensometer fixed across the exposed surface of a joint
28、(asdemonstrated in Fig. 5), and are used to measure displacementsalong or across joints. The joint movements to be measuredmay be the opening or closing of the joint or slippage along thejoint. Rod-type extensometers are generally used as jointmeters with both ends fixed across the joint. Preset lim
29、itswitches are often mounted on the joint meter to serve as awarning device in problem areas such as slopes and founda-tions.3.2.5 Wire ExtensometersSuch devices utilize a thin stain-less steel wire to connect the reference point and the measuringpoint of the instrument (as shown in Fig. 6). This al
30、lows agreater number of measuring points to be placed in a singledrill hole. The wire or wires are tensioned by springs orFIG. 1 Rod ExtensometerFIG. 2 Bar ExtensometerD 4403 84 (2005)2weights. The wire is extended over a roller shiv and connectedto a hanging weight. Wire extensometers tensioned by
31、springshave the advantage of variable spring tension caused by anchormovements. This error must be accounted for when reducingthe data. Wire-tensioned extensometers have been used tomeasure large displacements at drill hole depths up to approxi-mately 500 ft (150 m). The instruments used for deep me
32、a-surements generally require much heavier wire and greaterspring tensions. Although wire extensometers are often used inopen drill holes for short-term measurements, in areas of poorground or unstable holes it is necessary to run a protectivesleeve or tube over the measuring wires between the ancho
33、rs.3.3 Anchor Systems:3.3.1 Groutable AnchorsThese were one of the firstanchoring systems used to secure wire extensometer measuringpoints in the drill hole. Groutable anchors are also used for rodtype extensometers. Initially PVC (poly(vinyl chloride) pipesclamped between the anchor points were emp
34、loyed to isolatethe measuring wires from the grout column (as shown in Fig.7), however, this arrangement was unreliable at depths greaterthan 25 ft (7.5 m) because the hydrostatic head pressure of thegrout column often collapsed the PVC tubing. To counteractthis condition, oil-filled PVC tubes were
35、tried. The use of oilenabled this method to be used to depths of over 50 ft (15 m).As an alternative to this system, liquid-tight flexible steelconduit is used to replace the PVC pipe. This alternativeFIG. 3 Tape Extensometer with Vernier Readout and DeadweightFIG. 4 Tape Extensometer with Dial Gage
36、 and Tension SpringFIG. 5 Joint MetersD 4403 84 (2005)3system seems to work well and can be used in most applica-tions. Resin anchors fall in this category and are very success-ful.3.3.2 Wedge-Type AnchorsThese consist of a mechanicalanchor that has been widely used for short-term anchoringapplicati
37、ons in hard rock. Fig. 8 shows the two basic types ofwedge anchors: (1) the self-locking spring-loaded anchor, and(2) the mechanical-locking anchor. Self-locking anchors, whenused in areas subject to shock load vibrations caused byblasting or other construction disturbances, may tend to slip inthe d
38、rill holes or become more deeply-seated, causing thecenter wedge to move. Another disadvantage of the wedgeanchor is that no protection is offered, if using wires, to themeasuring wires in the drill hole against damage that might becaused by water or loose rock.3.3.3 Hydraulic AnchorsThese anchors h
39、ave proven to besuccessful in most types of rock and soil conditions. Fig. 9shows the two basic types of hydraulic anchors manufacturedfor use with extensometer systems: (1) the uncoiling Bourdontube anchor, and (2) the hydraulic piston of grappling hookanchor, which is limited to soft rock and soil
40、s. Both anchorshave the disadvantage of being rather costly. The Bourdon tubeanchor works well in most rock and soil conditions and thecomplete anchor system can be fabricated before installing it inthe drill hole. There have been other specialized anchorsystems developed, however, these systems hav
41、e proven to betoo costly and unsuccesful for most applications.3.4 Extensometer TransducersThese extensometers con-vert displacements occurring in in-situ materials between twoanchored points to mechanical movements that can be mea-sured with conventional measuring devices such as dial gages,LVDTs,
42、strain gages, and the like.3.4.1 Depth-Measuring InstrumentsA dial gage, or adepth micrometer are the simplest and most commonly usedmechanical measuring instruments. Used in conjunction withextensometers, they provide the cheapest and surest methods ofmaking accurate measurements. When using the di
43、al gage ordepth micrometer, the operator is required to take readings atthe instrument head, however, local readings may not bepractical or possible due to the instrument location or areaconditions.3.4.2 Electrical TransducersFor remote or continuousreadings, electrical transducers are used rather t
44、han dial gages.LVDTs are often used because of their accuracy, small size, andavailability. LVDTs require electrical readout equipment con-sisting of an a-c regulated voltage source and an accuratevoltmeter, such as a digital voltmeter or bridge circuit. The useof linear potentiometers or strain gag
45、es is often desirablebecause of the simplicity of the circuitry involved. Thedisadvantage of using linear potentiometers is their inherentlypoor linearity and resolution.3.4.3 When very accurate measurements are dictated bycertain excavations, for example, the determination of thetension arch zone a
46、round a tunnel opening, extensometerswhich can be calibrated in the field after installation shall beused. In all cases, the accuracy of extensometers, eitherdetermined through calibration or estimation, should be givenin addition to the sensitivity of the transducers. The strain-gaged cantilever ex
47、tensometer (shown in Fig. 10) has beenused successfully for many years. The strain-gaged cantileveroperates on the principles of the linear strain produced acrossa given area of a spring material when flexed. This type ofextensometer readout is normally used when rock movementsof 0.5 in. (12.5 mm) o
48、r less are expected. Strain gages producea linear change in resistance of 1 to 3% of their initialresistance, over their total measurement range. Because of thissmall change in resistance, it is absolutely necessary to provideextremely good electrical connections and cable insulationwhen using this
49、type of transducer. Standard strain-gagereadout equipment can be used with this type of extensometer,however, care must be taken to protect this equipment from thehostile environments found in most field applications. Vibrat-ing wire and sonic readouts are also reliable and are becomingmore common than strain-gage readouts. Provision shouldalways be made for mechanical readout capability.4. Procedure4.1 Preparatory Investigations:4.1.1 Select the location, orientation, length, and number ofanchors for each extensometer on the basis of a thoroughreview of both the constru
