SIST EN ISO 18674-2:2017

SLOVENSKI STANDARD
01-marec-2017
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Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation
- Part 2: Measurement of displacements along a line: Extensometers (ISO 18674-
2:2016)
Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 2:
Verschiebungsmessungen entlang einer Messlinie: Extensometer (ISO 18674-2:2016)
Reconnaissance et essais géotechniques - Mesures géotechniques - Partie 2: Mesure
de déplacement le long d‘une ligne par extensomètre (ISO 18674-2:2016)
Ta slovenski standard je istoveten z: EN ISO 18674-2:2016
ICS:
13.080.20 Fizikalne lastnosti tal Physical properties of soils
17.040.30 Merila Measuring instruments
93.020 Zemeljska dela. Izkopavanja. Earthworks. Excavations.
Gradnja temeljev. Dela pod Foundation construction.
zemljo Underground works
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18674-2
EUROPEAN STANDARD
NORME EUROPÉENNE
November 2016
EUROPÄISCHE NORM
ICS 13.080.20; 93.020
English Version
Geotechnical investigation and testing - Geotechnical
monitoring by field instrumentation - Part 2: Measurement
of displacements along a line: Extensometers (ISO 18674-
2:2016)
Reconnaissance et essais géotechniques - Mesures Geotechnische Erkundung und Untersuchung -
géotechniques - Partie 2: Mesure de déplacement le Geotechnische Messungen - Teil 2:
long d'une ligne par extensomètre (ISO 18674-2:2016) Verschiebungsmessungen entlang einer Messlinie:
Extensometer (ISO 18674-2:2016)
This European Standard was approved by CEN on 11 September 2016.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18674-2:2016 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
European foreword
This document (EN ISO 18674-2:2016) has been prepared by Technical Committee ISO/TC 182
“Geotechnics” in collaboration with Technical Committee CEN/TC 341 “Geotechnical Investigation and
Testing” the secretariat of which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by May 2017, and conflicting national standards shall be
withdrawn at the latest by May 2017.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18674-2:2016 has been approved by CEN as EN ISO 18674-2:2016 without any
modification.
INTERNATIONAL ISO
STANDARD 18674-2
First edition
2016-10-15
Geotechnical investigation and
testing — Geotechnical monitoring by
field instrumentation —
Part 2:
Measurement of displacements along
a line: Extensometers
Reconnaissance et essais géotechniques — Mesures géotechniques —
Partie 2: Mesure de déplacement le long d‘une ligne par extensomètre
Reference number
ISO 18674-2:2016(E)
©
ISO 2016
ISO 18674-2:2016(E)
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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Tel. +41 22 749 01 11
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copyright@iso.org
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ii © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 3
5 Instruments . 4
5.1 General . 4
5.2 In-place extensometer . 8
5.2.1 Measuring points . 8
5.2.2 Connecting elements . 8
5.2.3 Measuring head and read-out device . 9
5.3 Probe extensometer .10
5.3.1 Measuring points and guide tube .10
5.3.2 Probe .11
5.5 Tape extensometer (convergence tape) .12
5.6 Measuring range and accuracy .12
6 Installation and measuring procedures .13
6.1 Installation .13
6.1.1 Surface components . .13
6.1.2 Installation in boreholes and in fill .13
6.1.3 In-place extensometer .14
6.1.4 Probe extensometer .14
6.1.5 Tape extensometer .15
6.2 Carrying out the measurement .15
6.2.1 Instrumentation check and calibration .15
6.2.2 Measurement .15
7 Data processing and evaluation .15
8 Reporting .16
8.1 Installation report .16
8.2 Monitoring report .16
Annex A (normative) Measuring and evaluation procedure .17
Annex B (informative) Backfill materials .26
Annex C (informative) Geo-engineering applications .27
Annex D (informative) Measuring examples .28
Bibliography .46
ISO 18674-2:2016(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www.iso.org/iso/foreword.html.
The committee responsible for this document is ISO/TC 182, Geotechnics.
A list of all part in the ISO 18674 series, published under the general title Geotechnical investigation and
testing – Geotechnical monitoring by field instrumentation, can be found on the ISO website.
iv © ISO 2016 – All rights reserved

INTERNATIONAL STANDARD ISO 18674-2:2016(E)
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 2:
Measurement of displacements along a line:
Extensometers
1 Scope
This document specifies the measurement of displacements along a line by means of extensometers
carried out for geotechnical monitoring. General rules of performance monitoring of the ground, of
structures interacting with the ground, of geotechnical fills and of geotechnical works are presented in
ISO 18674-1.
If applied in conjunction with ISO 18674-3, this document allows the determination of displacements
acting in any direction.
This document is applicable to:
— monitoring the behaviour of soils, fills and rocks;
— checking geotechnical designs in connection with the Observational Design procedure;
— deriving geotechnical key parameters (e.g. from results of pile load tests or trial tunnelling);
— evaluating stability ahead of, during or after construction (e.g. stability of natural slopes, slope cuts,
embankments, excavation walls, foundations, dams, refuse dumps, tunnels).
NOTE This document fulfils the requirements for the performance monitoring of the ground, of structures
interacting with the ground and of geotechnical works by the means of extensometers as part of the geotechnical
investigation and testing in accordance with References [5] and [6].
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 18674-1:2015, Geotechnical investigation and testing — Geotechnical monitoring by field
instrumentation — Part 1: General rules
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18674-1 and the following apply.
3.1
extensometer
field instrument for monitoring changes of distance between two or more measuring points located
along a measuring line
Note 1 to entry: Monitoring of such changes allows the determination of displacements of measuring points
acting in the direction of the measuring line.
ISO 18674-2:2016(E)
Note 2 to entry: At a measuring point, the movements of the medium (e.g. soil, rock, concrete and steel structures)
being investigated are transferred to the measuring point by devices such as anchors, rings or bolts (see 5.1.6).
Note 3 to entry: In the ground, the measuring points are typically installed in boreholes. The measuring line then
coincides with the axis of the borehole.
3.2
in-place extensometer
permanently installed extensometer, essentially consisting of anchor(s), connecting element(s) and at
least one measuring head
Note 1 to entry: Each connecting element is affixed to an anchor and free to move along the measuring line.
Note 2 to entry: Measuring heads are commonly located at one end of the measuring line. When carrying out the
measurements, they function as reference measuring points.
Note 3 to entry: For in-place extensometers in boreholes, see Reference [7].
Note 4 to entry: See Figure 1.
3.3
rod extensometer
in-place extensometer where the connecting element is a rod
Note 1 to entry: Common rod materials are steel or fibreglass.
Note 2 to entry: See Figure 1 a).
3.4
wire extensometer
in-place extensometer where the connecting element is a wire
Note 1 to entry: See Figure 1 b).
3.5
single extensometer
in-place extensometer with one anchor only
Note 1 to entry: See Figure 1 b).
3.6
multiple-point extensometer
in-place extensometer with more than one anchor
Note 1 to entry: Up to six anchor points are common in geo-engineering practice.
Note 2 to entry: See Figure 1 a).
3.7
chain extensometer
in-place extensometer formed of a series of single extensometer elements
Note 1 to entry: See Figure 1 c).
3.8
probe extensometer
extensometer where the connecting element is a moveable unit
Note 1 to entry: Probe extensometers can be developed as single-point probe extensometer (3.9) or double-point
probe extensometer (3.10).
Note 2 to entry: See Figure 2.
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ISO 18674-2:2016(E)
3.9
single-point probe extensometer
extensometer, essentially consisting of a measuring probe and a guiding tube with measuring marks
and in which, at the measuring position, only one measuring mark interacts with the probe
Note 1 to entry: The connecting element is the unit consisting of a measuring cable and a probe. The measured
value is the distance between the measuring mark and the reference mark at the head of the guiding tube.
Note 2 to entry: Because of its design, function and usual geotechnical application, the single-point probe
extensometer is commonly designated as a “magnetic extensometer,” a “magnet settlement probe” or an
“inductance probe.”
Note 3 to entry: See Figure 2 a).
3.10
double-point probe extensometer
extensometer, essentially consisting of a measuring probe and a guiding tube with measuring marks
and in which, at the measuring position, two measuring marks interact with the probe
Note 1 to entry: The connecting element is the measuring probe. The measured value is the distance between the
two measuring marks which are in interaction with the probe.
Note 2 to entry: Because of its design and function, the double-point probe extensometer is commonly designated
as an “incremental extensometer” or a “sliding micrometer.”
Note 3 to entry: See Figure 2 b).
3.11
gauge length
L
nominal distance between the contact points of the double-point extensometer probe
Note 1 to entry: L is commonly 1,0 m.
Note 2 to entry: L is commonly verified in a calibration of the probe prior to the measurement.
3.12
tape extensometer
extensometer for distance measurements between two accessible measuring points by means of a
measuring tape, essentially consisting of a device for tensioning of the tape with a reproducible pulling
force, two end pieces for connecting the device to bolts (3.13) and of a read-out unit
Note 1 to entry: Traditionally, tape extensometers were used in tunnelling. By means of follow-up measurements,
the change of the distances of two tunnel wall measuring points (in tunnelling, termed “convergence”) is
determined. For this reason, tape extensometers are commonly designated as “convergence tapes.”
Note 2 to entry: See Figure 3.
3.13
convergence bolts
measuring bolts fitting to the type of tape extensometer used
4 Symbols
Symbol Name Unit
d depth of borehole m
d distance between measuring point i and measuring head m
i
F subscript for follow-up measurement —
h height of measuring head above sea level m
i number of a measuring point —
ISO 18674-2:2016(E)
Symbol Name Unit
K temperature correction term —
T
L gauge length of a double-point probe extensometer m
L length of the connecting element between measuring head and measuring point i m
i
l distance between measuring points m
l length of a measuring ring for probe extensometer m
M
n total number of measuring points along a measuring line —
P pulling force of wire extensometer kN
R subscript for reference measurement —
s displacement reading m
T temperature °C
t elapsed time s
u, v, w displacement component in x-, y-, z-direction, respectively m
w displacement component of measuring point i in z-direction relative to the measuring head m
i rel
w absolute displacement component of the measuring head in z-direction m
w absolute displacement component of measuring point i in z-direction m
i
Δw relative displacement between adjacent measuring points i and i-1 in z-direction m
i
x, y, z local coordinates of measuring points on a guide tube or in a borehole m
−1
α coefficient of linear thermal expansion K
T
ε strain in direction of the z coordinate —
z
5 Instruments
5.1 General
5.1.1 The following types of extensometer in-place, probe and tape should be distinguished from each
other (see Table 1 and Figures 1 to 3).
Table 1 — Extensometer types
Extensometer
Automatic data
Feature
acquisition
No. Type Subtype
Single-point/multiple-point in-place
all instrument components are per-
in-place
extensometer
1 manently installed in the ground or possible
(see 5.2)
at accessible surfaces
rod/wire extensometer
probe single-point/double-point probe
(see 5.3) extensometer
measuring unit sequentially moved
not common
into measuring positions
tape
3 steel/wire tape extensometer
(see 5.4)
5.1.2 Changes of the distances between measuring points shall be monitored by comparison of the
measured values with those of the reference measurement. Displacements of the measuring points along
the measuring line shall be deduced in accordance with Annex A.
5.1.3 An increase of the distance between two measuring points (=extension) shall be assigned a
positive value.
5.1.4 The point onto which the extensometer measurements are related shall be denoted the
“reference point.”
4 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
5.1.5 For absolute measurements, the coordinates of the reference point shall be independently
determined or assumed and verified as fixed.
NOTE If the reference point is assumed to be at the deepest anchor, surveying of the measuring head can
serve as a check.
5.1.6 Extensometer measuring points shall be marked by devices such as anchors, rings or bolts. The
measuring points of these devices shall be specified as follows:
— for anchors, the centre of an anchor;
— for rings, the centre of a ring;
— for bolts, the centre of a contact butt (for screwed couplings) or the centre of an eye (for eye/hook
couplings).
5.1.7 It shall be secured that the device, marking a measuring point, is set in such a way that it is
solidly connected to the medium so that any movement of the medium at the measuring point is fully
transferred to the device.
5.1.8 Instruments shall not significantly affect the conditions of the medium under investigation
and, in turn, shall not be significantly affected in their functionality by the medium (in accordance with
ISO 18674-1:2015, 5.1 and 5.2).
ISO 18674-2:2016(E)
a) Rod extensometer b) Wire extensometer c) Chain extensometer
Key
1 anchor
1 anchors 1 to 3
1.3
2 connecting element (wire)
2 connecting elements 1 to 3
1.3
3 measuring head
3 local measuring heads 1 to 3
1.3
4 borehole wall
5 read-out device
6 pulling device
P tension force
EXAMPLE 1 For subfigure a), triple-point rod extensometer with electrical displacement transducers.
EXAMPLE 2 For subfigure b), single-point wire extensometer with dial gauge read-out.
EXAMPLE 3 For subfigure c), triple-point chain extensometer with electrical displacement transducers.
Figure 1 — Examples of in-place extensometer types
6 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
a) Single-point probe extensometer b) Double-point probe extensometer
Key
1 measuring tube 6 probe (in measuring position with rings No. 2 and 3)
2 anchor plates 1 to 3 (with external measuring rings) 7 setting rods (or pulling rope)
1.3
2 measuring rings 1 to 5 8 read-out unit
1.5
3 probe (in measuring position with anchor Plate No.2) 9 backfill
4 measuring tape 10 borehole wall
5 measuring head with reference mark
EXAMPLE 1 For subfigure a), magnetic probe extensometer in telescopic tubing.
EXAMPLE 2 For subfigure b), sliding micrometer.
Figure 2 — Examples of probe extensometer types
ISO 18674-2:2016(E)
Key
1 convergence bolt
2 measuring tape (or measuring wire)
3 device for tensioning of tape (or wire) and read-out
4 coupling element
Figure 3 — Principal sketch of a tape extensometer
5.2 In-place extensometer
5.2.1 Measuring points
The measuring points should be similar in their function to those common in rock nailing and
anchoring works.
EXAMPLES Wedge, straddle packer, spring-activated clamp, cement- or resin-grouted borehole packer, anchor
grouting with non-shrinking cement.
NOTE The movement of a measuring point is also transferred to the attached connecting element.
5.2.2 Connecting elements
5.2.2.1 For rod extensometers, a string of interconnected steel rods or a continuous glass fibre-
reinforced resin rod should be used, and for wire extensometers, steel wires should be used.
5.2.2.2 The selection of the material and that of the cross sectional area of the connecting elements
should be guided by the measuring task, environmental conditions, measuring accuracy and the length
of the measuring section (see Table 2).
5.2.2.3 If a connecting element can be disconnected temporarily from its fixing device at the measuring
point, it shall be established that the coupling tolerance does not exceed the intended measuring accuracy
of the system.
EXAMPLE Screw couplings or bayonet locks of the connecting element at the anchors.
NOTE Movements across the borehole axis or closure of the borehole can block the connecting element
and can affect the functionality of the extensometer. The functionality of an extensometer can be checked by
intermittently uncoupling a connecting element.
5.2.2.4 The coefficient of thermal expansion of the connecting elements shall be specified. Temperature
variations within the system should be taken into account.
NOTE Temperature-induced changes of the length of the connecting elements can have a substantial
influence on the accuracy of an extensometer system. The measurement of thermal gradients by a series of
temperature sensors along the extensometer can be useful in developing a suitable correction for temperature
changes (see measuring example in D.2).
8 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
5.2.2.5 The free movement of the connecting elements against each other and the backfill shall be
ensured by placing the connecting elements inside protective tubes.
EXAMPLE See Figure 4.
5.2.2.6 Friction between connecting elements and protection shall not affect the measurement.
5.2.2.7 For wire extensometers, the connecting elements shall be tensioned prior to the measurement.
A constant tensioning force shall be applied. The calibration of the read-out device shall have been made
with regard to the specified tension force. In the case that the tensioning force is changed, the measured
values shall be corrected accordingly.
Key
1 anchor 7 grout
2 connecting element attached to (1) 8 borehole backfill
2 connecting element to adjacent anchor (not shown)
3 coupling anchor/connecting element a borehole diameter
4 protective tube b filled borehole section
5 rubber sleeve c grouted section of anchor ≥30 cm
6 clamp
Figure 4 — Example of a cement-grouted anchor of a multiple-point borehole rod extensometer
with the connecting element attached to the anchor and a passing connecting element
5.2.3 Measuring head and read-out device
5.2.3.1 The connecting elements terminate at the measuring head. The axial distance between
the measuring butt of the measuring head and the measuring butt of the connecting element shall be
measured.
EXAMPLE See Figures 1 and 5.
NOTE Common distance meters are mechanical dial gauges, electric displacement transducers, vibrating
wire displacement transducers and topographic levels.
5.2.3.2 In certain applications, it may be necessary to shorten, or to extend, the connecting elements
(including the protection tubes) in the course of the monitoring project. If such a situation is likely
to occur, provisions should be made in the monitoring plan and the extensometer system designed
accordingly.
ISO 18674-2:2016(E)
EXAMPLE Shortening, respectively extension, of the connection element is required when the measuring
range of the distance meter is exceeded.
Key
1 head unit (head plate or recessed head) 5 protection tube
2 measuring butt at connecting element 6 connecting element (rod; wire)
3 measuring butt of head plate 7 fixation of head unit (dowelling; cementation)
4 displacement sensor
Figure 5 — Types of in-place extensometer measuring head layouts (schematic)
5.3 Probe extensometer
5.3.1 Measuring points and guide tube
5.3.1.1 Each measuring point should be marked by a ring which is embedded in, or attached to, the
medium. The ring may be a part of the guide tube [see Figure 6 b)].
5.3.1.2 The guide tube shall not affect the movement of the measuring rings.
5.3.1.3 For single-point probe extensometers [see Figure 2 a) and PrEx1 in Table 2], the measuring
points can be set at any location along the measuring line.
5.3.1.4 For double-point probe extensometers [see Figure 2 b) and PrEx 2-1 and PrEx 2-2 in Table 2],
measuring rings shall be used which are compatible with the type of probe used. The measuring points
shall be equally spaced according to the gauge length (see 3.11), with a tolerance depending on the
measuring range of the probe (see Table 2).
5.3.1.5 Hydraulic pressures or ground pressures which may develop during the installation and
throughout the measuring period shall be considered in the selection of the guide tube.
10 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
5.3.1.6 Ground excavation procedures may require temporary or permanent cutting or interruption
of guide tubes. It is permissible to continue the probe extensometer survey in the remaining parts of the
guide tubes.
a) Continuous guide tube, inductive b) Discontinuous guide tube, mechanical
measurement measurement
Key
1 guide tube
2 precision measuring ring of length l
M
3 setting screw
4 backfill (mortar)
5 mechanical high-precision coupling
Figure 6 — Possible measuring ring fixations for probe extensometers
5.3.2 Probe
5.3.2.1 The extensometer device shall allow a controlled positioning of the probe in the measuring
points. Reading of the measured value shall be made with the probe at rest.
5.3.2.2 At a measuring location of a single-point probe extensometer, the probe shall uniquely interact
with one measuring point. The measured value shall be the distance between the measuring point and
the reference mark of the measuring head.
NOTE A tension-resistant graduated measuring cable is commonly used for the measurement of that
distance.
5.3.2.3 At a measuring location of a double-point probe extensometer, the probe shall uniquely interact
with two adjacent measuring points. The measured value should be the difference between the base
length L of the probe and the distance between the two measuring points.
NOTE For a measuring line, the number of double-point probe extensometer measuring points is n-1, where
n is the total number of measuring rings installed.
ISO 18674-2:2016(E)
5.5 Tape extensometer (convergence tape)
NOTE Convergence measurements can also be performed by optical geodetic methods.
5.4.1 For tape extensometer measurements, the following components are required (see Figure 3):
— convergence bolts;
— tensioning and read-out devices (convergence device);
— measuring tape or measuring wire;
— coupling elements to the convergence bolts.
5.4.2 Convergence bolts of the measuring systems CV 1 and CV 2 (see Table 3) shall be equipped with
a contact butt for the coupling elements. The contact butt should consist of a durable and corrosion-
resistant material (e.g. stainless steel or galvanized steel). The contact butts and coupling elements
should be protected from dirt and against damage.
5.4.3 Convergence bolts may be equipped with survey targets to allow geodetic measurements.
5.4.4 The convergence measuring system shall be equipped with a device for assuring a constant and
reproducible pulling force of the measuring tape or wire.
5.4.5 The coefficient of linear thermal expansion of the measuring tape or measuring wire shall be
specified by the instrumentation manufacturer and documented (see 8.1).
5.6 Measuring range and accuracy
Measuring range and accuracy of extensometers depend, amongst other factors, on the measuring
length. Table 2 provides information which should be taken into consideration when selecting
extensometers.
NOTE Annex C provides an overview of the various extensometer types in some common geo-engineering
applications. Examples of the various types of extensometers and typical applications are presented in Annex D.
12 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
Table 2 — Types, common measuring lengths, ranges and accuracies of extensometers
Extensometer type In-place Probe extensometer Tape extensometer
extensometer (convergence tape)
2-point (L = 1,0 m) Cardan joint with Hook and
measuring butt eye
Technical feature Rod Wire 1-point
Non- Mechan- Invar Steel tape
mechan- ical wire (punched or
ical graduated)
Designation (abbreviated) Ex-rod Ex-wire PrEx 1 PrEx 2-1 PrEx 2-2 CV 1 CV 2 CV 3
common (extreme)
1 lengths of measuring 30 (300) 10 (300) 30 (200) 30 (150) 30 (150) 15 (100) 15 (20) 15 (30)
lines [m]
common measuring ±1 000 ±10
range of or ± 10 % ±20 per to ± 50
a a
2 ±50 ±250 ±50 ±20 ±50
extensometer of tube metre per
sensor [mm] length metre
typical accuracy of
installed
±0,05
b
extensometer [mm] ±0,2 ±2 ±5 ±0,3 ±0,05 ±0,1 ±0,5
to ± 0,5
over a measuring 30 10 30 30 15 15 15
length
of . [m]
a
Measuring range, adjustable to higher values.
b
For relative displacements.
6 Installation and measuring procedures
6.1 Installation
6.1.1 Surface components
Extensometer components mounted to accessible surfaces shall be protected from direct sunlight,
aggressive environment, construction works, fly rocks from blasting and vandalism.
NOTE Protection can be achieved by sun-shade covers, lockable protective covers, recessed measuring
heads or covered extensometer shafts.
6.1.2 Installation in boreholes and in fill
6.1.2.1 Knowledge of the ground profile is required for the installation of extensometers in the ground.
NOTE Drilling of extensometer boreholes with sample recovery, providing direct information on the ground
conditions, allows a better placement of the anchors and enables a better interpretation of the measuring results.
6.1.2.2 In soft ground and in fill, a single-point extensometer may be installed by sounding techniques
such as CPT or dynamic probing (in accordance with ISO 22476-1 and ISO 22476-2).
6.1.2.3 Extensometer boreholes should be back-filled with a suitable material.
NOTE 1 One of the purposes of backfilling is to re-establish separations between different aquifers in
accordance with ISO 22475-1:2006, 5.5.4.
NOTE 2 Common backfill materials are low-strength cement-based mortar and cement-bentonite suspensions
(see Annex B).
ISO 18674-2:2016(E)
6.1.2.4 The composition of the backfill material shall be documented and its properties considered in
the relation to the surrounding medium.
6.1.2.5 The axial stiffness of an extensometer, including its protective tubes and cured backfill, shall
not exceed the stiffness of the medium.
NOTE 1 This requirement is particularly relevant for settlements in soil, fill and soft rock.
NOTE 2 The following mitigating measures are considered acceptable:
a) for in-place borehole extensometers, selection of low-strength backfill (see Annex B) in combination with
spot fixation of the anchors such as mechanical clamps, bladder-type hydraulic anchors or cement-activated
packer anchors;
b) for embedment extensometers, selection and installation of telescoping tubing or corrugated guide tubes;
c) for probe extensometers, selection and installation of telescoping tubing, corrugated guide tubes and/or
selection of low-strength backfill.
6.1.2.6 Sufficient time should be allowed for the backfill to cure so that anchor fixity and stability is
observed prior to the zero and baseline measurements being undertaken in accordance with ISO 18674-
1:2015, Figure 1.
6.1.3 In-place extensometer
When selecting and installing in-place extensometers, attention shall be paid to the following.
a) Friction between connecting elements and protection tubes can be reduced by using spacers.
b) The extensometer assembly should be installed in the borehole without twisting the rods.
c) The protective tubing shall withstand the pressures from the ground, groundwater, grouting and
back-filling operations.
NOTE For very long vertical extensometers (e.g. 100 m), common installation practice is sequential back-
filling over a limited height. The next back-filling step is then carried out after curing of the previous stage.
d) For multiple-point extensometers, the connecting elements shall be uniquely identified at the
measuring head.
e) Adjustments of the length of the connecting elements must be recorded and considered in the
evaluation procedure.
NOTE Adjustments are common if the measuring range is likely to be exceeded.
6.1.4 Probe extensometer
When selecting and installing the measuring tubes of the probe extensometer, attention shall be paid to
the following.
a) Possibility that the measuring tube may float in a water-filled borehole.
NOTE Floating of the measuring tube can be avoided by measures, or a combination of measures, such as
filling the tube with water, loading the bottom by dead weight and initial cement grouting the toe of the tube.
b) Possibility of excessive external pressure, which may lead to a collapse of the measuring tube.
c) Difficulty of recovering the drilling casing without undue influence on the measuring tube.
NOTE An installation rod inside the measuring tube can assist to keep the tube stretched during the
recovery of the drilling casing.
14 © ISO 2016 – All rights reserved

ISO 18674-2:2016(E)
d) For double-point probe extensometer systems, the measuring rings are to be firmly and durably
connected to the medium. For that purpose, the entire annulus between the measuring tube and
the borehole wall can be back-filled with a suitable material (see 6.1.2.3 and 6.1.2.4).
e) In cast-in-place concrete piles, the measuring tube shall be fixed to the reinforcement cage prior
to the placement of the cage and the concrete. Special attention shall be paid during lifting and
lowering the cage to prevent any damage to the tubes.
NOTE High-precision double-point extensometers of the Type PrEx 2–2 (see Table 2) are often installed in
concrete piles to assist in determining the distribution of skin friction along the pile, e.g. in pile load testing.
6.1.5 Tape extensometer
When installing the measuring bolts of a tape extensometer, attention shall be paid to the following.
a) The bolts should be placed in holes and fixed with cement- or resin-based material or by mechanical
expansion anchors.
b) The bolts may be temporarily secured at the steel mesh or, if permissible, welded onto steel arches
or reinforcement bars.
6.2 Carrying out the measurement
6.2.1 Instrumentation check and calibration
6.2.1.1 For general function checks and calibrations, refer to ISO 18674-1:2015, 5.6.
6.2.1.2 For in-place extensometers, portable measuring instruments (such as dial gauges and read-out
devices) and electric displacement transducers (where accessible) shall be regularly calibrated. If not
specified by the manufacturer, the maximal interval shall be two years.
6.2.1.3 Single-point probe extensometers shall be function checked at least once a year. It shall be
confirmed that the measuring cable does not change its length (stretching or shrinking) in course of the
monitoring project.
6.2.1.4 Double-point probe extensometers shall be calibrated at least once a year. Before and after
each series of measurements, they shall be checked against a known reference. Differences between the
reference and the measurement shall be recorded and accounted for in the data processing.
6.2.1.5 Convergence instruments shall be checked against a known reference before and after each
series of measurements. Differences between the reference and the measurement shall be recorded and
accounted for in the data processing.
6.2.2 Measuremen
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