EN ISO 18674-7:2025
(Main)Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 7: Measurement of strains: Strain gauges (ISO 18674-7:2025)
Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 7: Measurement of strains: Strain gauges (ISO 18674-7:2025)
This document specifies the measurement of strain by means of strain gauges and strainmeters 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.
This document is applicable to:
— performance monitoring of
— 1-D structural members such as piles, struts, props and anchor tendons;
— 2-D structural members such as foundation plates, sheet piles, diaphragm walls, retaining walls and shotcrete/concrete tunnel linings;
— 3-D structural members such as gravity dams, earth- and rock-fill dams, embankments and reinforced soil structures;
— checking geotechnical designs and adjustment of construction in connection with the observational design procedure;
— evaluating stability during or after construction.
With the aid of a stress-strain relationship of the material, strain data can be converted into stress and/or forces (for 1-D members; see ISO 18674-8) or stresses (for 2-D and 3-D members, see ISO 18674-5).
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 strain measuring instruments as part of the geotechnical investigation and testing in accordance with References [1] and [2].
Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 7: Dehnungsmesszellen (ISO 18674‑7:2025)
Dieses Dokument legt die Dehnungsmessung mit Hilfe von Dehnungsmessstreifen und Dehnungsmessern für die geotechnische Überwachung fest. Allgemeine Regeln für die messtechnische Überwachung von Baugrund, von Bauwerken, die mit dem Baugrund in Wechselwirkung stehen, von geotechnischen Auffüllungen und von geotechnischen Arbeiten sind in ISO 18674 1 festgelegt.
Dieses Dokument ist anwendbar für:
die messtechnische Überwachung von
tragenden 1-D-Bauteilen wie Pfähle, Streben, Stützen und Ankerzugglieder;
tragenden 2-D-Bauteilen wie Fundamentplatten, Spundwänden, Schlitzwände, Stützwände und Tunnelauskleidungen aus Spritzbeton/Beton;
tragenden 3-D-Bauteilen wie Schwergewichtsmauern, Erd- und Felsschüttdämme, Dämme und bewehrte Schüttkörper;
Überprüfung geotechnischer Entwürfe und Konstruktionsanpassung in Verbindung mit der Beobachtungsmethode bei der Planung;
Bewertung der Stabilität während oder nach dem Bau.
Mit Hilfe einer Spannungs-Dehnungs-Beziehung des Baustoffs können Dehnungsdaten in Spannungen und/oder Kräfte (für 1-D-Bauteile; siehe ISO 18674 8) oder Spannungen (für 2-D und 3-D-Bauteile, siehe ISO 18674 5) umgewandelt werden.
ANMERKUNG Dieses Dokument erfüllt die Anforderungen an die messtechnische Überwachung von Baugrund, von Bauwerken, die mit dem Baugrund in Wechselwirkung stehen, und von geotechnischen Arbeiten mittels Dehnungsmessgeräten als Teil geotechnischer Erkundung und Untersuchung nach Literaturhinweis [1] und Literaturhinweis [2].
Reconnaissance et essais géotechniques - Surveillance géotechnique par instrumentation in situ - Partie 7: Mesure des déformations : jauges de déformation (ISO 18674-7:2025)
Geotehnično preiskovanje in preskušanje - Geotehnične meritve - 7. del: Merjenje deformacij: Merilniki deformacij (ISO/FDIS 18674-7:2025)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN ISO 18674-7:2024
01-april-2024
Geotehnično preiskovanje in preskušanje - Geotehnične meritve - 7. del: Merjenje
napetosti: Merilniki napetosti (ISO/DIS 18674-7:2024)
Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation
- Part 7: Measurement of strains: Strain gauges (ISO/DIS 18674-7:2024)
Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 7:
Dehnungsmesszellen (ISO/DIS 18674-7:2024)
Reconnaissance et essais géotechniques - Surveillance géotechnique par
instrumentation in situ - Partie 7: Mesure des déformations : jauges de déformation
(ISO/DIS 18674-7:2024)
Ta slovenski standard je istoveten z: prEN ISO 18674-7
ICS:
93.020 Zemeljska dela. Izkopavanja. Earthworks. Excavations.
Gradnja temeljev. Dela pod Foundation construction.
zemljo Underground works
oSIST prEN ISO 18674-7:2024 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN ISO 18674-7:2024
oSIST prEN ISO 18674-7:2024
DRAFT INTERNATIONAL STANDARD
ISO/DIS 18674-7
ISO/TC 182 Secretariat: BSI
Voting begins on: Voting terminates on:
2024-01-17 2024-04-10
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 7:
Measurement of strains: Strain gauges
ICS: 93.020; 13.080.20
This document is circulated as received from the committee secretariat.
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
ISO/CEN PARALLEL PROCESSING
THEREFORE SUBJECT TO CHANGE AND MAY
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WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 18674-7:2024(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
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NOTIFICATION OF ANY RELEVANT PATENT
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PROVIDE SUPPORTING DOCUMENTATION. © ISO 2024
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2024(E)
DRAFT INTERNATIONAL STANDARD
ISO/DIS 18674-7
ISO/TC 182 Secretariat: BSI
Voting begins on: Voting terminates on:
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 7:
Measurement of strains: Strain gauges
ICS: 93.020; 13.080.20
This document is circulated as received from the committee secretariat.
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
© ISO 2024
ISO/CEN PARALLEL PROCESSING
THEREFORE SUBJECT TO CHANGE AND MAY
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
NOT BE REFERRED TO AS AN INTERNATIONAL
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NATIONAL REGULATIONS.
Website: www.iso.org ISO/DIS 18674-7:2023(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
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NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
ii
PROVIDE SUPPORTING DOCUMENTATION. © ISO 2023
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 3
5 Instruments. 4
5.1 General . 4
5.2 Strain gauges . 6
5.2.1 Surface-mounted strain gauges . 6
5.2.2 Embedded strain gauges . 10
5.3 Strainmeters . 11
5.4 Instruments for specific applications . 11
5.4.1 Monitoring of 1-D structural members . 11
5.4.2 Monitoring of 2-D structural members . 13
5.4.3 Monitoring of 3-D structural members . 15
6 Installation and measuring procedure .17
6.1 Installation . 17
6.1.1 Installation of strain gauges . 17
6.1.2 Installation of strainmeters . 20
6.2 Measuring procedure . 20
6.2.1 Instrumentation check and calibration . 20
6.2.2 Measurement . 20
7 Data processing and evaluation .20
8 Reporting .20
8.1 Installation report . 20
8.2 Monitoring report . 20
Annex A (normative) Data processing and evaluation.21
Annex B (informative) Distributed Fibre Optic Strain Sensing (DSS) .27
Annex C (informative) Temperature effects on strain measurements .33
Annex D (informative) Geo-technical applications.35
Annex E (informative) Measuring examples .37
Bibliography .48
iii
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(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/TC182, Geotechnics.
A list of all parts 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
oSIST prEN ISO 18674-7:2024
DRAFT INTERNATIONAL STANDARD ISO/DIS 18674-7:2023(E)
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 7:
Measurement of strains: Strain gauges
1 Scope
This document specifies the measurement of strain by means of strain gauges and strainmeters
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 18674-1:2015.
This document is applicable to:
— performance monitoring of
— 1-D structural members such as piles, struts, props and anchor tendons;
— 2-D structural members such as foundation plates, sheet piles, diaphragm walls, retaining walls
and shotcrete/concrete tunnel linings;
— 3-D structural members such as gravity dams, earth- and rock-fill dams, embankments and
reinforced soil structures
— checking geotechnical designs and adjustment of construction in connection with the Observational
Design procedure;
— evaluating stability during or after construction.
With the aid of a stress-strain relationship of the material, strain data may be converted into stress
and/or forces (for 1-D members; see ISO 18674-8:2022) or stresses (for 2-D and 3-D members, see
ISO 18674-5:2019).
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 strain measuring instruments as part of
the geotechnical investigation and testing in accordance with References [1] and [2].
2 Normative references
The following documents, in whole or in parts, 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.
EN ISO 18674-1:2015, Geotechnical investigation and testing – Geotechnical monitoring by field
instrumentation – Part 1: General rules
ISO 18674-2:2016, Geotechnical investigation and testing — Geotechnical monitoring by field
instrumentation — Part 2: Measurement of displacements along a line: Extensometers
ISO 18674-3:2017, Geotechnical investigation and testing — Geotechnical monitoring by field
instrumentation — Part 3: Measurement of displacements across a line: Inclinometers
ISO 18674-4:2020, Geotechnical investigation and testing — Geotechnical monitoring by field
instrumentation — Part 4: Measurement of pore water pressure: Piezometers
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
ISO 18674-5:2019, Geotechnical investigation and testing — Geotechnical monitoring by field
instrumentation — Part 5: Stress change measurements by total pressure cells (TPC)
ISO 18674-8:2022, Geotechnical investigation and testing – Geotechnical monitoring by field
instrumentation – Part 8: Measurements of loads: Load Cells
3 Terms and definitions
For the purposes of this document the terms and definitions given in ISO 18674-1:2015 and the following
apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
strain gauge
field instrument for measuring strain in structural members
Note 1 to entry: The strain is sensed over the full length of the gauge, commonly by a vibrating wire sensor, an
electrical resistance strain gauge sensor or an FBG sensor (Fibre Bragg Gratings with optical sensing).
Note 2 to entry: Typical configurations are strain gauges mounted to a surface of a steel member (see 3.3 and
Figures 1a,1b, 3a,3b), strain gauges embedded in concrete (see 3.4 and Figure 1c) or FBG integrated into the
structure or fixed onto the surface (see Figure 3b).
Note 3 to entry: A series of FBG sensors with a single lead cable is named FBG array (see Figure 4).
Note 4 to entry: For mechanical strain gauges, see Reference [3].
Note 5 to entry: Distributed fibre optic strain measurements (DFOS) are not subject to this standard, as this new
technology still is under intensive development and change. Usage principles and examples for DFOS are given in
the Annex B (informative)
3.2
strainmeter
strain gauge for measuring strain by means of a displacement measurement
Note 1 to entry: The strain is sensed over the defined gauge length of the strainmeter (see Figure 2)
Note 2 to entry: An extensometer with a defined gauge length, e.g. a mobile extensometer (see ISO 18674-2:2016)
has the function of a strainmeter
Note 3 to entry: A typical configuration is a continuous chain of strainmeters embedded in fill, soil or concrete.
Note 4 to entry: Alternative terms for a strainmeter used in practice are “fill extensometer”, “soil strainmeter”,
“soil extensometer”, “embankment extensometer” or “linear continuous extensometer“
Note 5 to entry: The term strainmeter is sometimes (incorrectly) used for specific strain gauge sensors, e.g. rebar
strainmeter
3.3
surface-mounted strain gauge
strain gauge designed for attachment at the surface of a structural member
Note 1 to entry: There are different types of instruments for surface-mounting: spot-weldable, arc welded and
adhesive bonded strain gauges
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
3.4
embedment strain gauge
strain gauge for the embedment in a medium
Note 1 to entry: Typically, the medium is mortar, grout, reinforced concrete, shotcrete or mass concrete
Note 2 to entry: see Figure 1c.
3.5
instrumented reinforcement bar
Piece of reinforcement bar into which a strain gauge is integrated.
Note 1 to entry: when installed alongside the structural reinforcement it is commonly known as a sister bar
Note 2 to entry: when installed as part of the structural reinforcement it is commonly known as a rebar strain
meter
Note 3 to entry: sister bars and rebars measure the same parameter, overall strain in a reinforced concrete
element. Difference in using sister bars or rebars is that when using sister bars the overall steel area in the
structural element increases slightly leading to a reduction of the actual strain at the measuring section.
Note 4 to entry: see Figure 5.
3.6
gauge length
initial length over which the strain is measured by the strain gauge or initial length over which the
displacement is measured by a strainmeter
Note 1 to entry: For vibrating wire strain gauges and strainmeters the length is defined by the mounting blocks /
end plates or anchors
Note 2 to entry: For a FBG strain gauge, depending on the mounting, the gauge length is either the length of the
grating or the distance between the mounting blocks / end plates, anchors or spots of adhesive (see Figures 3 and
4)
Note 3 to entry: For instrumented reinforcement bars that are installed alongside the structural reinforcement,
the gauge length is the length between the anchoring zone of the bar, which is difficult to define precisely.
4 Symbols
Symbol Name Unit
A area m
E Young’s modulus Pa
F normal force N
L gauge length m
n number of instruments -
S shear force N
s spacing between strain sensor and axis m
T temperature °C
-1
α coefficient of linear thermal expansion K
T
ε normal strain -
µε micro strain µm/m
σ normal stress Pa
2a height of I-Beam m
2b width of I-Beam m
c distance to XX-axis m
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Symbol Name Unit
D Diameter m
d Distance to YY-axis m
t Thickness m
XX x-axis -
YY y-axis -
Subscripts
ax axial in relation to compression (sign +) / extension (sign -) of a 1-D member -
ax’ axial in relation to bending of a 1-D member -
corr corrected value -
F follow-up value -
G gauge instrument -
i measuring point i -
R reference value -
T temperature, thermal -
5 Instruments
5.1 General
5.1.1 Distinctions should be made between
(a) the type of the strain measuring instrument (strain gauge type versus strainmeter), and
(b) the location of the measuring point (at the surface of a structural member versus embedded in a
medium).
(c) the measurement principle of the instrument (e.g. VW, electrical resistance or FBG)
NOTE See Table 1 and Figures 1 to 5.
Table 1 — Types of strain gauges and strainmeters in geotechnical monitoring
Strain measuring instru-
Location of meas- Member / Common gauge Common
ment and common configu- Section
uring point medium length [mm] sensor
ration
(1)
steel 5 – 150 vibrating
at the surface of a
surface-mounted strain gauge 5.2.1
wire (VW) or
structural member (2)
concrete 50 – 350
electrical re-
sistance strain
(3)
embedment strain gauge concrete 50 –500 5.2.2
gauges, FBG
inside medium
Displacement
(4)
strainmeter fill 1 000 – 3 000 5.3
transducer
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
EXAMPLES (1) reinforcing bar, steel pile, sheet pile, strut
(2) pillars, girder, beam
(3) concrete pile, diaphragm wall, gravity dam, shotcrete tunnel lining
(4) earth dam, rock-fill dam, embankment
5.1.2 In line with the geotechnical engineering sign convention, compressive strain should be taken
as positive.
NOTE 1 Within the ISO 18674 series, that sign convention is adopted in Part 4 (piezometers), Part 5 (total
pressure cells) and Part 8 (load cells), however not in Part 2 (extensometers). The latter adopts the sign
convention established in geodesy.
NOTE 2 In ISO 18674-1:2015, no unified rules were set on the sign convention for geotechnical monitoring
instrumentations. In Clause 5.1.2 it is stated in general terms that “the sign conventions and units shall be clearly
stated and adhered to”.
5.1.3 The instrument shall be selected such that it has negligible influence on the elastic stiffness of
the monitored structure.
5.1.4 The measuring range of the instrument shall be selected according to the expected strain range
within the project. The measuring range of the instrument shall be 25 % higher than the expected
strain in the member to measure.
NOTE Many types of vibrating wire strain gauges and strainmeters allow the adjusting of the range starting
point towards tension or compression.
5.1.5 If the stiffness of the medium is changing in time, e.g. in course of curing or consolidation of the
medium, an instrument should be selected which is designed for a low stiffness in the strain measuring
direction.
5.1.6 For an instrument installed in respectively on matrix or composite material, sturdiness and
the gauge length of the strain instrument shall be selected considering the size of the aggregates in the
medium. If possible, the ratio of the gauge length to the largest aggregate should be more than 5 or 6.
5.1.7 Strain instruments shall not be located at or near the ends of a structural member or connection
points with other structural members in order to measure homogenous strain. A distance of at least
three times the width, respectively the diameter, of the member can be considered as sufficient.
5.1.8 The temperature shall be monitored at the measurement point or at a representative location.
NOTE Commercially available vibrating wire strain gauge and FBG sensors have integrated temperature
sensors.
5.1.9 Temperature effects on the instrument as well as on the structure itself shall be taken into
account when reporting and analysing the data.
NOTE 1 Temperature effects on the stresses in the structural members is often large and insufficiently known,
see Annex C
NOTE 2 The use of strain gauges compensated for specific materials (steel, aluminium, concrete) avoids the
need to correct the measured strain to obtain the structural strain and calculate the stress.
5.1.10 When FBG sensors are used in series, FBG shall be manufactured with different Bragg
wavelengths.
NOTE The number of FBG that can be in series will depend the FBG interrogator as well as the amount of
strain to be measured.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
5.2 Strain gauges
5.2.1 Surface-mounted strain gauges
5.2.1.1 The fixing and the type of strain gauge should be selected depending on the surface onto
which it is to be mounted.
NOTE 1 Spot-welded gauges (see Figure 1a) commonly are applied to rebars, arc-welded gauges (see Figure 1b)
to larger steel members such as struts, steel sets or sheet piles.
NOTE 2 The advantages of the spot-welded gauge are its small size and minimum errors that result from
bending of the member due to the close proximity of the strain gauge to the surface of the member.
NOTE 3 If adequate space is available on a steel surface, an arc-welded version is commonly preferred, see
Reference [4].
NOTE 4 Adhesive bonding is commonly applied on compound material or for FBG sensors on steel. FBG strain
arrays are sometimes bonded into pre-manufactured grooves of steel members for better protection.
NOTE 5 Strain gauges based on the electrical resistance principle (foils sensors) are commonly applied with
adhesive bonding to a surface.
5.2.1.2 When arc-welding is applied, a dummy rod should be used as a spacer whilst the mounting
blocks are welded (see 1 in Figure 1b). After welding, the dummy rod can be replaced by the strain
gauge sensor (see 2,3 and 4 in Figure 1b).
NOTE This procedure is carried out to shield the sensitive strain sensor from arc-welding effects.
5.2.1.3 When mounting blocks are used, these can have set screws acting in different directions.
NOTE 1 See Figures 1b and 12.
NOTE 2 For examples with mounting blocks see Figures 3c, 3d and Figure 4a.
5.2.1.4 For gauge length installation of FBG strain arrays and FBG strain gauges, the cable / sensor
shall be pre-tensioned. The pretension level shall consider the strain to be measured.
a) spot-weldable strain gauge
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
b) arc-weldable strain gauge
c) embedment strain gauge
Key
1 anchor/flange, mounting block/tab 2 plucking coil
3 steel vibrating wire 4 protection housing
5 substrate/medium (e.g. steel/concrete) 6 screw nut on setting screw
7 spring
Figure 1 — Features of vibrating wire strain gauges
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Key
1 anchors (here illustrated as flanges) 4 telescopic tube
1,2
2 displacement transducer with electric cable 5 extension rod
3 guides 6 sleeve
1,2
Figure 2 — Features of a strainmeter
a) spot-weldable FBG strain gauge
b) epoxy bonded FBG strain gauge
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
c) bolt-on FBG strain gauge
d) grout-in FBG strain gauge
Key
1 medium (steel / concrete) 5 optical fibre
2 mounting block / end plate 6 FBG (strain or temperature)
1,2
3 welding (row or points) 7 epoxy bonding
1-4
4 protection housing 8 anchor / bolt
Figure 3 — Features of a Fibre Bragg Grating
a) clamp mounted FBG strain array (gauge length installation)
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
b) adhesive bonded FBG strain array (continuous or spot)
Key
1 medium (steel / concrete) 5 optical fibre
2 mounting clamp 6 FBG (strain)
1-3
3 cable ties 7 adhesive
1-3
4 cable jacket 8 anchor / bolt
1-6
Figure 4 — Features of a Fibre Bragg Grating strain arrays
Key
1 re-bar anchors 2 strain sensor body
3 strain gauge end blocks 4 plucking coil
5 vibrating steel wire
Figure 5 — Features of an instrument reinforcement bar
5.2.2 Embedded strain gauges
5.2.2.1 Instead of mounting blocks as per 5.2.1.2, embedment strain gauges can be equipped with
th th
flanges at both ends of the gauge. The diameter of the flanges should be between 1/5 to 1/10 of the
gauge length.
NOTE See Figure 1c.
5.2.2.2 Instrumented reinforcement bars should have a minimum anchor length of 15 times the
diameter of the bar.
5.2.2.3 Embedded FBG strain arrays / cables with a protective coating to avoid direct contact
between concrete and the optical fibre and gratings shall ensure that strain from the external coating/
jacket is fully transferred to the optical fibre.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
5.3 Strainmeters
5.3.1 Embedment strainmeters can be either fixed installed or moved between equally spaced
measuring marks embedded in the medium.
NOTE 1 For fixed-installed strainmeters, see Figure 2.
NOTE 2 For moveable strainmeters, see mobile high-precision extensometer, ISO 18674-2:2016, 5.3.
NOTE 3 In contrast to strain gauges, embedment strainmeters are often aligned in a continuous chain of single
strainmeter elements, see Figure 8.
5.4 Instruments for specific applications
5.4.1 Monitoring of 1-D structural members
EXAMPLES Concrete pile, steel pile, strut, prop, anchor tendon.
5.4.1.1 The strain sensing direction of the instrument shall be along the axis of the 1-D structural
member.
5.4.1.2 The location of the instrument shall be specified in relation to its distance from the axis.
NOTE In the case of Figure 6a, the measuring location coincides with the axis.
5.4.1.3 When apart from axial strain, bending moments are to be measured or occur, the
instrumentation design should consider the dimensions of the structural member to define the number
and location of strain gauges per measuring section.
EXAMPLE 1 Concrete pile, see Figure 8.
EXAMPLE 2 Steel I-Beam, see Figure 9.
NOTE 1 In the layout of Figure 6b, there are two opposite strain instrument locations at equal distance d to
the axis. That configuration allows, besides monitoring of axial strain, the monitoring of bending of the member
in a pre-specified direction.
NOTE 2 The configuration of Figure 6c allows monitoring of bending in the principal strain directions.
NOTE 3 Pairs of gauges are typically installed as far as possible from the axis of the steel member and opposite
across the axis (Figure 7: Cross section of an I-beam identifying axis) so as to detect the strain due to bending
when present.
5.4.1.4 Along the axis of a 1-D member, the strain measuring locations may be either discrete or
continuous. For discrete locations, strain gauges should be selected; for continuous measuring lines a
continuous chain of strainmeters.
NOTE 1 See Figure 8.
NOTE 2 Discrete measuring locations require interpolation of the strain measuring data, particularly
in respect to bending. They also might serve as a redundancy check, e.g. for inclinometer measurements, see
ISO 18674-3:2017.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
a) axial strain . in a pre-speci- b) . plus bending in principal
c) . plus bending
fied direction directions
Key
1 concrete pile + pile axis
2 strain measuring location d distance of strain instrument to axis
Figure 6 — Possible strain monitoring layouts in a concrete pile (cross section)
a) Strain in ZZ-axis and b) Strain in ZZ-axis and
bending around YY-axis bending around XX-axis
Key
1, 2 number of strain gauge
2a height of I-Beam
2b width of I-Beam
c distance of strain gauge to XX-axis
d distance of strain gauge to YY-axis
Figure 7 — Possible layouts for monitoring of axial strain and bending of a steel I-beam
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Key
F force
1 ground
2 pile head
3 pile
4 strainmeter (e.g. mobile extensometer), continuously aligned in pile axis
5 strain gauges, pair equidistant to pile axis at three discrete measuring locations
Figure 8 — Continuous and discrete strain measuring locations in a concrete pile
5.4.2 Monitoring of 2-D structural members
EXAMPLES Foundation plate, sheet pile, diaphragm wall, shotcrete/concrete tunnel lining.
5.4.2.1 The strain sensing direction of the instrument shall be in the plane of the 2-D structural
member.
5.4.2.2 For monitoring of the general 2-D strain state at a measuring location, an assemblage of
three independently oriented instruments shall be installed. A rosette adapter can be used to facilitate
installation in pre-specified directions.
NOTE 1 A rosette with three directions 120° apart is the optimal layout.
NOTE 2 More than three instrument orientations create a redundancy which might be useful for a check of the
consistency of the measurements.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
5.4.2.3 For specific engineering questions, the number of differently oriented instruments per
measuring location may be reduced.
EXAMPLE 1 Bending of a diaphragm wall towards an excavation: One pair of vertically oriented strain
gauges per measuring location (see 6 in Figure 9). That layout of strain instruments may be supplemented or
1.6
substituted by vertical inclinometers, see ISO 18674-3:2017.
EXAMPLE 2 Estimate of the circumferential stress developing in a shotcrete tunnel lining due to increasing
ground pressures acting onto the lining: Pair of strain gauges oriented in circumferential direction of the tunnel
(see Figure 10) and conversion of strain into stress by employing a stress-strain relationship of the shotcrete.
Key
F force 1 ground 2 excavation
3 2-D member: diaphragm wall 4 1-D member: strut 5 strut strain gauges
6 pair of strain gauges 7 optional load cell (ISO 18674-8:2022) 8 adaptor
1.6
9 base of excavation 10 topographic surface
Figure 9 — Example of strain measuring locations in 1-D and 2-D structural members (strut and
diaphragm wall)
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Key
1 excavation surface 2 shotcrete 3 strain gauges
1,2
4 outer lattice girder 4 inner lattice girder 5 surface of shotcrete lining
1 2
Figure 10 — Example of strain gauge layout in shotcrete lining
5.4.3 Monitoring of 3-D structural members
EXAMPLES Mass-concrete structure, earth dam, embankment.
5.4.3.1 Strain monitoring in gravity dams and arch dams can be carried out by strain gauges and
strainmeters.
NOTE 1 See Figure 11.
NOTE 2 High-precision strainmeters are instruments for monitoring the behaviour of the dam structure in
response to changing water levels of the reservoir.
NOTE 3 One purpose of combined strain and temperature measurements is monitoring of the concrete’s
curing process. Commonly, commercially available vibrating wire strain gauges have a built-in temperature
sensor.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Key
1 concrete embedment strain gauge with integrated temperature sensor
2 high-precision mobile strainmeter, gauge length 1 metre
Figure 11 — Strain measuring instruments in a gravity dam
5.4.3.2 For monitoring of the general 3-D strain state at a measuring location, an assemblage of six
independently oriented strain gauges shall be installed. A rosette adapter can be used to facilitate
installation in pre-specified directions.
NOTE Monitoring of the general 3-D state of strain by strain gauges is theoretically possible, however rarely
carried out in practice.
5.4.3.3 Strain measurements in earth dams and embankments should be carried out by means of a
strainmeter. The instrument can be embedded as a single element, as a cluster of differently orientated
elements or as a chain of several elements.
NOTE Common is a horizontal chain of strainmeter elements embedded inside and near the crest of an
earth-fill dam or embankment.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
6 Installation and measuring procedure
6.1 Installation
6.1.1 Installation of strain gauges
6.1.1.1 General
6.1.1.1.1 The installation work of strain gauges should be shielded from the activities of the
construction site and protected from adverse weather conditions.
6.1.1.1.2 Strain gauges and their cables shall be protected against mechanical damage which may be
caused by construction activities (e.g. pile driving; pouring of concrete; concrete vibrators), traffic or
vandalism.
6.1.1.1.3 Strain gauges shall be protected against temperature effects due to direct sunlight. The
temperature exposure should be similar as in the member to measure.
6.1.1.2 Spot-welding onto steel members
6.1.1.2.1 For the application of strain gauges to steel members, flat and clean surfaces shall be
secured at the measuring point, e.g. with the aid of a grinder and sandpaper.
NOTE If the surface has irregularities, an air gap can occur between the surface and the sensor. Spotwelding
on such surface might damage the sensor.
6.1.1.2.2 Spot-welding of the gauge carrier to the prepared surface shall be carried out according to
the procedures prescribed by the supplier of the instrument. The procedures shall be documented.
NOTE 1 Commonly, there are two to three rows of welding spots, spaced about 2 mm, which are applied on
each side of the vibrating wire or FBG carrier in a specific order.
NOTE 2 Following the welding procedure supplied by the manufacturer reduces the risks of locking-in welding
stress in the gauge.
6.1.1.2.3 Any contact between welder tip, the sensor and the lead cable shall be avoided. As a
precaution an insulation sheet may be held between welder tip and the sensor and cables.
6.1.1.2.4 When using vibrating wire sensors the plucking coil unit (see 2 in Figure 1b) shall be placed
on top of the vibrating wire element and fixed in its position, e.g. by cable-ties, straps or tapes.
6.1.1.2.5 The entire assembly should be protected against corrosion.
EXAMPLE Wrapping by self-vulcanising tape.
6.1.1.3 Arc-welding onto steel members
6.1.1.3.1 At the measuring point, a flat and clean surface shall be secured as per 6.1.1.2.1.
6.1.1.3.2 Arc-welding of the mounting blocks shall be carried out in the sequence shown in Figure 12.
6.1.1.3.3 During arc-welding, the strain gauge element between the two mounting blocks shall be
temporarily replaced by an inert setting element of equal external dimension.
NOTE This procedure prevents welding-induced damage to the strain gauge.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
6.1.1.3.4 The assembly consisting of strain gauge element, mounting blocks and welds should be
protected against corrosion.
Key
1 arc weld with sequence of application
2 mounting blocks (top view); Block 1 with a single set screw, Block 2 with two inclined set screws
1,2
3 setting element
Figure 12 — Sequence of arc-welding when installing strain gauges to a steel surface
6.1.1.4 Attaching to concrete surfaces
6.1.1.4.1 The strain gauges should be attached to the concrete surface by anchors or adhesion
NOTE 1 Commonly mounting blocks with groutable or expandable anchors inserted into predrilled holes are
used, see Figure 13.
NOTE 2 Epoxy gluing is a typical type of adhesion
6.1.1.4.2 When applying adhesive material, its specification should take into consideration the
characteristics, quality and durability during the complete monitoring period.
NOTE 1 Creep and strain transfer are characteristics to consider, as well as possible presence of micro
cracking.
NOTE 2 Calibration tests in regard to strain transfer provide good understanding of the physical properties of
the adhesive
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Key
1 vibrating wire strain gauge element with integrated temperature sensor
2 mounting blocks
1,2
3 concrete
4 grout, mortar, non-shrink cement
5 anchor
Figure 13 — Vibrating wire strain gauge attached to a concrete surface (example)
6.1.1.5 Embedding into concrete
6.1.1.5.1 The gauge may be precast in a briquette of concrete identical to that used in the member,
provided that the briquette is cast maximally 48 hours prior to the main concrete pour.
NOTE This procedure is a precaution against possible damage of the gauge when pouring and vibrating the
concrete.
6.1.1.5.2 Prior to a concrete pour of a reinforced structural member, the gauges, respectively the
briquettes, shall be placed and held in their pre-determined position and orientation by means of steel
wires wrapped around the instruments and connecting to the reinforcement cage.
6.1.1.5.3 In mass concrete applications, the gauge may be installed either before or immediately after
placement of the concrete. When, at a measuring point, placing several strain gauges in different pre-
determined orientations, use should be made of a rosette adaptor.
6.1.1.5.4 Embedded FBG strain cables shall be fixed to reinforcement with cable ties at intervals
no greater than two meters. The cable should be kept straight and taut all along the section to be
monitored.
6.1.1.5.5 Strain Gauges may also be installed by providing a reservation tube at the reinforcement
cage and embedding afterwards the gauges in this reservation tube with a grout/mortar.
EXAMPLE Array of FBG's on a reinforcement or a carrier bar
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
6.1.2 Installation of strainmeters
6.1.2.1 In stiff media (e.g. concrete, rock, stiff soil), mobile strainmeters should be employed. For
installation, refer to ISO 18674-2:2016, Clause 6.1.4.
6.1.2.2 In soft media (e.g. fill, unconsolidated soil), fixed-installed embedded strainmeters should be
employed. In embankments, strainmeters shall be installed in trenches which should be formed and
back-filled concurrently with the construction work.
NOTE A common configuration is a horizontal continuous chain of strainmeters for monitoring lateral
movements inside an embankment.
6.2 Measuring procedure
6.2.1 Instrumentation check and calibration
6.2.1.1 Before installation, the functionality of the instrument shall be checked by performing tests
suggested by the manufacturer. For general function checks and calibrations, reference shall be made
to ISO 18674-1:2015, 5.6.
6.2.1.2 A calibration certificate or batch factor shall be supplied by the manufacturer for each
instrument delivered.
6.2.2 Measurement
The measurement shall be carried out according to ISO 18674-1:2015, 7.
7 Data processing and evaluation
7.1 Data processing of the strain measurements and their evaluation shall be carried out in
accordance with Annex A.
8 Reporting
8.1 Installation report
The installation report shall be in accordance with ISO 18674-1:2015, 9.1.
8.2 Monitoring report
The monitoring report shall be in accordance with ISO 18674-1:2015, 9.2.
oSIST prEN ISO 18674-7:2024
ISO/DIS 18674-7:2023(E)
Annex A
(normative)
Data processing and evaluation
A.1 Data processing in terms of strain
A.1.1
Change of strain Δε in the period between reference and follow-up measurements at the measuring
i
point i:
Δε = ε – ε (1)
i i ,F i,R
where
ε strain reading of the follow-up measurement
i ,F
ε strain reading of the reference measurement
i,R
A.1.2
Change of the axial strain Δε due to loading/unloading of a 1-D member:
ax
Δε = (Δε + Δε + .
...
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