Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 5: Stress change measurements by total pressure cells (TPC) (ISO 18674-5:2019)

This document specifies the measurement of stress changes by means of total pressure cells (TPC). 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 18674‑4, this document allows the determination of effective stress acting in the ground.
This document is applicable to:
— monitoring changes of the state of stress in the ground and in geo-engineered structures (e.g. in earth fill dams or tunnel lining);
— monitoring contact pressures at the interface between two media (e.g. earth pressure on retaining wall; contact pressure at the base of a foundation);
— checking geotechnical designs and adjustment of construction in connection with the Observational Design procedure;
— evaluating stability during or after construction.
Guidelines for the application of TPC in geotechnical engineering are presented in Annex B.
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 total pressure cells as part of the geotechnical investigation and testing according to EN 1997-1[1] and EN 1997-2[2].

Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 5: Spannungsänderungsmessungen mittels Druckmessdosen (ISO 18674 5:2019)

Dieses Dokument trifft Festlegungen zu Spannungsänderungsmessungen mittels Druckmessdosen. Die allgemeinen Regeln für die messtechnische Überwachung des Baugrunds, der mit dem Baugrund in Wechselwirkung stehenden Bauwerke, geotechnischer Auffüllungen und geotechnischer Bauarbeiten sind in ISO 18674 1 angegeben.
Wenn dieses Dokument in Verbindung mit ISO 18674 4 angewandt wird, erlaubt es die Bestimmung der im Baugrund wirkenden effektiven Spannung.
Dieses Dokument gilt für:
- die Überwachung von Änderungen des Spannungszustands im Baugrund und in geotechnischen Bauwerken (z. B. Erddämme oder Tunnelschalen);
- die Überwachung von Drücken an der Grenzfläche zwischen zwei Bauteilen (z. B. Erddruck auf Stützwände; Flächenlast an der Sohle einer Gründung);
- die Überprüfung geotechnischer Entwürfe und die Anpassung von Konstruktionen im Zusammenhang mit der Beobachtungsmethode;
- die Bewertung der Standsicherheit während der Ausführung und im Endzustand.
Ein Leitfaden zur Anwendung von Druckmessdosen in der Geotechnik ist in Anhang B enthalten.
ANMERKUNG   Dieses Dokument erfüllt die Anforderungen an die mittels Druckmessdosen durchgeführte messtechnische Überwachung des Baugrunds, der mit dem Baugrund in Wechselwirkung stehenden Bauwerke und geotechnischer Bauarbeiten im Rahmen der geotechnischen Erkundung und Untersuchung nach EN 1997 1 [1] und EN 1997 2 [2].

Reconnaissance et essais géotechniques - Surveillance géotechnique par instrumentation in situ - Partie 5: Mesures de la variation de pression par cellules de pression totale (TPC) (ISO 18674-5:2019)

Le présent document spécifie la mesure des variations de pression à l'aide de cellules de pression totale (TPC). Les règles générales relatives à la réalisation d'un suivi du terrain, des structures en interaction avec le terrain, des remblais géotechniques et des travaux géotechniques sont présentées dans l'ISO 186741.
S'il est appliqué conjointement à la norme ISO 186744, le présent document permet de déterminer les contraintes effectives agissant dans le terrain.
Le présent document s'applique:
— à la surveillance des changements de l'état de contrainte dans le terrain et les structures géotechniques (p. ex. dans des barrages en terre ou des revêtements de tunnel);
— à la surveillance des pressions de contact à l'interface entre deux milieux (p. ex. poussée des terres sur un mur de soutènement; pression de contact à la base d'une fondation);
— au contrôle des calculs géotechniques et à l'ajustement d'une construction en lien avec la méthode observationnelle;
— à l'évaluation de la stabilité durant ou après la construction.
Les directives pour l'utilisation de TPC en génie géotechnique sont présentées à l'Annexe B.
NOTE Ce document satisfait aux exigences relatives à la surveillance du terrain, des structures interagissant avec le terrain et des travaux géotechniques au moyen de TPCs, dans le cadre de la reconnaissance et des essais géotechniques conformément à l'EN 1997-1[1] et l'EN 1997-2[2].

Geotehnično preiskovanje in preskušanje - Geotehnične meritve - 5. del: Merjenje sprememb napetosti s tlačnimi merskimi celicami (ISO 18674-5:2019)

Ta standard je 5. del skupine standardov ISO 18674, kot je opisano v standardu ISO 18674-1: 1. del: Splošna pravila. Določa metode in pravila za merjenje skupnih napetosti v geotehničnem inženiringu ali splošneje v inženirstvu temeljev. Z napetostmi v tleh ali kamninah se presoja obremenitve zgrajenih konstrukcij v tleh.

General Information

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Published
Publication Date
29-Oct-2019
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
30-Oct-2019
Completion Date
30-Oct-2019

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SLOVENSKI STANDARD
SIST EN ISO 18674-5:2019
01-december-2019
Geotehnično preiskovanje in preskušanje - Geotehnične meritve - 5. del: Merjenje
sprememb napetosti s tlačnimi merskimi celicami (ISO 18674-5:2019)

Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation

- Part 5: Stress change measurements by total pressure cells (TPC) (ISO 18674-5:2019)

Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 5:
Spannungsänderungsmessungen mittels Druckmessdosen (ISO 18674 5:2019)
Reconnaissance et essais géotechniques - Surveillance géotechnique par

instrumentation in situ - Partie 5: Mesures avec capteurs hydrauliques (ISO 18674-

5:2019)
Ta slovenski standard je istoveten z: EN ISO 18674-5:2019
ICS:
13.080.20 Fizikalne lastnosti tal Physical properties of soils
93.020 Zemeljska dela. Izkopavanja. Earthworks. Excavations.
Gradnja temeljev. Dela pod Foundation construction.
zemljo Underground works
SIST EN ISO 18674-5:2019 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 18674-5:2019
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SIST EN ISO 18674-5:2019
EN ISO 18674-5
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2019
EUROPÄISCHE NORM
ICS 13.080.20; 93.020
English Version
Geotechnical investigation and testing - Geotechnical
monitoring by field instrumentation - Part 5: Stress change
measurements by total pressure cells (TPC) (ISO 18674-
5:2019)

Reconnaissance et essais géotechniques - Surveillance Geotechnische Erkundung und Untersuchung -

géotechnique par instrumentation in situ - Partie 5: Geotechnische Messungen - Teil 5:

Mesures avec capteurs hydrauliques (ISO 18674- Spannungsänderungsmessungen mittels

5:2019) Druckmessdosen (ISO 18674 5:2019)
This European Standard was approved by CEN on 11 October 2019.

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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, 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: Rue de la Science 23, B-1040 Brussels

© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18674-5:2019 E

worldwide for CEN national Members.
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SIST EN ISO 18674-5:2019
EN ISO 18674-5:2019 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 18674-5:2019
EN ISO 18674-5:2019 (E)
European foreword

This document (EN ISO 18674-5:2019) 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 April 2020, and conflicting national standards shall be

withdrawn at the latest by April 2020.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN 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, France, Germany, Greece, Hungary, Iceland,

Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of

North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the

United Kingdom.
Endorsement notice

The text of ISO 18674-5:2019 has been approved by CEN as EN ISO 18674-5:2019 without any

modification.
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SIST EN ISO 18674-5:2019
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SIST EN ISO 18674-5:2019
INTERNATIONAL ISO
STANDARD 18674-5
First edition
2019-10
Geotechnical investigation and
testing — Geotechnical monitoring by
field instrumentation —
Part 5:
Stress change measurements by total
pressure cells (TPC)
Reconnaissance et essais géotechniques — Surveillance géotechnique
par instrumentation in situ —
Partie 5: Mesures de la variation de pression par cellules de pression
totale (TPC)
Reference number
ISO 18674-5:2019(E)
ISO 2019
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols .......................................................................................................................................................................................................................... 5

5 Instruments ............................................................................................................................................................................................................... 6

5.1 General ........................................................................................................................................................................................................... 6

5.2 Deformation measuring method ............................................................................................................................................. 7

5.3 Compensation measuring method ......................................................................................................................................... 7

5.4 Stiffness of the pressure compartment .............................................................................................................................. 8

5.5 Shape of the pressure compartment .................................................................................................................................... 8

5.6 Accuracy ........................................................................................................................................................................................................ 9

6 Installation and measuring procedure ......................................................................................................................................10

6.1 Installation ...............................................................................................................................................................................................10

6.1.1 Installation in the ground ......................................................................................................................................10

6.1.2 Installation in fill ...........................................................................................................................................................11

6.1.3 Installation in concrete/shotcrete .................................................................................................................12

6.1.4 Installation in contact planes .............................................................................................................................13

6.2 Carrying out the measurement ..............................................................................................................................................14

6.2.1 Instrumentation check and calibration .....................................................................................................14

6.2.2 Measurement ...................................................................................................................................................................14

7 Data processing and evaluation .........................................................................................................................................................14

8 Reporting ...................................................................................................................................................................................................................14

8.1 Installation report .............................................................................................................................................................................14

8.2 Monitoring report ..............................................................................................................................................................................14

Annex A (normative) Evaluation procedure ..............................................................................................................................................15

Annex B (informative) Geo-engineering applications .....................................................................................................................17

Annex C (informative) Measuring examples ..............................................................................................................................................18

Bibliography .............................................................................................................................................................................................................................27

© ISO 2019 – All rights reserved iii
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(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 of the voluntary nature of standards, 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 www .iso

.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 182, Geotechnics.
A list of all parts in the ISO 18674 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved
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SIST EN ISO 18674-5:2019
INTERNATIONAL STANDARD ISO 18674-5:2019(E)
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 5:
Stress change measurements by total pressure cells (TPC)
1 Scope

This document specifies the measurement of stress changes by means of total pressure cells (TPC).

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 18674-4, this document allows the determination of effective stress

acting in the ground.
This document is applicable to:

— monitoring changes of the state of stress in the ground and in geo-engineered structures (e.g. in

earth fill dams or tunnel lining);

— monitoring contact pressures at the interface between two media (e.g. earth pressure on retaining

wall; contact pressure at the base of a foundation);

— checking geotechnical designs and adjustment of construction in connection with the Observational

Design procedure;
— evaluating stability during or after construction.

Guidelines for the application of TPC in geotechnical engineering are presented in Annex B.

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 total pressure cells as part of the

[1] [2]
geotechnical investigation and testing according to EN 1997-1 and EN 1997-2 .
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. 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

ISO 18674-4, Geotechnical investigation and testing — Geotechnical monitoring by field instrumentation —

Part 4: Measurement of pore water pressure: Piezometer

ISO 22475-1, Geotechnical investigation and testing — Sampling methods and groundwater

measurements — Part 1: Technical principles for execution
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 18674-1 and the following apply.

© ISO 2019 – All rights reserved 1
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)

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 http: //www .electropedia .org/
3.1
total pressure cell
TPC
field instrument for stress change measurements

Note 1 to entry: Typically, a total pressure cell system consists of a pressure compartment, a pressure tubing, a

pressure measuring device, a measuring line and a control and readout unit (see Figure 1 and Reference [3]).

Note 2 to entry: The pressure compartment consists of two steel platens, welded together around their

peripheries, where the intervening cavity is filled with a liquid. The cavity is connected to the inner chamber

of a pressure measuring device via a liquid-filled pressure tubing. Inner and outer chambers of the pressure

measuring device are separated by a flexible diaphragm.

Note 3 to entry: Total pressure cells are permanently installed either in fill or soft ground (embedment pressure

cells) (3.2), in contact planes between any two media (contact pressure cells) (3.3) or in boreholes (borehole

pressure cells) (3.4).

Note 4 to entry: The target of the measurement is the change of the total normal stress Δσ of the medium acting

onto the flat side of a pressure compartment (see 1 in Figure 1).
Key
1 pressure compartment
2 pressure tubing
3 pressure measuring device
3a inner chamber
3b outer chamber
3c diaphragm
4 height of the cavity of the pressure compartment
5 height of the pressure compartment
6 measuring line (electric cable or twin hydraulic tubing)
7 control and readout unit
8 medium investigated
Figure 1 — Principal components of a TPC measuring system
2 © ISO 2019 – All rights reserved
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)
3.2
embedment pressure cell
total pressure cell (3.1) which is fully embedded within a medium

EXAMPLE Push-in cell in soft soil; “tangential cell” in shotcrete tunnel lining (see 4 in Figure 2); embedment

cell in fill (see Figure 3).
3.3
contact pressure cell
total pressure cell (3.1) which is placed in a contact plane between two media

EXAMPLE Cell at the base of a slab foundation; “radial cell” (see 3.9) in shotcrete tunnel lining.

3.4
borehole pressure cell
total pressure cell (3.1) which is installed in a borehole
Note 1 to entry: See 2 in Figure 2.
3.5
aspect ratio
ratio of height to the smallest lateral dimension of the pressure compartment

Note 1 to entry: For rectangular compartments, the smallest lateral dimension is the width, for circular

compartments the diameter.
Note 2 to entry: Typical aspect ratios are of the order of 1:20 to 1:40.
3.6
total stress

stress in the ground carried by the solid portion (skeleton) of the ground and the pore water

Note 1 to entry: One only stress component can be monitored by a total pressure cell (3.1) (which is the change of

the total normal stress Δσ ).

Note 2 to entry: Changes of 2-D and 3-D stress states can be monitored by a cluster of a sufficient number of

independently oriented TPC compartments installed at a measuring location: Three compartments for a 2-D

stress state, and six compartments for a 3-D stress state.

Note 3 to entry: By placing a TPC compartment with its sensing side towards the vertical, the vertical normal

stress component σ can be directly monitored.
3.7
effective stress
stress in the ground carried by the solid portion (skeleton) of the ground
Note 1 to entry: It is σ’ = σ - u
where
σ’ is the effective stress tensor;
σ is the total stress tensor;
u is the porewater pressure.
The formula above is only applicable to saturated soil.
3.8
contact stress
stress component which acts normal to a contact plane

EXAMPLE Normal stress acting in the interface between a slab foundation and the ground.

© ISO 2019 – All rights reserved 3
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)

Note 1 to entry: Shear stresses acting within the contact plane cannot be measured by a TPC (3.1).

3.9
radial stress
specific contact stress (3.8) between the ground and a tunnel lining

Note 1 to entry: Radial TPCs (3.1) (“radial cells”) are especially designed for monitoring radial stresses.

Note 2 to entry: See 3 in Figure 2.
3.10
tangential stress
hoop stress monitored within shotcrete or concrete tunnel linings

Note 1 to entry: Tangential TPCs (3.1) (“tangential cells”) are especially designed for monitoring tangential

stresses in tunnel linings. An alternative term is “concrete TPC”.
Note 2 to entry: See 4 in Figure 2.
Key
1 borehole (vertically down-dipping; back-filled)

2a, 2b, 2c array of three differently oriented borehole TPCs for monitoring horizontal ground stresses

3 radial TPCs at the ground/shotcrete lining interface
4 tangential TPCs in the shotcrete lining
5 shotcrete lining
6 tunnel excavation contour
Figure 2 — Example of a TPC layout in near-surface tunnelling
4 © ISO 2019 – All rights reserved
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)
Key
A clay core 1 TPC (single or cluster)
B filter zone 2 piezometer
C rock fill
NOTE Zones A and C have independent cable routing systems (see 6.1.2.5).
Figure 3 — Example (schematic) of a TPC layout in an earth dam
4 Symbols
Symbol Name Unit
C edge correction factor —
E Young’s modulus MPa
h elevation of measuring station in compensation method m
h elevation of the TPC in compensation method m
p pressure in the outer chamber of the measuring device MPa
p pressure in a follow-up measurement MPa

p hydrostatic pressure difference between the external measuring station and TPC MPa

pressure of the liquid in the compartment and in the inner chamber of the measur-

p MPa
ing device
p pressure loss in the compensation delivery line MPa
p pre-tensioning pressure MPa
p-t
p pressure in reference measurement MPa
p pressure reading taken at the outside measuring station MPa
read
u pore water pressure MPa
γ specific weight of compensation fluid N/m
fluid
σ ; σ ’ normal stress (total; effective) MPa
n n
Δσ difference of total normal stress MPa
σ vertical stress MPa
σ maximum horizontal stress MPa
σ minimum horizontal stress MPa
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)
5 Instruments
5.1 General

5.1.1 It shall be noted that TPC measurements are prone to substantial errors as the presence of

the cell in the medium tends to create significant changes in the stress field which is the target of the

measurement.
NOTE 1 See Figure 4 (Reference [5]).

NOTE 2 The selection of appropriate instruments, adherence to their range of application and adequate

installation procedures are critical to reduce these errors to acceptable levels (see 5.4 and 5.6).

a) TPC stiffer than medium b) TPC softer than medium
(over-registering TPC) (under-registering TPC)
Key
I stress trajectories around a TPC 1 pressure compartment embedded in a medium.
II normal stress profile A — A’
Figure 4 — Registering effect of embedded TPCs

5.1.2 Deformation and compensation measuring methods should be distinguished from each other

(see Table 1).
Table 1 — Monitoring features associated with TPC measuring methods
Long-term
Measuring TPC Atmospheric pressure Automatic
stability of Logging speed
method stiffness compensation data acquisition
sensor signal

Deformation tends to be depends, amongst independent barometric amenable comparatively

(see 5.2) soft others, on the pressure monitoring may quick
type of electrical be needed
sensors used
vented TPC tend to be
unreliable

Compensation tends to be tends to be long- vented TPC tend to be cumbersome; comparatively

(see 5.3) stiff term stable reliable comparatively slow
costly
6 © ISO 2019 – All rights reserved
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)

5.1.3 Any change of the total normal stress Δσ acting onto the flat side of a pressure compartment (1

in Figure 1) shall be uniquely associated with a change of the pressure of the liquid in the intervening

cavity of the compartment.

5.1.4 The stiffness of the pressure compartment in sensing direction should be low in comparison with

the stiffness of the pressure tubing and the housing of the pressure measuring device.

5.1.5 The shape and location of the pressure measuring device shall not affect the total normal stress

σ of the medium acting onto the pressure compartment.

NOTE A common technical solution is a TPC where the measuring device is located sufficiently far away from

the pressure compartment, and where the pressure compartment and measuring device are interconnected by a

stiff pressure tubing.

5.1.6 The pressure measuring device (3 in Figure 1) typically is a diaphragm pressure transducer. The

cavity of the interconnected components compartment, tubing and measuring device shall be completely

filled with, in engineering terms, an incompressible and de-aired liquid. The difference in elevation

between compartment and measuring device should be so small that it can be neglected in the evaluation

procedure (see A.1.1).

5.1.7 The housing of the pressure measuring device should be sufficiently stiff so that even high

ground pressures acting onto the outer side of the device do not affect the mechanical behaviour of the

diaphragm, in particular its calibration characteristics.

NOTE Experience in high embankments has shown that the earth pressure, acting on the housing of a

pressure transducer, can cause a substantial shift of the zero-point and a change in the linearity of the transducer.

5.2 Deformation measuring method

5.2.1 The measurement of the deflection of the diaphragm of the pressure measuring device (see 3c in

Figure 1) can be used as a method for measuring the pressure of the liquid in the intervening cavities.

NOTE Commonly, the diaphragm separating the inner chamber and outer chamber coincides with the

measuring diaphragm of an electric pressure transducer.

5.2.2 The pressure in the outer chamber of the measuring device (see 3b in Figure 1) shall be either

constant or atmospheric.

5.2.3 If TPC measurements are influenced by changes of the atmospheric pressure, thesee should be

monitored separately.

NOTE Attempts to circumvent this issue by integrating a small venting tube into the measuring line (see 6

in Figure 1) are often marred with difficulties, as such tubes tend to become blocked by condensed water. This

feature is in contrast to the compensation measuring method (see 5.3 and Table 1).

5.2.4 Deformation measurements carried out directly at the platens of the pressure compartment,

e.g. by means of strain gauges or built-in vibrating wire sensors, should be avoided as this measuring

procedure will typically result in compartment dimensions with high aspect ratios leading to

unfavourable embedment conditions (see 6.1) and ill-defined edge correction factors (see A.1).

5.3 Compensation measuring method

5.3.1 In TPC compensation measuring systems, any changes of the distance between the platens of the

pressure compartment caused by Δσ shall be compensated by an externally applied pressure p .

n a

NOTE The common practice is hydraulic application of p at comparatively high pressure levels and

pneumatic application of p at comparatively low pressure levels.
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SIST EN ISO 18674-5:2019
ISO 18674-5:2019(E)

5.3.2 Compensation should be carried out at the diaphragm (3c in Figure 1) of the pressure measuring

device. Any deflection of the diaphragm, as described in 5.2.1, shall be compensated by a pressure p

acting in the outer chamber of the device.

5.3.3 The compensation point shall be clearly defined and well identifiable when making the

measurement. Pressure valve or electric diaphragm switch techniques may be employed.

5.4 Stiffness of the pressure compartment

In sensing direction, the stiffness of the pressure compartment should conform to the stiffness of

the medium.

NOTE 1 Stress concentration effects influence the measuring results yielding either systematically too low or

too high values (see Figure 4).

NOTE 2 Amongst the factors which influence the stiffness of the TPC system are the following:

— measuring principle (deformation, see 5.2, versus compensation, see 5.3; see also Table 1);

— aspect ratio;
— height of liquid-filled cavity (4 in Figure 1);
...

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