oSIST prEN ISO 18674-6:2026

• Abstract:

  This standard specifies the measurement of settlement of geotechnical structures/works or structures influenced by geotechnical works by means of hydraulic settlement systems. General rules of performance monitoring of the ground, or structures interacting with the ground, of geotechnical fills and of geotechnical works are presented in ISO 18674-1:2015. ; This document is applicable to: ; — monitoring of settlement acting onto, or within, geotechnical structures such as embankments, excavations, compensation grouting, tunnel lining, railways, roads and other civil structures; ; — checking geotechnical designs and adjustment of construction in connection with the Observational Design procedure;; evaluating (subsoil) stability during or after construction.

• Status: Not Published
• Public Enquiry End Date: 21-Jun-2026
• ICS: 13.080.20 - Physical properties of soils
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: KON - Structures
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 21-Apr-2026
• Due Date: 08-Sep-2026
• Ref Project: prEN ISO 18674-6 - Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 6: Measurement of settlement: Hydraulic settlement systems (ISO/DIS 18674-6:2026)

Overview

oSIST prEN ISO 18674-6:2026:2026 - Geotechnical investigation and testing – Geotechnical monitoring by field instrumentation – Part 6: Measurement of settlement: Hydraulic settlement systems is a draft European and International standard developed by CEN and ISO. This standard describes procedures for measuring the settlement of geotechnical structures or structures affected by geotechnical works using hydraulic settlement systems. Accurate measurement and monitoring of settlement are essential for ensuring the safety, stability, and performance of various civil and geotechnical works, such as embankments, excavations, tunnels, roads, and railways.

This standard complements ISO 18674-1:2015, which outlines general provisions for geotechnical performance monitoring. Part 6 focuses specifically on hydraulic settlement measurement systems, defining their setup, installation, operational requirements, and reporting protocols.

Key Topics

• Hydraulic Settlement Systems: The standard defines hydraulic settlement systems as instrumentation methods that use pressure sensors and liquid columns to monitor the vertical movement (settlement or heave) of ground or structures. The three main types include:

  • Hydrostatic profilers (with/without reservoir)
  • Hydraulic settlement cells
  • Liquid level systems (open or closed)

• Components and Setup: Detailed description of system components such as profile tubes, pressure sensors, reservoirs, reference points, data collection equipment, and protective measures.

• Installation Guidance: Recommendations for installing measurement systems, such as proper tube embedding, air pocket elimination, and reference marker placement. For precision, the reference point must be in a stable, isolated area and regularly surveyed.

• Data Collection and Accuracy: Guidance on performing accurate measurements, calibration, and periodic data verification. A consideration of factors like liquid density, temperature effects, and atmospheric compensation is critical to ensure reliable settlement monitoring.

• Applications and Ranges: Tables and guidance on selecting the appropriate type of hydraulic settlement system based on the expected settlement range, site conditions, and required accuracy.

Applications

Hydraulic settlement systems specified in oSIST prEN ISO 18674-6:2026:2026 have a wide range of practical applications within geotechnical engineering and construction. These include:

• Civil Infrastructure Monitoring: Continuous or periodic settlement tracking of embankments, tunnels, railways, roads, and other structures built on or interacting with the ground, helping to manage risk and ensure structural safety.
• Subsoil Stability Evaluation: Assessment of soil movement and stability during and after construction activities such as earthworks, compensation grouting, or foundation building.
• Observational Design Verification: Using real-time data to validate or adapt geotechnical design assumptions and construction processes, especially in accordance with the Observational Method.
• Long-term Structure Maintenance: Supporting the safety and serviceability of critical infrastructure through life-cycle monitoring of settlements.
• Dam and Levee Projects: Measuring settlement at key points beneath or within large earthen or concrete structures to inform maintenance and hazard mitigation.

Related Standards

oSIST prEN ISO 18674-6:2026:2026 is part of the broader ISO 18674 series on geotechnical monitoring by field instrumentation, which collectively define international best practices for various measurement tasks. Relevant related standards include:

• ISO 18674-1:2015: Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 1: General rules
• Other ISO 18674 parts addressing instruments for deformation, displacement, and inclination measurements
• Standards covering physical properties of soils (ICS 13.080.20) and earthworks/excavations (ICS 93.020)

Practical Value

Adopting oSIST prEN ISO 18674-6:2026:2026 supports engineers, contractors, and project owners in:

• Ensuring consistency and technical reliability in settlement measurements
• Improving safety and optimizing the performance of geotechnical structures
• Supporting compliance for regulatory and contractual requirements in geotechnical investigations
• Enhancing project risk management through robust, traceable instrumentation and data reporting practices
• Facilitating harmonization of geotechnical monitoring practices across international projects

For project owners, designers, and contractors involved in earthworks, tunnels, and infrastructure, ISO 18674-6 is an essential reference for implementing modern, effective hydraulic settlement monitoring systems.

SLOVENSKI STANDARD
01-junij-2026
Geotehnično preiskovanje in preskušanje - Geotehnične meritve - 6. del: Meritve
posedanja: Hidravlični sistemi za merjenje posedanja (ISO/DIS 18674-6:2026)
Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation
- Part 6: Measurement of settlement: Hydraulic settlement systems (ISO/DIS 18674-
6:2026)
Geotechnische Erkundung und Untersuchung - Geotechnische Messungen - Teil 6:
Messung von Setzungen: Hydraulische Setzungsmesssysteme (ISO/DIS 18674-6:2026)
Reconnaissance et essais géotechniques - Surveillance géotechnique par
instrumentation in situ - Partie 6: Mesure de tassement: systèmes hydrauliques de
mesure de tassement (ISO/DIS 18674-6:2026)
Ta slovenski standard je istoveten z: prEN ISO 18674-6
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 18674-6
ISO/TC 182
Geotechnical investigation and
Secretariat: BSI
testing — Geotechnical monitoring
Voting begins on:
by field instrumentation —
2026-04-07
Part 6:
Voting terminates on:
2026-06-30
Measurement of settlement:
Hydraulic settlement systems
Reconnaissance et essais géotechniques — Surveillance
géotechnique par instrumentation in situ —
Partie 6: Mesure de tassement: systèmes hydrauliques de mesure
de tassement
ICS: 93.020; 13.080.20
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
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USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
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BE CONSIDERED IN THE LIGHT OF THEIR
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RECIPIENTS OF THIS DRAFT ARE INVITED
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NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 18674-6:2026(en)
DRAFT
ISO/DIS 18674-6:2026(en)
International
Standard
ISO/DIS 18674-6
ISO/TC 182
Geotechnical investigation and
Secretariat: BSI
testing — Geotechnical monitoring
Voting begins on:
by field instrumentation —
Part 6:
Voting terminates on:
Measurement of settlement:
Hydraulic settlement systems
Reconnaissance et essais géotechniques — Surveillance
géotechnique par instrumentation in situ —
Partie 6: Mesure de tassement: systèmes hydrauliques de mesure
de tassement
ICS: 93.020; 13.080.20
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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Published in Switzerland Reference number
ISO/DIS 18674-6:2026(en)
ii
ISO/DIS 18674-6:2026(en)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 3
5 Instruments. 3
5.1 General .3
5.2 Hydrostatic profiler with reservoir .5
5.3 Hydrostatic profiler without reservoir .6
5.3.1 Manually operated .6
5.3.2 In place hydrostatic system .6
5.4 Hydraulic settlement cell .7
5.5 Liquid level system .8
5.5.1 General .8
5.5.2 Open liquid level system .8
5.5.3 “Closed” liquid level system .9
5.6 Measuring range, accuracy and repeatability .10
6 Installation and measuring procedure . .11
6.1 Installation .11
6.1.1 Hydrostatic profiler with reservoir .11
6.1.2 Hydrostatic profiler without reservoir . 12
6.1.3 Hydraulic settlement cell . 12
6.1.4 Open liquid level system . 13
6.1.5 Closed Liquid level system.14
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) Evaluation procedure . 17
Annex B (informative) Geotechnical applications .24
Annex C (informative) Measuring examples .25
Bibliography .44

iii
ISO/DIS 18674-6:2026(en)
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,
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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).
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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
DRAFT International Standard ISO/DIS 18674-6:2026(en)
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 6:
Measurement of settlement: Hydraulic settlement systems
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This document specifies the measurement of settlement of geotechnical structures/works or structures
influenced by geotechnical works by means of hydraulic settlement systems. General rules of performance
monitoring of the ground, or structures interacting with the ground, of geotechnical fills and of geotechnical
works are presented in ISO 18674-1:2015.
This document is applicable to:
— monitoring of settlement acting onto, or within, geotechnical structures such as embankments,
excavations, compensation grouting, tunnel lining, railways, roads and other civil structures;
— checking geotechnical designs and adjustment of construction in connection with the Observational
Design procedure;
— evaluating (subsoil) stability during or after construction.
Not subject of this document are horizontal inclinometers, which are dealt with in reference [1] nor
monitoring devices based on a geodetical principle such as total stations or other optical level instruments,
which are dealt with in reference [2].
NOTE 1 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 hydraulic settlement systems as part of the
geotechnical investigation and testing in accordance with References [3] and [4].
NOTE 2 Throughout the text, settlement is used as a general term, referring to both heave and settlement.
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18674-1 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

ISO/DIS 18674-6:2026(en)
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
Hydraulic settlement system
system consisting of one or more sensors measuring levels or pressures of a hydraulic circuit, resulting in a
settlement measurement
Note 1 to entry: The vertical movement of the measuring point, located on a structure or within the subsoil can be
determined by measuring the relative vertical level of a liquid column with respect to the liquid level in a reference
reservoir.
Note 2 to entry: There are 3 main types of hydraulic settlement systems, namely hydrostatic profiler (see 3.2 and 3.3),
hydraulic settlement cell (see 3.4) and liquid level system (see 3.5).
3.2
Hydrostatic profiler with reservoir
system where a probe is moved along a line in a pre-installed sub horizontal tube and where the elevation of
the probe is related to the elevation of the liquid in a reservoir
Note 1 to entry: The sensor is typically manually pulled through the tubing.
Note 2 to entry: The pressure sensor can be vented or non-vented (connected to atmospheric pressure or not).
Note 3 to entry: See 5.2.
3.3
Hydrostatic profiler without reservoir
system where a sensor or series of sensors measures the pressure within a pre-installed sub horizontal
tube, filled with a liquid, and where the elevation of the tube is related to the pressure of the liquid in the
tube
Note 1 to entry: For a manual system, a probe is manually pulled through the tubing.
Note 2 to entry: For an automatic system, a series of sensors is placed fixed within a filled tube.
Note 3 to entry: See 5.3.
3.4
Hydraulic settlement cell
Hydraulic settlement gauge
cell with a pressure sensor, permanently installed into the ground and connected to a reservoir, measuring
the pressure at the sensor’s location in order to calculate vertical displacement
Note 1 to entry: The pressure sensor can be vented or non-vented (connected to atmospheric pressure or not).
Note 2 to entry: See 5.4.
3.5
Liquid level system
a series of cells with a pressure sensor, installed on a structure and connected to a reservoir, measuring the
pressure at the sensor‘s location in order to calculate vertical displacement
Note 1 to entry: See 5.5.
ISO/DIS 18674-6:2026(en)
4 Symbols and abbreviated terms
Symbol Name Unit
d Distance m
g Gravitational acceleration m/s
h Absolute vertical position m
i
h Absolute vertical position of reference point m
ref
p Measured pressure [N/m ]
i
RL Reference level (e.g. sea level) m
w Displacement at a certain date m
i,t
z Reading of a hydrostatic profiler with reservoir m
i
Δz Relative vertical position m
i
ρ Density of the liquid kg/m
5 Instruments
5.1 General
5.1.1 Different hydraulic settlement systems should be distinguished from each other (see Table 1).
NOTE Within the measuring range of the sensors, settlement is monitored as function of a liquid column.

ISO/DIS 18674-6:2026(en)
Table 1 — Hydraulic settlement systems
No. Type Sub-type Principle measuring procedure
A sub horizontal tube is installed at the level where the
settlements need to be identified.
Hydraulic
profiler with
A pressure sensor, hydraulically connected to a reservoir
1 — Manual system
reservoir (see
with constant liquid level, is manually pulled through the
5.2)
tube and measurements are made at fixed intervals within
the tube.
A sub horizontal tube is installed at the level where the
settlements need to be identified.
The tube is filled with a liquid
Hydraulic pro-
— Manual system In a manual system, a pressure sensor is manually pulled
filer without
through the tube and measurements are made at fixed
reservoir (see
— In place system
intervals within the tube.
5.3)
In an in place system, several pressure sensors are in-
stalled on a fixed interval and continuous measurements
are made during the monitoring period.
A plate with a pressure sensor is installed in the ground at
the level where the settlements need to be identified.
Hydraulic A liquid filled tube connects the pressure sensor to the ref-
— Single point
3 settlement cell erence reservoir, which is installed outside the settlement
— Multiple point/array
(see 5.4) sensitive area.
Multiple pressure sensors can be connected to the same
reference reservoir.
System of pressure sensors or level measuring systems,
hydraulically interconnected and typically installed on a
structure.
Liquid level
— Open system
4 system (see In an open system the pressure sensor or level measuring
— Closed system
5.5) system measure the absolute liquid level.
In a closed system the sensor measures the relative pres-
sure of the liquid compared to a reservoir.
5.1.2 Settlements shall be deduced by comparison of the measured values with those of the reference
measurement.
5.1.3 The point to which the measurements are related shall be denoted the “reference point”.
5.1.4 The reference point shall be located in a stable area outside the influence of settlement and/or shall
be monitored regularly in order to obtain absolute displacements.
NOTE 1 The accuracy by which the reference measurement is made, has a strong influence on the accuracy of the
absolute derived displacement.
NOTE 2 When making manual reading, it is good practice to independently determine the absolute coordinates of
the reference point for each measurement survey.
5.1.5 The connection between the reservoir and the measuring device shall be continuous and the liquid
column shall be completely clear of air bubbles or air pockets.
NOTE Air pockets within the tubing will influence the measurements, as they are compressible.
5.1.6 The influence of the temperature should be considered, if possible, based on the correlation between
temperature and vertical position measurements.
NOTE 1 Temperature has an influence on the measuring results, as the density of the liquid changes with
temperature. This effect is particularly noticeable when the temperature changes along vertically laid pipes.

ISO/DIS 18674-6:2026(en)
NOTE 2 Temperature might affect the behaviour of the structure. It is not easy to separate the effect on the
structure from the effect on the measurement.
5.1.7 The density of the liquid shall be considered.
NOTE If additives are used, this might affect the density .
5.1.8 Initial liquid column and expected settlement shall be considered when selecting the measuring
range of the pressure sensors.
5.2 Hydrostatic profiler with reservoir
5.2.1 A hydrostatic profiler with reservoir shall include the following components (see Figure 1): a sub
horizontal profile tube, a probe containing a pressure sensor, a permanently marked pull cord, a reel with an
internal reservoir, a hydraulic tube connecting the transducer to the liquid reservoir, a read-out device and a
stable base for the reel
NOTE 1 This is a manually operated system where the sensor records settlement at a fixed interval inside the
tubing.
NOTE 2 The horizontal tubing is installed at the level where one is interested in the settlement behaviour. This
implies that, although it is typically installed near the ground level, it can also be installed at different heights within
the ground, e.g. during the construction of an embankment.
NOTE 3 Typically 1 base is used, but it is possible to have a base at each end of the horizontal tubing.
NOTE 4 If the tube is only accessible from one end, a dead end pulley system or a pushing rod can be used to move
the sensor along the tube.
5.2.2 The level of the reservoir (inside the reel) shall remain constant during the survey.
NOTE 1 To obtain this, it is best to keep the reel in the same position during the survey.
NOTE 2 A level or pressure change in the reservoir can be caused by leakage.
5.2.3 The system design shall ensure that the height difference between the reservoir and the tube is
within the instrument range.
5.2.4 The tubing shall be installed avoiding unnecessary or local bending.
5.2.5 In between readings, the head of the tube shall be protected by a cap to prevent material getting into
the tube.
5.2.6 The pressure sensor can be vented or non-vented. In case a non-vented pressure sensor is used, the
atmospheric variation during the measurement should be considered.
NOTE In general, the atmospheric variations within the timeframe of a measurement are negligible.

ISO/DIS 18674-6:2026(en)
Key
1 pull cord
2 reference datum pin
3 base plate
4 embankment
5 cable reel
6 pressure sensor
7 profile tubing
8 hydraulic tube
Figure 1 — Example of a hydrostatic profiler set-up
5.3 Hydrostatic profiler without reservoir
5.3.1 Manually operated
5.3.1.1 A manually operated hydrostatic profiler without reservoir shall include the following components:
a sub horizontal profile tube filled with a liquid, a pressure sensor, a permanently marked pull cord, a read-
out device, and a base at the end of the horizontal tube.
NOTE 1 The sensor records the pressure head continuously while the sensor is pulled through the tubing and
paused at a fixed distance inside the tubing. A time log is made of the handling.
NOTE 2 The liquid level in the tubing is refilled to the maximum level possible before each survey and maintained
at the same level during the survey.
NOTE 3 It is good practice to measure the atmospheric pressure before and after each survey in case of a non-
vented pressure sensor.
5.3.2 In place hydrostatic system
5.3.2.1 An in place hydrostatic system without reservoir shall include the following components: a sub
horizontal profile tube filled with a liquid, an array of pressure sensors and a reading device.
NOTE 1 When the tube is open, the liquid level in the tube is checked and refilled periodically.
NOTE 2 The pressure sensors are installed at fixed positions.
NOTE 3 The pressure sensors can be installed on a river or seabed, taking into account waves, tidal and other water
level variations.
ISO/DIS 18674-6:2026(en)
NOTE 4 An in place hydrostatic system may be used when there is no access to the sensors
5.3.2.2 In case of a non-vented pressure sensor, the measurement shall be compensated for atmospheric
pressure.
5.3.2.3 When an in place hydrostatic system is located under water, the water pressure variation above
the sensors shall be measured and used as a reference.
NOTE 1 Typically, a pressure sensor is installed outside the tube on a location where no settlement is expected, to
measure the pressure variation above the tube.
NOTE 2 When measuring in the sea, the presence of waves and tides can affect the measurements. The influence
depends on the water depth, the wave height and the wave period.
5.4 Hydraulic settlement cell
5.4.1 A hydraulic settlement cell should include the following components: a settlement plate, a pressure
sensor attached to the plate, a data cable, a read-out unit or a logging device, a liquid filled tube and a
reference reservoir (see Figure 2).
NOTE 1 A reference survey is typically made and measured to ensure the stability and position of the reservoir.
NOTE 2 Multiple pressure sensors can be connected to the same reservoir.
NOTE 3 A hydraulic settlement cell is typically installed within the ground, e.g. underneath an embankment. A
hydraulic settlement cell is especially used at locations with difficult access.
NOTE 4 An alternative system exists where the hydraulic settlement cell is installed in a borehole or pushed into
the ground. In this case, there is no settlement plate.
NOTE 5 It is recommended to install a temperature sensor both at the location of the settlement cell and in areas of
vertically laid hydraulic lines if temperature fluctuations are to be taken into account.
5.4.2 The reference reservoir shall be open to allow atmospheric pressure within the reservoir.
5.4.3 The reservoir shall be protected against direct sun exposure, damage, leakage and displacements.
5.4.4 The liquid level in the reservoir should be checked and refilled periodically when necessary.
NOTE 1 A reference sensor within the reservoir is typically used to compensate for changes in the liquid level
within the reservoir.
NOTE 2 It can be necessary to fill the reservoir, e.g. to keep the liquid level above the reference sensor.
5.4.5 In case of a non-vented pressure sensor, the measurement shall be compensated for atmospheric
pressure.
ISO/DIS 18674-6:2026(en)
Key
1 embankment
2 mechanical protection
3 reference reservoir
4 liquid
5 settlement plate
6 pressure sensor
7 hydraulic tube
Figure 2 — Example of a hydraulic settlement cell set-up
5.5 Liquid level system
5.5.1 General
Two types of liquid level systems should be distinguished from each other:
— Closed liquid level system
— Open liquid level system
NOTE 1 Liquid level systems are typically installed on a structure, rather than in the ground.
NOTE 2 The readings of liquid levels system are significantly affected by temperature, and open systems are also
affected by liquid flow and evaporation of the liquid.
5.5.2 Open liquid level system
5.5.2.1 An open liquid level system shall include the following components: one or more liquid level
vessels, a liquid line, a vent line, a data cable, a level measuring system and a reference vessel (see Figure 3).
NOTE 1 Multiple liquid level vessels can be connected to each other.
NOTE 2 An open liquid level system is based on the principle of communicating vessels. When vertical displacements
occur on a location, there will be an active flow of liquid between the vessels. Due to the flow of the liquid there will
be a small delay between the movements and the stabilization of the liquid level. This delay prevents the system to be
used for dynamic measurements.
NOTE 3 Generally these systems have a limited measuring range with a high accuracy.
NOTE 4 The vent line is open to the atmosphere to allow liquid flow. The vent line interconnects the different
vessels, to guarantee an equal atmospheric pressure at all locations.

ISO/DIS 18674-6:2026(en)
Key
1 transducer housing
2 transducer vent line
3 vessel vent line
4 liquid level
5 simulated crack in wall
6 liquid line
7 liquid level vessel
Figure 3 — Example of an open liquid level system
5.5.2.2 The reference vessel shall be used to obtain relative settlements.
5.5.3 “Closed” liquid level system
5.5.3.1 A “closed” liquid level system shall include the following components: one or an array of pressure
sensors installed on a structure, a reservoir, a liquid line, a vent line and a data cable (see Figure 4).
NOTE 1 Temperature sensors help in the validation and analysis of the results and are typically included in the
sensor.
NOTE 2 A “closed” liquid level system has only one open end, located at the top of the reservoir. When vertical
displacements occur on a location, there will be no flow of liquid between the vessels. The displacement will result in a
pressure change at the location of the vessel.
NOTE 3 The vent line interconnects the pressure sensors to the open atmosphere at the location of the reservoir, to
guarantee an air pressure equal to the air pressure at the location of the reservoir.

ISO/DIS 18674-6:2026(en)
Key
1 reservoir
2 liquid
3 reference cell
4 pressure chamber
5 liquid line
6 relative position of cells
7 atmospheric compensation chamber
8 relative pressure sensor
9 vent line
a reference point
b cell without settlement
c cell with settlement
d cell with heave
Figure 4 — Example of a “closed” liquid level system
5.5.3.2 In case of a permanent installation, the liquid level in the reservoir shall be monitored to ensure that
a sufficient liquid level is maintained to a level higher than the highest sensor at any time, also considering
the maximum measuring range at the lowest sensor.
NOTE 1 The reference cell is used to measure the level in the reservoir.
NOTE 2 Any cell within the system, located in a stable area, can be used as a reference cell.
5.6 Measuring range, accuracy and repeatability
Table 2 gives an overview of the typical range and accuracy of the different hydraulic settlement sensors, as
well as some typical applications. The selection of hydraulic settlement system shall be in accordance with
Table 2 taking into account site conditions.
NOTE Table 2 does not represent the overall system accuracy. The total system accuracy is subject to site specific
conditions.
ISO/DIS 18674-6:2026(en)
Table 2 — Typical range, accuracy and application for hydraulic settlement sensors
Type Range Typical applications
Hydraulic profiler with Monitoring of large settlements profiles, e.g. underneath embank-
5 m - 20 m
reservoir (see 5.2) ments on soft soil
Hydraulic profiler with- Monitoring of large settlements profiles in case of significant
5 m - 50 m
out reservoir (see 5.3) height differences in the installed tube
Hydraulic settlement cell Monitoring of moderate and large settlements at point locations,
2 m – 10 m
(see 5.4) e.g. underneath dams
Open liquid level system Accurate settlement measurements on sensitive structures
50 mm – 600 mm
(see 5.5.2)
“Closed” liquid level Accurate monitoring of small and moderate settlements at point
100 mm-1 000 mm
system (see 5.5.3) locations on structures
NOTE 1 The range indicates the allowable absolute difference in height along the measurement profile
NOTE 2 The accuracy of the sensors is typically ±0,1 to 0,25 % FS.
6 Installation and measuring procedure
6.1 Installation
All measurements are relative and shall be measured related to a reference point.
6.1.1 Hydrostatic profiler with reservoir
6.1.1.1 The profile tube shall be able to withstand the pressure (from the construction) which will be
exerted on the tube during the full monitoring period.
6.1.1.2 The tube should be embedded in sand to avoid puncturing the tube.
6.1.1.3 The tube shall be sealed along its length to prevent entrance of material in the tube.
NOTE Typically, a continuous tube in one piece is unrolled, to avoid weak spots and obstructions at the location of
the (outer) couplings.
6.1.1.4 The diameter of the profile tube shall be chosen in relation to the diameter of the probe and the
expected (local) settlements.
NOTE Typically the inner diameter of the tube is chosen to be twice the diameter of the probe.
6.1.1.5 The profile tube should be installed in a trench, backfilled with sand to protect the tube from
damage, puncture or squeezing. Both ends of the tube shall be open, accessible and (slightly) higher than the
rest of the tube.
NOTE 1 A trench is dug to prevent damage during construction works.
NOTE 2 After digging the trench, a layer of sand at the bottom of the trench (5 to 10 cm) is added and compacted to
even out the trench and to prevent damage from anomalies within the ground.
6.1.1.6 The profile tube should be installed as straight as possible. Possible bends shall have a wide
curvature, to allow the profiler to pass under all circumstances.
6.1.1.7 The ends of the profile tube shall be protected during installation and covered in between the
measurement surveys to avoid damage and prevent any debris entering the profile tube .

ISO/DIS 18674-6:2026(en)
6.1.1.8 For best precision, the measurements should be taken at the same position during the full
monitoring project.
NOTE An alteration in the length of the profile tube can have an influence on the position of the probe and thus on
the measurement accuracy.
6.1.1.9 A reference point shall be created at least at one end of the profiling tube. This reference point
shall be accurately surveyed geodetically at each measurement survey.
NOTE 1 It is good practice to have a levelling marker at each side of the horizontal tubing, especially when 2 bases
are present.
NOTE 2 The movement of the reference point is used to calculate the movement of the base and to obtain absolute
displacements compared with the initial reading.
NOTE 3 Preferably the geodetic measurements are performed at the same time as the settlement measurement.
NOTE 4 The accuracy of the geodetic measurement has an influence on the measurement result.
6.1.2 Hydrostatic profiler without reservoir
6.1.2.1 The same conditions as in 6.1.1.1 to 6.1.1.3 shall be applicable for the profiling tube.
6.1.2.2 The profile tube should be installed in a trench, backfilled with sand to protect the tube from
damage, puncture or squeezing.
6.1.2.3 During installation extra care shall be taken to the shape of the profiling tube. Both ends of the
tube shall be open, accessible and at equal height and higher than the rest of the tube. The curvature of the
tube should avoid possible air pockets.
NOTE Air pockets within the tube influence the measurement results as different water pressures can be present
at each side of the air pocket.
6.1.2.4 The profiling tube shall be completely filled with a liquid before each survey. The liquid density
has an impact on the measurement and should be taken into account.
NOTE Depending on the situation, a choice can be made for a liquid which will not freeze.
6.1.2.5 A reference point shall be created near the lowest end of the profiling tube. This reference point
shall be accurately measured geodetically at each measurement survey and shall be coupled with the water
level in the profiling tube.
6.1.2.6 In case of (semi) permanent monitoring of an array of sensors, the sensors shall be installed at
fixed locations within the profiling tube.
NOTE 1 The ends of the tube are typically closed to prevent rapid evaporation of liquid. The closure should not be
tight, as this can lead to over or under pressure.
NOTE 2 In case of an installation on a riverbed or seabed, the sensors can be fixed to a heavy cable or chain and are
sunk to the bottom of the river. Another option is to fix the sensors to a cable at predetermined distances and to fix the
array in a tube. In the last case, the tube is sunk to the bottom.
6.1.3 Hydraulic settlement cell
6.1.3.1 The pressure sensor should be fixed on a base plate and shall be protected to withstand the
pressure of the backfill.
NOTE The base plate is typically placed in a local excavation and backfilled with sand to protect the sensor from
damage due to the construction works.

ISO/DIS 18674-6:2026(en)
6.1.3.2 Prior to installation of the hydraulic tube and the sensor cable, a trench shall be excavated. The
trench should slope upwards to the reservoir to allow air to escape.
6.1.3.3 The reference reservoir should be installed in an area outside the influence of settlement and shall
be geodetically monitored regularly to verify its stability.
6.1.3.4 The hydraulic tube and the reservoir should be filled gently to avoid air entrapment within the
system.
NOTE 1 A possibility to avoid air entrapment is to fill the system through the pressure chamber of the sensor, so it
is filled from the sensor towards the reservoir. Another possibility is to remove air bubbles from the system via the
sensor and to fill through the reservoir until liquid comes out at the sensor.
NOTE 2 Typically deaired water or a mixture of deaired water and antifreeze product is used to fill the system.
6.1.3.5 The tube and cable should be placed in a snake shape along the trench to help accommodate the
change of length due to excessive deformation.
6.1.3.6 The hydraulic tube and electrical cable shall be protected (e.g. with a protective tube and/or sand)
to prevent blockage or damage.
NOTE It is best practice to fill and test the tubing before protection, to confirm the proper function of the system
and to check for any air entrapment.
6.1.3.7 In case of a borehole installation, the sensor shall be fixed in the soil by a mechanical system or by
backfilling with grout or sand. Attention shall be paid to not modify the existing hydro-geological conditions.
6.1.4 Open liquid level system
6.1.4.1 When choosing the measuring range of the system, one should be aware of the feasibility of the
installation and the expected settlement of the structure.
6.1.4.2 The vessels shall be installed securely on the structure.
NOTE An open liquid level system is typically used on a structure.
6.1.4.3 Great care shall be taken during installation of the system, as an open liquid level system has a
limited measuring range (see Table 2).
NOTE Typically all sensors are installed at nearly the same elevation.
6.1.4.4 The reference reservoir shall be positioned slightly higher than the settlement points, taking into
account the limited measuring range. After settlement of the system, the reference vessel shall still be within
the measuring range of the system.
6.1.4.5 The reference vessel should be installed in an area outside the influence of settlement. The level of
the reference vessel shall be geodetically monitored regularly to verify its stability and to obtain absolute
settlements.
NOTE Independently verifying the level of one or more other vessels along the line helps to make a quality checks
and can be used to improve the accuracy of the measurements by applying a level adjustment/an error propagation.
6.1.4.6 The liquid line (interconnecting the vessels) shall be positioned lower than the vessels to avoid air
entrapment.
NOTE The diameter of the liquid line can influence the resistance of the liquid.

ISO/DIS 18674-6:2026(en)
6.1.4.7 The tubing should be installed as horizontal as possible to minimise temperature effects and liquid
flow. Vertical stretches in particular should be avoided.
6.1.4.8 Filling of the system shall be done on a very low flow rate to prevent overload of the pressure
sensors and additionally to prevent any air encapsulation along the line.
6.1.4.9 The end of the vent line shall be located at the reference vessel. Care should be taken to avoid any
obstruction of the vent line.
NOTE The vent line interconnects the vessel chambers to reach an equal barometric pressure at each location.
6.1.5 Closed Liquid level system
6.1.5.1 When choosing the measuring range of the system, one should be aware of the feasibility of the
installation and the expected settlement of the structure.
NOTE Typically the measuring range of a closed liquid level system is larger than that of an open liquid level
system, which results in a more flexible installation of the system.
6.1.5.2 The pressure sensors shall be installed securely on the structure.
6.1.5.3 The reservoir should be installed higher than the pressure sensors, taking into account expected
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