EN 1295-1:2019
(Main)Structural design of buried pipelines under various conditions of loading - Part 1: General requirements
Structural design of buried pipelines under various conditions of loading - Part 1: General requirements
This standard specifies the requirements for the structural design of water supply pipelines, drains and sewers, and other water industry pipelines, whether operating under atmospheric, greater of lesser pressure. In addition, this standard gives guidance on the application of the established methods of design used in CEN member countries at the time of preparation of the standard. - The Decision was decided for Annex B only. -
Statische Berechnung von erdüberdeckten Rohrleitungen unter verschiedenen Belastungsbedingungen - Teil 1: Allgemeine Anforderungen
Dieses Dokument legt Anforderungen für die statische Berechnung von Rohrleitungen für die Wasserversorgung, für Abwasserkanäle und -leitungen und andere in der Wasserwirtschaft verwendete Leitungen fest, die mit atmosphärischem oder einem höheren oder niedrigeren Druck betrieben werden.
Zusätzlich enthält dieses Dokument Anleitungen zur Anwendung der zum Zeitpunkt der Erarbeitung dieses Dokuments in den CEN-Mitgliedsländern national eingeführten Berechnungsverfahren.
Diese Anleitungen sind eine wichtige Quelle konstruktiver Erkenntnisse, können jedoch nicht alle möglichen Sonderfälle berücksichtigen; in solchen Fällen können Erweiterungen oder Einschränkungen der grundlegenden Berechnungsverfahren gelten.
Da in der Praxis zum Zeitpunkt der Berechnung nicht immer genaue Einzelheiten bezüglich der Boden- und Einbaubedingungen vorliegen, bleibt die Wahl der Berechnungsannahmen dem Ermessen des Planers überlassen. In diesem Zusammenhang können diese Anleitungen nur allgemeine Anhaltspunkte und Empfehlungen geben.
Dieses Dokument enthält die Anforderungen für die statische Berechnung und verweist auf die Quellen sowie die Grundlagen der national eingeführten Berechnungsverfahren (siehe Anhang A und Anhang B).
Calcul de résistance mécanique des canalisations enterrées sous diverses conditions de charge - Partie 1: Prescriptions générales
La présente Norme européenne spécifie les prescriptions pour le calcul de résistance mécanique des conduites d’alimentation en eau, des canalisations d’évacuation et d’assainissement et autres tuyaux de l’industrie de l’eau, fonctionnant soit à la pression atmosphérique, soit à des pressions supérieures ou inférieures.
En outre, la présente Norme européenne donne des indications pour l’application des méthodes de calcul nationalement reconnues, déclarées et utilisées dans les États membres du CEN au moment de sa préparation.
Ce guide est une source importante d’expertise sur le calcul de résistance mécanique, mais il ne peut inclure tous les cas spéciaux, pour lesquels des extensions ou des restrictions des méthodes de calcul de base peuvent s’appliquer.
Puisque dans la pratique des détails précis sur les types de sols et sur les conditions de pose ne sont pas toujours connus au stade du projet, le choix des hypothèses du projet est laissé à l’appréciation de l’ingénieur. De ce fait, le guide peut seulement donner des indications et des conseils généraux.
La présente Norme européenne spécifie les prescriptions pour le calcul structurel et indique les références et les principes de base des méthodes de calcul nationalement reconnues (voir Annexes A et B).
Projektiranje vkopanih cevovodov pri različnih pogojih obremenitve - 1. del: Splošne zahteve
Ta dokument določa zahteve za konstrukcijsko zasnovo vodovodnih cevi, odtokov in kanalizacije ter drugih industrijskih vodovodnih cevi, ki delujejo pri atmosferskem, večjem ali manjšem tlaku.
Poleg tega ta dokument podaja smernice za uporabo nacionalnih postopkov načrtovanja, ki so jih v času priprave tega dokumenta razglasile in jih uporabljajo države članice CEN.
Te smernice so pomemben vir strokovnega znanja o načrtovanju, vendar ne morejo vključevati vseh možnih posebnih primerov, v katerih se lahko uporabljajo razširitve ali omejitve osnovnih metod načrtovanja.
Ker v praksi podrobnosti o vrsti tal in pogojih namestitve niso vedno na voljo že v fazi načrtovanja, je izbira predpostavk pri načrtovanju prepuščena presoji inženirja. Zato lahko ta priročnik podaja samo splošne smernice in nasvete.
Ta dokument določa zahteve za konstrukcijsko zasnovo in podaja reference in osnovna načela nacionalnih postopkov načrtovanja (glej dodatka A in B).
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-julij-2019
Nadomešča:
SIST EN 1295-1:1998
Projektiranje vkopanih cevovodov pri različnih pogojih obremenitve - 1. del:
Splošne zahteve
Structural design of buried pipelines under various conditions of loading - Part 1: General
requirements
Statische Berechnung von erdverlegten Rohrleitungen unter verschiedenen
Belastungsbedingungen - Teil 1: Allgemeine Anforderungen
Calcul de résistance mécanique des canalisations enterrées sous diverses conditions de
charge - Partie 1: Prescriptions générales
Ta slovenski standard je istoveten z: EN 1295-1:2019
ICS:
93.025 Zunanji sistemi za prevajanje External water conveyance
vode systems
93.030 Zunanji sistemi za odpadno External sewage systems
vodo
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EN 1295-1
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2019
EUROPÄISCHE NORM
ICS 23.040.01 Supersedes EN 1295-1:1997
English Version
Structural design of buried pipelines under various
conditions of loading - Part 1: General requirements
Calcul de résistance mécanique des canalisations Statische Berechnung von erdüberdeckten
enterrées sous diverses conditions de charge - Partie 1: Rohrleitungen unter verschiedenen
Prescriptions générales Belastungsbedingungen - Teil 1: Allgemeine
Anforderungen
This European Standard was approved by CEN on 14 January 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, 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 1295-1:2019 E
worldwide for CEN national Members.
Contents
European foreword . 5
Introduction . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
3.1 Installation terms . 7
3.2 Design terms . 9
4 Requirements . 9
5 Basis of design procedures . 10
5.1 General . 10
5.2 External loads . 10
5.3 Limit states . 11
5.4 Longitudinal effects . 11
6 Additional considerations for pressure pipelines . 11
6.1 General . 11
6.2 Stresses and strains resulting from simultaneous loads . 12
6.3 Effect of pressure on deformation . 12
6.4 Buckling of pressure pipes . 12
6.5 Thrusts and longitudinal stresses . 13
7 Influence of construction procedures . 13
7.1 General . 13
7.2 Trenching procedures . 13
7.3 Pipe bedding . 13
7.4 Filling procedures . 14
8 Design philosophies and factors of safety . 14
Annex A (informative) Pipe definition according to cross-sectional behaviour . 15
Annex B (informative) Nationally established methods of design . 16
B.1 Identification of methods and addresses where they are available . 16
B.1.1 Austria . 16
B.1.2 Belgium . 16
B.1.3 Denmark . 16
B.1.4 Finland . 17
B.1.5 France . 17
B.1.6 Germany . 18
B.1.7 Netherlands . 18
B.1.8 Norway . 19
B.1.9 Poland. 19
B.1.10 Spain . 19
B.1.11 Sweden . 20
B.1.12 Switzerland . 20
B.1.13 United Kingdom . 21
B.2 Description of methods . 21
B.2.1 Austria . 21
B.2.1.1 Application . 21
B.2.1.2 Basic input data . 21
B.2.1.3 Structural design . 21
B.2.1.4 Loading . 21
B.2.1.5 Types of pipes . 22
B.2.1.6 Method of calculation . 22
B.2.1.7 Required analysis . 22
B.2.2 Belgium. 23
B.2.2.1 Application . 23
B.2.2.2 Basic input data . 23
B.2.2.3 Structural design . 23
B.2.2.4 Loading . 23
B.2.2.5 Type of pipes . 23
B.2.2.6 Method of calculation . 23
B.2.2.7 Safety factors . 24
B.2.3 Denmark . 24
B.2.3.1 Loads . 24
B.2.3.2 Safety . 25
B.2.3.3 Partial safety factors . 26
B.2.3.4 Calculations. 26
B.2.4 Finland . 26
B.2.5 France . 26
B.2.6 Germany . 27
B.2.7 Netherlands . 28
B.2.8 Norway. 28
B.2.8.1 Design of rigid pipes according to internal reports 1521 and 1554 . 28
B.2.8.1.1 Earth load . 28
B.2.8.1.2 Traffic load . 29
B.2.8.2 Design of buried plastic pipes according to VAV P 70 (Swedish standard) . 29
B.2.9 Poland . 29
B.2.9.1 Classification of pipes . 29
B.2.9.2 Limit states considered . 29
B.2.9.3 Assessment of loads . 29
B.2.9.4 Design of buried pipes . 29
B.2.9.5 Nomographs for simplified design . 30
B.2.10 Spain . 30
B.2.10.1 Concrete pipes. 30
B.2.10.2 Plastic pipes . 30
B.2.11 Sweden . 30
B.2.11.1 Design of buried plastics pipes according to Svenskt Vatten P92 . 30
B.2.11.1.1 Soil load . 30
B.2.11.1.2 Traffic load . 31
B.2.11.1.3 Short-term deflection . 31
B.2.11.1.4 Long-term deflection . 31
B.2.11.1.5 Strain . 31
B.2.11.1.6 Buckling . 31
B.2.11.1.7 Nomographs for simplified design . 31
B.2.11.2 Design of rigid pipes according to Svenskt Vatten P99 . 31
B.2.11.2.1 General. 31
B.2.11.2.2 The vertical loads considered are: . 32
B.2.11.2.3 Horizontal loads . 32
B.2.12 Switzerland . 32
B.2.13 United Kingdom . 32
B.2.13.1 Classification of pipes . 32
B.2.13.2 Design aids . 32
B.2.13.3 Assessment of loads . 33
B.2.13.4 Limit states considered . 33
B.2.13.5 Factors of safety . 33
Bibliography . 34
European foreword
This document (EN 1295-1:2019) has been prepared by Technical Committee CEN/TC 165 “Waste
water engineering”, the secretariat of which is held by DIN.
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 October 2019, and conflicting national standards shall
be withdrawn at the latest by October 2019.
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.
This document supersedes EN 1295-1:1997.
The principal change in this revision is the following:
a) Annex B “Nationally established methods of design” has been updated.
This standard is intended for use in conjunction with the series of product standards covering pipes of
various materials for the water industry.
This standard comprises two parts:
— Part 1, General requirements: it deals with the requirements for structural design of pipelines and
gives the basic principles of the nationally established methods of design;
— Part 2, Summary of the nationally established methods of design: it gives an overview of these
methods as prepared by the various countries where they are in use.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Introduction
The structural design of buried pipelines constitutes a wide ranging and complex field of engineering,
which has been the subject of extensive study and research, in many countries over a period of very
many years.
Whilst many common features exist between the design methods which have been developed and
established in the various member countries of CEN, there are also differences reflecting such matters
as geological and climatic variations, as well as different installation and working practices.
In view of these differences, and of the time required to develop a common design method which would
fully reflect the various considerations identified in particular national methods, a two stage approach
has been adopted for the development of this European Standard.
In accordance with this two stage approach, the Joint Working Group, at its initial meeting, resolved
“first to produce an EN giving guidance on the application of nationally established methods of
structural design of buried pipelines under various conditions of loading, whilst working towards a
common method of structural design”. This standard represents the implementation of the first part of
that resolution.
1 Scope
This document specifies the requirements for the structural design of water supply pipelines, drains
and sewers, and other water industry pipelines, whether operating at atmospheric, greater or lesser
pressure.
In addition, this document gives guidance on the application of the nationally established methods of
design declared by and used in CEN member countries at the time of preparation of this document.
This guidance is an important source of design expertise, but it cannot include all possible special cases,
in which extensions or restrictions to the basic design methods may apply.
Since in practice precise details of types of soil and installation conditions are not always available at
the design stage, the choice of design assumptions is left to the judgement of the engineer. In this
connection the guide can only provide general indications and advice.
This document specifies the requirements for structural design and indicates the references and the
basic principles of the nationally established methods of design (see Annexes A and B).
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply (see also Annex A).
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 Installation terms
The same définitions apply for trenches with vertical or sloping sides and pipes laid below
embankements. Some of these terms are illustrated in Figure 1.
Key
1 surface 10 depth of cover
2 bottom of road or railway construction, if any 11 depth of bedding
3 trench walls 12 depth of embedment
4 main backfill 13 trench depth
5 initial backfill
6 sidefill a thickness of lower bedding
7 upper bedding b thickness of upper bedding
8 lower bedding c thickness of initial backfill
9 trench bottom OD is the vertical outside diameter
v
NOTE The terms in Figure 1 are the same as in EN 1610.
Figure 1 — Trench installation
3.1.1
compaction
deliberate densification of soil during the construction process
3.1.2
consolidation
time-dependent densification of soil by processes other than those deliberately applied during
construction
3.1.3
embedment
fill around the pipe including bedding, sidefill and initial backfill
Note 1 to entry: it is the part of the trench which contributes to the structural performance of the buried pipeline.
3.2 Design terms
3.2.1
bedding factor
ratio of the maximum design load for the pipe, when installed with a particular embedment, to the test
load which produces the same maximum bending moment
3.2.2
design pressure
DP
maximum operating internal pressure of the system or of the pressure zone fixed by the designer
considering future developments but excluding surge
3.2.3
load bearing capacity
load per unit length that a particular combination of pipe and embedment can sustain without
exceeding a limit state
3.2.4
maximum design pressure
MDP
maximum operating internal pressure of the system or of the pressure zone fixed by the designer
considering future developments and including surge, where:
— MDP is designated MDPa when there is a fixed allowance for surge;
— MDP is designated MDPc when the surge is calculated
3.2.5
silo effect
effect whereby lateral earth pressure in trench backfill causes friction at the trench wall to carry part of
the weight of the backfill
3.2.6
soil-structure interaction
process whereby the deformations of soil and/or pipe caused by the contact and reaction pressures
between a pipe and the surrounding soil distribute the pressures to achieve equilibrium
3.2.7
system test pressure
STP
hydrostatic pressure applied to a newly laid pipeline in order to ensure its integrity and tightness
4 Requirements
4.1 All pipelines shall be designed to withstand the various loadings to which they are expected to be
subjected, during construction and operation, without detriment to their function and to the
environment.
4.2 The future owner of the pipeline is free to specify the appropriate method of design to be
adopted.
4.3 The designer shall determine whether or not the pipeline comes within the scope of the methods
covered by this European Standard.
4.4 The design adopted shall be such that construction may be carried out safely and so as to ensure
that the design assumptions regarding the influence of construction procedures and soil characteristics
will be satisfied.
4.5 Subject to the other requirements of Clause 4, design should be carried out preferably using in its
entirety one of the methods in Annex B of this European Standard.
4.6 Methods of design, in accordance with Annex B, when presented in the form of tables, charts or
computer programmes shall be deemed equivalent to a full calculation, provided that any simplification
does not reduce the level of safety below that which would be obtained by full design. Outputs from
computer programmes shall be capable of verification.
4.7 Whichever the design method used, the designer shall satisfy himself that the method constitutes
a coherent system and provides the accepted level of safety.
4.8 Account shall be taken of the probable consequences of pipeline failure in establishing the
acceptable level of safety.
4.9 The values adopted for all variables, including factors of safety, shall be in accordance with the
method used.
5 Basis of design procedures
5.1 General
Whilst there are differences between some of the established national design procedures, there are no
differences in respect of the fundamental basis of design, which is the interactive system consisting of
the pipe and the surrounding soil.
The external loadings to be considered shall include that due to the backfill, that due to the most severe
surface surcharge or traffic loading likely to occur, and those due to any other causes, producing a
loading of significant magnitude such as self-weight of the pipe and water weight, as appropriate. The
internal pressure in the pipeline, if different from atmospheric, shall also be treated as a loading.
The design of the pipeline, and its embedment, shall provide an adequate level of safety against the
appropriate ultimate limit state being exceeded. In addition, the design loading shall not result in any
appropriate serviceability limit state being exceeded.
5.2 External loads
Account shall be taken of the effect of the stiffness of the pipe and the stiffness of the surrounding soil.
Where appropriate, account shall be taken of the effects of trench construction, of groundwater and of
time dependent influences. The design should take into consideration, however, the possible effect on
trench conditions of any further planned works.
The effective pressure due to the backfill and any distributed surface loads shall be calculated on the
basis of the principles of soil-structure interaction.
The pressure exerted on pipelines by concentrated surface surcharges, such as vehicle wheels, shall be
calculated in accordance with a method based on Boussinesq, and account shall be taken of impact.
5.3 Limit states
The ultimate limit state for all types of pipe is reached when the pipe ceases to behave in the manner
intended in the structural design.
Serviceability limit states may be dictated by effects either on the performance of pipelines or on their
durability (for example leakage, deformation or cracking beyond allowable limits).
Additional serviceability limit states may apply to particular pipe materials, and reference shall be made
to the relevant standards.
The design of the pipeline shall ensure that these above limit states are not reached. This will include
consideration of one or more of the following factors:
— strain, stress, bending moment and normal force or load bearing capacity, in the ring or
longitudinal direction as appropriate;
— instability (e.g. buckling);
— annular deformation;
— watertightness.
Where fluctuating loads of significant magnitude and frequency or repetitive loads will exist,
appropriate consideration should be given in the calculation and the material behaviour
5.4 Longitudinal effects
Longitudinal effects include bending moments, shear forces and tensile forces resulting for example
from non uniform bedding and thermal movements and, in the case of pressure pipelines (see 6.5), from
Poisson's contraction and thrust at change of direction or cross-section.
These effects may be accommodated by the angular deflection and/or the shear resistance of flexible
joints and by the flexural strength of pipes, the serviceability limits of which should be obtained from
the different product standards.
The designer shall check that these provisions, together with the embedment design, are sufficient for
the project and, where needed, specify adequate additional measures.
6 Additional considerations for pressure pipelines
6.1 General
Pipelines operating at internal pressures above or below atmospheric are subjected to loadings in
excess of those at atmospheric pressure.
The application of internal pressure not only introduces additional stresses and strains in the
circumferential direction, but can also modify the deformation of flexible and semi-rigid pipes. In
addition, pressure pipelines, containing changes of direction or other discontinuities, shall be designed
for the longitudinal tensile loading, or the thrusts at the discontinuities.
Special consideration shall be given to pipelines which will be subject to transient surge pressures. Both
positive and negative transient pressures shall be considered, but it may not be appropriate for these to
be taken in combination with the full vehicle surcharge load.
The design shall take account of the design pressure, the maximum design pressure, and the system test
pressure (see 3.2).
Pressure pipelines shall also satisfy the design criteria which would apply if they were non-pressure
pipelines, in order to ensure their satisfactory structural performance for the initial period between
construction and the application of the internal water pressure, and subsequently when emptied for
maintenance.
6.2 Stresses and strains resulting from simultaneous loads
Internal pressures above or below atmospheric produce circumferential stresses and strains which act
simultaneously with bending stresses and strains due to external loadings.
Design cases to be considered depending on pipe material and/or type and respective load intensities,
can be one or more of the following:
— circumferential stresses resulting from combined loads;
— circumferential strains resulting from combined loads;
— separate analysis of circumferential stresses or strains.
Similar cases shall be considered for the longitudinal direction, when appropriate.
NOTE If the cross-section of the pipe is truly circular, circumferential stresses and strains due to internal
pressure will be purely tensile or compressive, but if the pipe cross-section is not truly circular or has been
deformed there will also be bending stresses and strains due to internal pressure.
6.3 Effect of pressure on deformation
When positive internal pressure is applied to a not truly circular pipe, it tends to re-round the deformed
pipe, i.e. to reduce the out-of-circle deformations.
The re-rounding process may have the beneficial effect of reducing the bending stresses and strains in
the pipe wall. The extent to which the re-rounding process reduces pipe deformation depends on pipe
property and on other various factors, such as the ratio of the internal pressure to the external pressure
and the amount of consolidation of the soil which has taken place around the pipe. Thus, the beneficial
effects of re-rounding are likely to be greater if the pressure is applied soon after backfilling, and less if
there is a longer delay until the first pressurization.
Although the application of internal positive pressure will always produce some degree of re-rounding,
the magnitude is difficult to predict. Also, although pipe ovalization benefits from internal pressure,
stresses and strains may not benefit to the same extent (e.g. when the deflected shape is not perfectly
elliptical).
6.4 Buckling of pressure pipes
Positive internal pressure assists pipes which are not rigid to resist any tendency to buckle, but since
there can never be complete certainty that the pressure may not be removed at some time during the
life of the pipeline, it is normal to design pipelines to resist buckling without this assistance.
Pipelines subject to hydraulic transients may experience sub-atmospheric pressures, and, although
these are usually of very short duration, they tend to increase the tendency to buckle.
Proper account shall be taken of this possibility in the design of such pipelines, and it is preferable to
rely on a conservative estimate of the sub-atmospheric pressure. When calculating stability, the sub-
atmospheric pressure shall be added to the external pressure caused by sustained loading.
6.5 Thrusts and longitudinal stresses
A further effect of the application of internal pressure in pipes is the generation of thrusts at bends and
other discontinuities. Depending on the type of provision made for resisting these thrusts, the pipes and
fittings may be subjected to additional longitudinal bending and/or tensile stresses, and to excessive
movement which could cause dislocation of joints.
7 Influence of construction procedures
7.1 General
Of the various factors to be considered in the structural design process, some, such as pipe diameter
and depth of cover, can be regarded as entirely under the control of the designer. Other factors, such as
the methods adopted for trench excavation and for filling around and above the pipeline, are only under
the control of the designer to the extent that they are specified in advance, and supervised during
construction.
7.2 Trenching procedures
The width of the trench can influence the extent to which the backfill load may be reduced by the silo
effect, and this effect is taken into account for certain applications.
The width of the trench can also influence the quality of the lateral soil support at the sides of the pipes.
This effect is variously covered in the design procedures, via the coefficient of lateral earth pressure, the
bedding factor, the soil modulus, etc.
The slope of the trench sides can affect the magnitude of the backfill load, and, if vertical trench sides
are employed, consideration shall also be given to the method of support.
If the trench supports are withdrawn after embedding and/or backfilling, voids are left which can cause
loosening of the soil, reducing the quality of the embedment and the friction on which the silo effect
relies, and also promote long term settlement.
The presence of groundwater, and the use of measures such as ground water lowering to remove it
during construction, can have important effects. The absence of groundwater assists in the compaction
of backfill, but the subsequent return of ground water after completion of backfilling can cause
movements of soil particles, possibly leading to increased loads and reduction of support to the sides of
the pipe.
7.3 Pipe bedding
If the nature of the ground at the base of the trench is such that it will not itself provide adequate
support, then, for all types of pipe, the thickness of lower bedding shall be designed to ensure adequate
support along the length of the pipeline.
Where pipes are installed in soft ground, the thickness of the lower bedding may need to be increased in
order to prevent excessive settlement of the pipeline.
The thickness of upper bedding should be such as to ensure that the bending moments in the pipe (as
calculated directly or covered by the bedding factor) are acceptable.
7.4 Filling procedures
In the vicinity of the pipe, the placing and compaction of the fill material have very great influence on
structural performance. They affect the distribution of soil pressure around the circumference of the
pipe, and hence the response of the pipe. The amount of compaction applied initially during installation
also affects the amount of settlement which will take place later, as a result of natural consolidation, or
consolidation accelerated by traffic. Usually, the larger such settlements, the greater the load which will
be transferred to the pipe.
When the soil around the pipe is being compacted in order to improve its structural quality, some of the
energy is diverted into the pipe (as strain energy of deformation) and some into the native soil. The
extent to which the total compaction energy is so diverted depends upon the pipe-soil stiffness ratio
and the type of native soil.
Prediction of these effects is difficult and is further complicated by the sensitivity of some soils to
moisture content. The use of soils which are easy to compact, and which have low sensitivity to
moisture content, can therefore greatly reduce the magnitude of strains developed in pipes as a result of
installation.
8 Design philosophies and factors of safety
Field and experimental studies of pipelines show variations in observed earth pressures and pipe
deformations, stresses and strains. The main cause of these variations is the inevitable inconsistency of
soil characteristics and construction practices, already described in Clause 7 of this European Standard.
The magnitude of the variation can be reduced by good supervision, control measurement and by the
use of fill materials which are easily placed and treated, but some degree of variation is inevitable.
Variations in pipe characteristics, such as strength or elasticity, also occur in practice.
Appropriate allowance for these variations should be made at the design stage and be in accordance
with one of the following design philosophies:
a) The design procedure shall aim to predict the mean values of loads, and shall compare these with
the load bearing capacity of the pipeline based on mean values of pipe strength or stiffness (for
example as derived by calculation), and on average earth pressure distribution assumptions.
b) The design procedure shall aim to predict the maximum possible (high fractile or upper bound)
values of loads, and shall compare these with estimates of the load bearing capacity of the pipeline
based on lower bound (or low fractile) values of pipe strength or stiffness (for example as
established by testing), and on unfavourable earth pressure distribution assumptions.
The factors of safety to be employed with designs following philosophy b) will be lower than those used
in a), to achieve the same probability of failure.
Annex A
(informative)
Pipe definition according to cross-sectional behaviour
The definition according to cross-sectional behaviour of pipes as rigid, semi-rigid or flexible, is
essentially based on consideration of the structural performance of the pipe cross-section under
external loads.
Some nationally established methods of design distinguish between “flexible”, “semi-rigid” and “rigid”
pipes on the basis of the relative pipe and surrounding soil stiffnesses. This distinction is particularly
useful in the evaluation of the backfill load for which the pipeline should be designed.
In other nationally established methods of design, the distinction between “flexible” and “rigid” is based
on the type of material from which the pipe is made, and the way in which the material is used. Thus
pipes whose material would fracture at only small deformations of the pipe cross-section are regarded
as “rigid”, whilst pipes whose cross-sections can deform substantially without fracture are regarded as
“flexible”.
Designers should take account of both considerations, and recognize that the definition of a pipe as
“rigid” or “flexible” according to one approach may not invariably be associated with the same
definition in the other approach. Having selected the design procedure to be employed, designers
should use the method of definition incorporated in that procedure.
Whilst materials can be defined as flexible or rigid according to their failure strain, a pipe made from a
material with a low failure strain will not necessarily be defined as rigid. Materials which fail at low
elongations, if used in thin-walled pipe, may produce very flexible pipes, because the deformation of the
pipe cross-section correspondin
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