EN 14801 2006 64 e stf BRITISH STANDARD BS EN 14801 2006 Conditions for pressure classification of products for water and wastewater pipelines The European Standard EN 14801 2006 has the status of a B[.]
General
Conditions for determination of allowable pressures (see clause 6) are differentiated in:
Fittings, ferrules and valves may have different loading parameters from the pipe to take into consideration, when determining the allowable pressures.
Constant parameters
General
For the purposes of this document the parameters given in Table 2 are considered to be constant for all combined conditions (see 5.4).
Design life
The components must have a design life of at least 50 years, in accordance with EN 805, which is also used to determine allowable pressures Additionally, Product Standards may provide re-rating factors or procedures for adjusting the allowable pressures of components for different design lives.
Temperature
The continuous operating temperature for the determination of the allowable pressures shall be assumed
20 ºC If applicable, Product Standards shall specify re-rating factors or procedures for re-rating for other temperatures.
Negative pressure
Components shall be designed to withstand, when installed, transient pressure of 80 kPa below atmospheric (see EN 805).
Unit weight of soil
The unit weight for native soil, embedment and backfill shall be assumed constant at 20 kN/m 3 for all determinations.
Native soil
The type of native soil and its Relative Standard Proctor Density, D Pr , shall be assumed constant for all determinations (see clause 6)
Negative pressure (transient, below atmospheric) 80 kPa
Unit weight of soil (native soil, embedment, backfill) 20 kN/m 3
Relative Standard Proctor Density of native soil (D Pr ) b 100 % a Classification of soil types, see Annex B b If appropriate, equivalent values of Modified Proctor Density may be used.
Variable parameters
General
The variable parameters are the combination of several parameters, resulting from loading (see 5.3.2, e.g soil loads, traffic loads, ground water loads) and installation (see 5.3.3, e.g bedding, embedment, backfill)
Under loading conditions A, B, and C1, C2, compaction is performed directly against the trench wall, with any sheeting being gradually removed as required by road owners and authorities Consequently, the removal of sheeting does not influence the variable parameters.
For loading condition C3 (see Table 4), an embankment condition with no sheeting is specified.
Loading parameters
The loading parameters of the loading conditions A, B and C given in Table 4 are:
depth of cover (not less than 0,7m)
In Table 4, the figures given for the loading parameters, derived from traffic loads, are differentiated for the loading conditions A, B and C
Loading condition A is primarily influenced by the traffic load on a main road with a shallow cover depth during construction The traffic load, detailed in Table 4, represents the additional soil pressure at the pipe crown This assessment facilitates the evaluation of the effects of both good and moderate embedment conditions.
1) The additional pressure, derived from traffic loads at the surface level, takes into account the method of Boussinesq and the influence of impact according to EN 1295-1:1997, 5.2 In accordance with the method adopted by the European
The product standard for determining allowable pressures must take into account pipe/soil interaction as outlined in EN 1295-1:1997, section 5.2 Quality considerations dictate that loading condition A is to be combined with installation conditions 1 and 2, leading to the resultant combined conditions A1 and A2.
Loading condition B is characterized by the traffic load on an urban road with a shallow cover depth, reflecting the conditions during road construction The traffic load, detailed in Table 4, represents the additional soil pressure at the pipe crown To evaluate the effects of good and moderate embedment quality, loading condition B is analyzed in conjunction with installation conditions 1 and 2, leading to the combined conditions B1 and B2.
Loading condition C is primarily influenced by earth load, with traffic load in rural areas having a minimal effect due to the significant depth of cover The traffic load, detailed in Table 4, represents the additional soil pressure at the pipe crown To evaluate the impact of varying embedment quality—good, moderate, and poor—loading condition C is analyzed in conjunction with installation conditions 1, 2, and 3, leading to the combined conditions C1, C2, and C3.
Table 4 presents the compaction levels of the primary backfill, reflecting the average standards set by road owners and authorities across Europe, including main roads, urban streets, and rural areas.
For guidance on longitudinal effects see Annex A (informative)
NOTE 1 PFA etc does not incorporate longitudinal effects The results of longitudinal effects cannot always be corrected by altering the allowable pressure (PFA etc.)
NOTE 2 Due to unfavourable site conditions (e.g uneven trench bottom), differential settlements along the pipeline, either a step or a settlement in the trench bottom can occur; such conditions can cause damage to the components of the pipeline.
Installation parameters
The installation parameters of the installation conditions 1, 2 and 3 according to Table 4 are given in Figures 2,
If appropriate, instead of Relative Standard Proctor Density (D Pr ), equivalent values of Modified Proctor
3 Embedment, average D Pr = 96 % for calculation purpose
Figure 2 — Installation condition 1 for type of compaction “WELL” in the embedment (see Column 1 of
3 Embedment, average D Pr = 92 % for calculation purpose
Figure 3 — Installation condition 2 for type of compaction “MODERATE” in the embedment (see
3 Embedment, average D Pr = 85 % for calculation purpose
Figure 4 — Installation condition 3 for type of compaction “NONE” in the embedment (see Column 3 of
Combined conditions
The combined conditions (i e A1, A2; B1, B2; C1, C2, C3) shall consist of the following:
all constant parameters according to Table 2 and
a combination of the variable loading conditions (A, B, C) and the variable installation conditions (1, 2, 3), as applicable
The parameters listed in Table 2 and Table 4 encompass the majority of scenarios encountered following the installation of buried water supply and wastewater pressure pipelines across Europe.
Annex B presents a unified classification system for soil, while Annex C offers an example of a specific condition (B2) Additionally, Annex D provides insights into how the type of compaction relates to construction procedures.
If certain calculation methods need more information regarding bedding reaction angle, see Annex E (informative)
6 Determination of allowable pressures PFA, PMA, PEA
General
For convenience, EN 805:2000, Table 2 is repeated here
Table 3 — Pressure conditions for specifying components (copied from EN 805:2000, Table 2)
≥ 80 kPa ≤ 80 kPa below atmospheric pressure
For the determination of the allowable pressures (PFA, PMA, PEA), the conditions according to Table 4 (i e A1, A2; B1, B2; C1, C2, C3) consist of the following parameters:
1) all constant parameters (see Table 2) and
2) a combination of the variable loading conditions (A, B, C) and installation conditions (1, 2, 3) (see Table 4), as applicable.
Allowable pressures
Allowable operating pressure
Product Standards shall specify the allowable operating pressure (PFA) of the components (see EN 805)
Table 4 — Variable loading conditions and installation conditions for pressure classification purposes
Loading condition A Installation conditions (see Figures 2, 3, 4)
Additional pressure at pipe crown, derived from traffic load at surface including impact kN/m² 80 c
A3 not applicable due to inconsistency in the type of soil and compaction
Loading condition B Installation conditions (see Figures 2, 3, 4) condition unit trench
Additional pressure at pipe crown, derived from traffic load at surface including impact kN/m² 50 c
B3 not applicable due to inconsistency in the type of soil and compaction
Loading condition C Installation conditions (see Figures 2, 3, 4) condition unit Trench/em- bankment Compaction of backfill % D Pr a 85
Additional pressure at pipe crown, derived from traffic load at surface including impact kN/m² 5 c
(in trench) – at pipe crown
In certain cases, Modified Proctor Density values can be utilized instead of Relative Standard Proctor Density (D Pr) It is important to note that the assumed traffic pressures do not take into account the response of the installed pipe material Additionally, the notional value is considered without full harmonization of traffic loads, as referenced in EN 1991-2:2003, section 4.3, and is independent of the depth of cover.
Allowable maximum operating pressure
Product standards must define the maximum allowable operating pressure (PMA) for components, including surge considerations as outlined in EN 805 When calculating PMA, a surge allowance of at least 200 kPa (2 bar) is required For reference, the first part of EN 805:2000, section 11.3.2, is reiterated here.
Test pressure according to EN 805:2000, 11.3.2
"For all pipelines the System Test Pressure (STP) shall be calculated from the Maximum Design Pressure (MDP) as follows:
surge calculated: STP = MDP c + 100 kPa
surge non calculated: STP = MDP a x 1,5 or STP = MDP a + 500 kPa whichever is the least
The fixed allowance for surge pressure included in MDP a shall not be less than 200 kPa."
NOTE Product Standards should give information regarding admissible surge (e g number of surge cycles per day), if applicable.
Allowable site test pressure
Product Standards shall specify the allowable site test pressure (PEA) of the components (see EN 805).
Methods for the determination of allowable pressures
Each Product Standard must specify the method used to determine allowable pressures, which may include calculation methods such as the structural design method outlined in EN 1295-1, as well as test methods, and should incorporate a safety factor.
Procedure for the determination of allowable pressures
Figure 5 describes the procedure to determine the allowable pressures
Figure 5 — Procedure for the determination of allowable pressures
Product Standards shall specify for each pipe size and pipe type the applicable combinations of loading conditions (A, B, C) with installation conditions (1, 2, 3) This is in order to determine the relationship between
Method for the determination stated by Product Standards (structural calculations and/or test methods)
Product Standards do not necessarily need to specify all seven combinations of loading and installation conditions However, at least one installation condition for each loading condition shall be specified
Classifications (e.g PN) may continue to be used in Product Standards, but the relationship between e.g PN and PFA etc shall be specified in the Product Standards
Longitudinal effects
General
Buried pipes experience various loading types, leading to different responses While many design methods focus on the 2-D deformation, stresses, and strain in the ring-shaped cross section, pipelines are inherently 3-D structures Longitudinal effects become significant when successive cross sections vary in properties or loading conditions Although the primary concern is the strength and stability of the cross section for pressure containment, longitudinal effects often contribute to pipeline failures This chapter provides guidelines for addressing these longitudinal loading effects.
Failure behaviour due to longitudinal effects
The longitudinal effects address the following limit state conditions (see Table A.1)
Table A.1 — Overview of limit state conditions to be checked for longitudinal effects
Longitudinal effect Pipe failure Joint failure Gradient loss
Failure modes
Failure modes in water or sewage pipelines can lead to leakage and environmental contamination, as well as local scour that threatens nearby structures and other pipelines In sloped areas, the stability of slopes may be compromised, and excessive deformation of the pipeline can negatively impact the overall operability of the system.
Pipeline failures often occur due to the rupture of pipe materials or joints within the system While appendages installed in the pipeline are not the focus of this discussion, they do not significantly impact the overall safety assessment of the pipeline The majority of failures are attributed to the rupture or leakage of joints.
A.1.3.1 Pipeline failure instability of part of the pipeline structure Special attention shall be given to stresses and strains at bends, especially small radius and mitre bends, Tee pieces and other discontinuities in the pipeline due to stress intensification New pipeline material may be appropriate for the application, but ageing and other decay phenomena may affect the material such that after time the pipeline may not be fit for purpose and failures may occur
The aspect of durability over the envisaged lifetime of the pipeline shall be considered
Third-party damage is a significant cause of pipeline failure To enhance the durability of pipeline structures, it is essential to consider measures such as increased cover, additional wall thickness, alternative pipe or coating materials, and different routing options to prevent such damage.
The durability and integrity of the pipeline system must be maintained at the same quality level throughout its lifespan, as originally intended during the design and construction phases.
Through a preservative inspection program, the required quality level can be proven or through timely maintenance or operational measures the pipeline may be upgraded to the required quality level
For a continuous pipeline constructed from interconnected pipes, it is essential that the joints maintain the pipeline's properties and meet the minimum strength requirements specified for the pipes Additionally, special care must be taken to address any local discontinuities in the stiffness of the pipeline.
In mechanically jointed or articulated pipelines, joints may exhibit some movement; however, they must remain tight to ensure local stresses do not exceed the strength of the joint material It is crucial that axial displacements and angular rotations stay within the joint's design limits, allowing the joint to endure internal forces such as axial, shear, and bending moments, as well as local reaction forces from surrounding soil or structures Particular care should be taken to maintain joint tightness, especially in areas near bends, T-pieces, or transitions between different materials.
Significant pipeline displacements caused by external factors, such as mining subsidence, consolidation settlements, or soil shrinkage, can alter the pipeline's gradient It is crucial to account for these potential gradient changes in the design, especially when the pipeline's operation or discharge relies on maintaining an appropriate gradient.
Longitudinal effects
Longitudinal effects in pipelines, caused by pressure and local loadings, can lead to additional stress and deformation, known as the beam effect These effects may compromise the overall integrity of the pipeline and potentially result in failure modes.
The primary longitudinal effects are highlighted, but it's important to consider additional effects that may be relevant to specific cases Design and construction of the pipeline system, which includes straight pipes and all associated components, should be guided by sound engineering judgment.
Trench bottoms are rarely uniform, and the soil characteristics beneath pipelines can differ due to variations in soil formation and disturbance Transitions between construction methods or the presence of local rigid supports can make pipelines sensitive to these changes Ground anchors and local piled supports may act as rigid supports, necessitating consideration of construction subsidence based on workmanship quality to mitigate these effects.
Directional changes in piping systems can be achieved through manufactured bends, field bends, T-pieces, or by utilizing the rotational flexibility in joints during construction Cold spring bending, a form of elastic bending, is often employed to meet specific spatial constraints Additionally, elastic bending is commonly observed in towing siphons and horizontal directional drilling crossings.
However, unintended directional changes may result from (larger) soil movements
The operational loadings of the pipeline, including pressure and temperature variations, must be considered to guarantee adequate strength and local stability, while fully acknowledging the interaction between the pipe and the surrounding soil.
Analyzing internal forces in pipelines and joints is essential to ensure that the stressing and straining of the pipe material and joints remain within specified limits, especially when the joint's freedom of movement is restricted.
The pressure within a pipeline is primarily managed by the circumferential strength of its cross section, leading to hoop strain that causes axial contraction Conversely, axial thrust forces from pressure at bends, closed valves, or T-pieces can result in pipeline elongation In articulated pipelines, tensile-resistant joints counteract these thrust forces, transferring them to the soil through friction In straight articulated pipelines, only contraction remains significant Both thrust forces and contraction strains contribute to the overall load on the pipeline system, necessitating careful consideration to ensure tightness and safe operation.
Operational temperature may differ from the temperature at which the pipeline has been installed, the so- called construction temperature
The cooling of a buried pipeline causes the pipe material to contract, which can lead to the separation of joints However, the surrounding soil resists this contraction, resulting in a tensile axial force that reinforces the tensile stress caused by pressure loading.
An increase in temperature causes the pipeline to expand, leading to a reduction in tensile axial force or even the development of compressive forces due to soil counteraction High compressive forces can result in axial instability, particularly if there are initial alignment imperfections in the pipeline Additionally, free spans can contribute to this instability This issue is primarily significant in pipelines operating at elevated temperatures or in materials with a high thermal expansion coefficient.
Short radius bends in buried pipelines can lead to concentrated bending moments and increased soil pressure due to their shape and reduced bending stiffness It is advisable to avoid using bends with a radius less than R = 5D, as they require special attention to prevent potential issues.
When a pipeline is connected to rigid structures or T-connected to another pipeline, any local restraints can significantly increase axial displacements at the connection point.
In stiff soils expansion thrust forces at bends may cause concentrated lateral soil reactions that laterally and axially stress the cross section
Temperature influence on strength and stiffness properties of the pipe material shall be considered
The soil continuum surrounding buried pipelines can experience internal deformation due to factors such as mining activities While subsidence typically occurs vertically, significant deep subsoil deformations can also lead to horizontal shifts at the surface Sudden shear deformations, particularly in earthquake-prone regions, pose a risk to pipelines crossing fault lines Additionally, landslides and slow soil flow on slopes can further endanger pipeline integrity However, pipelines can be engineered to withstand these deformation loads effectively.
Consolidation settlements can occur due to additional loading from soil fills on compressible, water-saturated soil layers with low permeability When a pipeline's dead weight significantly exceeds that of the replaced soil, it imposes extra load on the subsoil, resulting in consolidation settlement Additionally, heavy soil backfill in a trench, compared to the specific weight of the surrounding native soil, can also contribute to subsoil consolidation.
In general consolidation settlements are a one-time soil body deformation There are, however, circumstances where there is a season dependent shrinkage and swelling
When assessing pipelines, it is crucial to focus on the variations in soil deformation along their length Special attention must be given to the use of rigid supports in regions prone to subsidence or settlement The downward movement of soil can increase the load on the pipeline due to the weight of the overburden topsoil, leading to significant bending at the junction where the pipeline transitions to a non-supported section.
Local lateral soil reactions exert forces on the cross section of pipelines, particularly affecting low pressure, thin-walled, large diameter pipes, which can experience significant stress as a result.