CD 533 Determination of pipe and bedding combinations for drainage works

This document is published by National Highways.

This document supersedes HA 40/01, which is withdrawn.

This document forms part of the works specification. It does not purport to include all the necessary provisions of a contract. Users are responsible for applying all appropriate documents applicable to their contract.

The requirements and advice given in this document are based upon the guidance in BS EN 1295-1 [Ref 22.N]. Further guidance for the structural design of buried pipelines in the UK can be found in BS 9295 2010 [Ref 7.N].

BS EN 1295-1 [Ref 22.N] contains a National Annex A which states the vehicle loadings to be used for pipeline design. These loadings were derived from BS 5400-2 2006 [Ref 21.N], which has since been revised. The loadings reported in BS 5400-2 2006 [Ref 21.N] are still employed in pipeline design as they give a more conservative result than loadings derived from newer standards.

The charts and tables given at the appendices are based on BS 5400-2 2006 [Ref 21.N] and BS 9295 2010 [Ref 7.N]. These charts and tables provide a simplified method for defining the pipe and bedding combinations required for permitted vehicle loading requirements and to be compliant with the range of permitted combinations given in MCHW Series 0500 [Ref 9.N] and MCHW HCD Series F [Ref 11.N].

MCHW Series 0500 [Ref 9.N] permits a range of pipes of different material properties to be used for drainage works in highways. MCHW HCD Series F [Ref 11.N] also allows a choice in the method of bedding pipes where the degree of structural support given to the pipe when it is laid in a trench varies. Not all possible combinations of pipe and bedding selected from the permitted options will be suitable for a particular design solution. Permitted combinations are therefore specified for each contract.The charts given in the appendices to this document are intended for a wide range of pipe bedding combinations for a range of manufactured pipe thicknesses, possible trench widths and bedding factors.

The assumptions made in GG 101 [Ref 8.N] apply to this document.

1.1 The design and selection of type of pipe and bedding to meet the loading requirements from the permitted alternatives in MCHW Series 0500 [Ref 9.N] shall be in accordance with:

NOTE The requirements presented in Sections 2 and 3, in addition to the associated design charts and guidance presented in Appendix A to this document, provide a simplified method for selecting pipe and bedding combinations.

1.2 This document shall apply to the following type of pipes:

NOTE Refer to Appendix A for Figures A.1, A.2 and A.3 which detail bedding/pipe combinations.

1.3 Hydraulic design requirements for pipes are not covered in this document and shall be in accordance with DMRB CG 501 [Ref 5.N].

1.4 This document shall apply to pipes up to 900 mm internal diameter.

1.5 Pipes greater than 900 mm diameter, or multi-cellular pipe structures, shall be designed with reference to DMRB CG 300 [Ref 23.N].

1.6 This document shall be implemented forthwith on all schemes involving the use of buried pipes on the Overseeing Organisations' motorway and all-purpose trunk roads according to the implementation requirements of GG 101 [Ref 8.N].

1.7 The requirements contained in GG 101 [Ref 8.N] shall be followed in respect of activities covered by this document.

3.1 Bed type materials shall be specified in accordance with MCHW Series 0500 [Ref 9.N].

3.1.1 Bed types A, B, F, N and S shown in the HCD drawing "Surface Water Drains" MCHW HCD Series F [Ref 11.N] should be specified for rigid pipes and bed types S and T with flexible pipes.

NOTE 1 Type Z is for use with any type of pipe for permanent protection against mechanical damage.

NOTE 2 Type T bedding for rigid pipes is less likely to be economic for the same degree of support.

NOTE 3 Pipe bedding detail is given in the drawings for carrier drains and filter drains MCHW HCD Series F [Ref 11.N].

NOTE 4 Different bedding combinations provide varying degrees of support to the pipe. In the case of combined surface and sub-surface drains the granular bedding and surround additionally functions as a filter medium.

3.2 Flexibly jointed pipes shall be specified with a compressible filler to break the Type Z surround at every joint or second joint for pipes shorter than standard proprietary length.

NOTE 1 All bed types shown in the drawing "Filter Drains" MCHW HCD Series F [Ref 11.N] are suitable for both rigid and flexible pipes.

NOTE 2 The specification requirements for bedding, backfill material and construction are given in MCHW Series 0500 [Ref 9.N].

3.3 Main road loading (which include motorways and all-purpose trunk roads) shall be applicable to pipes under pavements including cross drains and for pipes in verges where heavy loads can occur.

3.4 Main road loading shall consist of eight wheel loads, each of 112.5 kN including impact factor, in the HB loading pattern corresponding to 45 units of HB loading in accordance with BS 5400-2 2006 [Ref 21.N] as reflected by BS 9295 2010 [Ref 7.N].

3.5 Field loading shall apply to fields and lightly trafficked access tracks.

3.6 Field loading shall consist of two wheels 1.0 m apart each of a static load of 30 kN with an impact factor of 2.0 giving a total load for each wheel of 60 kN.

3.7 Filter drain loading shall be applicable to pipes in filter drains in verges and central reserves.

3.8 Filter drain loading shall correspond to outer verge loading specified with 30 units of HB loading with each wheel load being 62.5 kN.

3.9 Only two wheel loads shall be used for filter drain loading but with an increased impact factor of 1.4 giving a total load for each wheel of 87.5 kN.

3.9.1 Filter drains not immediately adjacent to the carriageway may be designed for field loads.

3.10 Water loads, that is the weight of the pipe content, shall only apply to pipes greater than 600 mm internal diameter.

NOTE The design charts, included in Appendix A, Figures A.1 to A.3, cater for three categories of loading conditions which include imposed surface loads together with soil and water loads as appropriate.

3.11 Where imposed loads during construction exceed normal in-service loads, the design charts in Appendix A of this document shall not be used.

3.12 Where pipes laid under and adjacent to roads are subject to higher loads during construction, guidance shall be followed from: "A Guide to Design Loadings for Buried Rigid Pipes" by Young & O'Reilly (TRRL) [Ref 1.N], BS 9295 2010 [Ref 7.N] or BS EN 1295-1 [Ref 22.N].

3.13 Where pipes are bedded in highly compressible soil, granular beds (types B, F and S) shall be used.

3.14 Where pipes are bedded in highly compressible soil, geotextile filter in accordance with MCHW Series 0600 [Ref 10.N] shall be used on the base of the trench to prevent contamination of the bedding material.

3.15 Rigid pipes shall be designed either in accordance with BS 9295 2010 [Ref 7.N] or with the design charts in Appendix B.

NOTE 1 Rigid pipes are those which fracture before significant deformation occurs (such as those of clayware, and concrete).

NOTE 2 The derivation of the design charts for rigid pipes in this document is in accordance with the principles given in "A Guide to Design Loadings for Buried Rigid Pipes" Young & O'Reilly (TRRL) [Ref 1.N] and "Simplified Tables of External Loads on Buried Pipelines" Young, Brennan & O'Reilly (TRRL) [Ref 3.I]).

3.15.1 Reference should be made to "A Guide to Design Loadings for Buried Rigid Pipes" Young & O'Reilly (TRRL) [Ref 1.N] and "Simplified Tables of External Loads on Buried Pipelines" Young, Brennan & O'Reilly (TRRL) [Ref 3.I] for the full theory and for the design of special cases not covered by the appendices to this document.

3.16 The safe supporting strength of the pipe/bed combination shall be given by Equation 3.16:

3.17 The allowable depth range of the pipes shall be determined such that , where is the total load on a particular pipe at a given depth.

3.18 The load on the pipe shall be found with the “narrow trench” equation, as defined in "A Guide to Design Loadings for Buried Rigid Pipes" Young & O'Reilly (TRRL) [Ref 1.N].

NOTE The load on the pipe is directly related to the trench width. Thus if the width of trench constructed exceeds the assumed design width then the load on the pipe is likely greater than the design load.

3.19 Where the "narrow trench" equation is used for pipes greater than 375 mm internal pipe diameter, the trench width shall be taken as at least 700 mm greater than the pipe outside diameter. See Young & O'Reilly (TRRL) [Ref 1.N].

3.20 For all pipes below 375 mm internal diameter the “wide trench” equation (see Young & O'Reilly (TRRL) [Ref 1.N] ) shall be used to calculate the load on the pipe, and in this case no restriction on the trench width is necessary.

3.25 Flexible pipes shall be designed in accordance with BS 9295 2010 [Ref 7.N] and BS EN 1295-1 [Ref 22.N], or in accordance with the MCHW Series 0500 [Ref 9.N].

NOTE 1 Flexible pipes are those which deform to a significant extent before collapse (such as those in plastics, or steel).

NOTE 2 Commonly, flexible pipes used for highway drainage are manufactured from thermoplastic materials.

NOTE 3 The design charts in Appendix A are based on a design method in BS EN 1295-1 [Ref 22.N] for flexible pipes which takes into account the relative stiffness of the pipe and the surrounding soil.

3.26 A maximum ring deflection (that is, change in diameter) of 5% and a factor of safety against ring buckling of 2 shall be used to obtain the safe depth ranges.

3.26.1 The pipe bed surround E values provided in Table 3.26.1N1 should be used in the design process for the design of flexible pipes provided in BS 9295 2010 [Ref 7.N] and BS EN 1295-1 [Ref 22.N].

NOTE 1 The degree of compaction of the pipe bedding material has a significant effect on the results and consequently the worst case, with the material in a loose condition, has been assumed. The corresponding E values used are provided in Table 3.26.1N1.

NOTE 2 The strength of a thermoplastic pipe is a function of the initial pipe stiffness and the modulus E of the native soil material through which the trench has been excavated, and that of the pipe bedding and surround. The compaction of the sidefill material is, therefore, fundamental to the performance of flexible/semi rigid pipes.

3.27 The design assumptions in this section shall be used when designing pipe/bedding combinations with the charts in Appendix A.

3.27.1 Class T bedding should not be specified for main road loading situations.

NOTE Granular bedding material is deemed to be relatively self compacting and provide adequate support with minimal compaction. It is the preferred bedding material

3.28 The embedment for semi-rigid and flexible pipes shall be as set out in Table NA8 of BS EN 1295-1 [Ref 22.N].

NOTE Thermoplastic pipes principally comprise those manufactured from PVC-u, polyethylene and polypropylene, though there are variations. The performance of each polymer is different and is dependent on the initial pipe stiffness, and the creep ratio.

3.29 Coefficients for soil modulus and bedding angle of friction relating to the removal of trench support systems shall be in accordance with BS 9295 2010 [Ref 7.N].

NOTE 1 The withdrawal of trench support systems is usually completed in stages.

NOTE 2 When pipes are laid in trenches where the sides are supported by trench support systems, the values of the soil modulus and the bedding angle of friction are reduced by a coefficient depending on the stage during the backfill process at which the support system is withdrawn.

3.29.1 Where trench support systems are withdrawn in stages before each layer of backfill is compacted, the friction coefficient should be taken as unity.

3.29.2 Where trench support systems are removed in stages after the backfill is compacted, the friction coefficient should be taken as 0.6.

NOTE A friction coefficient of 0.6, for when trench support systems are removed in steps after the backfill is compacted, is recommended by most pipe manufacturers.

3.29.3 Where the trench support systems are withdrawn after the trench is completely backfilled, the friction coefficient should be taken as 0.2.

3.29.4 Withdrawing trench support systems after the trench is completely backfilled should be avoided.

The charts of "bedding/pipe combinations" are shown in Figures A.1, A.2 and A.3 below. These are applicable to the types of pipe defined in Section 1, Scope.