CD 524 Edge of pavement details

This document is published by National Highways.

This document supersedes HA 39/98 and HA 83/99, which are withdrawn.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.

This document represents the combination of two former advice documents HA 39/98 'Road edge details' and HA 83/99 'Safety aspects of road edge drainage features'.

The document has rationalised any duplication between the two source documents and does not include any information in the two source documents that is not related to road edge drainage features.

The document compliments and expands on the information provided in the B and F Series of drawings in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N].

Section 3 of the document includes requirements and advice on surface water channels. Surface water channels are described in further detail in document CD 521 [Ref 5.N]. In order to prevent duplication of information, Section 3 of this document includes several cross-references to CD 521 [Ref 5.N].

Tables summarising scenarios for use and pros and cons of the types of drainage solution available are presented in CG 501 [Ref 3.N].

The assumptions made in GG 101 [Ref 6.N] apply to this document.

1.1 This document shall be used for the design of the various types of edge of pavement drainage details shown in the ‘B’ and ‘F’ Series drawings of MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N].

NOTE 1 The ‘B’ Series of MCHW HCD Series B [Ref 10.N] depict pavement edge details dealing principally with drainage aspects of highway verges and central reserves.

NOTE 2 Some of the drainage components shown indicatively in the ‘B’ Series drawings of MCHW HCD Series B [Ref 10.N] are shown in detail in the ‘F’ Series drawings MCHW HCD Series F [Ref 11.N].

NOTE 3 The list of ‘B’ and ‘F’ Series drawings in MCHW HCD Series B [Ref 10.N] and MCHW HCD Series F [Ref 11.N] are listed in Appendix A.

NOTE 4 MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] supplements MCHW Series 0500 [Ref 7.N] as does the MCHW Series NG 0500 [Ref 9.N]. The role of guidance for the ‘B’ and ‘F’ Series drawings of the MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] is fulfilled by this document.

1.2 This document shall be implemented forthwith on all schemes involving highway drainage on the Overseeing Organisation's motorway and all-purpose trunk roads according to the implementation requirements of GG 101 [Ref 6.N].

1.3 The requirements contained in GG 101 [Ref 6.N] shall be followed in respect of activities covered by this document.

1.4 Unless protected from direct traffic, road edge drainage features shall be capable of withstanding the structural loading imposed on them during normal usage and the structural loading that can be imposed on them during lane closures and consequent trafficking of hard shoulders.

1.5 Road edge drainage features shall be designed to mitigate the effects on errant vehicles leaving the carriageway.

NOTE The road edge drainage features can present a potential hazard to errant vehicles leaving the carriageway.

1.6 The selection of road edge drainage features shall be guided by the health and safety issues associated with the maintenance needs of the features which is particularly important for central reserve drainage where difficulty of access can exacerbate maintenance interventions.

NOTE 1 The road edge drainage feature plays a major part in road safety by removing surface water quickly.

NOTE 2 A suitable road edge drainage feature prevents damage to the structural foundations of the carriageway. This, in turn, reduces the requirement for maintenance.

NOTE 3 Maintenance on motorways and other high speed roads involves a higher than normal degree of risk, both to road users and maintenance operatives.

NOTE 4 Safety issues associated with rural roads differ from those associated with urban roads.

2.1 The edge of pavement and drainage details in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] shall be used as the basis of design for combined surface and sub-surface drains.

NOTE 1 The edge of pavement and drainage details included in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] are listed in Appendix A.

NOTE 2 Combined surface and sub-surface drains enable rapid removal of runoff from the road and verge surface.

NOTE 3 Combined surface and sub-surface drains in cuttings can manage a wide range of flows. They are particularly suited to dealing with sub-surface flows, which can vary as the water table fluctuates.

NOTE 4 As combined surface and sub-surface drains incorporate pipes with relatively large diameters they generally have an exceptionally large groundwater capacity which can extend as a cut-off below the capping layer.

NOTE 5 Combined surface and sub-surface drains are appropriate for draining roads with long lengths of zero longitudinal gradient.

NOTE 6 Combined surface and sub-surface drains with type Y surfaces as shown on drawing B15 of MCHW HCD Series B [Ref 10.N] are more suitable for roads with flatter gradients where flow along the surface into catchpits is less assured.

NOTE 7 Combined surface and sub-surface drains with type W surfaces as shown in Drawing B15 of MCHW HCD Series B [Ref 10.N] have a greater fall towards the centre of the drain because the reduced permeability of the thick surface layer can result in greater flows along the surface.

2.1.1 The use of surface and sub-surface drains in central reserves should be avoided in order to reduce health and safety risks during maintenance interventions.

2.1.2 Where combined surface and sub-surface drains are used in cuttings they may serve as carrier drains.

NOTE Separate fin or narrow filter drains are not normally required where combined surface and sub-surface drains in cuttings function as carrier drains.

2.1.3 The diameter of combined surface and sub-surface drains should be limited to 300 mm.

2.2 Combined surface and sub-surface drains shall not be used in narrow verges subject to use by equestrians.

2.3 The upper section of combined surface and sub-surface drains shall be designed to mitigate the risk of stone scatter.

NOTE 1 Managing risks associated with stone scatter is described in DMRB CD 525 [Ref 2.N].

NOTE 2 The treatment options depicted in Drawing B15 of MCHW HCD Series B [Ref 10.N] incorporate measures to reduce stone scatter.

2.3.1 Where possible, more then one type of upper section should be specified in the contract with the final choice being left to the contractor.

2.4 A lip shall be provided between the edge of carriageway and the top of the combined surface and sub-surface drain.

NOTE The purpose of the lip is to enable continuous over the edge drainage to be maintained and to reduce the effect of build-up of vegetation.

2.5 The depth of the lip between the edge of carriageway and the top of the combined surface and sub-surface drain depends on the the upper section type selected and shall be as shown in Drawing B15 of MCHW HCD Series B [Ref 10.N].

2.5.1 Where Type 7 chambers as shown in Drawing F11 of MCHW HCD Series F [Ref 11.N] are used to collect surface flows from combined surface and sub-surface drains, Note 7 of Drawing F11 should be amended to match the depth of the lip.

2.6 The top surface of a combined surface and sub-surface drain shall be dished to allow high flows to be channelled along the surface and into catchpits.

2.6.1 Catchpit grating levels should be calculated to suit the falls toward the centre of the drain.

2.7 The lower section of combined surface and sub-surface drains shall be designed as filter drains and generally in accordance with the drawings in MCHW HCD Series F [Ref 11.N].

2.7.1 Where possible, more than one type of lower section should be specified in the contract with the final choice being left to the contractor.

NOTE The lower section of combined surface and sub-surface drains are generally formed with Type B material.

2.7.2 To avoid migration of fine sands and silts into the filter material, the lower section of combined surface and sub-surface drains laid in fine sands and silts should be Type J or K as shown in Drawing F2 of MCHW HCD Series F [Ref 11.N].

2.7.3 Lightweight aggregate may be used as the filter material for combined surface and sub-surface drains Type G, H and I as shown in Drawing F2 of MCHW HCD Series F [Ref 11.N].

2.8 To prolong the life of the combined surface and sub-surface drain, a geotextile membrane in accordance with MCHW Series 0600 [Ref 8.N] shall be placed between the upper and lower sections of the drain for drains Type G, H and I as detailed in Drawing F2 of MCHW HCD Series F [Ref 11.N].

NOTE The upper and lower sections of the drain are terms that approximate to the upper and lower half of the drain, and do not refer to the point where the filter material (or concrete) changes from one type to another.

2.9 Combined surface and sub-surface drains shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

2.9.1 The combined surface and sub-surface drain asset record should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

3.1 The cross-sectional profiles of surface water channels shall be triangular, trapezoidal or rectangular.

3.2 Surface water channels with rectangular cross sections shall be as described in DMRB CD 521 [Ref 5.N].

3.3 The edge of pavement and drainage details in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] shall be used as the basis of design for surface water water channels with triangular or trapezoidal cross sections.

NOTE The edge of pavement and drainage details included in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] are listed in Appendix A.

3.3.1 Dimension Z in Drawing B14 of MCHW HCD Series B [Ref 10.N] should be a minimum of 200 mm in order to provide a robust section capable of withstanding occasional vehicle overrun.

NOTE 1 In rigid construction, dimension Z can be lower than 200 mm where the channel and slab are slipformed together. See Type 2 as depicted in Drawing B14 of MCHW HCD Series B [Ref 10.N].

NOTE 2 Depending on the pavement alternative chosen, the value of dimension Z can be increased to found it on a convenient pavement layer.

3.3.2 Dimension Y in Drawing B14 of MCHW HCD Series B [Ref 10.N] may be set so that flooding is avoided by allowing the water to flow onto the verge and down the embankment slope.

3.3.3 This optimum value of Y may vary with road geometry and discharge volumes for each outfall.

NOTE For uniformity of channel cross section over the scheme, dimension Y can be set at the level required for the most vulnerable flood section.

3.3.4 Where carriageway levels differ appreciably, the channel and central reserve profile should be modified from that shown in Drawings B6 and B7 of MCHW HCD Series B [Ref 10.N].

3.4 With rigid carriageway construction, a tie bar shall be included to limit differential movement between the channel and pavement.

3.5 A drainage path between the pavement layers that terminate above the formation level of the channel and the sub-surface drain shall be provided as shown in Drawing F21 of MCHW HCD Series F [Ref 11.N].

3.6 Cross-sectional details for surface water channels and the location of such channels relative to the position of safety fences shall be determined with due consideration to safety.

NOTE 1 Guidance on safety issues associated with the design of surface water channels is included in CD 521 [Ref 5.N].

NOTE 2 Surface water channels located behind a VRS are shown in Drawing B18 of MCHW HCD Series B [Ref 10.N].

3.7 The hydraulic design of surface water channels shall be as detailed in CD 521 [Ref 5.N].

3.8 Surface water channel outfalls shall be as detailed in CD 521 [Ref 5.N].

3.9 Surface water channels shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

3.9.1 The surface water channel asset record should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

4.1 The edge of pavement and drainage details in MCHW HCD Series B [Ref 10.N] shall be used as the basis of design for Type A and B drainage channel blocks.

NOTE 1 The edge of pavement and drainage details included in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] are listed in Appendix A.

NOTE 2 Block types A and B are intended as a relatively inexpensive solution in situations where positive drainage is desirable for dealing with smaller volumes of flow and which would not justify the use of the larger surface water channel.

4.1.1 The use of Type A and B drainage channel blocks in cuttings should only be necessary where the cutting surface has low permeability.

NOTE Surfaces with runoff coefficients of 0.21 or greater can be considered as having low permeability.

4.1.2 The cross falls within the central reserve as depicted in Drawing B8 of MCHW HCD Series B [Ref 10.N] may be increased to provide additional waterway area.

4.2 The distance from the edge of the carriageway to type A and B channels in central reserves shall be 1.0 m or greater (see dimension X in Drawing B8 of MCHW HCD Series B [Ref 10.N]).

4.3 The distance from the edge of the carriageway to type A and B channels in cuttings shall be 1.0 m or greater (see dimension W in Drawing B4 of MCHW HCD Series B [Ref 10.N]).

4.4 Type A and B drainage channel blocks as depicted in drawings B4 and F15 of MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] shall not be used as edge drains contiguous with hard shoulders, hardstrips, or carriageways.

4.5 Type A and B drainage channel blocks as depicted in drawings B4 and F15 of MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] shall not be used in verges subject to frequent equestrian use.

4.6 The hydraulic capacity of the channel shall be based on a 1 in 1 year storm with flow contained within the channel cross section.

4.7 In hard shoulders and hard strips, the maximum flooding width under a 1 in 5 year storm shall be 1.5 m and 1.0 m respectively.

4.8 In central reserves, flooding shall not encroach into the offside lane under a 1 in 5 year storm.

NOTE Where drainage channel blocks are used in central reserves the distance between outlets will be limited.

4.9 The edge of pavement and drainage details in MCHW HCD Series F [Ref 11.N] shall be used as the basis of design for Type C, D, E and F drainage channel blocks.

NOTE 1 The edge of pavement and drainage details included in MCHW HCD Series F [Ref 11.N] are listed in Appendix A.

NOTE 2 Drawing F16 of MCHW HCD Series F [Ref 11.N] gives typical dimensions for Type E and F drainage channel blocks.

NOTE 3 Type E and F drainage channel blocks in combination with type C drainage channel blocks are intended as an alternative to gullies on embankments where kerbs are used. They have a distinct advantage on high embankments, avoiding the difficulty of construction of long gully connections down the embankment slope.

NOTE 4 Type D drainage channel blocks are articulated and are particularly suited to use on steep embankment slopes where settlement is expected.

NOTE 5 Type D drainage channel blocks are suitable for use as outfalls for surface water channels and for types E and F drainage channel blocks.

NOTE 6 Safety aspects of rectangular channel blocks are considered in DMRB CD 521 [Ref 5.N].

4.10 Type D drainage channel block units shall be designed with anchorage at the top and bottom and with bedding support.

4.11 The design of drainage channel block systems shall assess the likelihood of the following factors when deciding whether to use them:

NOTE There are potential maintenance difficulties associated with the use of drainage channel blocks from the above factors.

4.12 Drainage channel blocks shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

4.12.1 The asset record for drainage channel blocks should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

5.1 Kerbed edge channels draining to gullies shall be as shown in Drawing B9 of MCHW HCD Series B [Ref 10.N] except as qualified in the advice and notes related to this clause.

5.1.1 Pipework connecting gullies to carrier pipes may pass through the sub-surface drain.

5.1.2 The use of drain type 5 in Drawing F18 as shown in MCHW HCD Series F [Ref 11.N] may not be possible where pipework connecting gullies passes through the sub-surface drain.

NOTE 1 The short interruption resulting from gully connection pipework that pass through of drain types 6, 7, 8 and 9 as shown in Drawing F18 of MCHW HCD Series F [Ref 11.N] will not generally affect the performance of the filter.

NOTE 2 The kerbs depicted in B9 of MCHW HCD Series B [Ref 10.N] are of the extruded type, bedded on to the carriageway surface; it is not intended to exclude the range of kerb/bedding combinations permitted in MCHW Series 0500 [Ref 7.N].

5.2 Kerbed edge channels draining to drainage channel blocks shall be as shown in Drawing B10 of MCHW HCD Series B [Ref 10.N] except as qualified in the note related to this clause.

NOTE The kerbs depicted in B10 of MCHW HCD Series B [Ref 10.N] are of the extruded type, bedded onto the carriageway surface; it is not intended to exclude the range of kerb/bedding combinations permitted in MCHW Series 0500 [Ref 7.N].

5.3 Hydraulic design of kerbed edge drainage shall be in accordance with DMRB CG 501 [Ref 3.N].

5.4 Outlet spacing for kerbed edge drainage shall be in accordance with DMRB CD 526 [Ref 12.N] .

5.5 The use and placement of kerbs shall be in accordance with DMRB CD 127 [Ref 1.N].

6.1 The edge of pavement details in MCHW HCD Series B [Ref 10.N] shall be used as the basis of design for over-the-edge drainage.

NOTE The edge of pavement details included in MCHW HCD Series B [Ref 10.N] are listed in Appendix A.

6.2 Embankments shall be designed to preclude topsoil instability.

6.2.1 Use of over-the-edge drainage should be confined to areas where there is a low risk of topsoil instability.

NOTE Over-the-edge drainage can cause soil erosion, topsoil slippage, softening of the side slopes and embankment instability.

6.2.2 Drainage of verges and carriageway over the edge of embankments should only be used where there is an open ditch or watercourse at the base of the embankment.

6.3 Where footways abut the carriageway or on structures, over-the-edge drainage shall not be used.

7.5 Edge of pavement drains shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

7.5.1 The edge of pavement drain asset record should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

NOTE Pipework that forms part of the edge-of-pavement drain is not recorded as a separate asset, but is considered an integral component of the drain.

8.1 The edge-of-pavement drawings in MCHW HCD Series B [Ref 10.N] shall be used as the basis of design for combined drainage and kerb systems.

NOTE 1 The edge-of-pavement details included in MCHW HCD Series B [Ref 10.N] are listed in Appendix A.

NOTE 2 Although combined drainage and kerb systems are useful on roads with relatively flat gradients, they are prone to the build-up of sediment and debris which can impede flow into and within the system.

NOTE 3 Combined drainage and kerb systems are useful in urban areas where there is a high incidence of utility services because they do not need as great a depth of excavation as piped systems.

NOTE 4 The dual function of combined drainage and kerb systems can offer cost savings in certain circumstances.

8.1.1 Drawing B16 of MCHW HCD Series B [Ref 10.N] shows the feature founded on the capping layer and where a capping layer is not present, the bedding concrete may be placed on fill, existing ground or the sub-base.

8.1.2 Where the bedding concrete is placed on fill, existing ground or the sub-base, the contract documents should indicate the position of the edge-of-pavement drain if present.

8.2 Combined drainage and kerb features shall be designed both for accidental loading and impact.

NOTE Combined drainage and kerb systems are, by their very nature, hollow. Consequently they can be less robust than solid kerbs equivalents of same general dimensions.

8.3 Placement of kerbs shall be in accordance with the provisions of DMRB CD 127 [Ref 1.N].

8.4 Combined drainage and kerb systems shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

8.4.1 The combined drainage and kerb system asset record should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

9.1 The edge of pavement details in MCHW HCD Series B [Ref 10.N] shall be used as the basis of design for linear drainage channel systems.

NOTE 1 The edge-of-pavement details included in MCHW HCD Series B [Ref 10.N] are listed in Appendix A.

NOTE 2 The linear drainage channels can be formed with proprietary precast units or with proprietary units cast in situ.

9.1.1 Drawing B17 of MCHW HCD Series B [Ref 10.N] shows a linear drainage channel founded on the capping layer and where a capping layer is not present, the bedding concrete may be placed on fill, existing ground or the sub-base.

9.1.2 Where the bedding concrete is placed on fill, existing ground or the sub-base, the contract documents should indicate the position of the sub-surface drainage.

9.1.3 Linear drainage channel systems may be used in both verges and central reserves on motorways.

9.1.4 Linear drainage channel systems in all-purpose trunk road verges should be placed behind the vehicle restraint system (VRS).

9.1.5 Linear drainage channel systems in all-purpose trunk road central reserves should be placed behind VRS fencing or immediately in front of vertical concrete barriers.

9.1.6 The impact of maintenance should be assessed for linear drainage systems.

NOTE Linear drainage systems can increase the burden on maintenance.

9.1.7 The use of linear drainage systems in locations where frequent maintenance interventions are required should be avoided.

NOTE Linear drainage channel systems which are used on roads with relatively flat gradients are prone to the build-up of sediment and debris which can impede flow into and within the system.

9.2 Linear drainage channel systems located in motorways and all-purpose trunk roads that are not protected from direct traffic load shall be Class D400 systems as detailed in the MCHW Series 0500 [Ref 7.N].

9.3 Linear drainage channel systems located in motorways and all-purpose trunk roads that are protected from direct traffic loading shall be Class D400 or Class C250 as detailed in the MCHW Series 0500 [Ref 7.N].

9.4 Slotted linear drainage systems shall not be used in areas subject to pedestrian and cyclist use.

9.5 Precast channel type linear drainage systems used in areas subject to pedestrian or cyclist use shall have mesh gratings.

9.6 All linear drainage channel system gratings and covers shall be of a type that is integral with the drainage channel.

9.7 Linear drainage systems shall not be used in areas subject to trafficking by high-speed heavy goods vehicles.

9.8 Linear drainage channel systems shall be recorded as continuous assets as detailed in DMRB CD 535 [Ref 4.N].

9.8.1 The asset records for linear drainage channel systems should include all the attributes for continuous assets as detailed in CD 535 [Ref 4.N].

The ‘B’ series drawings shown in MCHW HCD Series B [Ref 10.N] show indicative edge-of-pavement details pertaining to highway drainage. These drawings are indicative only and are listed in Table A.1.

Some of the drainage components shown in the ‘B’ Series drawings are shown in detail in the ‘F’ Series drawings in MCHW HCD Series F [Ref 11.N]. These drawings are indicative only and those pertinent to this document are listed in Table A.2.

Information required to be shown in the contract by virtue of the 'B' and 'F' series drawing in MCHW HCD Series B [Ref 10.N] & MCHW HCD Series F [Ref 11.N] are detailed in Table A.3.