3a. Road build design
New and improved roads shall have a 40 year design life and the build depth shall be determined by the expected traffic and the type of road, please refer to the Road build tables section.
Approved materials
All materials for roads shall be obtained from a source and supplier approved by us. See our Approved suppliers list.
Utility connections and service ducts
Utility services and service ducts shall be laid below the road, within the natural soil known as the subgrade with a minimum number of crossings.
Stabilising the subgrade
Where the strength of the subgrade is less than 1.5% CBR, different techniques can be used to stabilize the subgrade. The depth and type of technique to be used shall be submitted to, and approved by, us.
Ground stabilisation may include the addition of lime, or lime and cement to increase the strength of cohesive soils or, cement may be used to increase the strength of granular soils.
Geogrids is a geosynthetic material used for the reinforcement of unbound aggregate in low strength subgrade beneath the sub-base.
The ribs of the geogrid shall be of rectangular cross section in both the longitudinal and transverse directions with a nominal edge thickness of 1.35mm.
The Quality Control Strength of the geogrid, when tested in accordance with ISO 10319 and expressed in accordance with ISO 2602 1980 (BS 2846, Part 2, 1981), shall be 40kN/m with a peak strain of around 11% in both the longitudinal and transverse directions. In addition, typically the loads at 2% and 5% strain shall be 14kN/m and 28kN/m respectively in both the longitudinal and transverse directions.
The geogrid shall be manufactured in accordance with the Manufacturer Quality Assurance requirements BS EN ISO 9001 from polypropylene sheet, oriented in two directions so that the resulting ribs shall have a high degree of molecular orientation which continues through the area of the integral node. The geogrid shall have a minimum of 2% finely divided carbon black, as determined by BS 2782 Part 4, Method 452B: 1993, well dispersed in the polymer matrix to inhibit attack by ultraviolet light.
Geotextiles is a geosynthetic material used as a separation layer between sub-base and subgrade, and shall be laid with a minimum 500mm overlap between sheets.
The subgrade shall be smooth prior to the rolling out of the geotextile and no trafficking on the geotextile prior to placement of the sub-base or granular layers will be permitted.
Non-woven geotextiles shall have a minimum tensile strength of 15kN. This figure shall be ascertained by the Wide Width Strip Test in accordance with BS EN ISO 10319:2024. All woven geotextiles shall have a minimum tensile strength of 30kN/m in each direction. All Geotextiles shall have a minimum CBR puncture resistance of 3kN.
The geotextile shall have a permeability to water flow at right angles to its plane of not less than 10 litres per m² per second under a constant head of water of 100mm. The result shall be determined in accordance with BS EN ISO 11058:2019.
The pore O90 size shall be between 60 and 300 microns and shall be determined in accordance with BS EN ISO 12956:2020. The O90 value refers to the pore size below which lies 90% of the pore sizes in the geotextile. For woven geotextiles the pore size shall be 90% finer than 200/µm, non-woven geotextiles shall have a pore size 90% finer than 100/µm.
Geotextiles used in infiltration features shall be selected on the soil particle size confirmed in the ground investigation. This permeability of the geotextile shall be compatible with the infiltration rate used for the design.
Geotextiles shall be stored so that they will not be damaged by sunlight or ultraviolet light, or suffer mechanical damage.
Geo-composite material shall have a combination of the properties of both a geogrid and geotextile as described above.
Raising the ground
Where the subgrade beneath the road is to be raised, re-used material may be used or, new material shall be used, as a capping layer. The capping layer material shall be laid in layers not exceeding 225mm compacted thickness or appropriate to the compaction plant used, whichever is less.
Reused material shall be appropriately tested and classified as either 6F5 or sub-base material. A suitably qualified engineering consultant shall be instructed as early as possible to allow sufficient time for testing and classification and all works shall be in accordance with series SHW Series 600-Earthworks.
The following process is provided for guidance:
- Pre-testing - Samples should be taken and tested at UKAS accredited laboratory to classify the material in accordance with table 6/1 and table 6/2 in series 600 SHW. Contaminated or made ground cannot be used as an acceptable fill material.
- Earthworks method statement- should be prepared by a consultant and should detail the material to be used, the testing carried out to prove it meets a series 600 material, and how the material will be compacted on site.
- Earthworks validation report completed by consultant. This details the material used, where it has been used, thicknesses and compaction.
New material classified as 6F5 should comprise crushed rock, crushed concrete, crushed slag or combinations thereof.
Formation level
This is the finished level of the underside of the sub-base. Within 24 hours of the final preparation of the formation level it shall be covered with a minimum thickness of 150mm of compacted sub-base. Construction traffic shall not run on the prepared formation.
To prevent problems of weed growth, a suitable weed killer should be applied to the formation of a new footpath.
Frost susceptibility
Any materials used within the top 450mm of the road and footpath shall not be susceptible to frost.
If tests show the subgrade not to be susceptible to frost, a reduction in sub-base thickness may be acceptable and any proposed reduction in sub-base thickness shall be submitted to, and approved by, us.
For frost heave, materials shall be tested in accordance with BS 812: Part 124 as amended by
Clause 801 of the Specification for Highway Works Volume 1 May 2007, except that the use of the silica sand and limestone filler reference specimens is not mandatory. Materials are classified as non-frost susceptible if the mean heave is 15mm or less.
Alternative minimum depth road build
It may be acceptable to replace some of the granular foundation with bituminous materials and details shall be submitted to, and approved by, us. Consideration shall be made to the most appropriate procedures and machinery used due to the reduced strength of the granular foundation e.g. tracked rather than wheeled pavers to lay the binder course.
Should the granular foundation be replaced with bituminous material the in-situ CBR requirement on top of the sub-base is not appropriate.
Please refer to the Road build tables for minimum depth which presume the sub-grade is classified as frost susceptible and additional bituminous material has been included to provide protection.
Sub-base
Two types of sub-base shall form the granular foundation for new roads. Type 1 sub-base shall be used for non-permeable roads, and type 3 sub-base shall be used for permeable road and with voids of 15 to 20%.
Sub-base shall be made and constructed using unbound mixtures in accordance with BS EN13285:2018.
A minimum in-situ CBR value of 30% and/or a minimum stiffness of 35 MPa shall be required on all roads. CBR or stiffness values shall be determined by Plate Bearing Tests or Light Weight Drop Tests.
Binder course
This material shall be asphalt concrete of 20mm size aggregate known as AC 20 Dense Binder course in accordance with BS EN 13108-1 and the binder shall be in accordance with BS EN 12591:2009 grade 40/60.
Binder course materials shall be laid in two layers of 70mm and 60mm with a minimum thickness on 50mm and maximum thickness of 100mm.
Up to 20% of crushed recycled bituminous material may be included as an aggregate replacement in binder course mixes.
Binder course should not be left exposed for a prolonged period prior to the application of the next layer above. Exposure to weather and trafficking may result in the binder course becoming so adversely affected that replacement shall be required prior to the next layer being laid.
The binder course shall be thoroughly cleaned and a tack coat shall be applied prior to the laying of the next course.
Bond coat
Each bituminous layer shall be bonded to the bituminous layer below. Each layer should be thoroughly cleansed prior to application.
A bond coat shall provide adhesion between bituminous road layers and seal the layers to prevent water ingress ,in accordance with BS EN 13808:2013 and shall be applied hot in accordance with BS 594987:2024.
Surface course
For low trafficked roads, the material shall be asphalt concrete of 10mm aggregate size known as AC 10 Close Graded surface course in accordance with BS EN 13108-1:2016 and the binder shall be in accordance with BS EN 12591:2009 grade 70/100 or 100/150.
The aggregate shall be a coarse hardstone with a minimum polished stone value of 55, unless a higher value is stipulated by us, and shall have a maximum abrasion value of 16.
For high trafficked roads, the material shall be hot rolled asphalt known as, HRA 30/14F surf, with 20mm pre-coated chippings, in accordance with BS EN 13108-4:2016 and the binder shall be grade 40/60 (50 pen) bitumen in accordance with BS EN 13108-4:2016.
The coarse aggregate content of the mix shall be 30% with a minimum polished stone value of 45 and the pre-coated chippings shall be 14/20mm size with a minimum PSV of 60 unless a higher value is stipulated by us. The binder to coat the chippings shall be 40/60 grade in accordance with BS EN 12591:2009.
The mix shall be either LCC Special Mix or LCC Standard Mix.
Joints in bituminous material
All joints shall be made in accordance with BS 594987:2024 and shall be offset (stepped) by a minimum of 300mm from parallel joints in the layer beneath.
This shall apply to all transverse joints that have not been formed to a specific profile, joints where the asphalt abuts an existing surface and all longitudinal joints.
Joints in the binder course shall be treated in such a way as to enhance compaction and bonding.
The surface of the finished joint should not be painted/over banded due to the risk of differential skid resistance.
Before the adjacent width is laid, surface course joints shall be made by:
- Cutting back the edge to a vertical face for the full thickness of the layer
- Discarding all loosened material and painting or spraying the vertical face with a thin uniform coating of hot applied 40/60 or 70/100 paving grade bitumen, or cold applied thixotropic bituminous emulsion of similar grade or polymer modified bitumen emulsion bond coat. Surplus bitumen on the surface after the joint is made should be avoided.
Example junction with existing carriageway or permeable drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Block paving
Block pavers shall be either 60mm thick for impermeable roads or 80mm thick for permeable roads. Pavers shall be specifically designed for permeable roads.
Block pavers shall be laid in roads to a 45 degree herringbone pattern with a double stretcher to the channel and single stretcher to iron work accepting the fact that the pattern is not achievable around bends. A minimum of one third of a block cut will be required in all areas.
A transition is required when changing from block paving to traditional bituminous construction.
Example transition permeable to asphalt drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Only rectangular shaped blocks/pavers with a minimum Polished Paver Value (PPV) of 55 shall be accepted.
Pre-cast concrete paving blocks shall be chamfered and in accordance with BS EN 1338:2003 and laid in accordance with BS7533 Part 3.
Clay Pavers shall be chamfered and in accordance with BS EN 1344 and laid in accordance with BS7533 Part 3:2005+A1:2009.
Pavers for low trafficked roads shall be laid on laying course material (bedding sand) that shall be naturally occurring or crushed glass and in accordance with BS 7533-3:2005+A1:2009. Crushed rock shall be used for laying courses on high trafficked roads.
The properties of the laying course for impermeable roads shall be in accordance with Table D.3, category II of BS7533-3.
The jointing material for permeable roads shall be in accordance with the requirements of Table D.4, Table 10/1 and 10/2 of BS7533-3.
Backfilling of trenches
All trenches and other excavations beneath or within one metre of roads, footpaths and vehicle crossings, shall be refilled with sub-base.
The materials should be layered so that the compacted thickness of each layer is between 2.5 and 5 times the nominal size of the material.
Due to some trench widths alternative compaction equipment may be used.
When suitable material cannot be sourced and the width of excavation is greater than 850mm, and at an appropriate depth, approved capping material can be used.