Understanding Capping Layers in Highway Construction

A road is only as reliable as the ground beneath it, but that ground is not always firm or consistent enough to build on directly. Where improvement is required, a capping layer can be introduced to create a more stable platform for the road foundation.

It helps protect weaker ground during construction and provides a reliable surface on which the sub-base can be placed. However, capping is not required on every road. Its inclusion, thickness and material should reflect the ground conditions and foundation design.

This guide explains where a capping layer sits, when one may be needed and what is involved in selecting, constructing and checking it.

Where Does a Capping Layer Sit in the Road Foundation?

A road is constructed from several layers, with the capping positioned towards the bottom of the build-up:

Surface and structural pavement layers → sub-base → capping layer → subgrade

The subgrade is the natural or prepared ground beneath the road. Where the design shows that it needs additional improvement, capping material is placed directly above it.

Once the capping has been compacted and finished, its upper surface generally forms the formation on which the sub-base is constructed.

If no capping layer is required, the formation will usually be prepared at the top of the earthworks beneath the sub-base.

Although capping and sub-base sit next to one another, they perform different roles. Capping improves the platform over weak or variable ground, while the sub-base forms a more controlled foundation for the pavement above.

One should not be substituted for the other simply because the materials appear similar.

Our guide to sub-base aggregates explains more about the materials used in the layer above.

What Makes a Suitable Capping Material?

Capping material should be selected against the foundation design and project specification, rather than ordered because it looks suitable or is sold under a familiar product name.

Its grading and maximum particle size must be appropriate for the required layer thickness and compaction method.

Once placed, the material needs to compact into a stable platform and resist breaking down beneath construction traffic.

The amount and nature of the fines may also be controlled, particularly where plasticity or sensitivity to moisture could affect placement and performance. Durability, chemical suitability and consistency between deliveries may introduce further requirements depending on the project.

A suitable capping material is therefore one that meets the specified properties and continues to do so throughout supply, not simply one that has been used successfully elsewhere.

Are 6F2 and 6F5 the Only Options?

No. 6F2 and 6F5 are familiar material descriptions that may appear in highway earthworks and capping specifications, but they are not the only possible options.

Depending on the design, capping may be produced from primary quarried aggregate, approved recycled or secondary material, or site-won material that has been processed to meet the required properties. Some schemes may instead use soil or granular material improved through an approved stabilisation method.

The origin of the material does not establish whether it is suitable. Primary, recycled and site-won products must all satisfy the capping layer specification applying to the project.

Our guide to the differences between 6F2 and 6F5 explains these two materials in more detail.

How Thick Should a Road Capping Layer Be?

There is no standard capping thickness suitable for every road.

The required depth is determined through the foundation design and can be influenced by the strength and condition of the subgrade, the capping material and the loads expected during construction. The use of stabilisation or geosynthetics may also affect the chosen design.

How the material will be compacted must be considered as well. The total capping depth may need to be built up in separate lifts so that each can be compacted effectively using the specified plant.

The drawings and project specification should state the required finished thickness. This means the depth after compaction, which should not be confused with the greater loose depth initially spread on site.

How Is a Capping Layer Constructed?

The precise method will depend on the ground conditions, capping material and project specification. However, successful capping layer construction begins with protecting the ground beneath it and maintaining control as the material is placed.

Preparing and Protecting the Subgrade

Before capping begins, the subgrade should be prepared to the required levels, with any unsuitable areas removed or treated as directed by the design. Temporary drainage may also be needed to prevent water collecting on the exposed ground.

Weak or wet subgrade can be easily disturbed, so unnecessary trafficking should be avoided. Plant movements and material placement should follow the approved construction method.

Where a separator or reinforcement has been specified, it should be installed before the capping material.

A geotextile may be used to prevent the subgrade and aggregate from mixing, while a geogrid may provide reinforcement through interaction with the material above. They perform different functions and are not automatically required beneath every capping layer.

Placing the Capping Material

Capping material should be spread in a way that limits disturbance to the prepared subgrade. Care is needed to prevent soil being pushed into the aggregate or coarse and fine particles separating during tipping and spreading.

The material is normally built up in manageable layers, or lifts. Their thickness should suit the maximum particle size, the compaction plant and the requirements of the construction method. Placing too much material at once can leave the lower part of the layer inadequately compacted.

Controlling Moisture and Compaction

Moisture condition can affect how easily capping material is handled and compacted. Material that is excessively dry may not compact uniformly, while overly wet material can become unstable beneath plant.

Each lift should be compacted using the specified plant, layer thickness and number of passes. The aim is to create consistent support across the formation rather than a surface that merely appears firm.

Soft spots, rutting or pumping under construction traffic should be investigated before work continues. Covering them with another layer may hide the visible signs without resolving the underlying problem.

Finishing and Protecting the Formation

Once compacted, the capping should be trimmed to the required line and level. Suitable falls must be maintained so rainfall does not collect on the completed formation.

Until the sub-base is placed, the surface should be protected from contamination, excessive trafficking and weather damage. Any areas disturbed during this period may need to be repaired and checked again.

What Should Be Checked Before the Sub-Base Is Placed?

A completed capping layer should not be accepted simply because its surface looks firm.

Delivery and material records should first confirm that the specified product has been used. The finished level and compacted thickness can then be checked against the drawings, alongside the condition and uniformity of the surface.

Any rutting, soft areas, contamination or trapped water should be addressed before the next layer is placed. Where a geotextile or geogrid has been installed, its visible condition and continuity may also need checking.

The project may require proof rolling, plate testing, CBR testing or another assessment of compaction or foundation performance. These methods provide different information, so the required test and acceptance criteria must come from the design and specification. No single test applies to every capping layer.

Which Capping Layer Specification Applies?

The applicable requirements depend on the road, authority and contract.

Foundation design for National Highways projects may refer to DMRB CD 225, while construction requirements may be set through the Specification for Highway Works and works-specific documentation.

The Manual of Contract Documents for Highway Works was restructured and republished in September 2025. The relevant documents can vary according to the overseeing organisation and contract date, so references should always be checked against the version applying to the scheme.

Local-authority, adoptable-road and private projects may use different specifications. Describing an aggregate as “suitable for capping” is therefore not enough to confirm compliance.

Before supply begins, the supplier should be given the required material class, grading, permitted sources, testing requirements and supporting documentation. This helps ensure that the product delivered matches the foundation design.

A Reliable Road Foundation Begins With the Ground

A capping layer creates a controlled transition between weak or variable ground and the road foundation above. Its success depends on a suitable design, compliant material and construction that protects the ground while achieving consistent support.

Explore Armstrongs’ aggregate range or contact our team to discuss the capping material required for your project.

Capping Layer FAQs

What Is a Capping Layer in Road Construction?

A capping layer is material placed over weak or variable subgrade to create a more stable platform for the road foundation above.

Where Does a Capping Layer Sit in a Road?

It sits directly above the prepared subgrade and beneath the sub-base. The top of the completed capping generally forms the formation on which the sub-base is constructed.

Does Every Highway Require a Capping Layer?

No. The need for capping is determined by the ground conditions, pavement-foundation design and applicable project requirements.

What Is the Difference Between Capping and Sub-Base?

Capping improves the platform over weaker ground, while the sub-base provides a controlled foundation closer to the structural pavement. They perform different roles and should not be treated as interchangeable.

How Thick Should a Road Capping Layer Be?

There is no universal thickness. The required compacted depth depends on the subgrade, foundation design, capping material and construction method.

Is 6F2 Used for Capping Layers?

6F2 may be used as capping material where it meets the applicable project specification. The product description alone does not confirm that it is suitable for every scheme.

Can Recycled Aggregate Be Used as Capping Material?

Yes, where recycled material is permitted and satisfies the required grading, composition, performance and testing requirements.

How Is a Completed Capping Layer Tested?

Checks may include material records, level and thickness measurements, visual inspection and specified performance testing. The exact method and acceptance criteria depend on the project specification.