Designing road foundations to DMRB-CD225

by Jonathan Cook, on februari 03, 2022

The Design Manual for Roads and Bridges (DMRB) sets out the regulations for most of the highways in the UK. Within this framework, CD225 is the specific standard that deals with road pavement foundation design.

This guide explores DMRB-CD225, outlining what this means for road foundation design options and the mechanical stabilisation of foundation layers. Read on to learn more about:

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What is the DMRB?

The Design Manual for Roads and Bridges (DMRB) is a suite of documents that dictate how the majority of UK highways are appraised, designed, maintained and operated. Their use is mandatory for motorways and trunk roads in all four UK nations and is widely used by highway authorities across the world. 

In addition, many UK Local Authorities have chosen to follow DMRB to govern work on their highways. However, this is changing, as Local Authorities decide on the extent to which the requirements are appropriate - as they adapt their construction and maintenance practices to accommodate squeezed budgets.

In 2019, the DMRB standards underwent a thorough review. Various old standards were combined into a new suite of streamlined and improved documents. A new coding system was put in place where the first letter indicates the discipline, e.g. C=Civil Engineering and the second letter indicates the life-cycle stage, e.g. D =Design stage. The following three digits identify specific standards.

CD225 - the regulations for road foundations

CD225 is the standard within DMRB that contains the regulations regarding the design of road foundations. This explains the accepted process for subgrade assessments and foundation designs, as well as the procedure for performance foundation designs.

CD225 - Design for New Pavement Foundations replaces the old IAN 73/06 Revision 1 (2009) Design Guidance for Road Pavement Foundations and HD 25/94 Foundations. The requirements of CD225 form the basis – quite literally, of the following standard CD226 – Design for New Pavement Construction. The two are interlinked.

What is the road foundation?

The road foundation comprises the subgrade, capping layer and/or the subbase. Use of a capping layer that typically utilises a lower cost material is usually considered where the subgrade is weak and subbase costs are high, as a means of minimising the layer thickness of expensive subbase.

The capping layer is an improvement layer placed over weaker subgrade to protect the subgrade and improve the formation stiffness. 

The subbase is a platform on which the main structure of the pavement is constructed. Notably, the subbase is considered to form part of the pavement as well as the foundation. Where no capping layer is present, the subbase will be placed directly over the subgrade formation.

Foundation Layers and Pavement Layers - from CD225

Figure 1: Foundation Layers and Pavement Layers - from CD225

Function of the road foundation

The primary function of the road foundation is to distribute vehicle loads to the underlying subgrade, without causing distress to the foundation layer or overlying layers. In addition, the foundation must protect the subgrade during construction, avoid excessive deformation, and provide long-term structural support to the overlying pavement layers.

During construction

The applied loading from construction vehicles is high, the stiffness and strength of the foundation must be sufficient to prevent damage to the subgrade that may influence the long-term performance of the pavement.

During service-life

The foundation must continue to provide adequate stiffness and bearing capacity to avoid deterioration of the upper pavement layers.

Foundation design options

CD225 sets out two approaches for the design of pavement foundations, with a third special case for road widening.

Restricted design

Performance is assured by using a limited range of well understood materials, tried and tested over time. CD225 provides a series of Design Charts to obtain design thickness of subbase and capping layers, based on subgrade surface modulus.

Performance design

Designers have a greater flexibility in their choice of materials and methods. Performance design can offer economic and environmental benefits through the use of innovative solutions and materials. CD225 set limits on vertical strain in the subgrade and maximum surface deflection for each Foundation Class. Performance is assured by on-site testing.

Pavement widening design

Uses either the Restricted or Performance approaches above but with the addition of ensuring continuity of sub-surface drainage between existing pavement and the widened section.

Foundation classes

CD225 requires that road foundations shall be designed to achieve a specific foundation class. The foundation class will then be utilised in the design of the overlying pavement in accordance with CD226.. 

Table 3.7 of the specification defines four foundation classes in terms of their assumed long-term surface modulus.

Foundation Classes - from CD225 Table 3.7

Figure 2 - Foundation Classes - from CD225 Table 3.7

Mechanical stabilisation of foundation layers

CD225 includes the option to use mechanical stabilisation with a suitable geogrid.

Weak subgrades with a surface modulus of less than 30MPa (CBR values less than 2.5%) are considered to be unsuitable for construction of the pavement foundation. For these conditions, CD225 allows the use of mechanical stabilisation with geogrids as an alternative to excavation and replacement of the weak soil. CD225 sets an upper limit on the design surface modulus for subgrade improvement at 50MPa.

For higher Foundation Classes, mechanical stabilisation may be used in a Performance Design to achieve the required design surface modulus. Limits on subgrade strain and surface deflection defined in CD225 must be adhered to.

Mechanical stabilisation with Tensar geogrids

The mechanics and principles of mechanical stabilisation are well established: Tensar geogrids have been utilised in this application for 40 years.

Tensar geogrids are designed specifically for stabilisation. They interlock with the aggregate to confine the aggregate particles and create a mechanically stabilised layer (MSL) that evenly distributes the load, increases bearing capacity and ensures longer-term stability and performance. Strengthening the aggregate layer enables layer thickness to be reduced.

The performance benefit of Tensar mechanically stabilised foundation layers has been proven by full-scale testing and multiple monitored trials. The design characteristics of the Tensar mechanically stabilised layer have been defined for a range of material types, loading and subgrade conditions. 

Designers can utilise this proven performance in the design of foundation layers to meet a specific Foundation Class in accordance with CD225. The benefit is - typically - significant reductions in foundation layer thickness with the associated savings in materials costs and construction carbon emissions.

Tensar foundation design proposal to CD255

Tensar Designers are ready to support your project in the following ways:

- They can propose a mechanically stabilised foundation layer for weak subgrade improvement, delivering a maximum design surface modulus of 50MPa, or

- They can support a Performance Design with a proposal for a mechanically stabilised foundation, providing a design that will achieve the target surface modulus for a specified Foundation Class and comply with CD225 limits maximum deflection under a standard wheel load. 

A supplementary check is also carried out using Tensar’s own software to ensure that traffic induced deformation of the foundation layer during construction remains within acceptable limits. A comparison with a non-stabilised design can be provided to confirm the savings achieved in materials, costs, and construction carbon emissions. By completing the Data Capture Form here, engineers can utilise this service free of charge for a proposal. Subsequent design responsibility by Tensar will be subject to a design fee.

Performance design delivered Telford & Wrekin Council real savings

Faced with the need to reconstruct a section of the A442 between Randlay and Stirchley, Telford & Wrekin Council was seeking ways to minimise the closure period to reduce disruption to road users.

Tensar proposed a mechanically stabilised foundation that would achieve Foundation Class 2 over a 1.5% CBR subgrade with a reduced foundation layer thickness requiring less excavation. Simon Wilkins, Senior Highways Engineer with T&W Council confirmed that:

“Tensar’s pavement design brought significant time and materials savings to the project, while delivering a long-lasting road surface that will dramatically reduce the maintenance requirements on this important section of the A442.”



A 45% reduction in pavement foundation thickness and 50% increase in traffic capacity was achieved at the A442 Randlay to Stirchley. Click here to read more and download the case study. 

Mechanical stabilisation in full-depth pavement design

In addition to foundation design, Tensar can provide a performance-based full depth pavement design utilising mechanically stabilised foundation to meet both construction and in-service life requirements. The improved performance of the stabilised foundation layer can result in considerable reductions in overall pavement thickness. By reducing the volume of non-renewable aggregates, costs, time, and emissions are reduced, contributing to sustainability objectives.

Importance of language in the DMRB

The revised DMRB sets out clearly stated ‘requirements’ conveying that a specific action is compulsory. Actions that ‘must’ be followed reflect legislative or statutory obligations, while actions that ‘shall’ be followed are required to comply with the document. Permissible options are preceded by ‘may’ and non-required actions by ‘can’. The use of Tensar geogrids in foundation layer design falls into this category of a proposed option and is fully within CD225 standards.

Due to the constraints of a particular scheme, it may not be possible to follow DMRB parameters. Some requirements can be relaxed provided the highway authority is notified where there may be cumulative effect of other relaxations or departures. Departures from standard are permissible where justified or where innovative methods or materials are proposed, provided the designer receives approval from the highways authority. 

While the design of mechanically stabilised foundations do not require a departure from standard, the use of a MSL in a Tensar full-depth pavement design to reduce overall pavement costs, would be a departure from CD226 standard and therefore require approval from a highway authority following DMRB in full. Tensar engineers have experience in gaining approvals for a departure from standard and can support the application.

Next steps

This guide has introduced DMRB and CD225, the standard for road foundation design in the UK. We’ve touched on the relevant requirements for designing road pavement foundations and mechanical stabilisation, as well as outlining how Tensar can help with your projects.

Our page on geogrids for road and pavement stabilisation provides further information about how this form of mechanical stabilisation can speed up construction, reduce costs, and contribute towards your sustainability goals.

If you are ready to get started with designing road foundations that adhere to DMRB-CD225, submit your project details and we will be in touch shortly.

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