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Chapter 5: Structural design of pavements

The first part of this chapter is based on a contribution to the Archive by John Porter and Maurice Maggs that draws on many contributions to the Archive as well as other contemporary sources to provide an account of motorway pavement design from ad hoc beginnings to the highly developed state of the art that exists today. The overview takes a chronological approach using the evolution of the Department’s ‘Specification for road and bridge works’ and related documentation as the thread that unites the narrative.

In the second part the overview is supported and enlivened by a series of essays, each focussing on a key event or issue as seen through the eyes of an eminent contributor to the motorway programme. The contributors come from different areas of the highway community, including specifiers, designers, contractors, materials suppliers and researchers, and a measure of overlap between their contributions allows the reader to compare different views. The essays cover:

  • Specifications and Materials Branch of MoT.
  • Early contributions to the motorway programme by RRL.
  • Development of the ‘Specification for road and bridge works’.
  • Concrete and blacktop: a contractor’s view of the specification.
  • The rise and rise of the heavy goods vehicle.
  • Pavement design at the University of Nottingham.
  • The move to specification by performance rather than recipe.
  • Development of pavement management technology.

The chapter ends with a brief reflection on lessons learned during the construction of the motorway system and challenges yet to be confronted as we strive to get the best value for money from a largely complete network where maintenance and traffic management will become increasingly important.

But before we begin our story it may help readers not well versed in the sometimes arcane world of pavement design to run over a few of the concepts and terms that appear in the narrative.

First of course we have to note that in technical circles the term ‘pavement’ is used not to describe the part of a road that is used by pedestrians, the footway, but the material interposed between the native soil used to construct the earthworks and the surface on which the traffic is to run. We have also to contend with ‘flexible’ and ‘rigid’ to describe different types of pavement construction. A flexible road has a crushed rock or other granular base, usually strengthened by the addition of a bitumen binder; a ‘rigid’ road is constructed in concrete. ‘Flexible’ and ‘rigid’ are of course relative terms, even the so-called flexible motorway pavement will deflect under the heaviest wheel load by no more than a tenth of a millimetre. These terms and others that appear in this chapter are illustrated here along with a description of the construction of the pavement layer by layer.

The primary function of a motorway pavement is to provide an economic, safe and durable running surface suitable for high speed traffic; the surface must also protect the underlying pavement structure and earthworks from ingress of surface water. The structural quality of many materials used in pavement construction, particularly soils and unbound aggregates, can deteriorate markedly when wet. The pavement structure and the underlying earthworks must therefore be protected from ingress of ground water by cut-off drains on each side of the structure, and usually in the central reserve too. All materials used in pavement construction should be unaffected by frost or should be protected from frost by a sufficient depth of cover.

To ensure longevity and thence economy, the pavement materials must themselves not crack or deform under the repeated loading imposed by traffic; they must also protect the underlying earthworks by diffusing the concentrated loads imposed by traffic, particularly heavy goods vehicles, over a larger area of the underlying earthworks.

The pavement is placed and compacted in layers with the highest quality material closest to the road surface where the traffic loading is most intense. The vertical compressive stress caused by the passage of wheel loads reduces within the pavement as the vertical load spreads out until, at the surface of the earthworks, the stress applied to the subgrade is at a level that will not cause an undue build-up of deformation. The lower layers of the pavement have also to resist the horizontal tensile stresses generated by repeated traffic loading.

Structural Design of PavementsConcrete pavements present a further set of requirements. The basecourse and wearing course over a lean concrete roadbase have to resist cracking over the regular transverse cracks that open as the concrete cools as it hardens and the roadbase contracts.

The transverse joints that are provided to accommodate daily and seasonal thermal movements in a jointed concrete pavement must be sealed with a flexible sealant to prevent water entering the joint and corroding the dowel bars that connect adjacent slabs.

Finally and most importantly all materials used in pavement construction must be capable of being produced and laid to a uniformly high standard under the widely variable operational and weather conditions encountered on site

This figure goes through the pavement from the earthworks to the final running surface explaining the primary function of each layer of the pavement and the materials most often used in its construction. It is included to serve as a ready reference for any reader who wishes to go ‘back to basics’ the better to understand the narrative. A further more extensive set of definitions is provided in the Glossary at the back of this volume.

The following subjects are covered in the printed document (Volume 2):

OVERVIEW

The Early years
The Road Note 29 Years (1969-70)
Departmental Standards after Road Note 29
Pavement Management

EXTRACTS FROM THE ARCHIVE

Ministry of Transport Specifications and Materials Branch
Early contribution to the Motorway Programme by RRL
Development of the Specification for Road and Bridge Works
Concrete and Blacktop: A Contractor's view of the Specification
The Rise and Rise of the Heavy Goods Vehicle
Pavement Design at the University of Nottingham
The move to Specification by Performance rather than Recipe
Development of Pavement Management Technology

LESSONS LEARNED....AND CHALLENGES YET TO COME