Levelling course and subgrade

Published on 12 February 2008

Highway or railway structures are constructed on a foundation called a roadbed platform.

Road surface terminology
Road surface terminology
Accotement = Shoulder ; Plateforme support de chaussée = Roadbed
Arase de terrassement = Levelling course; Partie supérieure des terrassements = Upper part of the earthworks
Roulement = Traffic; Liason = Interlinkage; Base = Base
Fondation = Foundation; Couche de surface = Surface layer; Couches d’assises = Foundation layers
Couche de forme = Subgrade; Sol support = Supporting soil

Road surface underpinnings:

Highway or railway structures are constructed on a foundation called a roadbed platform.
This is made up of:

•Soil that is in place or materials brought in (about 1.00 m) to create the upper part of the earthworks from stocks of rubble and embankments - its surface is the levelling course

•Materials often prepared to lie just above this part, to form the subgrade.

The dimensions of the subgrade and the roadbed or trackbed are determined by the mechanical properties of the soils that form the upper part of the earthworks. The soil classification standard (NF P 11-300) provides this specification, and this is based on:

•Hydric state on realisation

•Mechanical behaviour

Based on these items, different cases for the upper part of the earthworks are defined and described in the French Highway Construction Guide (GTR), and this description is supplemented by the long-term load-bearing capacity on the levelling course, for which four categories of levelling course are distinguished.

The subgrade is a transitional element allowing matching of the properties of the infill materials or the in situ soils to the main requirements of a roadbed or trackbed. It therefore has to meet both short-term objectives for the building of the road or track (levelling, load bearing capacity, protection of the supporting soil and traffic-supporting capabilities) and long-term ones relating to the use of the structure (homogenisation, guarantee of minimum load bearing capacity, drainage characteristics of the roadbed or trackbed).
The subgrade materials can be used in their natural state or made suitable by modifying their characteristics or state (treatment with binders or other preparation) to meet criteria relating to:

•Water resistance

•Largest individual item size

•Resistance to the site construction traffic

•Frost resistance

The upper part of the earthworks together with the subgrade determine the long-term load-bearing capacity of the roadbed.