Road Tunnels Manual

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6. Geometry

As mentioned extensively in the Chapter Strategic issues of this Manual, the geometric characteristics have to be defined at the most early stage of the conception of a tunnel, and even of a road link comprising possibly one or more tunnels.

These characteristics are of very different natures, and can be grouped in the following categories:

  • the relation between construction method and cross-section
  • the theoretical notions related to traffic capacity
  • the general alignment of the road comprising the tunnel : number of carriageways and lanes, off-carriageway provisions (lateral and possibly central), headroom, maximal slopes, minimal horizontal and vertical radiuses, transverse slopes, the detailed characteristics of the transverse profile inside the tunnel : width of lanes and off-carriageway provisions, headroom taking into account the construction method and the equipment to be installed the space needs for safety features as part of the cross-section : lay-bys, emergency stopping lanes, emergency service recesses, safety fences and barriers, safety recesses, etc.
  • the specific geometric characteristics of other features located out of the cross-section: emergency exits, evacuation galleries, by-passes, cross-connections, etc.
  • the influence of the geometrical characteristics on safety.

This chapter is mainly based on the Technical Reports 05.11.B "Cross section geometry in Unidirectional road tunnels" and 05.12.B "Cross section design for bi-directional road tunnels".

Section Relation between Construction method and Cross section recalls the relation between construction method and cross section.

Section Theoretical and practical tunnel traffic capacity gives a summary of the theoretical notions related to traffic capacity.

Section General alignment of roads and national examples recalls the main rules concerning the general alignments of roads, including the main figures used in some countries, and insists on the need to maintain the largest geometrical characteristics of the outside road in the tunnel itself (with the important exception of the maximum slope, which has to be limited).

Section Carriage way geometry deals specifically with the transverse profile of the carriageway of road tunnels, for uni- as well as bidirectional ones.

Section Vertical Clearance concerns the vertical clearance of the tunnel.

Section Emergency lanes, off-carriageway geometry and special elements concerns the emergency lanes and the off-carriageway features, as well as the various safety features that have to be found along the tunnel.


This chapter of the manual was written by Willy De Lathauwer (Belgium), associate member of C4 committee as ITA representative.

Fathi Tarada (UK), reviewed this Chapter.

Reference sources

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