Construction and Geometry

For a tunnel to operate sustainably and ensure good comfort and safety to users, it is imperative that the entities which will be in charge of its operation are closely involved in the design phase. This chapter reviews the main elements which ensure good design of a tunnel.

In 1974, the ITA (International Tunnelling and Underground Space Association) was created to address construction aspects of all types of underground works, including road tunnels. In 2005, a Memorandum of Understanding was signed between PIARC and the ITA to ensure that their actions are and remain complementary and do not overlap. PIARC deals with tunnel geometry while the ITA takes care of purely constructional aspects. 

The page on Construction presents the main tunnel construction methods and their interfaces with geometry and safety aspects. For more information concerning pure construction aspects, the reader is invited to consult the ITA Website.

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

The page on 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).

The page on Carriageway geometry deals specifically with the transverse profile of the carriageway of road tunnels, for uni- as well as bidirectional types.

Emergency exits are provided in all except the shortest tunnels to allow tunnel users to evacuate on foot from the traffic tube to a place of safety. The different types of emergency exits for pedestrians are considered in the page on Emergency exits. These include cross-connections and cross-passages between tubes, and safety galleries (passages) constructed alongside the traffic tubes or perhaps under the carriageway and leading to the surface.

The page on Facilities for vehicles considers the facilities provided for vehicles. These include lay-bys, turning bays and cross-connections between tubes for vehicles, which cater for situations such as vehicle breakdowns or allow vehicles to turn around or cross into an adjacent tube, which could be useful for maintenance, for manoeuvring emergency vehicles during an incident, or for traffic management following an incident.

The page on Other facilities describes other facilities that may be provided within or at the portals of a tunnel.