The Salang Tunnel in Afghanistan: Road, Snow Galleries, Ventilation

The record table

How this archive measures: lengths, bores, dates and ordering

How this table is compiled

A viaduct is measured along its deck and a bridge by its main span, and confusing the two is the commonest error in a bridge list. A freight route's ruling gradient is set by the heaviest train it must lift, and that single figure decides whether a tunnel is needed at all.

Salang is recorded in this archive at 2.7 km, opened in 1964, on the list it belongs to rather than only on this page. Geology decides cost and programme far more than distance does; a short bore through squeezing ground can take longer than a long one through sound rock.

This page records the structures catalogued for Afghanistan in a tunnel almanac maintained at this address since the late 1990s. The tail of a list is where the ordinary infrastructure sits, and it is more representative of what gets built than the head is. A record is a statement about what has been published, not about what exists.

A tunnel boring machine produces a circular bore, which is why cross-section area and length do not scale together across entries. A disused tunnel is expensive to fill and cheap to seal, which is why so many survive behind a wall. The number of tubes is recorded because it changes both capacity and the safety regime that applies.

Wind, not weight, governs the design of a long-span bridge, and the deck section is shaped in a wind tunnel before anything is cast. Water management is the maintenance problem that never ends, and drainage design decides how expensive it will be. Smoke behaves differently on a gradient, and the uphill portal of a burning tunnel is the dangerous one.

Reading the figures

A scheme abandoned half-built leaves a heading that shows on no map and turns up during the next survey. Legging a boat through an unlined canal bore was normal practice for a century and left marks on the walls that survive. The first railway tunnels were driven by hand and gunpowder, and their alignments still carry the marks of both.

Sequential excavation takes the face in slices, which is slower per metre and far more tolerant of ground that changes. Route numbering changes over time, so a road reference in the table reflects the designation at the time of recording. Gradient inside a road tunnel is kept gentle for heavy vehicles, which lengthens the bore to gain the same height.

Traffic signals inside a tunnel exist to stop vehicles entering behind an incident, and they are the cheapest safety system on the list. Fixed links change settlement patterns, so the traffic a crossing carries after ten years is not the traffic it was justified on. Base tunnels replaced summit routes that climbed with spirals and switchbacks, and several of those older lines are still in place above them.

A national administration publishes what it maintains, so a crossing that changes owner can vanish from one list and appear in another. Tunnels built for one purpose are regularly repurposed for another, and a disused rail bore is the commonest cycleway in some countries. Water ingress is pumped out continuously on a subaqueous crossing, and the pumps are the one system that can never be off. A tunnel closed for maintenance is a route severed, so single-bore crossings are worked on at night and in short windows. The oldest entries in any national list are usually canal or rail, because roads were the last mode to need long bores.