How tunnels are built: excavation methods, machines and the ground

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The record table

The method is chosen by the ground, not by preference. Each row gives the conditions the method suits and the conditions that rule it out.

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

How this table is compiled

A machine specified for the wrong ground does not simply work slowly; it can stop for months while the face is treated. Immersed tubes sit outside both categories: the tunnel is built on land and sunk, so the ground is dredged rather than excavated. Dates use the day, month and year the event occurred.

The excavation method is chosen by the ground, and getting that choice wrong is the most expensive mistake available in tunnelling. The table is short because the sourcing bar is high. Corrections have historically arrived by letter rather than by form, and that is still how most errors in it have been found.

Drive length is the other axis, because a boring machine has to be designed, built and delivered before it cuts anything. Cost figures are excluded for a harder reason: currency, inflation and differing project scope make them close to meaningless in comparison. Comparing one country's practice against a neighbour's is more informative than reading either alone.

Hard, self-supporting rock can be drilled and blasted cheaply, which is why Norwegian and Alpine road tunnels are so numerous. Cross-passage spacing, fixed firefighting and emergency ventilation control are what modern safety regulation actually governs. Where a structure belongs to two classes it appears on both tables, which is not double counting but two different measurements.

Urban work is almost entirely machine-driven regardless of length, because blasting under buildings is not acceptable. Twin-tube crossings frequently show two slightly different lengths, because the two carriageways rarely follow the same line through the rock. This archive has recorded tunnel data since the late 1990s, compiled from operator publications, national administrations and the engineering press.

Reading the figures

Below roughly two to three kilometres that up-front cost rarely pays back, and above it the machine usually wins. Gradient limits are the main reason railway tunnels run longer than road tunnels on the same crossing. Subaqueous tunnels are governed by rock cover above the bore rather than by the depth of water above that.

Figures are checked against one another where sources disagree, and the disagreement is recorded rather than resolved silently. Tunnel engineering is a field where the qualifications attached to a number matter more than the number itself. A base tunnel trades a much greater length for a far lower summit altitude and a gentler ruling gradient.

The archive grew through correspondence with engineers and administrations in more than twenty countries, and still does. Rows are ordered so that the head of the table answers the question most readers arrive with. Reading the qualification before the figure is usually the faster route to understanding what a claim actually says.

Disputed values carry the dispute in the note column. The note column is worth reading before the number. Every figure here is traceable to a named source. A contained fire is more instructive than an uncontained one. Traffic figures are excluded throughout this archive, because they date far faster than the structures themselves.

Deaths are the figures established by official inquiry. Minor incidents are listed alongside the serious ones. Metres are used for structures, kilometres for routes. Secondary summaries were not used as a source. Promoter figures are used where no independent one exists. The date of verification is published with the table.