TBM versus drill and blast: how the choice is made, and what it costs

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The record table

A comparison on the axes that decide real contracts. Neither method is better in the abstract; the crossover point is set by drive length, geology and how much the surface above can tolerate.

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

How this table is compiled

Drill and blast mobilises in weeks; a boring machine takes a year or more to design and build for its drive. Corrections have historically arrived by letter rather than by form, and that is still how most errors in it have been found. Where a figure could not be corroborated it was left out.

Once the machine is paid for, its cost per metre falls below drill and blast, so long drives favour it heavily. Comparing one country's practice against a neighbour's is more informative than reading either alone. Where a structure belongs to two classes it appears on both tables, which is not double counting but two different measurements.

This comparison is set out on the axes that decide real contracts rather than on which method sounds more modern. Blasting can produce any shape, including the enlargements needed for laybys, cross-passages and ventilation plant. Cost figures are excluded for a harder reason: currency, inflation and differing project scope make them close to meaningless in comparison.

This archive has recorded tunnel data since the late 1990s, compiled from operator publications, national administrations and the engineering press. Twin-tube crossings frequently show two slightly different lengths, because the two carriageways rarely follow the same line through the rock. Cross-passage spacing, fixed firefighting and emergency ventilation control are what modern safety regulation actually governs.

Neither is better in the abstract, and most large schemes use both on different sections of the same alignment. That asymmetry is why short tunnels are still blasted even in countries with a deep machine-tunnelling industry. Subaqueous tunnels are governed by rock cover above the bore rather than by the depth of water above that.

Reading the figures

In cities the decision is usually made for the engineer: vibration and noise limits rule blasting out entirely. 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. Figures are checked against one another where sources disagree, and the disagreement is recorded rather than resolved silently. Reading the qualification before the figure is usually the faster route to understanding what a claim actually says.

Gradient limits are the main reason railway tunnels run longer than road tunnels on the same crossing. Traffic figures are excluded throughout this archive, because they date far faster than the structures themselves. Rows are ordered so that the head of the table answers the question most readers arrive with.

Ground conditions belong with every advance rate. A contained fire is more instructive than an uncontained one. Dates use the day, month and year the event occurred. The table is short because the sourcing bar is high. Promoter figures are used where no independent one exists. The note column is worth reading before the number.

Minor incidents are listed alongside the serious ones. Two independent sources were required for each row. Opening year means the year traffic was admitted. Deaths are the figures established by official inquiry. Disputed values carry the dispute in the note column. Every figure here is traceable to a named source.

Nothing below is interpolated or rounded up. Segmental linings are erected behind the shield. Secondary summaries were not used as a source. Height is measured above mean sea level. Metres are used for structures, kilometres for routes. Forecast dates are forecasts, and they move. Spoil is removed back through the machine.