Tunnel boring machines: how a TBM works, the types and their records

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The record table

Diameter is the shield diameter, which is what governs the bore. A machine's type is set by how it holds the face up, and that is set by the ground, not by the size of the tunnel.

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

How this table is compiled

Corrections have historically arrived by letter rather than by form, and that is still how most errors in it have been found. Dates use the day, month and year the event occurred. Ventilation type constrains how long a bore can be driven without intermediate shafts, and its absence is often what caps a tunnel's length.

The largest machine yet built is 17.63 m across the shield, which is roughly the height of a five-storey building. Comparing one country's practice against a neighbour's is more informative than reading either alone. Cost figures are excluded for a harder reason: currency, inflation and differing project scope make them close to meaningless in comparison.

An earth pressure balance machine keeps the excavated soil itself under pressure in the cutting chamber to hold the face. 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.

Bertha, at 17.5 m, remains the largest earth pressure balance machine and drove the Seattle State Route 99 replacement. Traffic figures are excluded throughout this archive, because they date far faster than the structures themselves. This archive has recorded tunnel data since the late 1990s, compiled from operator publications, national administrations and the engineering press.

Subaqueous tunnels are governed by rock cover above the bore rather than by the depth of water above that. The archive grew through correspondence with engineers and administrations in more than twenty countries, and still does. Twin-tube crossings frequently show two slightly different lengths, because the two carriageways rarely follow the same line through the rock.

Reading the figures

The type is defined by how it holds the face up, and there are four in general use. An open hard-rock machine needs no face support at all and grips the rock walls to push forward. A base tunnel trades a much greater length for a far lower summit altitude and a gentler ruling gradient.

A slurry or mixshield machine pressurises a bentonite suspension against the face, which suits water-bearing sands and gravels. 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.

Gradient limits are the main reason railway tunnels run longer than road tunnels on the same crossing. 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. Diameter here means the shield diameter. The date of verification is published with the table. Where a figure could not be corroborated it was left out. The table is short because the sourcing bar is high. A contained fire is more instructive than an uncontained one.

Secondary summaries were not used as a source. Every figure here is traceable to a named source. The note column is worth reading before the number. Forecasts on schemes this size move by years. Promoter figures are used where no independent one exists. Minor incidents are listed alongside the serious ones.