TECHNICAL | SUSTAINABLE TUNNELLING
in Rome to London, that represents at the very least an extra 32.5 metric tonnes of CO2 produced per day of
operation.” (The figures are calculated via
roadnet.com Carbon Emissions Calculator). There is more. Harding goes on to consider comparisons between
TBM-bored tunnels and those constructed through tunnelling methods, such as drill and blast. “Mechanically excavated tunnels result in smoother
tunnel walls and a 40 to 90 percent reduction in installed ground support. The smoother cross section results in less excavated material that must be hauled out of the tunnel, and mechanised tunnelling eliminates the risk of nitrous run off and plastic waste that are present in material from drill and blast.”
RE-USE PERSPECTIVE FOR CONTRACTORS On that basis, then, using a reconditioned TBM can deliver significant carbon savings on a tunnel project. Contractors will be reluctant to use them, though,
if there may be a downside of reduced reliability and tunnelling progress rates; metres excavated per day is the almost-universal measure of a project that is progressing well and profitably, and a TBM breakdown can be an absolute disaster. How confident can contractors be about using what a
used-car salesman might call a pre-loved TBM? Harding offers some figures. “In terms of metres per day, more than one third –
36% to be precise – of currently standing world records have been broken using a refurbished TBM; some of those machines had been in service for decades before setting their records.” Key to that, of course, is wear and tear on the machine
in its earlier usages. Some parts can be refurbished between projects, some can be replaced altogether, some are harder to change; the cost savings – and the embodied energy and emissions savings – will depend on the extent of rebuilding required. This will be highly variable, says Harding, but generally the simpler the project, in terms of geology and special conditions, the greater the savings. “They can range from 75% cheaper for simple
Top right: 20” disc cutter assembly Top & Centre: Main Beam TBM was built in 1980 for a subway job in New York and most recently was refurbished and modified for DigIndy project tunnels in Indianapolis Bottom: Inspection and maintenance are vital
12 | April 2023
machine with a project with tested ground conditions to 20% cheaper for a project with complex requirements, such as high pressure EPB.” But, generally speaking, there is no real limit to the
number of projects that a TBM can be used on, nor on the number of kilometres an individual machine can bore, he says.
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