TBMs are getting bigger. Herrenknecht will set a new record with a 15.55m EPBM on order for the A1 motorway between Bologna and Florence in Italy (page 6). Any discussion about the ever-increasing size of TBMs leads to the questions: how big can a TBM be?

The development of larger diameter TBMs has become the tunneling industry’s answer to the space race with the current target – Seattle’s Alaskan Way Viaduct Replacement Tunnel (subject to approval of course). According to Alistair Biggart in his Harding Prize lecture earlier this year, there is no obvious limit to TBM growth.

There are barriers to overcome such as increasing torque with stronger gearing and higher-powered motors. The thrust might need to be addressed, introducing more rams or the segments being thrust from will need to be developed to withstand the greater thrust.

More durable cutters are needed, especially on the outside edges of the TBM where wear is greatest, that still keep the TBM and the project economical. Developments will need to minimise maintenance to cut down on face interventions, which can be high risk. This may mean that the larger diameter TBMs are restricted to shorter tunnel drives.

There are also transportation issues. Shipping of TBMs is usual done by heavy loading companies that use a mixture of barges and trucks to get the machinery to site. The TBM is segmented for transportation. Earlier TBMs had the cutter face shipped as a single element but the new larger machines have had these segmented. The crucial factor here is how small manufactures can make the largest single shippable component.

The main bearing would need to be segmented or it could limit the future size of TBMs. On EPBMs the screw conveyor could also pose challenges in its segmentation.

But as the TBMs grow larger so must the supporting infrastructure. Muck disposal systems will need to keep up with the growth. If using a slurry machine, the slurry treatment plants will need to grow, as will the number and size of slurry pumps. In an urban situation like Seattle, you need to review how the muck is removed from the site, as the city cannot become congested with trucks. Neither do many have much space left for landfill, at least not at a reasonable price.

TBM manufactures wear their records with pride so I am sure they will be going all out to build the biggest. It will be interesting to see how long Italy’s A1 motorway will hold the title for. Not long we suspect.


Jon Young