Largest tunnel boring machine
Herrenknecht Mixshield S-880
China (Hong Kong)

The largest tunnel boring machine is the Mixshield S-880 "Qin Liangyu", built by German engineering firm Herrenknecht and operated by Bouygues Construction, which was used to excavate the Chek Lap Kok to Tuen Mun subsea road tunnel in Hong Kong, China. The Mixshield S-880 has a shield diameter of 17.63 metres (57 ft 10 in), an overall length of 120 metres (393 ft 8 in) and weighs 4,850 tonnes (5,346 US-tons). The 17.63–m tunnel section was excavated between 25 March and 3 November 2015, at which point the TBM's shield was converted into one of 14-m to complete the rest of the tunnel alongside another Herrenknecht TBM. The two 14-m TBMs broke through to complete the tunnels on 27 February 2019.

The tunnel connects the Hong Kong mainland to one of the world’s most ambitious 21st century construction projects – the Hong Kong-Zhuhai-Macau Bridge, with three cable-stayed bridges, three artificial islands and an underground section carrying a new 50-kilometre-long road eastwards from Macau, through the Chinese prefecture of Zhuhai to Hong Kong where it terminates by Hong Kong’s Chek Lap Kok airport on the western island of Lantau.

Tunnel Boring Machines (TBMs) are, as the name suggests, powerful machines that excavate tunnels. They consist of a circular rotating cutting face, with enclosed machinery behind that transports away the spoil and stabilizes the newly-dug tunnel. The S-880 "Qin Liangyu"was designed to operate in the waterlogged soil beneath seas and rivers. Such machines have to be pressurized to prevent the tunnel flooding, and line the walls behind them with a watertight combination of grout and precast interlocking concrete blocks.

The second largest tunnel boring machine – and the largest ever earth pressure balance machine - goes by the name of Bertha. In summer 2013, her colossal 17.5-metre-diameter (57.5-ft) cutting head started drilling out the State Route 99 tunnel beneath Seattle, on the northwestern coast of the USA. The cutter consists of a large steel face, into which are mounted no less than 600 small cutting disks that grind away at the rock in its path. The $80-million device, manufactured by Japanese engineering firm Hitachi Zosen, measures 91 metres in length (300 ft) and weighs 6,900 tons. The SR 99 tunnel is 3.2 km long (2 miles). Bertha’s boring work was completed in April 2017 with break-through of the tunnel, and the tunnel was opened to traffic on 4 February 2019

An even larger tunnel boring machine, a Herrenknecht TBM with a 19.25 m diameter cutting wheel was ordered for the Orlovsky Tunnel in St Petersburg, Russia, but the project was cancelled soon afterwards in 2011. Herrenknecht’s website suggests 19 m diameter machines can be built but none have yet been ordered.