A 17.5m diameter Herrenknecht mixshield TBM is being prepared for launch on a road tunnel in Jinan, China later this year.

Herrenknecht says a machine of this diameter has been used only twice before. The largest in 2015 in Hong Kong was also a Herrenknecht mixshield, with a 17.63m diameter.

The latest TBM is currently being dismantled and prepared for reassembly on the construction site for the tunnel in eastern China, around 400km south of Beijing. The tunnel will have three lanes in both directions and cross under the Yellow River.

TBM Shanhe, manufactured by Herrenknecht subsidiary Herrenknecht Tunnelling Machinery (HTM) in China, had to meet the challenges of boring under the river in an environment with high groundwater pressure and creating only one tunnel tube for bi-directional traffic.

A large diameter mixshield TBM was selected to cope with the clay, sand and silt geology and the 7.5 bar groundwater pressure under the Yellow River.

The mixshield technology developed by Herrenknecht is an advance on conventional slurry technology. The support pressure in the excavation chamber is managed precisely using an automatically-controlled air cushion so heterogeneous geologies and water pressures of more than 15 bar can be controlled safely even with very large excavation diameters. An individual, new slurry circuit was designed especially for TBM Shanhe.

Herrenknecht project manager Zhao Bin said the machine was also equipped with a telescopic camera to better monitor the tool change and face. He has supervised several projects for clients in China, including two TBMs on the Jinan Jiluo Road North Extension Tunnel project, which are crossing under the Yellow River just a few kilometres away from the new construction site. Two 15.71 diameter mixshield TBMs are excavating the twin bore tunnel.