The Making of the Mixshield - Part 1

This is the first of a two-part series on the development of the Mixshield TBM since its introduction in 1985. It is widely used because of its ability to operate in standard slurry-shield mode or switch to an Open or Earth Pressure Balance (EPB) mode in mid-drive. Its origins date back to the early 1960s, when experiments were conducted in England. Next, a German contractor developed an air bubble principle to control and regulate face support pressure. This was mainly used in gravel and sand under low water pressure. Concurrent developments in lining technologies permitted further refinements. The final key elements of a Mixshield are listed. The first use of a first true Mixshield, at the HERA tunnel in Hamburg, Germany, in 1985, is described. A timeline gives details of key benchmarks, culminating in 2006 with the fabrication of a Mixshield with the world’s largest diameter shield (15.43 meters) and improved cutterhead access for atmospheric cutter tool change. Additional features such as jaw crushers and agitators are described.