Digger Shield

Definition - What does Digger Shield mean?

A digger shield is a specialized form of tunnel boring machine. The digger-shield type of machine is a hydraulic-powered digger arm which excavates ahead of a shield. Its protection is extended forward by hydraulically operated polling plates acting as retractable spiles. The arm extends beyond an open shield and may use attachments such as ripper teeth or a hydraulic hammer. Later versions have included computerized excavator arms.

Trenchlesspedia explains Digger Shield

The extent of use of tunneling shields with digger arms in the current market is unclear. In his 2005 book “Innovation and Rise of the Tunnelling Industry”, Graham West describes the 1979 Zokor digger shield. He calls the digger shield a “composite tunneling machine” along with the boomheader shield. He said the Zokor digger shields are made in diameters from 7.5 to 12 meters. West writes that these machines are designed “to excavate tunnels in ground that is unsuitable either for full-face hard rock tunneling machines or for soft ground tunneling machines.”

The 26-foot-diameter Saugus-Castaic Tunnel near Los Angeles was constructed with a mole of this type in 1967-70. Purdue University recounts the use of a digger shield on a sewer project in New York City. Innovative shield tunneling methods using computerized excavator arms were in use in Japan in the 1990s.

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