HIGHLY specialised tunnelling units known as jumbos are being used on the Cheung Ching Tunnel part of Route 3. As the work progresses, the fully computerised twin-boom jumbos drill two holes simultaneously. The holes are then filled with explosives which, when detonated, produce a ''muck pile'' of rubble that will be removed with a special front-end loader designed for tunnel use. Depending on the quality of the rock, the French engineers from Dragages may choose to employ a stair-stepping method, in which one jumbo drills into a forward tunnel face, while another drills into a face located a few metres back. Both the jumbos and the loaders used on the Cheung Ching job are from Tamrock, in Turku, Finland.