There is a terrific very short video from the Wall Street Journal that explains how the tunnel boring machines work. These tunnel boring machines (TBMs) will be used to build the proposed Delta water conveyance. Fascinating.
The proposed tunnels will start at the intakes at Hood and travel 150 feet below ground to Clifton Court Forebay. Currently, two 40 foot side-by-side tunnels are planned to move the water using gravity flow.