The tail thus continually enveloped the last constructed portion of
this permanent lining. The actual excavation took place in advance of
the cutting edge. The method of accomplishing this, varied with
conditions. At times the material would be rock for a few feet from
the bottom, overlaid with soft earth. In such case the latter would
be first excavated and then the roof would be supported by temporary
timbers, after which the rock portion would be attacked. When the
workmen had excavated the material in front of the shield it was passed
through the heavy steel plate diaphragm in center of the shell out to
the rear and the shield was then moved forward so as to bring its front
again up to the face of the excavation. As the shell was very unwieldy,
weighing about eighty tons, and, moreover, as the friction or pressure
of the surrounding material on its side had to be overcome it was a
very difficult matter to move it forward and a great force had to be
expended to do so. This force was exerted by means of hydraulic jacks
so devised and placed around the circumference of the diaphragm as to
push against the completed steel plate lining of the tunnel. There
were sixteen of these jacks employed with cylinders eight inches in
diameter and they exerted a pressure of from one thousand to four
thousand pounds per square inch. By such means the shield was pushed
ahead as soon as room was made in front for another move.

The purpose of the shield is to prevent the inrush of water and soft
material while excavating is going on; the diaphragm of the shields
acts as a bulkhead and the openings in it are so devised as to be
quickly closed if necessary. The extension of the shield in front of
the diaphragm is designed to prevent the falling or flowing in of the
exposed face of the new excavation.