At the start the shields were fitted with movable platforms, but no hoods of any kind were placed until after the rock excavation was completed.
Methods of Excavation.
The distribution of materials to be excavated, as previously outlined, divided the excavation into three distinct classes, for which different methods had to be developed.
These three classes were:
First.—All-rock section.
Second.—Rock in the bottom, earth in the top.
Third.—All-earth section.
The extent of the second and third classes was much greater than that of the first, and they, of course, determined the use of the shield. Shields had not previously been used extensively in rock work, either where the face was wholly or partly in rock, and it was necessary to develop the methods by experience. The specifications required that where rock was present in the bottom, a bed of concrete should be laid in the form of a cradle on which to advance the shield.
All Rock.—At different times, three general methods were used for excavating in all-rock sections. They may be called: The bottom-heading method; the full-face method; and the center-heading method.
The bottom-heading method was first tried. A heading, about 8 ft. high and 12 ft. wide, was driven on the center line, with its bottom as nearly as possible on the grade line of the bottom of the tunnel. It was drilled in the ordinary manner by four drills mounted on two columns. The face of the headings varied from 10 to 30 ft. in advance of the cutting edge. After driving the heading for about 10 ft., the bottom was cleared out and a concrete cradle was set. The width of the cradles varied, but was generally from 8 to 10 ft.
The excavation was enlarged to full size as the shield advanced, the drills being mounted in the forward compartments of the shield, as shown by Fig. 1, [Plate LXVII], which represents the conditions after the opening had been cut in the bulkhead, but before the new methods, mentioned later, had been developed.