. Shield and compressed air tunneling . ctwith such obstacles as may be in the way of the shield, it must bestrong enough to receive such shocks without serious importance of this is illustrated by an accident that happenedto the shield of the Blackwall tunnel (E-7) under the RiverThames. The cutting edge of this shield consisted simply ofthe skin plates reinforced about 12 in. from the front edge byinclined plates. By driving the bottom part of the cutting edgeagainst large pieces of rock, embedded in the ground, it collapsedand it was necessary to drive a heading 19 ft. wide in ad
. Shield and compressed air tunneling . ctwith such obstacles as may be in the way of the shield, it must bestrong enough to receive such shocks without serious importance of this is illustrated by an accident that happenedto the shield of the Blackwall tunnel (E-7) under the RiverThames. The cutting edge of this shield consisted simply ofthe skin plates reinforced about 12 in. from the front edge byinclined plates. By driving the bottom part of the cutting edgeagainst large pieces of rock, embedded in the ground, it collapsedand it was necessary to drive a heading 19 ft. wide in advance of 142 SJIH^JLD TUNNELING the shield. In this heading a concrete cradle was built for theshield to shde upon, but to keep it dry enough to work in anotherheading had to l)e driven at the top to intercept the water. { C. E., vol. 130, p. 65.) 15. Construction of Cutting Edge.—For strength the cuttingedge is made wide at the back, but for convenience of excava-tion it usually tapers toward the front as shown in Fig. Fig. 61.—The cutting edge of a shield. In some shields the front portion, or shoe, of the cuttingedge consists only of the skin plates as in the case of the Black-wall tunnel shield just described. More often the shoe is madeof castings of iron or steel. The best construction is obtained
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