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Longitudinal Section.
Cross Section.
Fig. 131.—Shield for Blackwall Tunnel.
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With the foregoing brief review of the early development of the shield system of tunneling, we have arrived at a point where the methods of modern practice can be studied intelligently. In the pages which follow we shall first illustrate fully the construction of a number of shields of typical and special construction, and follow these illustrations with a general discussion of present practice in the various details of shield construction.
Transverse Section.
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