Fig. 133.—Semi-elliptical Shield for Clichy Sewer Tunnel.
Mr. Raynald Légouez, in his excellent book upon the shield system of tunneling, considers that tunnel shields may be divided into three classes structurally, according to the character of the material which they are designed to penetrate. In the first class he places shields designed to work in a stiff and comparatively stable soil, like the well-known London clay; in the second class are placed those constructed to work in soft clays and silts; and in the third class those intended for soils of an unstable granular nature. This classification will, in a general way, be kept by the writer. As a representative shield of the first class, the one designed for the City and South London Railway is illustrated in [Fig. 129]. The shields for the London Tower tunnel, the Waterloo and City Railway, the Glasgow District Subway, the Siphons of Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the same general design and construction. To represent shields of the second class, the St. Clair River and Blackwall shields are shown in [Figs. 130] and [131]. The shields for the Mersey River, the Hudson River, and the East River tunnels also belong to this class. To represent shields of the third class, the elliptical and semi-elliptical shields of the Clichy tunnel work in Paris are shown by [Figs. 132] and [133]. The semi-circular shield of the Boston Subway is illustrated by [Fig. 134].
Half Transverse Section A-B.
Half Rear-End Elevation.
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Details of Casting Supporting Ends of Jacks.
Details of Castings under Ends of Girders.
Longitudinal Section C-D.