A tunneling shield consists essentially of a tube or cylinder slightly larger in diameter than the tunnel it is intended to build, which slides over the exterior of the finished lining like the tubes of a telescope. The front end of this cylindrical shield is provided with a diaphragm or bulkhead in which are apertures which may be opened or closed at will. Behind this diaphragm are placed a number of hydraulic jacks, so arranged that by thrusting against the last erected iron ring the entire shield is pushed forward. The hind end of the shield is simply a continuation of the cylinder which forms the front end, and this hind end, or tail, always overlaps the last few feet of the built-up iron-shell tunnel.
When the openings in the bulkhead are closed, the tunnel is protected from the inrush of water or soft ground, and the openings may be so regulated that control is maintained over the material passed through. After a ring of iron lining has been erected within the tail of the shield, excavation is carried out ahead. When sufficient excavation has been taken out, the jacks are again extended, thus pushing the shield ahead, and another ring of iron is erected as before.
One of the Sixty-seven-Ton Tunnel Shields
For the erection of these heavy plates, a hydraulic swinging arm, called the “Erector,” is mounted, either on the shield itself or on an independent erector platform, according to conditions. This erector approaches closely the faculties of the human arm. It is hydraulically operated and can be moved in any desired direction. This method of construction can be followed in almost every kind of ground that can be met with, and it is especially valuable in dealing with soft, wet grounds. In passing through materials saturated with water, the shield is assisted by using compressed air in the working chamber.
Cutting Shield Head