Tunneling Shield

a movable prefabricated metal structure that provides for the safe driving of an underground excavation and makes it possible to install a lining in the excavation. A tunneling shield was first used in 1825 in Great Britain by M. I. Brunei to build a tunnel under the Thames River. In the USSR, tunneling shields have been used since 1932. They were used to build most of the subway tunnels in Moscow, Leningrad, Kiev, and other cities. Tunneling shields are used in the building of various types of tunnels and in the underground mining of mineral deposits.

Unmechanized tunneling shields are classified according to the shape and size of the cross section and the method used to separate the face from the rest of the tunnel. The cross section may be circular, elliptical, horseshoe-shaped, or rectangular. The size of the cross section may be 8.5–9.75 m (large), 5.7–6.7 m (medium), or 2.6–3.6 m (small). The face may be separated from the rest of the tunnel with or without the use of a cutoff wall, by means of dividing walls, or by means of a watertight bulkhead.

In an unmechanized tunneling shield (see Figure 4 in TUNNEL), the cutting edge of the shield prevents cave-ins and is used for the partial cutting of soft or friable rock. Propelling jacks, face jacks, piping, and controls are located on the shield’s support ring. The overhanging part of the shield protects the site where the lining is erected. Horizontal and vertical bulkheads divide the shield into work compartments, which are required for tunnel driving by the full-face method. The shield is moved by means of the propelling jacks, which push against the forward end of the lining. The face jacks support the face; jumbo jacks advance the jumbo, or drill carriage.

Mechanized tunneling shields are used throughout the world. They are classified according to the area of application, the method used to break the rock, and the type of tool employed. Mechanized tunneling shields may be used in unstable, weak, or stable rock of varying hardness. The rock may be broken by means of shaving, rotary cutting, drilling, or spalling. The tool may be of the rotary, swinging, planetary, or combined type. The main dimensions of a mechanized tunneling shield depend on the diameter of the lining, geological conditions, and the type of mechanized equipment used.

The rate of advance depends on the type and cross-sectional size of the tunneling shield used. Under favorable conditions, the rate of advance may be as high as several hundred meters per month. For example, during the construction of the Leningrad Metro, tunnels were driven at a rate of 600 m/month.

The designs of tunneling shields may be improved by developing both interchangeable tools and standardized cutting tools and loading buckets.