I INTRODUCTION

Tunnel, passage, gallery, or roadway beneath the ground or underwater. Tunnels are used for highway traffic, railroads, and subways; to transport water, sewage, oil, and gas; to divert rivers around dam sites while the dam is being built; and for military and civil-defense purposes. Also considered tunnels are subterranean galleries, usually a series of horizontal passageways on different levels, as in a mine. Construction sites of powerhouses, blasted out of solid rock near dams, are also in the tunnel category.

Chesapeake Bay Bridge-Tunnel Chesapeake Bay features a piece of construction that may startle unprepared travelers. The 28.2 km (17.5 mi) crossing between Norfolk and Cape Charles, Virginia, begins as a bridge, but disappears into the water midway. A combination structure, the Chesapeake Bay Bridge-Tunnel combines two bridges with two tunnels that pass under major shipping channels.Chesapeake Bay Bridge and Tunnel District

II MODERN TUNNELING METHODS

Tunneling Machine Tunneling machines, sometimes called moles, make an initial cut into rock with a cutterhead. Individual disk-shaped cutters are set into the face of a powerful, rotating head, which may be more than 5.5 m (18 feet) in diameter. As the machine bores through rock, conveyor belts transport rock particles, muck, and any other refuse away from the head. Concrete segments are erected to line and protect the tunnel as each area is hollowed out. The segments also offer a firm surface for the machine to grip as it continues through the rock, sometimes at rates of more than 5 m (16.4 ft) an hour.© Microsoft Corporation. All Rights Reserved.

The building of a tunnel is known as driving a tunnel and involves advancing the passageway by blasting or boring and excavating. Tunnels through mountains or underwater are usually worked from the two opposite ends, or faces, of the passage. In the construction of a very long tunnel, vertical shafts may be dug at convenient intervals to excavate from more than two points. Improved boring and drilling machinery now allow a tunnel to be driven four or five times faster than was possible with older techniques.