A tunnel is an elongated, narrow essentially linear underground opening with a length greatly exceeding its width or height. Most tunnels are nearly or exactly horizontal but for special purposes, tunnels may be driven at angles up to 30 degree from the earth’s surface. The one which is greater than 30 degree from horizontal are designed as shafts. When rocks in tunnels are highly in-competent, especially when underground water is present, tunneling becomes a very costly and hazardous operation, and excavation and containment of such rocks present a challenge that requires maximum use of highly technical skills and ingenuity.
History of Tunneling
There is abundant archaeological evidence that in Europe stone age man sank shafts and drove tunnels to recover flint for the fabrication of sharp-edged implements such as knives, axes etc. later as an elementary knowledge of metallurgy was acquired by premature people, possibly for the first time central Asia, underground excavation became necessary to supply the increasing demands for metals and alloys. Very early underground excavations for metal-bearing have been identified in Caucasia, between the Black and Caspian Sea, and date back to approximately 3500 BC. Many tunnels were built in ancient times by the Babylonians, Indians, Persia and Egypt in search for precious metals.
Stone-age man used very primitive tools in underground excavation. Particularly useful to him were picks made of deer antlers, flint axes and hammers and wedges made of bone and wood. The production of metals and alloys provided materials for increasingly efficient rock excavation. Later on explosives were used in the seventeenth century. For hundreds and perhaps thousands of years underground working in hard rocks, especially those containing few fractures and fissures, were advanced by building fires against rock faces to cause expansion and spalling. In some operations spalling of the heated rock was accelerated by dowsing it with water. The fractured rock was than separated from the working face with picks, gads and wedges.
With the increasing use of explosives first the black powder later nitroglycerin, steel temping techniques were perfected and permitted efficient and economical hand drilling of holes for explosives. Tunneling machines have been used to excavated tunneling with diameters of about 6 ft to more than 36 ft. Rate of excavation of over 400 ft per day have been recorded in soft ground. In hard rocks it can be less as 100 ft per day. It includes a rotating cutter head and provision for controlling forward thrust and alignment.
In the hardest of rocks, near the middle of the nineteenth century steam powered piston drills and later percussion drills, powered by compressed air, made their appearance and at the same time several tunneling machines such as moles were invented.
Tunnels have been driven in a variety of natural materials ranging from unconsolidated water-soaked clay, sand and gravel to dry very hard un-fractured rocks. It is one of the most costly and at the same time one of the most hazardous of all engineering under-takings. In case of long tunnels in area of geological complexity, all types of uncertainties arise, including design and construction techniques and including estimate of cost. The location of a tunnel like the site of bridge often does not allow much freedom of choice. It becomes necessary at given place to maintain an alignment. Before designing and planning a tunnel the undesirable underground conditions must be anticipated. Tunnels through massive un-fractured granite or through horizontally layered sandstones that are well cemented and un-joint present no special problems in design and preparation of cost estimation; whereas in geological complex areas it is an art and intelligent guess work.
Purpose of Tunneling
Tunnels have been constructed for great variety of purpose, and they are classified as follows:
- Tunnels driven to gain access to economic mineral deposits and to provide haul-ways for extracted minerals. In some mining operations tunnels are driven to provide adequate circulation of air in underground workings.
- Transportation Tunnels, including pedestrian highness navigational and rail road tunnels. These are among the largest and at time the most difficult of all tunnels to excavate.
- Water or Sewage Tunnels: These tunnels may or may not be constructed so as to transport liquid under pressure, and a distinction is made between gravity flow tunnels and pressure tunnels. The latter are designed to contain without leakage water under hydrostatic pressure or force-pressure head.
- Military Tunnels: These tunnels are driven in connection with underground military operations.
- Tunnels to provide protection from atomic explosion.
- Utility Tunnels. Built to contain power and communication transmission line, gas line, etc.