02-11-2012, 01:01 PM
TUNNELS
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INTRODUCTION :-
A tunnel is an underground passage through a mountain, beneath a city or under a waterway. It may be for pedestrians and/or cyclists, general road traffic, motor vehicles, rail traffic, or for a canal. Some tunnels are constructed purely for carrying water (for consumption, hydroelectric purposes or as sewers); others carry services such as telecommunications cables. A tunnel is relatively long and narrow; in general the length is more (usually much more) than twice the diameter. Some hold a tunnel to be at least 0.160 Kilometer long and call shorter passageways by such terms as an "underpass" or a "chute".
TRANSPORTATION TUNNELS
It comprise of railways and highways, they are used for transportation from one place to another in a shorter path as well as fast and efficient, they are used to avoid a long detention around a mountain or a spur, the use of tunnel in such cases reduces the length of the highway/railway line and may be economical.
MINE TUNNELS
Mine Tunnels are used during ore extraction, enabling laborers or equipment to access mineral and metal deposits deep inside the earth. These tunnels are made using similar techniques as other types of tunnels, but they cost less to build. Mine tunnels are not as safe as tunnels designed for permanent occupation.
PUBLIC WORKS TUNNELS
It carry water or sewage across great distances. The earliest tunnels were used to transport water to, and sewage away from, populated regions. These tunnels were part of aqueduct systems, which also comprised underground chambers.
ADVANTAGES & DISADVANTAGES OF TUNNELS :-
ADVANTAGES :
• Economical than open cuts beyond certain depth
• Protected the system (Railway track / Highway etc.) from weather
• Best alternative to provide means of transportation in congested urban area
• Reduces Road / Railway Track length to reach the other side of mountain
• Long life & little maintenance reduces life-cycle costs
DISADVANTAGES :
• Initial cost of Tunnel Construction is more
• Tunnel Construction requires skilled labour & technical supervision
• Construction time is more
• Construction of Tunnel requires sophisticated and specialized equipments
CONSTRUCTION OF TUNNEL :-
The Procedure is as follows:
i. Requirement of tunnel v/s open cut
ii. Tunnel Alignment
• Study of Alternative Alignments of the Tunnel
• Selection of one Alternative for further Studies
• Survey of the Area at and around the Alignment of the Selected Alternative
iii. Geotechnical Studies and Tests for assessment of Tunneling Conditions
iv. Geometry and Design of the Tunnel
v. Design of Tunnel Supporting Systems
vi. Methodology of Construction of the Tunnel
REQUIREMENT OF TUNNEL V/S OPEN CUT
Where the alignment has to pass through high ground, the passage is normally taken through open cuts because tunneling is more expensive than open excavation. But, when depth of the cut exceeds thrice the maximum dimension of a tunnel that can be provided therein, tunneling generally proves to be economical. Of course, for commencement of tunneling from any point, good rock cover approximately equal to thrice the maximum dimension of the tunnel is necessary above the crown of the tunnel. While the tunnels are almost maintenance-free, open cuts are susceptible to landslides during heavy rains, causing passage disruption. At the same time, long tunnels require artificial ventilation and lighting, which are perpetual expenses. While the land occupied by the open cuts is lost forever, tunnels pass below the land without permanently disturbing it. This consideration becomes of vital importance in agriculturally rich and fertile areas and in populated areas. The decision - tunnel or open cut - therefore depends on
the overall economics and environmental considerations.
TUNNEL ALIGNMENT
The next aspect that needs to be decided for a tunnel is the alignment, that is, the route layout of the tunnel in plan. Firstly, the Reconnaissance Survey is carried out to examine the general characteristics of the area for determining the most feasible route/s for further investigations. After that, the Preliminary Survey is carried out to determine short, easy, safe and economical route. Lastly, the Final Survey is carried out around and along the selected alignment of Tunnel for design and preparation of working drawings. Figure shows the possible alignment for the tunnel water conveyance system for a hydropower system using tunnel.
GEOTECHNICAL STUDIES
How a tunnel is built depends heavily on the material through which it must pass. Hence, Geotechnical Investigations are essential for proper planning of a tunnel. Selection of the alignment, cross section, and construction methods is influenced by the geological and geotechnical conditions, as well as the site constraints. Tunneling through soft ground, for instance, requires very different techniques than tunneling through hard rock or soft rock, such as shale, chalk or sandstone.
D-SECTION
This type of section would be found suitable in tunnels located in massive igneous, hard, compacted, metamorphic and good quality sedimentary rocks where the external pressures due to water or unsound strata upon the lining is slight and also where the lining is not required to be designed against internal pressure. The principal advantages of this section over horse-shoe section are the added width of the invert which gives more working floor space in the heading during driving and the flatter invert which helps to eliminate the tendency of wet concrete to slump and draw away from the tunnel sides after it has been cast.
HORSE SHOE SECTION
These sections are a compromise between circular and D sections. These sections are strong in their resistance to external pressures. Quality of rock and adequate rock cover in terms of the internal pressure to which the tunnel is subjected govern the use of these sections. Modified horse-shoe section offers the advantage of flat base for constructional ease and change over to circular section with minimum additional expenditure in reaches of inadequate rock cover and poor rock formations.
TUNNEL LINING
This is a protective layer within the tunnel made of plain or reinforced concrete. Tunnels may be completely lined, partially lined, or even unlined. Tunnels in good sound rock may be kept unlined. However, lining is recommended when:
a) The internal water pressure exerted by water conveyed by the tunnel is high, say above 100m of water head. For very good competent rock, tunnels may be kept unlined for pressures even up to 200m water head.
b) The rock strata through which the tunnel passes has low strength and where the rock is anisotropic.