A tunnel is long, narrow, and horizontal to sub horizontal essentially linear excavation that is open to the ground surface at both ends. It is different from a cavern, which is an underground opening whose length and width are nearly similar. Those two types of openings are end members and every underground excavation is a combination to the two.
Related terms: A shaft is a near vertical or vertical excavation that is open to the surface only at the top whereas an edit or drift is similar to tunnel but open at one end only. In practice a drift precedes a tunnel. Stope or raise is an inclined excavation driven from the main tunnel or drift in an upward direction, generally for exploratory purposes.
Overburden: Any and all material overlying the tunnel is called overburden. It may be consolidated or semi consolidated material.
Invert / Floor: Is the bottommost portion of tunnel. In railroad tunnels it is the surface on which the vehicles ply.
Roof/Crown/Back: It is the upper portion of the tunnel above tunnel walls. The shape of the roof may be flat or curved depending upon the tunnel, however, most of the tunnels have curved roof.
A SHAFT - LOOKING VERTICALLY DOWNWARDS
Walls: Both sides of a tunnel extending from floor to invert are called tunnel walls.
Spring line: It is the line connecting all the points in a tunnel where the curved portion of the roof intersects the top of the tunnel walls.
Ground: Ground is the material through which the tunnel has to be driven and muck or tailing is the material excavated from the tunnel. Depending upon their composition and structure the grounds are of the following type.
Few tunnels are excavated without the use of some kind of artificial support. Terzaghi (1946) developed a classification scheme to describe rocks and their load on steel supports. Although this classification scheme is fifty years old, and is limited with today's technology, the classification scheme it is still quite useful for basic descriptions.
Intact rock: Intact rock contains neither joints nor hair cracks, and thus breaks across sound rock. Spalling conditions, which is when thin slabs of rock fall of the roof or walls of the tunnel, and popping conditions, where rock slabs on the sides or roof of the tunnel spontaneously and violently detach, may occur for several hours or days after blasting.
Stratified rock: Stratified rock consists of individual strata with little or no resistance against separation along strata boundaries. Spalling conditions are quite common.
VARIOUS PARTS OF A TUNNEL
Moderately jointed rock: Moderately jointed rock contains joints and hair cracks, but blocks between the joints are locally grown together or so intimately interlocked that vertical walls do not require lateral support. Again, spalling and popping conditions may be encountered.
Blocky and seamy rock: This consists of chemically intact or nearly intact rock fragments, which are entirely separated from each other and imperfectly interlocked. The vertical walls of the tunnel may require support.
Crushed rock: Crushed rock is chemically intact, but extensively fractured. If the crushed rock is fine grained and below the water table, it will exhibit the properties of a water bearing sand.
Squeezing rock: Squeezing rock slowly advances into the tunnel without a perceptible volume increase. This condition requires a very high percentage of microscopic and submicroscopic minerals or clay minerals with a low swelling capacity.
Swelling rock: Swelling rock advances into the tunnel primarily by the expansion of the rock itself. This condition seems to be limited to rocks containing clays, such as montmorillonite, which have a high capacity to swell when hydrated.
Rock mass classification schemes abound, and nearly every different author has different method of classifying rock masses for excavation purposes. The following table 1 includes three more types in soft ground category.
Firm / Intact
Crushed / Shattered
Raveling: In reveling ground chunk or flakes of materials drop from the exposed
Surface. This process may start soon after excavation especially under water.
Running: Running ground is clear, loose gravel or coarse sound. Medium to
Fine sound may also run if dry enough.
Flowing: Flowing ground is wet soil that moves like a viscous liquid and tends to enter the tunnel through every gap in the lining.
Geological Survey for a Tunnel
1. (a) Geological profile along tunnel line and (b) Geological surface map including soil, lithology, contacts, structure (fold, faults, joint etc) and water table with water bearing structures.
2. Pitting, drifting and/or drilling up to the invert or deeper in case of soft ground or even drilling a pilot tunnel for long and large diameter tunnels.
3. Geophysical Investigation including
a) The extent of faulting and fracture of major magnitude.
b) Depth of bedrock under deep soil cover.
c) Location of particular rock formation and their possible intersections with the tunnel invert.
4. Laboratory Tests
b) Structural analysis
Geological Comments on a Tunnel Design
1. Is the material (ground) hard/soft for tunneling?
2. What part of tunnel needs support and what type of supports is required.
3. Is groundwater likely to be encountered, it so, in what proportion of the tunnel and in what quantities.
4. Grouting (Screen / Consolidation) is required or not.
Tunneling methods, as obvious depend upon the type of tunnel and type of ground. Following are the methods associated in the tunneling:
1. Full face method: For small tunnels the whole tunnel section is blasted at once. However, in the longer tunnels if sufficient bridging capacity is there, this method can be applied.
2. Heading & Benching Method: For grounds having shorter bridging capacity, the tunnel is driven step by step. First heading is done and then benching. We proceed in this manner for whole length of the tunnel. The length of head and bench to be blasted in one go is determined by the bridging capacity (or standing time or stand up time) of the ground.
3. Side-drift method: In relatively softer ground and where the tunnel diameter is rather large, this method is useful. In this method drifting from sides are done first. The central portion is left for the time being which also acts as a support for side-drifted tunnel. After erecting required supports on the driven sides, the central portion is tunneled.
4. Multiple drift method: For more softer ground and longer tunnels this method is very effective. Instead of having only side drift we may have more drifts at first stage and after giving required support to the excavated portion the remaining ground is tunneled out.
5. Shield method: In very soft ground (raveling, squeezing, running) this method is very effective. The tunneling is done by an automatic drill head, which keeps on providing instant support after each excavation step.
The Excavation Cycle: The excavation cycle of a tunnel is as follows:
b) Charging the holes with explosives
c) Shooting / blasting
f) Erecting supports and roof-bolting (where required)
g) Lining (where required)