TUNNELS
Introduction
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.
Tunnel
Terminology
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.
Table: 1
|
Soft Ground |
Hard Ground |
|
Raveling Running Flowing Squeezing Swelling |
Firm
/ Intact Stratified Moderately
Jointed Blocky Seamy 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
a) Microscopy
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
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:
a) Drilling
b) Charging the
holes with explosives
c) Shooting /
blasting
d) Shortcreting
e) Ventilating
f) Erecting
supports and roof-bolting (where required)
g) Lining
(where required)
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