Transcript 15. Tunnels P.ppsx

Teaching Aids Service by
KRRC Information Section
Tunnel
A tunnel is an underground passageway,
completely enclosed except for openings for
ingress and egress, commonly at each end.
TUNNEL DESCRIPTION
1.
2.
3.
4.
5.
6.
Made into natural material (rocks)
Empty inside
Carry the loads itself
Both ends are open to atmosphere
Generally horizontal
Thick walled structure looks like cylinder
1- Key stone
2- Arch
3- Wall
4- Floor
5- Bench
6- Bench line or plane
7- Top heading
8- Invert
9- Ano (unit)
Types of Tunnels
• A tunnel may be for foot or vehicular road traffic, for rail traffic, or
for a canal.
• Some tunnels are aqueducts to supply water for consumption or for
hydroelectric stations or are sewers.
• Other uses include routing power or telecommunication cables,
some are to permit wildlife such as European badgers to cross
highways.
• Secret tunnels have given entrance to or escape from an area, such
as the Cu Chi Tunnels or the smuggling tunnels in the Gaza Strip
which connect it to Egypt.
• Some tunnels are not for transport at all but rather, are
fortifications, for example Mittelwerk and Cheyenne Mountain.
EXPLORATION & INVESTIGATIONS RELATED of
SLOPE STABILITY
• Geomorphologic maping and preparation of longitudinal
& cross sections
• Geological maping & surveyings (aerial photographs)
• Geophysical surveyings
• Underground explorations, boreholes
• Ground water surveyings
• Laboratory tests
• Model studies
SUBSURFACE EXCAVATIONS
GEOLOGY
1. Soil profile or hard rock geology
2. Structure
3. Ground water (hydrogeology)
4. Stability
INVESTIGATIONS
1. Mapping (Topographic, geologic, etc...)
2. Geophysical surveying (especially seismic
velocity of rocks)
3. Trial pits & boreholes
4. General and local stability analysis
5. Decide to excavation method
FACTORS EFFECTING
EXCAVATION of ROCKS
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Mineralogical composition of rocks
Texture & fabric
Petrographic features
Structure
Rock mass
Strike & dip of beds in relation to face of excavation
Intensity of tectonic disturbances
Degreee of weathering
1. Describe the following geological structures
that influence the tunnel design, stability and
cost.
• ( i ) Tunnels in Horizontal Strata
• (ii) Tunnel axis parallel to the dip direction.
• (iii) Tunnel axis driven parallel to the strike
• (iv) Tunnels in folded rocks
• ( i ) Tunnels in Horizontal Strata
• Horizontal strata : Such a situation is rare in
occurrence for long tunnels.
• When encountered for small tunnels or for short
lengths of long tunnels, horizontally layered rocks
might be considered quite favourable.
• In massive rocks, that is, when individual layers are
very thick, and the tunnel diameter not very large, the
situation is especially favourable because the layers
would overbridge flat excavations by acting as natural
beams.
• However, when the layers are thin or fractured, they
cannot be depended upon as beams; in such case,
either the roof has to be modified to an arch type or
has to be protected by giving a lining.
Horizontal strata : Such a situation is rare in occurrence for long tunnels.
When encountered for small tunnels or for short lengths of long tunnels,
horizontally layered rocks might be considered quite favourable.
In massive rocks, that is, when individual layers are very thick, and the
tunnel diameter not very large, the situation is especially favourable
because the layers would overbridge flat excavations by acting as natural
beams.
Tunnel axis parallel to the dip direction
• When the tunnel axis is parallel to the dip direction
(which means it is at right angles to the strike
direction), the layers offer uniformly distributed load
on the excavation.
• The arch action where the rocks at the roof act as
natural arch transferring the load on to sides comes
into maximum condition. Even relatively weaker rocks
might act as self- supporting in such cases. It is a
favourable condition from this aspect.
• However, it also implies that the axis of tunnel has to
pass through a number of rocks of the inclined
sequence while going through parallel to dip.
• (iii) Tunnel axis driven parallel to the strike
• When the tunnel is driven parallel to strike of the
beds, the pressure distributed to the exposed
layers is asymmetrical along the periphery of the
tunnel opening : one half would be bedding
planes opening into the tunnel and hence offer
potential planes and conditions for sliding into
opening.
• The bridge action, though present in part, is
weakened due to discontinuities at the bedding
planes running along the arch.
Tunnels in steeply inclined strata
In vertical beds when the
tunnel axis is parallel to the
dip direction the formation
stand along the sides and on
the roof as massive girdersan apparently favorable
condition.
Conversely
in
tunnels
running parallel to the strike
of vertical bed a number of
bedding
planes
are
intersected at roof and along
the arch so the natural beam
action gets considerably
weakened.
Tunnels in folded rocks
• Folded rocks show bends and curvatures and store a
lot of stain energy in the rock.
• Their influence on design and construction of tunnel
is important at the their position of angles.
• Folded Rocks are under peculiar rock pressure when
excavations are made the store energy is likely to be
released causing the rock fall or rock burst.
• Effect of the folds: While tunnel is excavated in an
area that contains folded rocks, different stresses and
conditions may occur depending on the fold type.
Fold axis and the tunnel direction is parallel
Fold axis and the tunnel direction is vertical
• Effect of faults: the relation between the fault
slope direction and the tunnel direction, width of
the fault zone, type and thickness of the fill
material and the hydrostatic pressures in both
sides of the fault are some problems in the
tunnelling.
Relation between the fault zone and the tunnel
• Tunnel excavations in the slopes: the discontinuities
(layers, fissures) inclined inside or outside of the
slope are very important regarding the stress and
strength of the tunnel.