TUNNEL BASICS AND CLASSIFICATION & SUPPORTING SYSYTEM
Tunnels:
A tunnel is
an underground or underwater passageway, enclosed except for entrance and exit,
commonly at each end.
A tunnel may be for foot or vehicular road traffic, for rail traffic,
or for a canal. The central
portions of a rapid transit
network are usually in tunnel. Some tunnels to supply water for consumption or
for hydroelectric stations or are sewers. Utility tunnels are used for
routing steam, chilled water, electrical power or telecommunication cables, as
well as connecting buildings for convenient passage of people and equipment.
Tunnels are generally grouped in four broad categories,
depending on the material through which they pass: soft ground, consisting of
soil and very weak rock; hard rock; soft rock, such as shale, chalk, and
friable sandstone; and subaqueous.
While these four broad types of ground condition require very
different methods of excavation and ground support, nearly all tunneling
operations nevertheless involve certain basic
procedures: investigation, excavation and materials transport, ground support,
and environmental control. Similarly, tunnels for mining and for
civil-engineering projects share the basic procedures but differ greatly in the
design approach toward permanence, owing to their differing purposes. Many mining tunnels have been planned only for minimum-cost temporary use
during ore extraction, although the growing desire of surface owners for legal
protection against subsequent tunnel collapse may cause this to change.
By contrast, most civil engineering or public-works tunnels involve
continued human occupancy plus full protection of adjacent owners and are much
more conservatively designed for permanent safety. In all tunnels, geologic
conditions play the dominant role in governing the acceptability of
construction methods and the practicality of different designs.
7.2GEOLOGICAL CLONTROl
Thorough geologic analysis is
essential in order to assess the relative risks of different locations and to
reduce the uncertainties of ground and water conditions at the location chosen.
In addition to soil and rock types, key factors include the initial defects
controlling behaviour of the rock mass; size of rock block between joints; weak
beds and zones, including faults, shear zones, and altered areas weakened by
weathering or thermal action; groundwater, including flow pattern and pressure;
plus several special hazards, such as heat, gas, and earthquake risk. For
mountain regions the large cost and long time required for deep borings
generally limit their number; but much can be learned from thorough aerial and
surface surveys, plus well-logging and geophysical techniques developed in the
oil industry. Often the problem is approached with flexibility toward changes
in design and in construction methods and with continuous exploration ahead of
the tunnel face, done in older tunnels by mining a pilot bore ahead and now by
drilling. Japanese engineers have pioneered methods for prelocating troublesome
rock and water conditions.
The dominant factor in all phases of
the tunneling system is the extent of support needed to hold the surrounding
ground safely. Engineers must consider the type of support, its strength, and
how soon it must be installed after excavation. The key factor in timing
support installation is so-called stand-up time—i.e., how long the ground will safely stand
by itself at the heading, thus providing a period for installing supports. In
soft grounds can vary from
seconds in such soils as loose sand up to hours in such ground as cohesive clay
and even drops to zero in flowing ground below the water table, where inward
seepage moves loose sand into the tunnel. Stand-up time in rock may vary from
minutes in raveling ground (closely fractured rock where pieces gradually
loosen and fall) up to days in moderately jointed rock (joint spacing in feet) and
may even be measured in centuries in nearly intact rock, where the rock-block
size (between joints) equals or exceeds size of the tunnel opening, thus
requiring no support.
7.3ENVIRONMENTAL CONTROL
In all
but the shortest tunnels, control of the environment is essential to provide
safe working conditions. Ventilation is
vital, both to provide fresh air and to remove explosive gases such as methane
and noxious gases, including blast fumes. While the problem is reduced by using
diesel engines with exhaust scrubbers and by selecting only low-fume explosives
for underground use, long tunnels involve a major ventilating plant that
employs a forced draft through lightweight pipes up to three feet in diameter
and with booster fans at intervals. In smaller tunnels, the fans are frequently
reversible, exhausting fumes immediately after blasting, then reversing to
supply fresh air to the heading where the work is now concentrated.
7.4ROCK SUPPORT
Most common loading on the support of a
tunnel in hard rock is due to the weight of loosened rock below the ground
arch, where designers rely particularly on experience with Alpine tunnels as
evaluated by two Austrians, Karl V. Terzaghi, the founder of soil mechanics, and Josef Stini, a pioneer in engineering geology. The support load is greatly increased
by factors weakening the rock mass, particularly blasting damage. Furthermore,
if a delay in placing support allows the zone of rock loosening to propagate
upward (i.e., rock
falls from the tunnel roof), the rock-mass strength is reduced, and the ground
arch is raised. Obviously, the loosened rock load can be greatly altered by a
change in joint inclination (orientation of rock fractures) or by the presence
of one or more of the rock defects previously mentioned. Less frequent but more
severe is the case of high geostress, which in hard, brittle rock may result in
dangerous rock bursts (explosive spalling off from the tunnel side)
or in a more plastic rock mass may exhibit a slow squeezing into the tunnel. In
extreme cases, squeezing ground has been handled by allowing the rock to yield
while keeping the process under
control, then remining and resetting initial support several times, plus
deferring concrete lining until the ground arch becomes stabilized.
7.5ROCK MASS CHARACTERIZATION
Tunnel engineers generally classify rocks on the basis of resistance to
deformation (strength), amount of weathering, and general resistance to
weathering. (Parker, 1996) These last two are not the same, although they may
seem so at first glance. A rock may be very resistant to weathering, but may
have also have been subjected to a very long period of time, which may result
in significant weathering. In general, the strength and resistance to
weathering of a rock is derived from the class of rock. Igneous and metamorphic
rocks, in general, are more resistant to deformation and weathering than
sedimentary rocks.
Class
|
Descriptor
|
Unconfined
Stress
Range (psi) |
Unconfined
Stress
Range (MPa) |
R0
|
Extremely
soft
|
20-100
|
0.2-0.7
|
R1
|
Very
low strength
|
100-1000
|
0.7-7
|
R2
|
Low
strength
|
1000-4000
|
7-28
|
R3
|
Moderate
strength
|
4000-8000
|
28-55
|
R4
|
Medium
high strength
|
8000-16,000
|
55-110
|
R5
|
High
strength
|
16,000-32,000
|
110-220
|
R6
|
Very
high strength
|
>32,000
|
>220
|
7.6 Importance of Geology in Planning of Tunnels
Ø Geology
dominates the feasibility, behavior and cost of tunnel.
Ø To
know type of material soil and rock
type.
Ø Good
geological planning allows Engineer to plan for various uncertainty risk right
from the beginning, decreasing the likely hood of an unexpected delay in the
middle of project.
Ø Weak
beds & Zones including faults and shear zones.
Ø Ground
water , including flow pattern and
pressure.
Ø Helps
in selection of alignment and construction method.
7.7 Rock Mass Rating
Determined on basis of 5 parameters :
Ø U.C.S.(Uniaxial
Comp. Strength).
Ø R.Q.D.(Rock
Quality Design).
RQD =
solid core having length more than
�1
Ø Joint
Parameter.
Ø Weathering
Condition.
Ø Water Condition.
Ø R.M.R.=U.C.Q.+R.Q.D.+J.P.+W.C.+Water
Cond.
7.8 RMR & Rock Classification
RMR
|
ROCK
QUALITY
|
0 – 20
|
Very Poor
|
21 – 40
|
Poor
|
41 – 60
|
Fair
|
61 – 80
|
Good
|
81- 100
|
Very Good
|
7.9Tunnel 5
Specifications (at CH:157+660) (PRACTICAL EXAMPLE)
Since
the road exists in hilly terrain there are 5 tunnels along the newly aligned
plan.I worked at Tunnel 5 which falls in Village Bhuwana of Distt Bilaspur,
Himachal Pradesh & is 740metre long.
Ø Location
of Tunnel :
.......................
Ø Chainage CH
:
From �157+66
Ø Length
of tunnel : 74
Ø Finished
Size : �1
Ø Formation
level
Portal 1 : 6�19.95m.
Portal 2 : 634.78m.
Ø Shape
of Tunnel : Horseshoe.
Ø Days
of Completion : 8
Ø Mechanical
Ventilation : Apllied
Ø Excavation
Method : Drill & Blast
(NATM)
Ø Type : Single tube,two lane,bi-directional traffc tunnel.
7.9.1Cross Section of
Tunnel
7.9.2 Geotechnical
Profile of Tunnel
7.10 SUPPORT
SYSTEM CLASSIFICATION
By
studying the RMR value of the rock masses present at Tunnel 5 Portal 2, 7
support systems were defined:
i.
A1
ii.
A2
iii.
B1
iv.
B2
v.
C1
vi.
C2
vii.
L
Support systems A1, A2, B1,
B2 are used to support the strong rock mass, while the C1
C2 and l classes are used to provide support for the loose rock
mass.
7.11PRE-EXISTING
STRUCTURE
Ø Portal
segmented into 4 benches
Ø Height
of top three bench-7m
Ø Height
of bottom bench-12m
Ø Slope-1:5
per bench
Ø Stabilising
method
·
Shotcrete lining
·
Rock bolting
·
Wire mesh
Ø Shotcrete
Details
·
M25 Grade
·
Min quantity of cement-400kg/m3
·
Aggregate size-10mm
·
Thickness-0.15m
Stabilising of slopes by shortcreting
Ø Wire
mesh
·
Fy-415N/m2
·
Dimension-150mmx150mmx6mm
Ø Rock
Bolts
·
32mm dia
·
Fully grouted
Grouting mortar M20 grade with water cement ratio:0.3
7.12Cycle Process for Excavation of Tunnel
A.For Good Rock
Condition
1. Profile Marking.
2. Drilling.
3. Blasting.
4. Defuming.
5. Mucking.
6. Rock Support- Shotcrete/ Rock Bolt.
B.For Poor Rock
Condition
1. Profile marking.
2. Drilling.
3. Blast.
4. Defuming.
5. Shotcrete.
6. Mucking.
7. Rock Support-Swellex Drill & Insert,Rock Bolt, Ribs, wiremesh,
Shotcrete, lattice Girder.
Profile Marking
Surveyor marks the profile of
drilling on face of Tunnel with the help of Total
Station
Drilling
Ø
Drilling Method is decided on type of geological
conditions and necessary drill hole diameter.
Ø
Holes shall be either drilled by rotary method with
water flush or pneumatic percussion method with air/water or any other type
such that the disturbances to the adjoining rock is minimal.
Ø
Use of
bentonite, rod grease or other lubricants is not allowed.
Blasting
Blasting Technique is considered as pre
specifications for controlling the complete rock surfaces.
Specifications
Atleast 50% of drill hole traces of each round must be visible in the
final rock surface distributed uniformly after the scaling of all loose and
shattered rock that is liable to fall
before or during rock reinforcement installation.
Specifications of Explosives used in
Tunnel.
Quantity in Box – 25 kg
Length (mm)
|
Dia. of Explosive(mm)
|
Weight(gm)
|
300
|
25
|
125
|
300
|
32
|
196
|
300
|
40
|
390
|
Ø
Mass of explosive per hole (Kg) = Volume of hole
length Charged x Explosive Density.
Ø
After Explosion, Powder Factor is calculated :
Ø Powder Factor = Explosive Used/ Muck Generated.
Ø Powder Factor should be as low as possible.
Defuming
After Blasting, Face is left
idle for Defuming. Normally it takes time upto
60min. But with
Exhaust fan it takes upto 30
min.
Mucking
Removal of the excavated
matter is called mucking.Mucking is done after defuming using
excavator and dumper. Normally it takes upto
3-4 hrs.
Wire Mesh
Ø Wire Mesh consists of welded
wires in a fabric. Wire Mesh is installed in surface and underground excavation
as reinforcement for shotcrete usually in combination with rock reinforcement.
Ø Steel of minimum yield
strength : 4�15N/mm2
Ø
Dimensions : �15
Shotcreting
Shotcrete for tunnel
supports may be used by itself as a thin skin type reinforcement or used in
combinations with rock bolts, wire mesh and other more conventional tunnel reinforcements. All
loose rocks shall be sealed out and washed before applying shotcrete. Shotcrete is
forced into open joints, fissures, seams and irregularities in the rock surface and in this way it serves
the same binding function as mortar in a stone wall.
Shotcrete hinders
water seepage from joints and seams in the rocks and thereby prevents
piping of joint filling materials and air and water deterioration of rock.
Type of
Shotcrete-
Ø Dry Shotcrete
Ø Wet Shotcrete
Shotcrete
Material :
Ø Minimum cement content:
400Kg/m3.
Ø Maximum size of aggregates:
10mm.
Ø Minimum strength (MPa)
: �1 day………�1�1
7 days……..�17.5
8 days……..5
Ø FosRock was used as
accelerator in Shotcrete.
Ø Accelerator- 3% by weight of
cement.
Rock Bolt
Types of Rock Bolt
Ø Expansion Shell Rock Bolt
Ø Resin Grouted Rock Bolt
Ø Rock Anchor Bolt
Ø Grouted Anchor Bar
After completion of the
drilling in rock the entire length of the hole shall be washed out with clean
water.
Swellex Pipe
Ø S75JR/SSteel quality 355MC.
Ø It swells when high water
pressure is applied from opening.
Ø Diameter :32mm
7.13
Rib Specification
It is used to support the rock along the circular portion inside the
tunnel.
Ø Hot rolled steel ISBH
Ø Installed at 1m Distance.
Ø A=5
5 Parts are joined togetherLength : L1 = 6.79m; L2=L3 = 3.364m; L4=L5 = .33m.
With �16mm Dia, Bolt, Grade 4.6 ,Length = 3inch.Connected to steel
pipes by welding.
7.14 Forepolling
The support class at portal 2 is L type hence
forepolling is carried out before excavation to form umbrella cover. The
forepolling was carried out by two techniques:
Ø Dry
drill
Ø Wet
drill
DRY
DRILL
Dry Drill method
is used drill has to be carried out in a loose rock soil mass. Dry drill is
done by
ROC machine.
WET
DRILL is used to drill a hard rock mass. It is done by
boomer machine.
FOREPOLLING
PATTERN
Ø 12m
long drill of 104mm dia
Ø MS
pipes inserted in drilled hole and fully grouted
Ø MS
pipes having dia 76/89mm
Ø Max
yield load-1200KN
Ø 40mm
dia rockbolt inserted in mild steel pipes
Ø Grouted
with M20 grade mortar having water cement raio-0.3
Ø Inner
crown forepolled at 3.5o and outer crown forepolled at 4.5o
Ø An
overlap of 3m provided for further forepolling inside the tunnel
GROUTING THE FOREPOLL
Forepoll is grouted by M20 grade mortar having water
cement ratio-0.28 and it is done by grout
pump(20barr).
ROCK MOVEMENT ANALYSIS
The
portal was equipped with several instruments to study various rock movements to
analyse the safety and stand up time of tunnels. Major instruments used:
Ø Optical
survey targets
Ø Extensometers
Ø Inclinometer
OPTICAL SURVEY TARGETS
Optical
survey targets are used to measure settlements if any due to the failure of support
systems provided.
EXTENSOMETERS
Extensometers
are used to measure the forward sliding of tunnels if any due to the relative
movements of rock masses.
