Tunneling

(1)

Tunneling

(2)

Tunnels

• A tunnel is an underground passage for the

transport of passengers, goods, water, oil

etc.

• Railway Tunneling in India started in 19

th

Century.

• The first Indian Railway tunnel was Parsik

Tunnel

• Today we have 348 railway tunnels in India,

excluding the tunnels constructed on the

(3)

Necessity and Tunnel

Advantages

• A tunnel eliminates the need for a long and circuitous route for reaching the other side of a hill. Thus an economical method.

• In hills with soft rocks, a tunnel is cheaper than cutting because of support systems required for cutting works.

• In urban setup, tunnels provide additional work-space without disturbing the existing infrastructure. For instance, underground railway system.

• Tunnels are more economical than open cuts beyondcertain depths • Maintenance cost of a tunnel is considerably lower than that of a

bridge or deep cutting.

– However, Initial Construction Cost of a tunnel is very high, as it requires skilled workforce and heavy machinery

– It is a time consuming exercise.

– Risks involved like water-cavity formation or encountering unexpected rock-soil behavior are very high.

(4)

Selection of Tunnel Route

• Alignment restraints:

– Underground space is a heterogeneous mass and in

addition, problems like water table, position of fractured

rocks etc. are to be tackled.

– A thorough detailed inspection and evaluation of the

existing alignment restraints of underground space

should therefore be made & correlated with the

tunnelling technology to be adopted for the project

• Environmental considerations:

– The site of tunnel should be selected in such a way that

the least difficulty is experienced for various

environmental factors such as disposal of exhaust gas,

groundwater, muck, etc.

(5)

Shapes of Tunnels

• Primarily there are four shapes

– Circular

– Egg Shaped

– D Section

(6)

Circular Tunnel

• The circular section is most suitable from structural

considerations.

• However, it is difficult for excavation, particularly

where cross-sectional area is small.

• For tunnels which are likely to resist heavy inward

or outward radial pressures, it is desirable to adopt

a circular section.

• In case where the tunnel is subjected to high

internal pressure, but does not have good quality of

rock and/or adequate rock cover around it, circular

section is considered to be the most suitable.

(7)

(8)

D Section Tunnel

• This type of section would be found suitable in tunnels

located in massive igneous, hard, compacted,

metamorphic and good quality sedimentary rocks

• 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 (discussed in next slides) are the added width of

the invert which gives more working floor space in the

heading during driving

• 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.

(9)

(10)

Shoe and Modified

Horse-Shoe

• 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.

(11)

(12)

Egg Shaped and Egglipse

Sections

• Where the rock is stratified, soft and very closely

laminated (as laminated sand stones, slates,

micaceous schists, etc)

• Where the external pressures and tensile forces in

the crown are likely to be high so as to cause

serious rock falls, egg shaped and egglipse sections

should be considered.

• These sections afford advantage in cases of sewage

tunnels and tunnels carrying sediments.

• Egglipse has advantage over egg shaped section as

it has a smoother curvature and is hydraulically

(13)

(14)

Tunnel Alignment

• A small error in setting the alignment would result in the two

ends never meeting at all.

• At the start of excavation work, both ends of the tunnel as

well as the center line are marked with precision.

• Following factors play an important role in tunnel alignment

– Alignment should be straight as far as possible because a straight route is mostly the shortest and economical route.

– Minimum possible gradient should be provided for tunnel approaches – There must be provision of ventilation and lighting which must be

decided at the time of tunnel alignment

– Side drains in the tunnels must have a minimum of 1 in 500

gradient. In longer tunnels, this gradient should be from the centre towards the both ends.

(15)

Size and Shape of Tunnel

• Size and Shape of a tunnel depends upon

– Nature of Ground

– Type of Railway Line to be constructed (single line or double) – Rock Quality

• If ground is made up of hard rock, tunnel can be given any shape then

• Tunnels in rocky terrains have semi-circular arch and vertical side walls.

• For soft grounds, pressure from the top and sides has to be resisted

• Theoretically, a circular section provides the largest cross-sectional area for the smallest diameter, provides greater resistance to external pressure.

• For a railway track, the circular section at the bottom has to be leveled to lay the track.

(16)

(17)

Size of Tunnel

Gauge

(mm) Single Line Double Line Breadth

(mm) Height (mm) Breadth (mm) Height (mm) MG (1000) 4270-4880 6100-6700 8530-9140 6100-6700 BG (1676) 4880-5490 6700-7320 8530-9140 6700-7320

Size of a railway tunnel depends upon _{Gauge of Railway Track}

(18)

Methods of Tunneling

• Broadly, tunneling methods can be

divided into two groups

– Hard Rock Tunneling

– Soft Rock Tunneling

• Selection of a method depends upon

– Size of the bore

– Equipment available

– Condition of the ground

(19)

Full Face Method

• The whole tunnel face is blasted at the same time.

• It allows tunnelling in one operation and is efficient.

• Large mechanical equipment is required for large

tunnels.

• This method is not suitable for unstable rock where

large opening will induce significant stress on the rock

mass.

• Usually suited for tunnels where dimensions don’t

exceed 3m. (dia)

• Vertical columns are erected at the face of the tunnel

and a large number of drills are mounted or fixed on

these columns at a suitable height.

(20)

(21)

Heading and Benching

• In large tunnels and when the quality of the rock is not

satisfactory, heading and benching method is often used.

• This method involves the driving the top portion of the

tunnel in advance of the bottom portion.

• The lining of roof arch can then be constructed first by

using the bottom bench as temporary supports.

• Another advantage of this method is that when cutting

the bottom bench, the blasting becomes more effective

by using vertical blast holes behind the tunnel face and

less explosives can be used.

• The heading portion lies about 3.7m to 4.6m ahead of

the benching portion

(22)

(23)

Drift Method

• In very large tunnel or weak rock, the attacking of the

tunnel face can be further subdivided into several stages.

• Similar to heading and benching, tunnelling is carried out

in smaller section first and then widened subsequently.

• Drift method can be further classified into centre drift,

side drift, top drift and bottom drift.

• A drift is a small tunnel measuring 3mX3m, is driven into

the rock in phases, and then widened in subsequent

process.

• A number of drill holes are provided all around the drift

and are blasted with the help of explosives so that size of

drift can be expanded to that of the tunnel cross section

• Location of drift depends upon local conditions.

(24)

(25)

Pilot Tunnel Method

• This method involved digging of two tunnels,

namely a pilot tunnel and a main tunnel.

• Pilot Tunnel cross section usually measures

about 2.4mX2.4m.

• PT is driven parallel to the main tunnel and

is connected to the centre line with cross

cuts at many points

• Cross cuts serve many functions like

removal of muck, simultaneously excavation

at multiple positions etc.

(26)

(27)

Hard Rock Tunneling

Method Advantages Disadvantages

Full Face Method Entire section in one go – effective method

Mucking tracks, tracks used for collecting muck, can be laid on the tunnel floor and expanded as work progresses

Drill Carriage makes this method suitable for large tunnels too

Requires Heavy

Mechanical Equipment No very suitable for unstable rocks

Successful only for smaller tunnel sections

Heading and Benching _{Work of drilling holes}

for explosives and removal of muck can be carried

simultaneously Requires lesser

blasting material than Full Face Method

(28)

Hard Rock Tunneling

Method Advantages Disadvantages

Drift Method If poor quality rock or excess water is

encountered, it can be detected in advance to take corrective measures A drift assists in

ventilation of tunnels Quantity of explosives required is less

A side drift allows the use of timber to support the roof

Time consuming process, as main tunnel excavation has to wait until the drift has been completed

Cost of drilling and Muck Removal is very high.

Pilot Tunnel Method __{Helps in removal of}

muck from main tunnel quickly

Helps in providing proper ventilation and lighting of the main tunnel

Not very cost effective

Slow progress in the initial phases

(29)

Tunneling in Soft Rock

• Tunneling in soft rock requires precautionary tactics and minimal (negligible) use of explosives.

• Excavation work is done using hand tools, such as pickaxes and shovels.

• In recent times, compressed air at high pressure and velocity is also employed to excavate the soft rock mass.

• During excavation in soft rock, support at the sidewalls is the primary requisite.

• Support could be in the form of timber or steel plates. • Soft Rock Tunneling is performed as

– Excavation or Mining

– Removal of excavated material – Scaffolding or shuttering

– Lining of Tunnel Surface

• Nature of ground plays the most important role in deciding the method used for tunneling in soft rocks.

(30)

Tunneling in Soft Rocks

Nature of Ground Typical Quality of Ground Running Ground Requires instant support

throughout the excavation. Dry sang, gravel, water bearing sand grounds

Soft ground Requires instant support for the roof but the walls can do without support for a few minutes. Damp sand, soft earth etc.

Firm ground Sidewalls and face of the tunnel can do without support for one or two hours, but the roof can last only a few minutes without

support. Firm clay, dry earth, gravel etc.

(31)

Tunneling in Soft Rock

• Forepoling Method

– An old method of tunneling through soft rocks

– A frame is prepared and inserted near the face of the

tunnel

– Frame is covered with suitable planks as support system

– Poles are then inserted at the top of the frame upto a

suitable depth.

– The tunnel excavation is then carried out beneath these

poles.

– Excavation is carried out on the sides, and the excavated

portion is suitably supported by timber throughout the

length of the tunnel.

(32)

(33)

Needle Beam Method

• Adopted in terrains where the soil permits the roof of the

tunnel section to stand without support for few minutes.

• A small drift is prepared for inserting a needle beam

consisting of two rail steel joints or I sections.

• The roof is supported on laggings carried on the wooden

beam.

• The needle beam is placed horizontally with its vertically

end supported on the drift.

• And the rear end supported on a vertical post resisting the

lining of the tunnel.

• Jacks are fixed on the needle beam and tunnel section is

excavated.

(34)

(35)

Tunneling through Water Bearing

Strata

• Tunneling through subaqueous or water-bearing strata is a

difficult job

• Shield tunneling is preferred in such cases

• Shield is a movable frame that is used to support the face of

the tunnel. The tunnel is excavated and lined under this shield

protection.

• A shield consists of a cutting edge, a skin plate in the form of

a shell structure, and a hood of jacks and stiffening steel

plates.

• Shield Method usually results in circular section because

– Rotation of shield is easy in circular section – Grants protection to primary lining

– Circular section provides maximum cross-sectional area – Semi-fluid pressure is best resisted by a circular section

(36)

Methods of Shield Tunneling in

Different Soils

Soil Type Method of Tunneling Silt One or Two port doors are

opened. Material is excavated and deposited at the bottom of the tunnel

Clay One or two ports are opened, material flows continuously into the tunnel. Excavation is carried out and soil is removed

immediately after excavation Sand Tunneling if of open type. Sand

settles on the floor of the shield and has to be continuously

removed. Material shouldn’t block propelling of jacks and other

(37)

Compressed Air Tunneling

• Most modern method of tunneling in soft rocks

• Compressed Air, with a pressure of about

1kg/cm

2

is forced into the enclosed space

within the tunnel so that the sides and top of

the tunnel do not collapse.

• Equipment consists of a bulk-head, and an

airtight diaphragm airlock.

• The pressure varies from strata to strata

depending upon the moisture content of the

work area.

(38)

Different Methods of

Tunneling

• Cut and Cover Tunneling

• New Austrian Tunneling

• Tunnel Boring Methods

(39)

Cut n Cover Method

• Simple method of construction for shallow tunnels where a trench is excavated and covered with an overhead support system.

• The excavation sides are vertical and temporary supports are provided.

• The main problems associated with cut and cover method are the stability of the soft ground, impact on the existing underground services & utilities and traffic disruption in urban areas.

• Temporary steel decks may be used to maintain the traffic while the construction works proceeds underneath.

• This method is also only suitable for high level tunnels. • Primarily divided into two sub-methods

– Bottom Up Method – Top Down Method

(40)

Cut n Cover Method

• Bottom Up Approach

– A trench is excavated with ground support

– Tunnel is then constructed in-situ

– Trench is then backfilled and carefully surface is

reinstated

• Top Down Approach

– Side support walls and capping beams are constructed

from ground level by slurry walling or bored piling

– Surface is then reinstated except for access openings.

– Finally excavation takes place under permanent tunnel

(41)

(42)

Tunnel Ventilation

• Tunnel ventilation is important because

– To provide fresh air to the workers during construction

– To remove dust created by drilling, blasting, and other

tunneling operations

– To remove dynamic fumes and other objectionable gases

produced by the use of dynamic and explosives

• Methods normally adopted for the ventilation of a

tunnel are

– Natural method of ventilation

• Achieved by drilling a drift through the tunnel from portal to portal.

• Not sufficient and artificial ventilation is still required.

(43)

Mechanical Ventilation

• Blow –In Method

– Fresh air is forced through a pipe or fabric duct by

means of a fan and supplied near the drilling face

– This method has the advantage of regular supply

of fresh air is available

– Disadvantage is that there is only exit for dust or

flumes to travel out of the tunnel, which

increases its travel distance unnecessary.

• Other method employed is blow-out method

in which foul air and fumes are pulled out

(44)

(45)

Lighting of Tunnels

• Tunnel operations involve working with undulating

terrain, waterlogged conditions, and dealing with

blasting equipment

• It becomes very important to maintain proper lighting to

ensure safety of personnel involved

• Electric lights, acetylene gas lights, lanterns etc. are

mostly used inside tunnels as lighting equipment.

• Electric lights are considered best as they do not leave

any smoke and provide bright light of required intensity

• Operation points, equipment stations, bottom of shafts,

storage points, and underground repair shops must

always be lighted properly.

(46)

Drainage of Tunnels

• Drainage of tunnels is important during the

construction period as well as afterwards

• Water accumulates inside the tunnel because of

groundwater seepage or water used during drilling

operations.

• Water seeping through all these sources is collected

in sump wells and pumped out.

• For a longer tunnel section, many sump points are

provided.

• After the construction is complete, drainage ditches

are provided along the length with a slope that leads

all the water into the sump wells.

(47)

Tunnel Shafts

• Shafts are vertical walls or passages constructed

along the alignment of a tunnel at one or more

points between two entrances

• A shaft is provided for the following reasons

– Working Shafts

• provided for speedy construction of tunnels by tackling the same section at a number of points