Tunneling

Tunneling Section D

Tarun Goel, JNGEC Sundernagar

Tunnels • A tunnel is an underground passage for the transport of passengers, goods, water, oil etc. • Railway Tunneling in India started in 19th Century. • The first Indian Railway tunnel was Parsik Tunnel • Today we have 348 railway tunnels in India, excluding the tunnels constructed on the Udhampur – Quazikund Sector in J&K. Tarun Goel, JNGEC Sundernagar

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 rocksoil behavior are very high.

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Shapes of Tunnels • Primarily there are four shapes – Circular – Egg Shaped – D Section – Horse Shoe Shape

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

Horse-Shoe and Modified HorseShoe • 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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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 more efficient. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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.

Tarun Goel, JNGEC Sundernagar

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 crosssectional 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. Tarun Goel, JNGEC Sundernagar

Typical Railway Tunnel

Tarun Goel, JNGEC Sundernagar

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 Number of Lines

Tarun Goel, JNGEC Sundernagar

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 – Extent to which timbering is required Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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

Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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

Slow Progress

Tarun Goel, JNGEC Sundernagar

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

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

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.

Self Supporting Ground

Excavation can be carried without any support or minimal support. Tarun Goel, JNGEC Sundernagar Sand stone, hard clay etc.

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. – It is slow and tedious process, requires skilled manpower. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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. • Economical method of tunneling Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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 Tarun Goel, JNGEC Sundernagar

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 equipment.

Running Sand

Bulk head shield is used in this case.

Tarun Goel, JNGEC Sundernagar

Compressed Air Tunneling • Most modern method of tunneling in soft rocks • Compressed Air, with a pressure of about 1kg/cm2 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. Tarun Goel, JNGEC Sundernagar

Different Methods of Tunneling • Cut and Cover Tunneling • New Austrian Tunneling • Tunnel Boring Methods

Tarun Goel, JNGEC Sundernagar

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

Tarun Goel, JNGEC Sundernagar

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 roof and base slab is constructed.

Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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.

– Mechanical Ventilation by Blow-In Method Tarun Goel, JNGEC Sundernagar

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 through a pipe and expelled by a fan. Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

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. Tarun Goel, JNGEC Sundernagar

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 • Vertical shafts of minimum size 3.7m X 3.7m with 4.3m diameter

– Ventilation Shafts • To ensure better ventilation • Shafts are usually inclined and have a girder size of about 1.2m diamter Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

Lining of Tunnels • Tunnels are liable to disintegrate and thus lining is important to strengthen their sides and roofs. • Primarily lining serves the following objectives – Enabling sides and roofs to withstand excessive pressure – Providing correct shape and cross-section to the tunnel – Checking the leakage of water from tunnel sides and the top – Binding loose rock and providing stability to the tunnel – Reducing the maintenance cost of the tunnel Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar

Tarun Goel, JNGEC Sundernagar