The topic under discussion has two aspects: (1) Construction of road tunnel, (2) its operation after completion of the job. It is understood that the present issue which is under examination pertains to Rohtang Pass Highway Tunnel in Himachal Pradesh whose executed diameter could be 11.5 metre, horse-shoe, or 10.0 metre diamter if done by tunnel boring machine. The tunnel starts at an elevation of 3055 metre and ends at an elevation of 3080 metre. The anticipated length of the tunnel is about 9.0 km and because of the difficult working condition at the exit of the tunnel, it may be worthwhile considering to bore the tunnel from one end. There are two alternatives: (1) Use of Drill & Blast Method, and (2) Use of Tunnel Boring Machine.
Use of Drill & Blast Method
Use of conventional Drill & Blast Method for tunnel excavation by heading and benching - The heading could be about 7.0 metre high and balance depth to be executed by benching. The height of heading will be governed by geometrical control of drilling equipment and facility required for implementation of supporting system to cater for bad ground conditions. The exact type of supporting system will depend on the type of rock. In case the tunnel is excavated by drill & blast method, the muck will be carried by rubber tyred equipment. Loading of the muck will be done either by rubber tyred loaders or crawler excavators/ loaders.
In order to see that the working conditions at the face of the tunnel being excavated are best, a very effective ventilation system is to be designed. The factors which go into the design of the ventilation system are given below:
Criteria of Ventilation System
Air per person working in tunnel: 4.5 m3/ minute
Air per HP of diesel equipment: 4.0 m3/ minute
Diversity factor: 0.7
Return velocity: 0.4 to 0.67/ second
In case of use of TBM, ventilation system is forced to TBM ducting cassette.
Duct friction factor: 0.015 to 0.018
Leakage: 5.0 to 6.4 mm2/ m2
Figure 1: Ventilateur Æ 1600 C1 - 9 pales vitesse de rotation 1500 tr/min Fan Æ 1600 C1 - 9 blades rotation speed 1500 rpmAir being a compressible material, the calculation for discharge from fans is based on fan characteristic curves. In case more than one fan is used in series, then we need to refer to the characteristic curves to find out the discharge. A typical discharge curve is given in Figure-1.
Ventilation system during construction will be replaced by another system during operation of the tunnel. The ventilation system during operation of the tunnel will take into account the traffic conditions, emission of gases and temperature at any given time in the tunnel. Good ventilation during construction will lead to a speedy execution of tunnel and good ventilation during operation will not create any health hazard.
Activities Involved In Construction of a Tunnel
As on date, Total Station Theodolites are available which are simple to operate with high accuracy.
Drilling could be done by use of 2 or 3 boom jumbos with additional boom for a basket. The average speed of drilling as on date per boom is about 50 metre. This would take into account the time required for collaring, flushing, withdrawal of drifter rod and all related activities. The geometrical control of the boomers will be such that it will have the total coverage of the tunnel profile. For loading explosives, we can use boomer boom basket. In addition, we can also use the loader bucket with or without platform to have accessibility at the required places on the face.
Sketch 1: Drilling patern for adit to bottom of P.S. & collection galleryThe speed of excavating the tunnel will depend on the geology of the tunnel. The only item which cannot be accurately assessed is time required for supporting. The time required for support will depend on the type of the rock. Presently, the supports which are in use are:
(a) Shotcrete with or without steel/ accrelic fibres
(b) Rockbolts with expan- sion shell and slit & wedge with or without grouting
(c) MAI self-driven anchors
(d) Grouted anchors or rockbolts
Over a period of time, the technique of grouting and rockbolts is perfected by making a grout of 0.32 : 0.35 water-cement ratio which is pumped into a drilled hole for erecting rockbolt. This grout does not come out of the hole because of high viscosity of grout. The friction of grout against the walls of the rock is higher than the weight of grout, hence grout remains in place. The rockbolt with anchorage is pushed by mechanical means upto the designated depth through the grout. In this operation, the grouting becomes most effective. Due to low water-cement ratio, there is minimum shrinkage. This makes the rock bolt effective.
In case of MAI anchors, the drill bit is left in the hole and the drill string which consists of high tensile steel is used for tightening the anchor against the end plate. It may also be necessary to use steel rib supports in locations where Q-value of rock is low for an effective support system. As the tunnel advances, logging of tunnel rock needs to be done based on Barton Q-system or RMR rating classification. This is necessary to have a reference for any future eventuality. This record becomes very useful for working out solutions for any distress condition which may arise as the tunnel proceeds ahead.
Technique of Blasting
There are various publications on this issue. An extract of a publication of Mr Stig O. Olofsson titled 'Applied Explosives Technology for Construction and Mining' as used for Nathpa Jhakri Hydroelectric Project is given in the following pages. Adoption of this method gives most effective result and could be followed by people doing tunneling.
Blasting Techniques Used for Jhakri Undergorund Power House
For execution of Nathpa Jhakri underground power house, most modern equipment was used for the work of drilling, rockb- olting and shotcreting. This equipment consists of Atlas Copco boomers 352, 351 and automatic rockbolting boomer. In view of this, the drilling performance was of a world class level. Also for shotcrete, wet shotcrete machines BPA-380 with robot arms were used.
The hydraulic boomers have electrical drives for operation of hydraulic pumps. There were fears of stray current from the electrical drives getting transmitted to the face under charging. Hence charging of the face by use of half second and milli-second detonators was not carried out as a parallel activity with drilling. It is executed as a parallel activity only when NONEL (non-electric sensitive) detonators are used. When drilling was done by use of pneumatically driven machines, charging of face was carried out as a parallel activity since possibility of stray electric current/ short circuit was not possible. Charging of holes could be done by placing explosives with detonators in long polythene pipes and then placing them in holes by the help of the basket boom available on drill jumbos. This helps in reducing the charging time to about an hour approximately.
When hydro-booms are used, it is convenient to follow parallel cut for drilling. Details of this cut are given in Sketch-1. The cut is placed approx in the middle of the cross-section and quite low down to get minimum consumption of explosives.
Mucking of the blasted rock will be done by use of rubber tyred loaders of 4 M3 capacity coupled with off-the-highway dumpers of 25 T capacity. Number of dumpers will be worked out on the basis of quantity of muck, time provided in cycle and the maximum lead. A rubber tyred dozer/ grader can keep the road properly graded to have an effective speed of the muck carrying vehicles.
Since the tunnel length is big, it will be necessary to transmit electricity at high voltage of 11 KV in the tunnel and provide transformers of 11 KVA/ 440 at regular intervals to avoid voltage drop beyond permissible levels. This will ensure efficient use of equipment.
Water supply lines, dewatering, electrical cables etc. will have to be placed on racks made on one side of the tunnel and will be protected properly during the course of benching.
Excavation of Tunnel by TBM (2nd Alternative)
Depending on the geological features of the rock, it could be decided to do tunneling by Tunnel Boring Machine. The options for the type of TBMs are as under:
- Hard rock open type TBM
- Single-shielded TBM
- Double-shielded TBM
- Shield machines – matching the machine to the job
- EPB shield machines
- Slurry shield machines
- Pressure bulk head machines with use of compressed air
Figure 2:In today's technology, it is possible to design a TBM to meet a given circumstance to match the requirements of the job. One can have an over-boring arrangement provided in the TBM to take care of deformation upto 200 mm. You can also have a mix-shield machine which can cater for both soft ground as well as hard ground. Hence selection of TBM of correct type should be jointly deliberated by the owner and the manufacturer of TBMs. This becomes most important job to do effective and timely execution of a tunnel. Decision of what type of TBM is to be used will depend on the probable geology along the alignment and water seepage. It will also depend on the in-situ stress of the rock which has relation to the cover of the mountain top of the tunnel to prevent their movement.
In all cases, a properly designed precast concrete invert segments with four rails – two for rail traffic and two for back up are provided. The invert segments are grouted immediately after placing in position.
Concrete Lining of The Tunnel
This will be done by use of continuous lining shutter which can do up to 1 km of lining per month. Hence, the lining will take less than one year. Jaiprakash Associates Ltd have done this at Chamera Hydroelectric Project and Baspa-II Hydroelectric Project.
Pavement could be done as given below. A typical sketch of the pavement is also enclosed at Figure-2.
The pavement crust is suitably designed keeping in view the anticipated growth of traffic over the design period.
- Laying of granular sub-base layer upto the minimum line of excavation
- Laying of 100 mm thick dry lean concrete over the GSB
- Laying of 320 mm thick high performance concrete over the DLC
2. Laying of DLC - A layer of 100 mm thick DLC was laid over the GSB and compacted by a 10 T double drum vibro roller. The design mix of DLC comprised coarse aggregate, fine aggregate, cement and water for '0' slump in the proportions of:
Quantity of materials for 1 m3
20 mm (25%) = 487.5 kg
12.5 mm (35%) = 682.5 kg
6 mm & down (40%) = 780 kg
Cement = 150 kg
Optimum moisture content = 8.70%
Maximum dry density = 2.41 g/cc
Water = 182.7 lit.
3. Laying of HPC – The laying of HPC will have to be done in each lane of 3.5 m width separately with a transverse joint at 90.0 m where an expansion joint is provided with 32 mm dowels 500 mm long half length embedded in the concrete panel being cast while the other half length of the dowel will be encased in pipe sleeves and embedded in the concrete of the adjoining panel. In the longitudinal joint, 16 mm dia dowel bars 800 mm long are provided such that half the length of the dowel is embedded in the panel being concreted and the other half will be embedded in the concrete of the adjoining panel. The two 500 mm wide strips at the two ends of the carriageway are concreted thereafter. The high performance concrete is of grade M40 and the design mix is as shown below:
For Dry Condition of Aggregate
Cement = 350 kg
Micro Silica – 28 kg
20 mm = 650.78 kg
12.5 mm = 533.16 kg
N.S. = 368.25 kg
C.S. = 367.13 kg
Water = 1750.01 kg
Admixture Fosroc SP 500 @ 1.2% = 4.54 kg
The use of silica fume is necessary for making high strength concrete more homog- enous and thereby increasing the workability required for achieving the high strength. Proper vibration with needle vibrators is carried out and the top surface is compacted and leveled with the help of a screed vibrators which is moved on the channels provided at the two ends of the shuttering. The top surface of the concrete is then given a broom finish before the concrete sets. A groove 6 mm thick and 100/120 mm deep is then cut by machine at every 4.5 m in transverse direction and also along the longitudinal joints of the 3.5 m lane widths. The grooves are then filed with saw-dust and the top 12 mm depth is filled with a sealant polysulphide compound.
Crash Barriers, Kerbs & Walkways
There are walk ways provided on both sides of each tunnel above the crash barriers and kerbs with a 1.0 m high hand railing comprising MS angles vertical posts welded to the plate embedment fixed in the top of the side drain concrete with horizontal G.I. pipe as hand railing.
It is necessary to make a provision of fire-fighting system which comprises two water tanks of 30,000 litres capacity erected above each of the tunnel portals with a 4" to 6" G.I. pipe line running from the tanks to the tunnels on one side for taking care of any fire hazards and even for washing and cleaning of the tunnels etc. Fire hydrants are fixed at every 30m.
Lighting & Ventilation
It is necessary to make provision of ventilation and lighting all through the tunnels. The lighting system is specially designed such that there is more lighting at the entry and gradually reducing inside the tunnel with an increase at the exit point.
The ventilation system comprises unidirectional jet fans with silencers. Though the jet fans are unidirectional, these operate in the reverse direction also in case of emergencies. The motors are provided with moisture oil and fungus resisting insulation of a type specifically designed and constructed to withstand severe humid condition and to operate after a long period of idleness without drying out.
There are advantages and disadvantages in execution of tunnels by Drill & Blast Method and by Tunnel Boring Machines. Use of either of the methods will depend upon immaculate geological investigations. In case it is decided to use TBM, the selection of TBM to match rock conditions will have to be specified so that all the competent companies quote on agreed premises. This premise needs to be sorted out well in advance before the tenders are floated. In case of Drill & Blast Method, besides the issue of over-break and damage is caused to rock joints during blasting, the speed of excavation by D&B method is much slower than that of TBM.