The responsibility for construction and maintenance of Balipara-Charduar-Tawang (BCT) Road has been entrusted to Border Roads Organisation (BRO). The 317.520 km long road connects West Kameng and Tawang districts of Arunachal Pradesh to the rest of the Nation. The axis is of immense strategic importance to the Armed Forces apart from being a lifeline for the civilian population as well as for the administration of West Arunachal Pradesh.
Arunachal Pradesh has a hydroelectric potential exceeding 50,000 MW. In order to tap the electric power generation potential of Tawang Chu River, NHPC had proposed two Mega Projects: Tawang I and Tawang II with a combined potential of 1400 MW in the Tawang Region.
Sela pass, which is the only axis to the Tawang region, remains closed for heavy traffic during winters due to heavy snowfall for considerable time of the year. NHPC had planned to bypass this formidable obstacle by constructing a tunnel under the Sela Pass. However, the cost of the tunnel project was very high and added significantly to the overall cost of the power projects. There were other local and environmental concerns as well. Consequently, no work on the tunnel was taken up and Sela remained an obstacle for all-weather connectivity on this axis.
The tunnel was again proposed in 2010 to provide all-weather connectivity to the border areas, which would enhance operational capability as well as provide succor to the local population. Since Sela remained snowbound for about eight weeks in a year, being the highest pass on the axis, it was decided to take up work on Sela Tunnel at the earliest. The tunnel was to be constructed in a strategically acceptable timeframe.
Keeping in mind the above factors, a pre-feasibility study for the tunnel across Sela was carried out by BRO and the DPR was submitted. Through the DPR, BRO proposed two tunnels of 475 m and 1790 m length with approach roads aggregating to 9.075 km. This finalized alignment remained well below the snow line and allowed for an all-weather passage of vehicles without major challenges of snow clearance. The alignment reduces the travel distance by more than 8 km and cuts down the travelling time by an hour, while obviating the treacherous Sela Pass climb which is prone to many accidents due to its peculiar topography.
Foundation stone of Sela Tunnel was laid by the Hon’ble Prime Minister Shri Narendra Modi on 09 Feb 2019 and on 15 Feb 2019 a contract under EPC mode for design and construction of Sela Tunnel Project was awarded. Project Vartak of BRO was entrusted to oversee its construction. Construction commenced on 01 April 2019 with the first blast taking place at Nuranang side Tunnel Portal on 31 Oct 2019.
Breakthrough of Escape Tube on 23 July 2021 by DG through e-ceremony
Geology of the area
The Sela Group has been referred to as Pari Mountain gneiss and Pidi Formation. These are considered equivalent to the central crystalline Western Himalayas, Kangchenjunga gneiss of Sikkim-Darjeeling and Thimpu Group of Bhutan Himalayas. Though the Sela Group was first described in the Sela section, it is extensively studied in Subansiri valley, upstream of Taliha, where it constitutes part of Takpasin Group.
Rocks of Sela Group can be divided into Lower Taliha Formation and Upper Galensiniak formation based on the difference in lithology. The rocks exposed along Sela-Tawang-Bumla Section consists of migmatite, garnetiferous gneiss, lit-par-lit biotite gneiss, cale-gneiss/marble (diopside and scapolite bearing), staurolite bearing schist, tourmaline bearing leucogranite, quartzite, and pegmatite.
Drilling while charging to save time
Salient Features of the Project
The initial contract of the Sela Tunnel included two tunnels, viz 1790 m and 475 m long and 9.075 km approach road. The lengths of the tunnels and approach roads were subsequently changed based on site conditions and to overcome a few challenges. The current configuration of the project is as follows:
- Tunnel 1 (T1): This tunnel is now planned for 993 m length and as a single tube tunnel.
- Tunnel 2 (T2): The length of Tunnel 2 is now 1591 m. This is a twin tube tunnel with one main and one escape tube.
- Approach Road to Tunnel 1 is 7260 m long which takes off from km 229 on BCT road.
- Road between two tunnels is 1200 m long.
- Approach Road to Tunnel 2 is 740 m long which takes off from km 247 on BCT road.
The main tubes of both T1 & T2 are 12 m wide with overhead clearance of 5.5 m. The escape tube with Tunnel 2 is 7.26 m wide with an overhead clearance of 4m being constructed parallel to main tunnel at a distance of 20 m and is connected with five cross passages. This project has been designed for traffic density of 3000 petrol cars per day and 2000 diesel trucks per day with max speed of 80 km/hr.
Electro-Mechanical and Safety Provisions
The tunnels are being provided with a state-of-the-art electro-mechanical systems including Jet Fan based longitudinal ventilation system, firefighting devices, CIE norms based illumination system, and SCADA controlled monitoring systems.
Challenges Faced and Overcome
Sela Tunnel is being constructed at an altitude of more than 13,000 feet above MSL across Sela Pass which is located more than 250 km away from a rail head. The area witnesses very high rainfall and heavy snowfall during the winter months. The temperatures drops up to minus 20 degrees which leads to drying up all water sources, posing technical issues for concrete works and hampering the efficiency of manpower and machines alike.
The considerable distance of this region from the nearest big township and lack of a stable mobile network has adverse repercussions on the persons deployed on the project work site. Himalayas are one of the youngest mountain ranges in the world and owing to the subduction phenomenon; tunnel construction in the Himalayan region is probably the most challenging construction activity in the field of civil engineering. Sela Tunnel Project is undeniably the most complex and difficult project being taken up in the nation today. Some of the challenges BRO faced and how the Karmyogis overcame them are elucidated below.
Initially, it was decided to start work from Nuranang side which would have allowed for early start of the tunnel works and excavation of the longer tunnel (T2) could be been taken up promptly. However, when the work to prepare the portal, as shown in the DPR, was taken up, it was realised that the rock mass at the portal location was very poor and unsuitable for locating the tunnel mouth.
BRO engineers discussed the issue at length and after brainstorming, it was decided to move the portal location 100 m to the west, and tunnel excavation was started. However, this had ramifications for the complete alignment as the other tunnel and roads had to be now realigned to meet the requirement of permissible gradients. The task involved doing a de novo reconnaissance of the complete Sela Ridge and locating portals at suitable locations. This task was carried out by means of a detailed foot reconnaissance using Total Station.
The task was not only technically challenging due to lack of line of sight, but was also extremely risky due to the steep and rugged terrain. After working out various combinations, the alignment was finalised with the changed lengths of tunnels and approach roads based on geology and topography to construct stable roads and tunnels with acceptable gradients. A seemingly unfeasible alignment was brought to life and executed. On 15 Jan 2021, the first blast at T1 from Baisakhi side was done by Lt Gen Rajeev Chaudhry, VSM, DGBR himself, after giving onsite directives. The tunnel was then excavated within a year of the first blast.
Dynamic Tunnel Support Systems
The Himalayas constitute a combination of metamorphic rocks. The phenomenon of subduction makes the situation even more dynamic as the mountains are literally growing each day. This poses challenges for design of tunnel supports as the excavation process encounters heterogeneous rock mass with varying stress conditions. This necessitates a dynamic approach to tunnel support design as tunnel supports have to be calibrated as per encountered conditions. During the excavation of Sela Tunnel, this became extremely critical as time for excavation of tunnels had to be reduced drastically to make up for the 18 months lost due to Covid-19.
The site team constituted a geotechnical group which ensured that the rock and stress conditions were analyzed everyday based on the 3D monitoring data and load cells. The group then optimised the tunnel supports by using a combination of friction rock bolts, grouted rock bolts and passive supports to achieve the shortest possible excavation cycles. The project achieved four km of tunnel excavation in less than two years, a remarkable feat at 13000 feet above MSL.
The rock mass under which Sela Tunnel project was being excavated has water bodies like Sela Lake perched on top, due to which joints in the rock-mass are filled with water. The water bearing joints undergo a freeze thaw cycle which opens up the joints and weakens the rock mass. A similar phenomenon triggered a massive cavity in the main tube of the longer tunnel. Fortunately, no one was hurt but the excavation work came to a total standstill.
The geotechnical group was pressed into action to analyse the cause and suggest solutions. In no time, it was decided to isolate the cavity area by progressing the excavation from the escape tube and approach the cavity area from both sides: one from the main excavation face and the other from a cross passage ahead. This way the excavation could progress normally while the cavity was being treated simultaneously.
The cavity though, solicited an intricate approach for restoring the original shape of the tunnel. As a first step, a drainage chute was constructed to drain the water into the tunnel drainage system after removal of rock debris. The cavity surface was then treated with local grouting and pre-excavation supports in the form of fore-poles were installed. The affected area of almost 40 m required a deliberate procedure of fore-poling, fixing of additional rock bolts and steel ribs to support the poor rock mass. This took almost three months. However, it was ensured that the cavity treatment did not affect the critical path of the project by ensuring that the cavity was addressed from both sides while normal excavation progressed across the main and escape tube.
PQC Works under progress in tunnel
Erection of Ribs during Cavity Treatment in Extremely Low Temperatures
The project site experiences extremely low temperatures going down to minus 20 degrees celsius at times. Such severe temperatures affected the manpower and machinery alike. Some of the challenges faced and on-site solutions engineered are as follows:
- Manpower: Tunneling work is both mechanised and labour intensive. During the winter months, the human output reduces drastically as workers are not able to continue for long hours at a stretch. The site work in these months was carried out by organising four shifts of six hours each. The workers were provided special clothing, work warming regimens, and local heating arrangements on the sites for better efficiency and good health.
- Machinery: The equipment on site faced major challenges wherein the filters of BS-IV engines would get choked and the equipment would remain off-road for days together. These issues were obviated by using winter grade diesel and using insulation layers on plants. Many of the vehicles and equipment would have their engines kept running throughout the winter nights.
- Concrete Setting Cycles: The final arch lining of the tunnel was executed using 12 m long gantry based concrete shutters. Each arch lining segment consisted of 150 - 250 cum of concrete dependent on over breaks. Ordinarily, the concrete setting time is around 12-18 hrs; however, at the Sela Tunnel site, these setting times went upto 48 hours due to the low ambient temperatures and wind chill. The setting times were brought down by using halogen lamps for local heating and wind screens placed on portals. Additional gantry-based formworks were inducted to further compress the timelines.
- Freezing of Water: Water is required at a tunnel construction site for drilling, concrete works and other miscellaneous activities. However, natural sources get frozen in the winter months and sometimes during night, even in summer months. This posed serious challenges for progressing works at the desired pace. The issue was resolved by making sumps inside the tunnel and keeping the water in circulation to prevent freezing. Some artesian sources were identified within the tunnels to tap water as they were perennial due to the inherent rock mass heat.
- Lack of Local Quarries: There is a dearth of local quarries in the region for sourcing suitable quality and quantity of aggregates for road and tunnel concrete works. This meant that most of the aggregates had to be recycled from within the site. The site team ensured that suitable tunnel muck and excavated material from the road were identified and stacked properly, and recycled for aggregates.
- Rarified Atmosphere: The tunnels are located above an altitude of 13,000 feet which causes the atmosphere to be rarified. This poses serious challenges for ensuring the required fresh air and oxygen content in the tunnels. The design of the ventilation system had to be carried out deliberately, wherein twin jet fans of one meter diameter have been placed at every 400 m in each traffic tube to ensure adequate air volume inside tunnels throughout the year for the design traffic. Fans have also been planned in all cross passages to help evacuation in the eventuality of a tunnel fire.
- Lack of Technical Manpower: The tunnel industry of India is currently overstretched with construction of a vast number of tunnels for railways, roads, and metro systems. The core technical manpower continues to be very limited. Sela Tunnel project was competing with such soft sites in the hinterland where people don’t prefer to work, and it was very difficult to recruit and motivate technical manpower to operate at altitudes above 13,000 feet in such a remote location.
Though the project had witnessed significant delays, it has picked up pace and is one of the fastest progressing tunnel works in the challenging and remote Himalayan regions. The Sela Tunnel once completed will offer the world the longest bi-lane tunnel at an altitude above 13,000 ft - another world record for BRO. The project will go down in the annals of history as one of the most challenging projects executed in India.