Sela Tunnel Project: Overcoming the Impossible

 
The current configuration:

Tunnel 1 (T1): It is a single tube tunnel for 993m length

Tunnel 2 (T2): This is a twin tube tunnel with one main and one escape tube with a length of 1591m

Roads: – Approach to Tunnel 1 is 7260m which takes off from Km 229 of BCT road.

• Road between two tunnels is 1200m.
• Approach to Tunnel 2 is 740m which takes off from Km 247 of BCT road.

Portal 1 of Tunnel 1 is located at an altitude of 3910m above MSL, while the Portal 4g of the Tunnel 2 is located 3973m above MSL. Drill & Blast technique for excavation as part of New Austrian Tunneling Method (NATM) is being used for the construction of the tunnels.

The main tubes of both T1 & T2 are 12m wide with overhead clearance of 5.5m. The escape tube with Tunnel 2 is 7.26m wide with an overhead clearance of 4m being constructed parallel to main tunnel at a distance of 20m 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.

Challenges Faced

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 drop 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 distance of the region from a nearest big township and lack of a stable mobile network has its adverse repercussions on the persons deployed on the project work site.

The work was initially decided to be started from Nuranang side as that would have allowed for early start of tunnel works and excavation of the longer tunnel (T2) could be taken up promptly. However when the work to prepare the portal as shown in the DPR were taken up, it was realised that the rock mass at the portal location was very poor and unsuitable for locating the tunnel mouth.

It was finally decided to move the portal location 100m 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 suitable portals. 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.

Dynamic Tunnel Support Systems

Himalayas are one of the youngest mountain ranges and 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 situation poses challenges for design of tunnel supports as the excavation process encounters heterogeneous rock mass and varying stress conditions. This demanded 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. The site team made a geotechnical group who ensured that the rock and stress conditions were analysed everyday based on the 3D monitoring data and load cells.

Cavity Formation

The rock mass if under which Sela Tunnel project is being excavated, has water bodies like Sela Lake perched on top. This leads to the joints in the rock-mass to be 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 group was pressed into action to analyse the cause and suggest solutions. 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 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 form of fore-poles were installed. The affected area of almost 40m 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.

The geotechnical group optimised the tunnel supports by using a combination of friction rock bolts, grouted rock bolts and passive supports to achieve 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 project site experiences extremely low temperatures going down to minus 20 degree Celsius at times. Such severe temperature affected the manpower and machinery alike.