Avinaash Mehra
‘There is no success without pain and at the end of pain is successes’
Successful breakthrough story of India’s longest Railway Tunnel is applauded all across the country as it is going to connect Kashmir valley to rest of the India. This Tunnel is one of the Longest Tunnels in Udhampur-Srinagar-Baramulla-Rail Link Project.
Out of 272KMs length of USBRL Project,161 KMs has already been commissioned. The Work of intervening stretch of 111 Kms between Katra and Banihal is under progress. Northern Railways (Ministry of Railways, Government of India) is leaving no stone un-turned to complete the Project by the end of year 2022.The project after completion will play a vital role in national integration and helpful for common masses in conducting activities like business, sightseeing, pilgrimage along with transportation of Army goods. Besides, this is going to strengthen the development of the industry and agriculture
Almost every section of USBRL has gone through challenges. Tunnel 49 is the longest among the sections/segments of USBRL.I feel proud to be a part of the design and supervision team and pleased to share the hardships faced during the excavation of India’s Longest Railway Tunnel.
Introduction
Himalayas is young mountain range with complex geology and tectonically active zone created by the collision of Indian and Sino-Tibetan plates during Eocene-Oligocene period. The tunnelling activity in various projects in Himalayas are suffered by diverse geological problems. There is hardly any tunnel in the Himalayan region where challenges have not been faced.
India’s longest 12.76 KMs Railway Tunnel T-49 made its successful breakthrough on 15/02/2022 which is reported in leading newspapers and Channels.T-49 tunnel has two tubes: Main Tunnel and Escape Tunnel. It is constructed by New Austrian Tunnelling Method (NATM) having modified horseshoe-shaped cross section profile.
This Tunnel is going to Connect Aprinchala Station towards North and Sumber station towards South. Northern Railway entrusted IRCON International Limited for construction of this section. The Tunnel during excavation was divided into 2 packages Tunnel 49(A) 5 KMs (HCC being Contractor) and Tunnel 49 (B) 7.76 KMs (AFCONS being Contractor). Switzerland based LOMBARDI Engineering India Limited is Detailed Design Consultant. Excavation work for T 49 started in year 2014.
To increase number of working fronts, three ADITS were excavated at different locations ie, Urnihal Adit,Higni Adit and Kundan Adit.
Tunnel T-49 is excavated through Ramban Formation and Ramsu Formation with presence of Quartzite, Gneiss, Phyllite of Precambrian age with presence of Major Boundary Thrusts. Stretch of approx.2400 (between Kundan Site and Arpinchala) most affected and undergone Tunnel deformations due to Complex Geology.
MAJOR CHALLENGES:
* INCREASING DEFORMATION TRENDS AND SUBSEQUENT ADDITIONAL SUPPORT DURING EXCAVATION
If the stresses are low and are well within the in – situ strength of the rock mass there are no tunnel stability problems. However, the stresses exceeding the in-situ strength of the rock mass initiate the process of progressive failure around the periphery of the tunnel opening. The failure of a rock mass is associated with volumetric expansion called as Deformation.
In the Main and Escape Tunnels converging ground conditions were observed due to high ground stresses, reflected in the form of cracks in shotcrete, bending and buckling of installed Lattice Girders and reduction of tunnel section, tunnel closures being of the order of 300mm to 600 mm (Tolerance being 50 to 250 mm).
During excavation of Main Tunnel from Kundan to Arpinchala increasing trends of deformation crossed tolerance limits in aprox.900M stretch of the Main Tunnel and escape tunnel in January 2018.
To mitigate the problem Additional Support of 32mm dia,12 M long Fully grouted Self Drilling Rock bolts were installed. For complete tunnel stability more than 1500 metric tons Cement was used for Contact and Consolidation Grouting (Cement Injection) of deforming Tunnels. Stabilization of deforming stretch proved to be a challenge.
* REPROFILING OF STABILIZED ZONE AFTER DEFORMATION
After stabilizing the deforming ground, the next herculean task emerged out to reshape disturbed Tunnel profiles. Process of reprofiling includes removal of deformed Lattice girders and re-excavation. On getting desired designed Tunnel profile, Lattice Girder is again installed with shotcrete. Deformation in Tunnels resulted in reprofiling of approx.900 M cumulative length of Main and Escape tunnel.
* MAJOR CAVITY FORMATIONS IN MAIN TUNNEL DOUBLE TRACK
Towards Arpinchala Station Yard, there are two Main Double track Tunnels (255 M and 348 M long) which are having bigger dimensions, accommodating two parallel railway tracks.
Unforeseen, major cavity was formed on 19-01-2019 in Double Track Tunnel between Ch:124+025 to 124+017 M at Right side of advancing tunnel towards Kundan Side. This Cavity was around 10 M deep. Rock mass in close vicinity was incompetent and sheared phyllites. Backfilling of Cavity consumed approximately 400 M3 Plain Shotcrete.
On 04-05-2019, cavity was again formed at same side between Ch:124+009 M to Ch:124+005. To prevent recurrence of cavity formations due to weak rock mass condition, round length and Spacing of Lattice Girders was decreased to 0.75M and longer length of SDR (32mm dia,12 M long,10*12 pattern) was installed in 85.7 M remaining length of Double track Tunnel.Approx.1300 number of additional Self Drilling Rock bolts were installed in heading portion with additional Grouting. Pace of work also slowed down due to these problems.
CAUSE OF DEFORMATION
Actual encountered Shear Zones:
Encountered Shear Zone from Kundan Side and Arpinchala Side.
A shear zone is a zone of strong deformation (with a high strain rate) surrounded by rocks with a lower state of finite strain. Gouge is the core material of Shear Zone. In Simple words, Gouge is like Lose Soil. There were two shear zones of 9 M and 3 M of Core Thickness which was traced and predicted by writer in May 2019.Now the cause of deformation was clear. Due to presence of these weak zones, Major deformations, Cavity formations resulted.
* DESIGN REVIEW AND MODIFICATION
Generally, Major design reviews are very difficult when project is near completion. To Prevent Deformations and to save time in unexcavated part of Main Tunnel (450 M) IRCON’s NR’s and Lombardi Officials agreed to review the Design included use of Heavy Lattice Girders but with increased deformation tolerance up to 450 mm unlike 150 mm in previous design. Besides, Pillar width or parallel spacing between Main and Escape Tunnel was increased from 25 Meters to 50 meters to minimize deformations
* INCREASING LEAD LENGTH FROM SOUTH PORTAL/TUNNEL 49 A.
When tunnel Length exceeds 3 KMs Length it becomes time consuming for execution team to remove the excavated muck and to transport the Tunnel construction material inside the tunnel. Since the length of T49 A package was 5 KMs, only one working front was available from South Portal.
To fix the problem, NR’s, IRCON’s and Lombardi Officials agreed to the construct Urnihal ADIT (Access to Main and Escape Tunnel) at 3.8 KM in year 2018.
* TRANSPORTATION OF CONSTRUCTION MATERIAL DURING RAINY AND SNOWY SEASON.
Construction of Jammu-Srinagar Highway is in progress. Road closure problem remained prevalent during rainy and snowy season due to frequent landslides.This also remained a hardship for contractor to stock the bulk Construction Material.
Almost every netizen is set to enjoy the delightful ride from Jammu to Srinagar through USBRL. This article would commemorate the difficulties faced during the construction of this project.
(The author is Geologist Lombardi Engineering)
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