Urban Underground: Tunnel development in key infrastructure sectors

Tunnels play a crucial role in setting up hydropower projects, developing urban ma­ss rapid transit systems, improving road and rail connectivity and facilitating water and irrigation projects. Currently, over 3,500 km of tunnels are at various stages of implementation across various sectors in the country. Among these, irrigation tunnels hold the lar­­gest share in terms of length, followed by hydro, railways and roads.

Indian Infrastructure takes a look at the pro­gress of tunnel development across key in­fra­structure sectors in India…

Hydro tunnels

According to India Infrastructure Research, In­dia has over 550 hydropower tunnels, with a cumulative length exceeding 840 km, under various stages of implementation. Of these, more than 100 tunnels are under construction, approximately 25 are in the bidding process and over 400 are in the planning stage. A state-wise analysis reveals that Arunachal Pradesh presents the highest potential in terms of numbers, with 77 tunnels (127 km), followed by An­dhra Pradesh with 74 tunnels (35 km), Hi­ma­chal Pradesh with 68 tunnels (200 km) and Ja­mmu & Kashmir with 50 tunnels (90 km). Meanwhile, over 525 tunnels with a len­gth of over 1,200 km have already been completed in the hydropower sector.

The drill and blast method (DBM) has been the primary technique employed in India for excavating hydropower tunnels. As technology advances, tunnel boring machines (TBMs) and controlled blasting are being increasingly implemented for hydro tunnel excavations. For instance, in the PakalDul project, 2.1 km of excavation is being carried out using a DBM and 7.3 km is being excavated using a double shield TBM.

About three-fourths of India’s hydropower potential is stationed in the Himalayas. Signi­ficant hydro tunnelling projects such as Parbati II, Luhri I, Kutehr, PakalDul and Teesta IV are under way in the region. However, tunnel development in the Himalayan region presents challenges due to the complex topography and geology. The key challenges faced during construction include exceptionally high in-situ stress lea­ding to rock bursts and highly abrasive rocks resulting in increased cutter consumption.

Road tunnels

According to projects tracked by India Infra­structure Research, more than 150 tunnels in the road sector, with length exceeding 480 km, are at various stages of implementation. Of the­se, approximately 70 tunnels are under way, mo­re than 65 tunnels are in the planning sta­ge, and over 15 are currently under bidding. The majority of road tunnel construction is be­ing undertaken in the hilly regions of the country. Jammu & Kashmir is in the lead with over 35 tunnels (180 km) under implementation, followed by Himachal Pradesh with over 30 tunnels (30 km) and Uttarakhand with 10 tunnels (45 km). Meanwhile, more than 35 tunnels (50 km) are expected to come up in Maha­rashtra. Further, the construction of more than 30 tunnels spanning over 60 km has been completed in the road sector.

Road tunnel construction in the Himalayan region poses various challenges such as alignment crossing below glacial lakes, construction through crystalline thrust, varying geology, high water ingress, low temperatures and difficulti­es in muck disposal. Tunnel development in the­se regions encounters multiple unforeseen issues such as face collapse, chimney formation, wa­t­er inrush, hot water springs, gas explosion and squeezing. Moreover, a significant cha­llenge fa­ced in tunnelling operations in these regions is difficulty in the procurement of material and equipment due to the rugged terrain.

Despite considerable challenges, India has undertaken landmark tunnel projects in the region, with one of the most significant being the Atal Tunnel in Rohtang. This operational road tunnel stretches 9 km and is the world’s longest highway tunnel constructed at an altitude above 10,000 feet. It is a single-tube, double-lane, horseshoe-type tunnel with a design speed of 80 km per hour.

Mechanised methods such as TBM and the New Austrian Tunnelling Method (NATM) are gaining traction in the highway and road segments. Furthermore, contractors are experimenting with new techniques and methodologies such as the P5 system and ground freezing, which are suitable for challenging/special projects. They are also utilising new and innovative materials such as geosynthetics, geomembrane, steel anchors and self-drilling rock bolts. The ongoing tunnel development is expected to create a significant demand for equipment us­ed in DBM, TBMs, drillers, excavators, loaders, cranes, and other related machinery.

Railway tunnels

In the past few years, Indian Railways has executed several tunnelling works. According to In­dia Infrastructure Research, more than 650 rail­way tunnels, spanning over 250 km, have been completed. In addition, there are more than 350 tunnels with a cumulative length of over 600 km under various stages of implementation. Of these, about 235 tunnels are in pro­gress, 100 tunnels are in the planning stage and over 25 tunnels are in the bidding stage. In terms of the length of tunnels under implementation, Jam­mu & Kashmir leads with more than 110 tunnels spanning over 180 km, followed by Utta­rakhand with 17 tunnels (105 km), Manipur with more than 50 tunnels (60 km) and Maha­rashtra with over 25 tunnels (55 km).

Some of the key railway tunnels are the PirPanjal Tunnel, which is a part of the Jam­mu–Ba­ramulla railway line (11 km), the Jiri­bam-Tupul-Imphal railway project extending over 60 km and the Rishikesh-Karnprayag railway project  covering approximately 100 km. In the railway sector, the method for tunnel excavation is determined by several factors such as geological conditions, cross-sectional area, shape of the tunnel, length of the tunnel, vibration restrictions and allowable ground settlements. While DBM has traditionally been the dominant method of tunnel excavation in this sector in India, there has been a notable rise in the adoption of mechanised tunnelling methods like NATM and TBMs. The NATM is the primary technique for tunnelling, employed in over 200 km of railway tunnels.

National High Speed Rail Corporation Limited has recently signed a contract with Af­cons Infrastructure Limited to construct India’s first undersea rail tunnel, which is part of a 21 km-long tunnel network for the Mum­bai-Ah­medabad bullet train project. The undersea tunnel at Thane Creek will span 7 km,  and will be situated about 25-65 metres below the gro­und level. Of the total length, 16 km of the tunnel will be excavated using TBMs, while 5 km will be constructed using the NATM. Notably, this project will involve the deployment of one of the largest TBMs for tunnel excavation.

Irrigation, water supply and sewerage tunnels

Based on tunnel projects tracked by India Infrastructure Research, the total length of tunnels in the irrigation, water supply and sewerage sectors is over 1,400 km. These projects are under various stages of implementation. Currently, more than 40 projects with a cumulative length of over 300 km are under way, while four projects with a length of 25 km are in the planning stage. Furthermore, an additional 40 projects with a length of over 1,000 km are in the planning stage and more than 80 tunnels with length exceeding 700 km have already been completed across these sectors. The construction of tunnels in these sectors incorporates a combination of methods such as TBM and DBM. State-wise, Maharashtra tak­es the lead in tunnel development for the irrigation, water and sewerage sectors, with over 40 tunnels spanning a length of over 900 km at various stages of development.

In Mumbai, several water tunnels have been constructed for the augmentation of water supply, while several others have been built to improve the water distribution system by replacing old and damaged water pipes. These water tunnels, situated approximately 50-100 metres below the ground level, have been constructed by deploying full-face TBMs. As per the latest updates, a total of 83 km of tunnels have been commissioned, 20 km are under construction while 115 km of tunnel construction is in the planning stage. The key upcoming water tunnels in Mumbai include the 64 km Pin­jal-Gu­n­davali tunnel, the 2.5 km Gargai-Modak Sagar Tunnel and tunnels under the Daman­gan­ga–Pin­jal National River Link Project spanning 42.7 km. In the irrigation sector, the 56 km Kal­eshwaram irrigation project (Link 7) is one of the key ongoing tunnel projects.

Metro tunnels

With rapid urbanisation and the government’s focus on developing mass rapid transit syste­ms, the scope of metro tunnel projects is also expa­nding. According to India Infrastructure Re­­se­arch, over 80 tunnels with length exceeding 250 km are under various stages of implementation in the metro sector. Of that, cons­truction of more than 45 tunnels is ongoing, 30 tunnels are in the planning stage and more than five are in the bidding process. In terms of tunnels length un­der construction, Uttar Pradesh leads with more than 15 tunnels spa­nning approximately 45 km, followed by Delhi with more than 40 km and Tamil Nadu with over 35 km. Meanwhile, more than 240 km of tunnel works have been completed in the sector, with major projects being the Mumbai metro, Che­nnai metro and Delhi me­tro. Given that a significant portion of metro systems in the co­untry operate underground, tunnel construction is an important component in the sector’s progress.

Over the past few years, the metro rail sector has extensively deployed TBMs to underta­ke tunnelling works. More than 160 TBMs have been deployed or are planned to be deployed for the construction of underground metro tunnels across key metro projects. Some of the major metro projects are the Jaipur metro (Pha­se I), Delhi metro, Chennai metro (Phase I), and Mumbai Metro Line 3.