The growth of the tunnel construction in the country has been driven by a robust pipeline of projects and investments in setting up hydropower projects, developing urban mass rapid transit systems, improving road and rail connectivity, constructing underground crude oil storage, and upgrading water supply and sewerage systems.
Sunil Sharma, Chief Manager – Technical, Tunnelling Association of India
Tunnel construction in the country has picked up pace in the past four-five years on account of an increased number of projects involving longer tunnel lengths, and in some of the world’s most difficult terrain conditions. The scope of tunnel projects is also expanding as a result of growing urbanisation and the rising demand for better infrastructure. While the railway sector was the first to undertake tunnel development, the maximum number of tunnels have been developed in the hydropower sector. Tunnels for supply of water received a boost with the launch of programs such as the Jawaharlal Nehru National Urban Renewal Mission, the Pradhan Mantri Krishi Sinchayee Yojana, and the Interlinking of Rivers program for the exploitation of water resources. The roads and highways sector witnessed limited tunnel construction, with some in hilly regions. Metro rail tunnelling is a recent phenomenon and has been spurred by the decision of dense cities to develop efficient public mass transport systems.
There is currently a pipeline of 1641 tunnels spanning 3445 km, in various stages of development - either completed, under implementation, awarded, under bidding, announced, approved, planned/proposed or stalled. Of these, 77 per cent have been completed; 20 per cent are under construction; and the remaining 3 per cent have been recently awarded. About 280 tunnels spanning a length of over 890 km, are targeted for completion by 2021-22, and 137 tunnels (spanning over 630 km) with long gestation periods, are expected to be completed by 2026.
Huge investments in infrastructure development across various segments will give a push to tunnel construction. The railways’ capex target for 2018-19 is an all-time high of `1.48 trillion; hydropower capacity is expected to increase by 13 GW in the next five to six years; and around 10% of the upcoming length of metro rail projects is planned as underground. Apart from these, the Bharatmala, Chardham Connectivity, AMRUT, and the Smart Cities Mission will offer ample opportunities to tunnel contractors, consultants, equipment manufacturers, and technology providers.
Hydro tunnels account for the largest share, followed by railways, irrigation, metro rail, water supply, sewerage, roads and highways. One of the vital projects of this sector is the Upper Siang hydropower project in Arunachal Pradesh which involves construction of 30 tunnels in a horseshoe-shape; it is the most prominent tunnel design in hydropower projects across India. Other key tunnel projects in the hydro sector include Dibang, Sawalkote, and Subhanshri Middle hydroelectric projects - all of which will be constructed in the horseshoe shape.
Sandvik fully automatic tunnelling Jumbos working at Rishikesh–Karanprayag Railway line
Sandvik Mining and Rock Technology has introduced a computer-controlled fully automatic tunnelling jumbo ‘DT922i’ in India for achieving high levels of safety, productivity and precise tunnelling. The first DT922i machine was supplied to Rail Vikas Nigam Limited (RVNL) appointed contractor, APCO Infratech, to drill the first ADIT for access to the main tunnel, which is part of the 125-km Rishikesh–Karanprayag Railway line, and for which the work began in 2019, and the entire line is expected to be operational by 2024/25.
This next generation tunnelling jumbo, based on Sandvik’s patented iSURE® technology (Intelligent Sandvik Underground Rock Excavation software), has a unique intelligence to visualize the geological aspects of rocks and automates the entire drilling cycle. The flexibility, relatively compact size, and other features that enable environment-friendly performance have made this drill jumbo increasingly popular in tunnelling projects across the world, especially if the rock hardness/geological composition and other factors prevent effective use of TBMs.
Currently, many landmark and challenging highway tunnel projects in hilly areas are under various stages of planning and execution. These include the 14-km Zojila tunnel, 9-km Rohtang tunnel on the Leh-Manali highway, 11.55-km tunnel on the Jiribam Tupul-Imphal rail line, 4.5-km Char Dham tunnel, and the recently announced Sela Nechiphu Pass tunnel. An immersed tunnel under Bhrahmaputra is also in the offing. Important tunnels such as the Chenani-Nashri road tunnel, the Banihal-Quazigund rail tunnel, the Kashang hydro tunnel, Teesta 3 and Kishanganga Hydroelectric Power, have been commissioned while several have been completed for metro projects in Delhi, Kolkata, Chennai, Lucknow etc.
Other landmark and challenging tunnel construction projects under execution include the Katra-Banihal railway line in Jammu & Kashmir (with a total length of 163 km), the Kaleshwaram Lift Irrigation Scheme (total length of 203 km), the 33.5 km Mumbai Metro Line 3, and the 7 km undersea tunnel of the Mumbai-Ahmedabad high-speed rail project.
Tunnelling Equipment, Techniques & Methods
The Indian industry is beginning to use modern technology for tunnel construction, and there is now greater room for adoption of international standards in tunnel design and construction. There is a demand for high-tech tunnelling equipment as geological complexities are the biggest challenge in tunnelling, more so in the Himalayan region and the Western Ghats. Soil and rock investigation, analysis of ground behaviour during tunnelling, and assessment of the risks, are important considerations. While the conventional drill and blasts techniques were more prevalent in the past, mechanized methods such as TBMs and NATMs are being used more and more.
Contractors are also experimenting with new techniques and methodologies such as the P5 system, sequential excavation systems, and ground freezing for the more geological challenging/special projects and are using innovative materials such as fibre bolts, fibre reinforced shotcrete, lattice girders, lining stress controllers, pipe-roof pre-support systems, geo-synthetics, geo-membranes, steel anchors and self-drilling rock bolts etc. to improve the durability and strength of the tunnels.
Tunnel boring machines (TBMs), including earth pressure balancing machines, slurry machines and shield machines, and New Austrian Tunnelling Method (NATM) are gaining traction in urban areas. Another advanced method which is seeing increasing acceptance is micro-tunnelling for laying deep water supply and sewer lines in areas where open cut tunnelling is not feasible due to existing surface utilities.
However, conventional techniques such as the drill and blast method (DBM) continue to play a dominant role in the execution of tunnel construction projects. In over 38 per cent of hydro tunnel works the DBM will continue to be preferred. The rails and roads will move towards the advanced NATM - an observational method that takes a design-as-you-go approach – with use of software for predicting the oncoming geology based on deformation and other observations of measurements obtained during monitoring of the excavation. With NATM, the scope for software for automation and monitoring accessories has become huge.
As regards equipment, there is an increased demand for high-tech equipment as geological complexities and the ever-increasing need for faster paced work. Typically, about 25-30% of the tunnel construction cost is invested in procuring equipment and machinery. Over the next 3-4 years, about Rs. 1170 billion will be spent on procuring equipment and machinery for upcoming tunnel projects (cost will vary depending on the type of tunnelling method deployed, geological conditions, and level of technological advancements).
Largest Diameter CRCHI Slurry TBM for Mumbai Coastal Road Project
The 29.2-km Mumbai Coastal Road Project connects Marine drive with Kandivali. The tunnel construction faces complicated geological conditions that require excavation in deep overburden and passes through a compound stratum of basalt, breccia, and shale, with the maximum uniaxial compressive strength of up to 200 Mpa.
A CRCHI Slurry TBM is being used to bore the tunnel section of 1,920 meters. The TBM (80 meters long, weighing 2,300 ton, with an installed capacity of 7,280 kW and a gradeability of 5%) is designed with a mixed cutterhead with eight spokes and eight panels, to bore through the complicated strata for a long distance. It is fitted with a big-diameter slurry feeding port and several slurry flushing lines to increase the flow rate of slurry. 20-inch big-sized disc cutters are equipped on the TBM to improve its rock breaking capacity and prolong its lifespan.
In addition, this CRCHI Slurry TBM adopts many innovative systems, such as a dual-chamber indirect slurry control system, a dual-circuit automatic pressure system, high-torque and retractable main drive, and a high-power slurry circulation system – all of which are designed for successful tunnelling.
TBMs, EPBMs and slurry machines will continue to dominate the metro, irrigation and water supply tunnelling projects with a share of 24 per cent. Considering that the number of hydro projects using TBMs have stalled, there will be higher demand for simple equipment like drilling jumbos, excavators, loaders, cranes, forklifts, conveyors etc. With the use of mechanised methods like TBMs and NATM for tunnelling in congested urban spaces, demand for equipment such as cutter heads, shield machines, augers, arch lining gantry, pressure transducers, hydraulic filters, etc. will increase. Meanwhile, navigation systems, computerised jumbos and advanced drilling systems are being deployed for precision and better monitoring.
Issues & Roadblocks
The tunnelling segment continues to struggle with the following issues:
- Geological complexities: Geologic surprises are one of the biggest challenges. Complexities in the Himalayan region such as difficult terrains, thrust zones, shear zones, lack of mapping of in-situ stresses, high rock cover, ingress of water or gases, geothermal gradient, high level of seismicity etc. affect tunnelling activity.
- Inadequate investigations: Despite the availability of expert agencies in the country, there is usually an insufficient budget allocated for thorough investigations and less time given for site surveys and geological-geotechnical investigation affect tunnelling activities. Inadequate investigations lead to high risk in contractual costs.
- Mismanaged contracts: Ambiguity in the design of construction contracts affects the progress of tunnel construction. Any deficiency in the existing contract documents, and lack of risk sharing mechanisms lead to botched post contract practices.
- Risk management: Higher safety risks and inadequate safety measures are the other challenges in tunnel construction.
- Health and safety risk issues: Inadequate ventilation during construction of long tunnels and insufficient safety measures (in case of sudden ingress of water), will affect the construction progress.
- Other issues: Environmental impact, need for skilled manpower for manning and maintaining the sophisticated equipment, equipment related issues, opposition from local population during land acquisition.
Risks & Challenges During Tunnelling
Tunnel risks and challenges are often associated with unpredictable soil and groundwater conditions, difficult environments, rocks of various types, which may traverse zones of various complexities. Risk events are often interrelated: there can be untoward incidents such as fire, landslide, and flooding, which will lead to delays, environmental impact, and cost overruns, and there is a possible risk of damage to the surrounding properties and persons, especially in dense cities and towns.
TERRATEC TBMs breakthrough on Pune Metro
Maharashtra Metro Rail Corp and Gulermak-TATA Projects joint venture recently celebrated the breakthrough of a 6.61m-diameter Terratec earth pressure balance (EPB) TBM (Mula) for excavating the 16.56-km Line 1 (Purple Line) of the Pune metro in Maharashtra.
With the holing through at Civil Court underground station seen as a milestone, the joint venture has now completed twin-tube tunnels between the start of the ramp at chainage 10,950 and the NATM section at the start of Civil Court Station (12,600) - a total of 1,650m of TBM tunnelling.
TBM ‘Mula’ (S79) is one of three Terratec EPB machines excavating the line; another is the 6.61m-diameter S78 TBM (‘Mutha’) which broke through in September at the 155m NATM scissor crossover at Civil Court Station, having been launched from near the College of Agriculture at the end of 2019. Both machines have excavated in hard rock beneath a densely populated area, complicated by lockdown restrictions, which saw a best monthly progress of 301m. Terratec’s field service team was on site to support the effort.
In 2019, Maharashtra Metro Rail Corp (MahaMetro) announced that the JV had won both the twin-tube tunnel packages on the new 16.56km-long north-south metro corridor. The 5-km underground section – which runs from the College of Agriculture in Shivajinagar to Swargate and has five stations – is considered the most challenging section of the line, as it passes under the densely populated areas of Kasba Peth, Budhwar Peth and Mandai market.
The Terratec EPBMs have mixed-face, dome-style cutterheads designed to work effectively in the dense basalt that is expected, with pressures up to four bar. As the TBMs progress, they will install 1,400mm by 275mm-thick precast concrete lining segments in a five-plus-key configuration.
To mitigate risks, tunnel projects should be conceptualized and planned systematically to ensure smooth implementation; adequate investigations carried out, proper selection of tunnelling equipment made, appropriate contracting practices documented, statutory clearances taken, competent construction subcontractors deployed, and social issues duly considered. Every tunnelling project, from its conception to commissioning, is influenced by the geology of the area, so, reliability of the predicted geology is important. Some projects are primarily schedule driven, while others are cost or quality driven. Whether a specific risk event is perceived fundamentally as a cost risk or a schedule risk is governed by the project-specific context.
All these challenges may result in increased cost and extended completion period. So, one must carefully consider the likelihood of a risk occurrence and its impact in the context of a specific set of project conditions and circumstances. Herein lies the challenge between failure and success.
India is one of the fastest growing markets for tunnel construction, with the tunnelling industry witnessing high growth and ready to adopt advanced technologies. It is estimated that over Rs. 5.00 trillion worth of projects will be awarded in the next five years. Over the past few years, the size of tunnelling projects has witnessed a substantial increase. Almost all the upcoming tunnel projects are of longer lengths, larger diameters, and even higher contract values. Rising investments in tunnel construction have resulted in high growth in the tunnel equipment market as well. Going forward, as the pressure on land increases for productive economic and social uses, there will be greater need to construct underground structures in the metro, water and sewerage, and road sectors.
Tunnelling infrastructure holds immense promise for contractors, consultants, technology and equipment providers, material suppliers etc. over the long term. With more industry players tying up with international players, either for risk assessment, design or construction technology, the industry will witness reduced risks in project construction and timely completion.