Technology Impetus

Key tunnelling plans for India’s first high speed rail project

The development of high speed rail projects has been a key focus area of Indian Railways (IR) over the past few years. The ambitious Mumbai-Ahmedabad high speed railway (MAHSR) project is set to pick up pace in the current fiscal year after facing delays due to resistance from farmers over land acquisition. The Ministry of Finance has reiterated in Budget 2020-21 that the project will be actively pursued. Besides, the government has also identified other sections – high speed and semi-high speed corridors – for development.

Operation features of MAHSR

The 508 km MAHSR corridor project is being implemented by National High Speed Rail Corporation Limited (NHSRCL) with technical and financial assistance from Japan. The finalised route length has 12 proposed stations, at Mumbai, Thane, Virar, Boisar, Vapi, Bilimora, Surat, Bharuch, Vadodara, Anand, Ahmedabad and Sabarmati. All the stations are elevated except Mumbai station which is underground. Approximately 92 per cent of the high speed railway track will be elevated through viaducts and bridges. Of the total length, there will be 460.3 km (90.5 per cent) of viaducts, 9.22 km (1.8 per cent) of bridges, 25.87 km of tunnels (including the 7 km long undersea tunnel) and 12.9 km (2.5 per cent) will be on embankments. Of the total tunnel length of 25.87 km, the longest tunnel will be 21 km and will have an undersea stretch of 7 km.

The design speed of the high speed rail (HSR) is 350 kmph while its operational speed is 320 kmph. It is expected that there will be 40,000 daily boarding passengers in 2023 and 0.2 million by 2053. The expected travel time between Mumbai and Ahmedabad is 2.07 hours with limited stops and 2.58 hours with all 12 stops. Initially, trains will have 10 coaches with a seating capacity of 750 passengers, and this will be increased to 16 coaches with a seating capacity of 1,250 passengers. There will be 35 trains per day in one direction – one train every 20 minutes in peak hours and one train every 30 minutes in non-peak hours. The MAHSR will be equipped to handle 17,900 passengers one way daily and this will be increased to 92,900 passengers in the future. The fare is likely to be 1.5 times that of IR’s AC-I class fares. The project is expected to be completed by December 2023.

Tunnels under MAHSR

Tunnels constitute 5.1 per cent of the total length of the MAHSR project. Of this, the longest tunnel is 21 km long and has an undersea section. The 7 km undersea tunnel for the double-line, high speed railway will be built using tunnel boring machines (TBMs) and the new Austrian tunnelling method (NATM) between the Mumbai underground station at Bandra-Kurla Complex (MAHSR 0.773 km) and Shilphata(MAHSR 21.150 km) in Maharashtra. For undersea tunnel construction, the implementing agency had initially planned to use slurry TBMs that are also being used for tunnel construction in the Mumbai metro project. However, progress has been extremely slow in the case of the metro project. As a result, NHSRCL has shifted from the slurry TBM to the dual-mode TBM, keeping in mind the high water pressure conditions. Besides, a geotechnical investigation of the undersea tunnel area has already been carried out by a team of engineers from NHSRCL, RITES and Japan’s Kawasaki Geological Engineering Company. In order to study the seabed structure, a static refraction technique survey was carried out, involving firing a high energy sound wave towards the seabed from below the water surface as well as mapping the refracted sound wave in order to determine the density of the rock under the seabed.

The tunnel cross-section elements under the project will be determined based on various factors such as construction gauge, track structure (slab track system used in Japan will be used here), ventilation devices, electric devices, aisle for maintenance, regulation for safety and aerodynamic aspects. The project envisages the development of eight mountain tunnels stretching over a length of 5.84 km using NATM. Of these, the third (MT-3) and fourth (MT-4) tunnels have been combined into one tunnel of 2.56 km length. There will be an adit tunnel (horizontal or nearly horizontal) of 394 metres to ensure space for using NATM, which will also be used for maintenance purposes. Three TBMs will be used in Mumbai in order to shorten the construction time. However, the NATM cannot be used in that area due to the presence of Thane creek. Three NATMs will be deployed for the Ahmedabad station. Of the total tunnel length, 15.41 km will be constructed using TBMs whereas 4.88 km will be constructed using NATM. In addition, there will be a tunnel entrance hood to limit micro pressure wave due to the very high speed of trains. The peak value of the micro pressure wave at a distance of 20 metres from the centre line of the tunnel portal will be less than 50 pascals. Besides, the cross-sectional area of the tunnel hood will be 1.4 times the cross-sectional area of the tunnel. There is no requirement for an entrance hood for the undersea tunnel as the speed of the train will be around 280 kmph under the sea.

Upcoming tenders

Works under the project have been divided into several packages. For the C2 package, the technical bids are likely to open on July 14, 2020. Japan will be the lead member for the package. The notice inviting tender has already been issued on April 23, 2019. Earlier, the technical bids were planned to be opened in May 2020, but this has been postponed due to the ongoing pandemic. Works under the package will involve the deployment of three TBMs as the outer diameter of the tunnel is 13.1 meters. It will involve the use of 100 per cent new closed-mode TBMs with a dual-mode option and full backup arrangements, including lining erection equipment. The time for completion of works is expected to be 62 months. The work will include the development of a reinforced cement concrete bed and anchor for the slab track.

The way forward

While the survey, geotechnical investigations and utility mapping for the project have been completed, the land acquisition process is still ongoing. Of the total 1,380 hectares of land required for the project, so far only 647.6 hectares of land (46.93 per cent) has been acquired. The implementing agency needs to expedite land acquisition in order to meet the timelines set for the project. To this end, National Highways and Infrastructure Development Corporation has offered stamp duty on new land, higher payoffs and free skill training for landowners. Availability of skilled manpower, efficient construction contracting agencies and consultants will also be key determinants in the successful and timely implementation of the project.

With inputs from a presentation by Heera Lal Suthar, Executive Director, Design, NHSRCL, at a recent India Infrastructure webinar

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