Delhi Metro: Unique Features of the Best Railway Network in the World

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The need for a rail-based public transport system for the city of Delhi had been on the cards for a long time. However, the plans materialized in the mid-90s when the phase-I of the Delhi Metro project was sanctioned by the Government of India. The construction of a rail-based mass transport system was entrusted to the Delhi Metro Rail Corporation (DMRC).

During the planning phase of the project, the experience of building a modern metro system within the country was minimal. All that the country had witnessed before was a track line of 16.45 km made out of vintage technology operational in the city of Kolkata, that too constructed between 1974 and 1996. Therefore, this was not of any help for deciding the technology system for the proposed Delhi Metro.

The city of Delhi has the peculiar characteristic of hefty traffic demand on the selected corridor. It also has difficult climatic conditions ranging from sweltering, dry, and dusty summer to freezing winters and storming raining seasons. On the international front as well, not many metro projects were undertaken for similar climatic and traffic conditions. Hence, while deciding on technical parameters and systems, these issues were considered.

Further, the roads in the city are very congested that could not have accommodated construction on them for a long duration. The engineers also did not have any tunneling experience in the complicated and variable geology of Delhi, which ranges from soft silty clay to weathered fractured rock to tough quartzite rock. Therefore, the construction methodology, technology, and procedures were selected to suit the requirement for speedy construction.

1. History of Delhi Metro

The government sanctioned the phase-I of the project in the year 1995. However, the actual planning and execution started only in 1997 after the formation of the DMRC.

Phase-I of the project, consisting of three lines, totaling about 69 km, was completed in 2005.

Phase-II of the project, consisting of a track length of 125 km, was completed by October 2010, for the 2010 Common Wealth Games held in Delhi. The phase-II was completed within the targeted time and cost. This phase included an airport express line of 22.5 km. After the completion of phase-II, a network of about 190 km became operational.

Phase-III, involving the laying of tracks up to around 168 km, was completed by the end of 2019. Currently, the Delhi Metro is undergoing phase-IV development of 114 km, which is expected to be completed by 2024.

2. Construction of Underground Tunnels

Tunnels for the metro system are typically shallow with minimum overburden. To clarify, the deeper tunnels will need deeper stations which may cause inconvenience to the metro users, and also the station will consume more energy for deeper escalators and lifts. However, shallow tunneling in the urban areas poses difficulties in settlement control at the surface to protect the structure’s properties, life, traffic, and utilities.

In Delhi, specially designed Tunnel Boring Machines (TBM) have been deployed to suit the geotechnical conditions. The condition survey of structures in the influence of tunneling zone is carried out, and the effect of the projected settlement on the structures is examined in advance to ensure the safety of structures. Suitable modifications to the design are done based on the pre-condition survey and settlement analysis.

The precast concrete segmental lining is used for tunneling. The segment casting yards are located away from the busy central urban area. The segments are transported in large trailers during the night hours to avoid traffic congestion during day time.

Since tunnels in most of the locations are below the groundwater table, hydrophilic gasket is used to prevent leakage from the segment joints. Many TBMs were deployed so that the completion of tunnels matches with the completion of station structures, and optimum project completion period is achieved.

3. Construction of Underground Metro Stations

The underground stations are mostly located below major roads. Therefore, the occupation of roads for a longer duration causes inconvenience to the public as traffic has to be diverted in such cases. Thus, the top down method of construction was adopted instead of the cut and cover construction method.

In top down method, RCC diaphragm walls are first constructed with the highest level of quality. The verticality of diaphragm wall panels is ensured by a suitable trenching method and confirming trench verticality before reinforcement is lowered, and thereafter concreting is done. The verticality is checked by recording the trench with special equipment like CODEN. If the trenching is not being vertical to the expected level, the trench is refilled, and re-trenching is done.

In order to prevent the collapse of soil into the trench, special polymer slurry is used instead of bentonite to avoid the problem of disposal of environmentally hazardous bentonite slurry. Similarly, it has been possible to provide Polyvinyl chloride (PVC) water stops between the two diaphragm wall panels to prevent the leakage from the diaphragm wall joints.

Once the construction of diaphragm wall is completed along the perimeter of the station, the top slab of the station box is first completed by leaving an opening in the top slab for further excavation of the structures below the top slab. With this, it is possible to restore the roads quickly and drastically reduce the road occupation.

4. Elevated Viaduct

The use of segmental construction in elevated viaduct was not common in India till DMRC started metro construction in Delhi. If the construction of elevated viaducts had been done with the conventional cast-in-situ method, it could have caused great inconvenience as it needs the occupation of roads. Further, it takes much longer time to finish the viaduct works. Therefore, DMRC decided to go for precast segmental construction.

The advantages of precast segmental construction are:

1. Segmental construction is a cost-effective method for a vast range of span lengths and types of structures.
2. Structures with sharp curves and variable superelevation can be adjusted to suit the requirements.
3. Segmental construction allows the reduction of the construction period as segments are manufactured and assembled rapidly.
4. Segmental construction shields the environment as minimum space is required for foundation and substructure at the site.
5. The superstructure is manufactured at a place away from busy locations. Therefore, the placement of superstructure is completed with the system erected from piers at heights.
6. In case if the site situation demands, then it is easy to adjust last-minute changes in span configuration.
7. It is easier to ship smaller segments by road trailers on the city roads.
8. During construction, the intervention to road traffic is considerably reduced. Segmental construction results in aesthetically pleasing structures and good finishes.

5. Use of Precast U-Span Girders

The use of precast U-span girders not only sped up the construction but also improved the aesthetics of the viaduct because of the sleek shape and uniform quality. Precast pier caps and precast U-span girders are suitable for constructing long elevated viaducts for the railway system. The following points describe the key advantages of the system:

1. Integrated sound barrier system and cable support system is achieved.
2. The longitudinal profile of track could be lowered by 1.5-2 m compared to a regular design.
3. Integration of the walkway for upkeep and passenger evacuation on each side of the tracks.
4. Cost-effective from both structural and systemic point of view.
5. The system provides flexibility in construction along the metro line between two stations.

6. Use of Special Structural Forms like Extra-dosed Bridge

Extra-dosed bridges are the best structural solution in places where long spans are required to be constructed, and there is a space constraint. They are also more economical than the cable-stayed bridges. The pylon heights in the extra-dosed bridge are almost half in height as compared to cable-stayed bridges.

For the extension of the U-shaped viaduct of phase-3 of Delhi Metro, DMRC dealt with crossing five railway tracks with a number of constraints such as sharp plan curvature, railway vertical clearance, impossible location of intermediate piers, and unavoidable interruption of the railway traffic. DMRC provided extra-dosed bridges at crossing of railway tracks. Besides the first bridge in Japan, this is the second bridge of its kind built over a railway crossing.

The Pragati Maidan bridge is an extra-dosed railway bridge with a main span of 93 m. The deck cross-section has a U-shape, which allows a smooth integration of the metro system with the superstructure. The U-span of the extra-dosed bridge is formed by wrapping the cables around a concrete beam. These cables induce a prestressing force in the beam and improve the strength of the beam. This beam also enhances the stiffness of the main span.

7. Selection of Rolling Stock

For a public transport system like the metro, financial sustainability of the system depends on keeping the operation and maintenance costs low. The cost of energy normally constitutes around 30-40% of the operational cost. Therefore, it is necessary to have a very energy-efficient system.

DMRC has successfully deployed highly energy-efficient metro rolling stock with very high reliability, maintainability, and safety standards. Stainless steel car body has been chosen to achieve lightweight and less maintenance on account of corrosion. All the materials used in the stock are of very high fire ratings. Propulsion equipment design has been chosen to achieve high acceleration coupled with the highest possible energy regeneration during braking.

Passenger comfort has been kept in focus. The design incorporates a two-level suspension system with superior bogie design for improved ride quality. State of the art passenger announcement systems and passenger information system coupled with dynamic route map provides requisite information to commuters.

High capacity energy-efficient on-board air conditioners have been provided. Car body construction ensures excellent thermal and acoustic insulation to achieve interior conditions consistent with the requisite international standards. To improve the rail wheel interaction and for mitigation of noise and wheel wear, wheel flange lubricators are used.

DMRC is committed to achieve maximum indigenization and has achieved notable success. Almost 90% of the stock deployed in DMRC has been manufactured in India. By adopting a regenerating type of rolling stock technology, DMRC became the first organization in the world to be facilitated by the United Nations in rail-based transport sector and has earned carbon credit.

8. AC Traction System of 25 kV

Most of the metro systems in the world have direct current (DC) traction system, usually around 750 V. The low voltage DC system has limitations on the capacity of the system. Therefore, some metros started using 1500 V DC system with overhead catenary. The high voltage is necessary for higher capacity as more trains are required between two sub-stations drawing very high current.

Since Delhi Metro required to have a very high capacity ranging from 60,000 to 90,000 passengers per hour in each direction, the team explored the possibility of further high voltage and finally selected a 25 kV AC traction system. Mostly, the 25 kV AC traction system is commonly used for passenger railway trains operation throughout the world. Therefore, for the first time for a metro operation, the 25 kV AC traction system was adopted in Delhi Metro. Since then, the system has been working very satisfactorily.

The lightweight, efficient rolling coupled with high voltage AC traction system has proved to be a very energy-efficient system and helped DMRC in maintaining its financial viability.

9. Automatic Fare Collection System

It was also realized that having an efficient metro network necessarily required a passenger-friendly fare collection system.

The automatic fare collection (AFC) system has a lesser staff requirement, lesser possibility of leakage, amenable to quick fare changes, all the transactions are logged, and management information reports easily generated. Hence, the obvious choice between the manual and the automatic system was the AFC system.

Again, the choice was between a magnetic recyclable ticket media or a contactless media. The analysis done by DMRC led to the conclusion that contactless option is the optimum solution. In short, it provides better throughput, more recycling of the ticketing media, and is less prone to mechanical problems due to contactless interaction between the validator and the ticket.

Contactless smart media cards (CSMC) were considered ideal for the multi-journey. However, the single journey dilemma remained on cost considerations. Finally, CSMCs were found to be the right choice for single journeys. The single journey token themselves are initially more expensive to produce. However, they are recovered at gate exits and can be reused up to 50,000 times, reducing the unit cost per journey.

DMRC was the first metro system in the world to introduce this type of contactless smart token for single journeys. Today, however, many metro systems in the world have adopted this system.

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