The Delhi Metro’s underground construction in its third phase of expansion cannot be considered anything short of a revolution. Close to 80 kilometres of underground tunnels have been laid beneath the city in the last five years without causing any disruption to the din and bustle of the daily life above.
The sheer enormity of the task can be gauged from the fact that, the total length of the underground corridors in Delhi Metro’s Phase-3 is more than the total underground sections built by DMRC in both Phases 1 and 2, making it one of the most challenging construction phases for the organization.
Combining the first two phases, Delhi Metro has a total of 31 underground stations, while Phase-3 alone has 35 underground stations. The total length of the underground corridors in the first two phases was 48.06 kilometres and Phase-3 alone has underground corridors of 54 kilometres. The first phase of Delhi Metro’s construction had 13.17 kilometres of underground Metro lines while the second phase had 34.89 kilometres.
The decision to construct more underground corridors was taken to ensure that the construction work caused minimum inconvenience to the people. By constructing so many underground stretches, DMRC has also avoided causing any damage to the existing infrastructure such as flyovers and roads.
For the first time in the history of Delhi Metro’s construction, four different tunneling stretches in Phase-3 have passed below existing underground Metro stretches in the city. These four underground stretches are:
- Tunnel between Sarojini Nagar to INA on the Majlis Park – Shiv Vihar corridor below the underground section of the Samaypur Badli – HUDA City Centre line at INA.
- Tunnel between Indira Gandhi Domestic Airport (T1D) to Vasant Vihar on the Janak Puri West – Botanical Garden corridor below the underground section of the Airport Express Link near the Indira Gandhi International Airport (Domestic terminals).
- Tunnel between Mandi House to ITO on the Escorts Mujesar – Kashmere Gate Corridor below the underground section of the NOIDA – Dwarka corridor (Blue Line) at Mandi House.
- Tunnel between Hauz Khas and Panchsheel Park on the Janakpuri West – Botanical Garden corridor below the already operational HUDA City Centre – Samaypur Badli section at Hauz Khas.
|Details of underground stretches (in a nutshell):|
|Phase 3||54 kilometres|
|Corridors||Underground length (in kms.)||Underground stations|
|Majlis Park – Shiv Vihar||19.117||12|
|Janakpuri West – Kalindi Kunj||23.807||15|
|Central Secretariat -Kashmere Gate||9.370||7|
|Dwarka – Najafgarh||1.541||1|
Phase 1 and 2
Undergound length (in kms.)
Most of the tunneling was done using Tunnel Boring Machines. A record 30 TBMs were pressed into service to complete the work. It is indeed rare to come across the use of so many TBMs within one urban centre anywhere in the world. In Phase 2, 14 TBMs had been used but this time the number had to be more than doubled.
There were also stretches where it was not possible to use TBMs. For example, at Kalkaji Mandir, the New Austrian Tunneling Method (NATM) was used to carry out tunneling work for a stretch of 176 metres.
The tunnel between Kalkaji Mandir and NSIC Okhla passes below piers 135 and 136 of the operational violet line and the distance between the two piers was 28 mtrs. It would have been impossible to construct twin tunnels within such a short space. Therefore, a decision was taken to construct a single tube twin tunnel through which both the tracks will pass using the NATM technology. In another stretch at Kashmere Gate, NATM was used for a stretch of a 12 metre long tunnel.
Monitoring during tunneling work
DMRC has always laid great stress on ensuring that its construction activities cause minimum inconvenience to the general public. During the tunneling work of Phase-3, detailed instrumentation was done at various locations to monitor the condition of buildings and other structures above.
In areas such as Old Delhi, stretches where the tunnels were passing below already operational Metro tunnels or beneath flyovers and bridges, DMRC’s engineers had to be very careful. Various equipment for monitoring the movement of the TBMs were installed such as settlement markers, inclinometers, bi-reflex targets etc. Exhaustive building condition surveys were also conducted at all major locations for the same.
Mass Spring Systems (MSS)
A decision was also taken to install Mass Spring Systems in the tunnels to mitigate vibrations generated from tunnels during the passing of trains. Mass Spring System (MSS) is the solution which helps mitigate vibrations generated by the passing trains at the source itself.
MSS elastically separates the tracks slabs in the tunnels or on the viaducts from the supporting structure. The material used for isolation is a microcellular Polyurethane Elastomer (e.g.- Sylomer from Getzner- Austria). Use of MSS helps in minimizing the transmission of vibrations (structure-borne noise) to the surrounding establishments in the vicinity of tracks. In addition, MSS also effectively reduces the development of audible secondary airborne noise caused by the vibration of buildings and other infrastructure components.
Wherever the Delhi Metro carried out construction work, it engaged with the people through numerous Community Interaction Programmes. In these programmes, DMRC’s detailed construction plans were discussed with the residents of the area and all possibilities of road diversions, temporary or permanent displacements etc were deliberated upon. With such initiatives, DMRC could win the trust of the people and progress smoothly with its construction work.
Underground construction has its challenges irrespective of where it is carried out. However, if the construction takes place in a city with close to 20 million people, the challenges simply multiply. Constructing over 50 kilometres of underground lines in Delhi was a major challenge with innumerable buildings and structures in precarious conditions. However, DMRC’s engineers showed great commitment and presence of mind in ensuring that the work could be completed without any major hindrance.