Construction of the London Underground began in 1860 and between then and now the network has grown to 270 stations and 402km of track, giving an average of 2.7km of track each year. The New York subway has averaged 2.4km of new track since contruction began. By the end of Phase II in March next year the Delhi Metro will have averaged 14km of new track a year, five times faster than the London and New York systems and giving Delhi a world class metro inside 15 years.

Phase II, which is reaching completion, was sped on by the pressure of the Commonwealth Games held in the city this Autumn. The pressure of the games deadline was passed onto the contractor, making the planning and scheduling of works a crucial factor in meeting the demands of the contract.

Once Phase II is complete in March next year the Delhi Metro will consist of six lines including 31 underground stations. It has a combination of elevated, at-grade and underground lines and uses both broad gauge and standard gauge rolling stock.

Contract BC 24
Delhi Metro Rail Corporation (DMRC) began construction of Phase II between 2006 and 2007 to extend its existing lines and take them to adjoining states of Haryana and Uttar Pradesh. The extensions were scheduled to be completed in four years.

“Our package called BC 24 was one of the last in series to be awarded as design and construction,” says Pawan Bhasin of contracting joint venture ITD-ITD Cementation. The contract is made up of a twin-bore tunnel of 4.2km driven by TBM from Khan Market to Nehru stadium and on to Jangpura station, and an intervention shaft (IVS). It goes into cut-and-cover from Udyog Bhawan to Lajpat Nagar for underground works on Central Secretariat- Badarpur corridor of Delhi MRTS. The time period allocated was just 35.5 months.

BC 24 runs from Central Secretariat- Udyog Bhawan starting in a short cut and cover section heading south and quickly becoming twin-bored TBM tunnel for 1.8km to Khan Market station. The stretch to Khan Market station was excavated using TBMs numbers three and four. Heading north for Khan Market were TBMs numbers one and two on a 2.3km drive from Jangpura station and passing through Nehru Station en route. The southern most end of the contract between Jangpura Station and the elevated track of contract BC 25, a stretch of less than 1km, was driven by TBM numbers three and four after they had completed the northern tunnels. The stations and cross passages were excavated using the New Austrian Tunnelling Method (NATM).

The alignment is an extension of the existing Line II and designed to link the southern part of Delhi and to ultimately connect with the neighbouring industrial town of Faridabad, part of the NCR in Haryana state. The planners also took account of the main games Jwaharlal Nehru (JLN) Stadium and existing business centres of South Delhi namely Nehru Place and Jasola.

The alignment was kept underground until Lajpat Nagar largely due to aesthetic consideration—protecting the beautiful Lutyens Delhi. “Needless to say, initially up market South Delhi residents also protested and asked for an underground metro as they felt it will infringe upon their privacy,” says Bhasin.

Cutting under Delhi
Generally the geology of Delhi consists of clayey, silty sand with high permeability characteristics but the launching point for TBMs three and four heading south from Udyog Bhawan was found to have quartzite rock at a shallow depth. The contractor carried out site investigations that revealed the rock dipped sharply in the southerly and easterly directions.

“BC 24 alignment took off from Udyog Bhawan (Offices of Ministry of Industry) and we were to provide a shaft at chainage 0 to lower the TBM. However, detailed soil investigation showed rock strata (quartzite) at the depth of 4m resulting in shifting ahead of shaft by 200m along the alignment. So the first 200m of tunnel was constructed by cut and cover method,” explains Bhasin.

Four TBMs were deployed on the project. The contractor opted for Herrenknecht EPBMs as it believed they were the best choice for soft, silty, highly saturated soil conditions. “NATM or other tunnelling methods like drill and blast were not possible in these conditions and also part of the alignment passed under residential buildings, therefore TBM tunnelling was quite a safe and fast methodology,” says Bhasin.

Project client Delhi Metro Rail Corporation wanted a finished tunnel diameter of 5.8m so the contractor ordered TBMs with a 6.64m diameter cutterhead. The TBMs had a design thrust of 22 750kN, a main bearing diameter of 2.6m, main bearing power of 630kW, torque of 5,213kNm and a maximum advancement of 80mm/min. With five back-up gantries behind, the total length of complete assembly was 76m each.

Workshop testing and commissioning of all four TBMs was done at Guangzhou, China. They were shipped to Mumbai in July 2008, and arrived on the Delhi job site by road in September 2008—less than a year after the award of the contract.

“We have used precast segments for permanent lining. Each ring consists of six different segments. The rings are of two types, 1.5m length and 1.2m length. The outer diameter of each ring is 6.35m and inner diameter is 5.8m. The design strength of the concrete segments is M45,” says Bhasin.

Khan Market Station
In the centre of the contract lies Khan Market Station. It is the meeting point for the two TBMs heading north with the two TBMs heading south. Above ground the site in congested with busy streets and extensive buildings. Below ground the contractor had to navigate through building foundations and complex utility networks.

The area from Khan Market to Jangpura Station is an upmarket disctrict in Delhi, which meant the contractor had to be particularly careful in minimizing distrurbance while tackling the challenges. Logistically the site was very complicated. The congested area and the pressure to minimise disturbances meant managing traffic flows in the area was a priority for the contractor. The difficulty was compounded by night-time restrictions.

Many residents went further to protect their privacy by preventing surveys from being carried out on their buildings to monitor the effects of the tunneling activity. With many of the high-rise buildings unable to provide structural drawings, the contractor was forced to take extra precautions to ensure the work did not result in damage to the properties above. At Jangpura the contractor was faced with boring tunnels beneath multistoried residential apartments resting on individual spread footings and with no structural details available.

In these areas the contractor used pregrouting with bentonite and polymer grouts to stabilise the ground ahead of the TBMs. In some areas, such as the 5-star Hotel Ambassador at Khan Market, additional piles were driven.

Passing along both sides of Khan Market station, and in parts passing within the station, were two brick arch sewers dating from British ownership. The 7m and 5m-diameter sewers needed to be relocated in order for the station to be constructed. Torrential rain in 2008 caused one sewer to overflow resulting in complete submergence of Khan Market Station and the entire tunnel between Khan Market and Nehru Station including the TBM. This one day of rain put the project behind by more than a month.

Managing the water table was to be a long-term challenge for the tunnel. During TBM drives primary grouting was done. After completion of TBM drives generally secondary, including PU at some places, was done to prevent leakages. “During post-grouting we have used PU, silica gel, cement and mirosilica,” reported Bhasin.

Meeting the deadline
The main driver for constructing phase II of the metro was to provide a very good transport system for spectators of the Commonwealth Games.” So we had to be fast to finish our project well before the start of the games,” says Bhasin. The tight time schedule created several challenges.

According to the schedule the shafts and launch chambers needed to be ready prior to the arrival of the TBMs to site. This gave the contractor less than a year to complete the works. The challenge was complicated by the hard rock found on the northern launch site that led to further site investigations. But the shafts were successfully completed in time.

The mapping of utilities needed to be completed before tunnelling could start and the diversion of many was also a major draw on time. This included the timely relocation of large diameter trunk sewers, water lines and other utilities along alignment, especially at station locations.

As the contract was design and construct, the contractor and client had to develop a faster system for proofing and approving designs so that the work could fit inside the schedule. “The proactive approach of DMRC, their continuous monitoring and support, quick decision making of their consultants, and having the extensive experience and zeal of the site team could overcome all challenges of this difficult project,” says Bhasin.

When up and running, each TBM was to average an advance of 350m per month with a peak month of 450m of tunnel, despite the challenges the project faced. In the station excavations the diggers were able to remove 250m3 of soil a day or 100m3 of rock.

“The entire project was successfully completed by us and handed over to customer two months before contractual time period of 35.5 months. The line was made operational for public on 2 October 2010, the day of the opening of commonwealth games,” says Bhasin.

Project overview. Blue: TBM 1; Red: TBM 2; Pink: TBM 3; Green: TBM 4 Breakthrough of one of the 6.64m diameter EPBM Site investigation boreholes along the tunnel alignment with surface and groundwater levels The 1.5m by 1.2m segments form the tunnel lining in a 5:1 arrangement