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Tunnelling: Delhi Metro

Delhi’s ambitious metro construction programme continues at an incredible pace, with the latest section celebrating an unusual breakthrough, as Margo Cole reports.

Delhi Metro breakthrough

Celebrations: The construction team marks the simultaneous breakthroughs of TBMs “Riddhi” and “Siddhi”

 

Last month saw a double breakthrough of tunnel boring machines (TBMs) working on the latest phase of the Delhi Metro in India. The two machines, named Riddhi (meaning “prosperity”) and Siddhi (“accomplishment”), broke through simultaneously after each had excavated 1.15km of tunnel.

The earth pressure balance (EPB) machines, manufactured by Herrenknecht, are being used by a joint venture of Samsung and Hindustan Construction Corporation (HCC) to drive nearly 5km of twin bore tunnel in the south west of Delhi as part of the third phase of metro construction.

This phase will add a further 140km of lines to the existing 190km metro network by 2016.

In February 2013 Delhi Metro Rail Corporation awarded the HCC-Samsung JV a Rs8.66bn (£86.6M) design and build contract for a section of new metro on the Janakpuri West to Kalindi Kunj corridor, one of five new metro corridors being built in Phase III.

5km of running tunnel

The JV’s contract is for almost 5km of new twin running tunnels between the existing Janakpuri West station and a new station at Palam, as well as two new underground stations at Dabri Mor and Dashrath Puri, and an interchange with the existing elevated metro station at Janakpuri West.

All the tunnelling is being done using EPB machines, with full concrete segmental lining, while the two new underground stations are being built using cut and cover methods.

HCC-Samsung is using four TBMs - Riddhi and Siddhi, plus Jantar (meaning “instrument”), and Mantar (“formula”) to construct the 5.8m internal diameter running tunnels.

Riddhi and Siddhi were launched from a shaft at Janakpuri West, and headed south and east until they emerged in the twin breakthrough last month at the Dabari Mor station site.

They are now being lifted out and serviced before
being re-launched at the new Dashrathpuri station excavation in November, from where they will start boring northwards, and again emerge at Dabari Mor station.

At the same time, the second set of TBMs, Jantar and Mantar, were launched from a specially constructed shaft at Vikarpuri, north west of Janakpuri West, from where they have been heading towards the station.

Once they have emerged at Janakpuri West, they will be lifted out and taken to Dashrathpuri station, ready for the next boring sequence, heading south to emerge out at what will eventually be Palam station.

All the tunnelling is due to be completed in June 2015, with the entire project due for completion a year later.

HCC-Samsung’s consultant for detailed geotechnical, civil and structural engineering design is Mott MacDonald, which has been involved in the Delhi Metro for more than 10 years.

Biggest challenge

Mott MacDonald metro and railways director Mohan Gupta says the biggest challenge during the tunnelling is to control volume losses to limit ground subsidence, because of what he describes as Delhi’s “often challenging soft ground conditions”.

The excavation is being carried out at a depth of around 16.5m, predominantly in sandy, silty soils.

Gupta expects similar ground conditions for all the drives, but says the TBMs have been designed and built to handle a range of ground conditions, should they come across anything unexpected.

The team has installed considerable instrumentation on the buildings and services at ground level to monitor any movement caused by volume loss.

“Additionally,” says Gupta, “the water table is very shallow, and continuous grouting is being done to limit settlements.”

Other technical challenges on the project include carrying out the work in what is a densely populated residential area.

Tunnelling also takes place close to the piles that support the existing elevated metro viaduct at Janakpuri West.

HCC-Samsung claims Riddhi and Siddhi’s double breakthrough at Dabari Mor is only the second time two parallel TBM drives have broken through together.

The only other instance the JV has found occurred in 2012 on the 8.6km Toronto-York Spadina subway extension in Canada.

Fully operational

Janakpuri West to Palam is the westernmost section of the 36km Janakpuri West to Kalindi Kunj metro corridor, which is expected to become fully operational in 2016.

The corridor includes 21km of tunnel.

Around 53km of Delhi Metro’s third phase will be in tunnel, with 35 TBMs expected to be used at the various different locations, compared with 14 TBMs used to construct tunnels for the 125km of metro built in Phase II.

Delhi has been building and extending its metro network at a rapid rate over the last 12 years to meet the needs of its fast-expanding population.

The first phase, which consisted of 65km of railway on three lines and 58 stations, mainly serves the central business and residential areas.

The lines came into operation between 2002 and 2006.

Phase II began almost immediately after Phase I opened, adding another 125km of track, four more lines and 85 stations, including a 23km express link to the city’s international airport. The various sections of Phase II became operational between 2008 and 2011.

Phase III focuses more on the outer areas of the city, and on connect the long radial lines of Phases I and II with circular ring lines to help reduce travel distances and relieve congestion on existing radial lines.

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