Kolkata, formerly Calcutta, was once the Indian capital and a centre for modern education, science, culture and politics in India. But by 1911 under British rule the capital had been moved to New Delhi as nationalism grew in the region. Post independence, the city witnessed economic stagnation for nearly 50 years until at the turn of the century an economic rejuvenation led to an acceleration in the city’s growth.
Now as the capital of the West Bengal state, Kolkata is the commercial capital of eastern India. Located on the east bank of the Hooghly River the city’s metropolitan area has a population exceeding 15 million, making it the third most populous metropolitan area in India and the 13th most populous urban area in the world. The city is also classified as the eighth largest urban agglomeration in the world.
But like other metropolitan cities of India, Kolkata continues to struggle with urbanisation problems such as poverty, pollution and traffic congestion.
Relieving congestion
Heavy traffic and congested streets are hindering further economic growth in the city. Formerly three lane highways have been encroached upon by shop fronts and market traders, reducing traffic to a single lane.
To tackle the problem, the government put forward a plan to expand the city’s metro network, which at present has just one line running north-south that was built and extended over the past 30 years.
The new 14.67km east-west line will carry traffic between the busy Howrah Railway Station in the west, which serves continental rail, Central Station, which connects with the existing north-south line, and Sealdah Railway Station in the east, which serves local rail. Shortly after Sealdah the metro line becomes elevated and continues on to Salt Lake Stadium and Central Park. The line is 8.90km underground and 5.77km elevated (see figure 1).
UG1
The underground section has been broken into two contracts. UG1 runs from Howrah Maiden in the west, through a crossover box to Howrah Station, under the river to Mahakaran Station and ends just before Central Station. UG2 picks up with Central Station, runs through Sealdah Station, into a crossover box before Phool Bagan and ends at the interface with the elevated section.
Paul Verrall is senior manager for tunnels with the Transtonnelstroy-Afcons joint venture that was in March 2010 awarded contract UG1. “The program is to launch the TBMs by August/September but I don’t think it is realistic,” says Verrall. The project has hit difficulty because land needed to begin construction works has not been handed over by the client. The client has had trouble negotiating with the community on the hand over of the land.
The EPB TBMs for UG1 will be launched from the crossover box between Howrah Maiden and Howrah stations, into the silty clay ground. But only a fraction of the land needed to excavate the box has been handed over. Verrall explains, “The problem at the moment is we only have one small section and so we can only do one side of the box wall. The other side is in the middle of the street and on the other side of the street are a load of tenants that have to be removed by the authorities.”
The city is due to hold elections in May, which could see a new government taking charge of the project. Contractors across the project hope that the land issue is resolved ahead of the elections to avoid further delays.
“If we were given all the land as per tender we would need six months to do the diaphragm walls, put the shaft down; we’d need six to eight months to do the full site installation and we’ll be ready to launch the TBMs. We would need anywhere between eight and 12 months from total handover to when we start the TBMs. It’s a lot of work,” says Verrall.
Howrah and under Hooghly
When the 6.35m Herrenknecht TBMs do finally get going they will launch out of the crossover box toward Howrah Station and the Hooghly River. From launch the TBM will have approximately 700m to drive until it reaches Howrah Station on the riverbank. Verrall says, “we need the first 100-150m for the assembly of the TBM and the commissioning. Once one TBM is away we’ll start commissioning the second. They should run with approximately a 50-ring lag between them.
“Howrah station definitely will not be done when the TBM arrives. So we need to have the two diaphragm head walls done and we will just pass through.”
As the TBMs exit the east side of Howrah Station the depth of the base slab will be about 25m and the overburden approximately 18m. “We’ll dive down on a three per cent gradient under the Hooghly River,” says Verrall, “where we’ll have an overburden of 40-45m – about 20m of soil cover and 20m of water cover.”
Verrall doesn’t foresee any problems with crossing the river but adds, “once we get under the river I have no intention of stopping whatsoever, we will keep going until we are out the other side.” So before Verrall takes the dive under the river he will first make sure there is nothing that can stop the TBM once it gets going.
Muck disposal is vital to the approach Verrall takes to the crossing. The crossover box where the TBMs launch from is the logistics hub for the contract. Everything going in or out of the tunnel must pass through this box. But the box is not ideally located; it is locked in a congested area of the city with narrow, overcrowded roads located on all sides.
At the peak of TBM production some 1,800-2,000cu m per day will need to be removed via the crossover box and, due to heavy traffic conditions during the day, this all must be done at night. “Based on that, you need about 120 trucks to carry away the earth and two excavators to load the trucks otherwise you’ll never be able to load them fast enough” says Verrall.
He adds, “Before we get to the river both TBMs should be under full production and we’ll see how the logistics are working. If we are not able to keep up with muck removal because of traffic congestion in the city, the TBM that is not under the river will be slowed down completely and we’ll continue with the one under the river at full production. When out the other side of the river the TBM will slow and the second TBM will make its crossing at full production. If everything on the logistics is running very smoothly then I’ll let one get halfway across the river before the second enters the crossing.”
They will run for six days a week with two 12-hour shifts and on Sundays will shut down for maintenance. The TBMs will be set up for one ring an hour and should be capable of achieving those speeds but removing the muck from the site is going to be the main barrier to advance rate. Muck removal is often the bottleneck on all city projects. “We need to advance 12m per day to keep on schedule. Each shift could advance 15m, but achieving a 30m advance rate a day is not possible because we could never get the excavated material away from the site.” Cutting just 15 rings per day will produce 2,200cu m of muck.
Each ring is made up of five segments and a key. A ring’s internal diameter is 5.5m and it is 275mm thick. It is a universal ring constructed with M50 concrete, 1.4m wide and bolted together. An EPDM gasket with a separate hydrophilic strip will be used to seal the gap between segments.
Lining up
When the TBMs pull up and out of the river crossing they will need to turn south for Mahakaran station and then east out of Mahakaran for Central Station. Mahakaran Station is not expected to be constructed before the arrival of the TBMs so the operators will plough through the head walls when they reach them.
There are several buildings in the region of Mahakaran Station and Central Station that are dilapidated. Some of the oldest buildings are thought to be found on piles about 25m deep. The overburden on this stretch is only 15-18m.
The tunnel alignment through these turns has been restricted by utilities, the position of the stations, narrow roadways and sensitive buildings. The combination has forced the TBMs to make some tight curves along the route. In revisiting the alignment the contractor is hoping to renegotiate these radiuses.
“The TBM will theoretically allow a radius of 220m, but that is theoretical, I don’t believe that you would maintain 220m. If a mistake gets made or you encounter some kind of steering problem you cannot recover,” explains Verrall. “Currently the alignment demands a 220m radius. So you are totally on the limit. The ring design, with a 50m taper will allow a radius of 170m. We are asking them to realign the curves to a radius of 230m to give us some more breathing space.
“With all of these alignments there is a safety factor. On the horizontal alignment the tolerance is 100mm so to be on the safe side when we come into the curves we’ll start to steer on the inside of the curves staying within the 100mm tolerance.” This gives the TBM operator a greater margin to play with should anything go wrong.
Verrall adds, “For the realignment we cannot move the stations back or forth but we can maybe move the stations a little to the left or right to make room for the utility diversions. The problem here is how to do it. If we keep a narrow station box, which will allow them to do a utility diversion, and increase the radius, it brings the two tunnels very close together, just 1.5m in separation. That’s much too close for comfort. Usually we would want at least one and a half diameters between the two.”
The closer the two tubes are the more careful the contractor has to be with the pressure exerted on the ground. “The earth pressure in this case will not only be calculated upwards but also sideways. And you have to be careful of over excavation, that you are not sucking away the material from the tunnel next to you,” says Verrall.
Once successfully reaching Central Station the TBMs could be removed through the station or drawn back through the tunnel.
UG2
Central Station marks the interface between UG1 and UG2. A joint venture of ITD and ITD-Cem was awarded the contract for UG2 in February 2010. General consultant to the project is the Mycel joint venture of Maunsell (now AECOM), Yachiyo, CES, Egis and LHPA.
Chris Schulz of MYCEL explains that as with UG1 the stations on UG2 will be cut and cover with diaphragm walls. “Pool Bagan and Seadlah stations are each two level stations and will be 15-18m deep. Due to soil conditions at Pool Bagan the diaphragm walls will be deeper than originally planned. Central is a three level station excavated to a depth of 25m. The station boxes are 220m long and 21m wide with 1.2m thick walls.”
UG2 is also facing difficultly with gaining land from the client but Schulz was confident it would not affect the schedule. The TBMs will launch out of the crossover box located between Sealdah and Phool Bagan stations.
“The most difficult portions of the project will be passing under the Seadlah railway lines and passing under the north-south metro central station,” says Schulz.
Engineers constructing the north-south line some 30 years earlier had planned for the possibility of an east-west line and installed diaphragm walls under the station. They had left an available space of 6m wide but in fact the new project now needs a space of 12m wide and so these diaphragm walls are causing an obstruction, making the crossing quite complicated. “There are as-built drawings showing the location of walls but the reliability of these drawings is not known,” says Schulz.
He adds, “There is very little cover between the top of the new tubes and the north-south line central station above, approximately only one meter of cover. The contractor is likely to use pipe roofing to secure this for the drive but one possibility is removing that metre of material as the area already has to be excavated ahead of the TBM to remove the diaphragm walls.”
Due to delays a definite deadline for tunnel excavation is unknown. But the project is scheduled to open to the public in 2014.
The east-west metro should help ease congestion in Kolkata Figure 1, alignment of the east-west metro The UG1 work site sits in the shadow of Howrah Bridge Excavation work has begun on a small section of the crossover box Market traders flank the worksite creating logistical challenges when trucking muck
