On a hot summers day in Bangalore, ambassadors, politicians, lobbyists, financers, press and the cream of the Indian tunnelling community crowd excitedly into a large marquee erected alongside the Majestic Station shaft eagerly awaiting the launch of the first slurry TBM to be used in India. Police patrol the site and line the sides of the tent. With one hand resting on their batons they survey the throng as Karnataka state Chief Minister Shri Yeddyurappa enters to a hail of applause, flash photography and genuine adoration from the baying crowd.
The guest list for the TBM launching ceremony on Bangalore’s metro reads as a who’s who for the Indian construction industry. Addressing this crowd Yeddyurappa proudly stamps his name on the project by congratulating those who have brought it this far. He is gifted with flowers and presents from the joint venture construction team. “The gods of the underworld must be very pleased,” says the master of ceremonies. Following a ceremonial smashing of a coconut and lighting of fires the chief minister heads for the launch shaft and pushes the button that sets the cutter wheel in motion.
Yeddyurappa claims that the 6.44m Hitachi Zosen TBM is the first TBM to be used in a Southern Indian state and the first slurry TBM to be used in the country.
Bangalore east-west metro
As the third largest city in India, Bangalore faces congestion problems similar to those found in Delhi, Mumbai and Kolkata. Its population is growing at more than three per cent per annum with a record annual growth of 12 per cent. The 5.4 million residents place a massive demand on the limited public transport system. As a result, vehicle ownership is increasing. Some 1050 new vehicles are registered in Bangalore every day. But with high salaries in the 800 square km city, and multinational companies well established, the city can afford an upgrade.
There are two metro lines under construction in Bangalore, the north-south corridor and the east-west. A joint venture of Taiwanese contractor Continental Engineering Corporation (CEC) and Indian contractor Soma is executing the east-west line with the TBM starting works some 10 days after the launching ceremony last month. CEC’s Indian arm CICI is carrying out the tunnelling works while Soma is excavating the stations.
The 4.45km of twin bore tunnels will be fully excavated by TBMs with the east and west ramps, the stations and a short pocket track being excavated by cut and cover. The tunnels will have a finished internal diameter of 5.6m, lined with precast segments 1.5m wide and in a five plus one arrangement. The segments are manufactured near the work site by Ya Li, part of Far Eastern Group.
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By GlobalDataThe two TBMs will be staggered about one month or 50-100m apart and will have a minimum overburden of one diameter, though it is more typical along the alignment to have an overburden of around 10m.
Eastbound drive
Majestic station is the interchange station for the east-west and north-south lines. The contract for the station has not been awarded as the final costings for the project have not been agreed. There are some quiet concerns that this could slow the commissioning of the project but for now the focus is on the tunnelling works.
Project manager for CEC Russell Brown explains that the first 140m of the drive that TBM Helen is undertaking is among the most testing for the project. With a shallow overburden of just 1.5 diameters the TBM is carefully edging its way under the fragile buildings of Bangalore above. It is the only stretch of the project where the team were not able to keep all buildings outside the zone of influence. Nigel Butterfield, project manager for the general consultant says at this point the client, Bangalore Metro Rail Corporation (BMRC), will accept slight damage to local buildings, which must not exceed cracks of 5mm in width.
On the 960m drive eastbound from Majestic Station to Central College Station the TBMs will pass through stretches of silt, silty sand and highly weathered rock and hard rock at depths of 24m below ground. The drive should remain dry as the water table sits 4-7m below ground in this region.
To help keep within the permissible damage in the mixed face conditions, especially during the delicate first 140m, Ian Morrison from UK firm Mudtech has been called upon to develop a slurry capable of supporting the tunnel face during slow advance and that is able to keep the spoil in suspension.
TBM Helen and TBM Margarita will reach Central College Station in September and October, says Brown. Soma is constructing the station but when the TBMs arrive the main excavation will not have been completed. A recovery shaft on the west side and a launching shaft on the east will allow the TBMs to leap frog the station.
From Central College the TBMs drive to Vidhana Soudha. The 350t machines will battle a full face of hard rock up to 300MPa UCS. To tackle the rock the machines can generate a propulsion force of 1160kN/m2 and a torque of 6285kN-m (see figure 3). The cutterhead has been fitted with 17-inch rear loaded cutter discs. There are eight centre cutters, 34 inner cutters, two gauge cutters, one copy cutter, 16 scraper bits and 80 cutter bits.
The 727m drive to Vidhana Soudha Station should be completed between January and March 2012. Excavation of the station pit should have been completed, says Brown, and the TBMs will be dragged through to relaunch on the eastern end.
The final drive of the eastern leg for the two Hitachi Zosen TBMs will tackle the 479m run to Cricket Stadium Station. The drive will pass through clayey silt, highly weathered rock and hard rock. Brown explains, “At the drive from Vidhana Suda to Cricket Stadium there is lots of hard rock coming up into the face. It happens about five or six times and in particular one of the challenges for us is the last 200-300m. We come round a very tight 270m-radius curve by the GPO building – a very ornate building with a large set of steps and columns to it. We sit right under the front facade of that building, it’s right in our zone of influence.”
BMRC stipulates that only negligible damage, defined as hairline cracks, is acceptable on heritage buildings. Passing through the mixed ground in close proximity to the GPO building, the tunnellers are going to have to carefully monitor subsidence as they pass through the curve.
Westbound drive
By the end of June 2012 the TBMs should have broken into the receiving shaft at Cricket Stadium. They will be dismantled and shipped back past the Majestic Station site to the west end of the pocket track for launch on the final short 227m drive. The pocket track is reversing siding that allows the trains to park off the main line. The 170m long track will be constructed using cut and cover and will be excavated using drill and split.
The short 227m drive from the pocket track cut to City Railway Station is yet another delicate stretch for the TBMs. The machines have to pass under some eight active railway lines operated by Indian Railways. It is expected to take a month to negotiate the route under the railway carefully monitoring any settlement using an array of monitors over the tracks.
Ramps and stations
The shallow alignment allows each of the stations to be excavated by cut and cover. Each station is 272m long with the exception of the far west 242m long City Railway Station which is restricted by rail lines on the east side and the ramp on the west side.
Soma’s Shri R Richardson Asir explains that the stations are being excavated by drill and blast and constructed using either the top down or bottom up approach. Soldier piles and rock dowels are used to support the excavation and the station is cast within the excavated pit.
Scientist and head of rock blasting and excavation at the National Institute of Rock Mechanics in India Dr H S Venkatesh says, “For the success of any blasting, especially in an urban environment, one needs to work on the weaknesses rather than the strengths.” In this case the weaknesses are ground vibrations, air overpressure, control of flyrock and damage to adjacent rock mass. Venkatesh experimented with different blasting patterns, explosive types and volumes as well as cussions and barriers to develop an acceptable blasting method for the excavation. At some stations, buildings are in very close proximity to the blasting works so hoarding was needed to ensure no flyrock made it off the site. The details of the research to overcome these weaknesses will be the subject of a later paper.
The excavation of the east and west ramps that join the underground section to the elevated sections is being undertaken by CEC. Venkatesh says that, “Though blasting happens to be the safest, cheapest and fastest means of excavation, many-atime urban situations may not permit blasting, and alternate means of excavation need to be adopted, which may be expensive and slow.” CEC’s Brown adds that for the east and west ramp blasting was ruled out because of urban restrictions. He explains there are tough restrictions on the number of blasts allowed each day, the size of the blast and the days you are allowed to blast. To increase the flexibility in the working pattern Brown has opted for the slower process of drill and split that uses hydraulic splitters in place of explosives to break up the rock.
At the far east end of the project, linking Cricket Stadium with the east ramp is a short 151m drive. Here there is silt and sandy silt on the tunnel alignment with some chance of weathered rock. A refurbished Robbins EPB machine was chosen for the drive. The alignment passes very close to the cricket stadium’s lighting tower leaving the tunnellers having to tread carefully to avoid having to reposition the hundreds of bulbs that light the cricket pitch. Opting for the EPB machine makes it easier for the project to keep on schedule and is better able to tackle the soft ground.
Tunnelling work for the east-west line should be completed by the end of the summer next year.
Figure 1, a typical cross section through the tunnel, units in mm Figure 2, a map of the north-south and east-west line, above ground in blue and below in red Assembly of the TBM at Majestic Station Figure 3, Details of the 6.44m Hitachi Zosen TBM, units in mm Figure 4, the east-west alignment has more than 2.5km of twin bore tunnel
