On a hot summer’s day in Bangalore, ambassadors, politicians, lobbyists, financers, press and the cream of the Indian tunnelling community crowd gather excitedly in 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 adoration from the baying crowd.
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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.
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.
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By GlobalDataYeddyurappa claims 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%.
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 daily. 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 ten 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 the Far Eastern Group.
The 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 cost for the project has not been agreed. There are some quiet concerns this could slow the commissioning of the project but for now the focus is on the tunnelling works.
Project manager for CEC Russell Brown says 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.
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 and relaunch.
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 says “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 subsistence 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’ll 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 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 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 says 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 and 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-a-time 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.
Tunnelling work for the east-west line should be completed by the end of summer next year.
First published in the July 2011 edition of tunnels & tunnelling.
