THE ‘PINK CITY’ of India’s Rajasthan State, Jaipur is five years into its first mass rapid transit project: the Jaipur Metro. Appropriately called the Pink Line, this elevated route entered operation in June. And already an extension is underway. As with many Indian metro projects, the decision to go underground is only made when essential. Jaipur is no exception. The extension to the Pink Line is tunnelled, but only because it passes under the historical Chandpole Gate into a heritage area of the city. The elevated section of the Pink Line ends some 120m short of the gate, which also marks the start of the twin-bore TBM drive from the existing Chandpole Gate terminus, to the new end of the extension at Badi Chauper.
PROJECT OUTLINE
This extension, Phase 1B of the Pink Line, calls for 2.3km of twin bore tunnel at 5.8m internal diameter, lined with a 5+1 275mm-thick tapered ring; and two underground stations to be constructed by contractor CICI. Two Robbins EPBMs set up for soft ground conditions were refurbished from the contractor’s previous job on Delhi Metro’s Phase Two: package BC-16.
Geology consists of alluvium silty sand along the entire length, with some gravels (but not in consolidated patches) and the bore is entirely above the water table. These conditions are similar to the Delhi package, but a curve with a 392m radius prior to reaching the gate called for active articulation capabilities, whereas the previous job was just a straightforward drive. Jim Clark, Robbins project manager adds: "It’s quite a big job, cutting the machine in half around the centre section, putting another section in and forming the articulation joints. The parts were made overseas and installed at the contractor’s casting yard in Jaipur. We also installed a new a+b grouting system and an Enzan Tunnel Guidance & Logger System for position monitoring."
The two stations will be constructed top-down after the TBMs have passed through the stations’ diaphragm wall framework, which will be complete before they arrive. Overburden will range from 5.8m after the launch shaft then a constant downwards alignment to about 10m at the reception shaft. But the real challenge comes during the underpassing of the Chandpole Gate, its foundations just some 4.5m above the crown of the TBM.
Gate approach
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By GlobalDataChandpole Gate is 97 rings into the drive according to CICI project manager Chris Cooper, and the Zone of Influence is 10 rings either side. This is just enough time to get used to the boring parameters and make sure settlement is under control to pass the gate within the 5mm trigger levels.
However, there were initial concerns. "We only have one diameter (or less) above the tunnel," says Cooper, "and when we launched we experienced some friction over the top of the machine, which dragged the ground forward and caused heave."
Robbins’ Clark says, "On the alignment leading up to the gate we had monitoring points every 1m and we were getting heave of 25mm or more in places. The soil completely lacked elasticity, which combined with the low overburden, was causing the heaving. Clay is forgiving, but this silty sand is not. There was a bit of trial and error to get everything in balance." The teams had launched the first machine on umbilicals due to the small launch shaft, and the plan was to approach the gate, stopping short of the Zone of Influence, before dropping down the backup gantries. During this initial phase of boring, there was leeway to experiment with trial and error, and it was seen that when the articulation was used for vertical steering, it was worsening the ground heave.
Cooper says, "To solve this we basically put the articulation back into line, and we injected bentonite through the skin to reduce the friction between the sand and the TBM skin. We gate’s Zone of Influence. CICI employs a skilled but young crew of local engineers with expat managers and supervisors, so control of the TBMs during this time was given to Robbins. In India this can be a common procedure, with some sites having as many as 200 manufacturer personnel undertaking work, and others having just one or two supervisors. In this case, 18 were employed.
Clark elaborates on the work: "Due to the low cover we experienced heave of up to 25mm over the TBM when using the articulation for steering, so we closed up the articulation, and reverted to steering the machine traditionally, by use of the main thrust cylinders and copy cutters. Bentonite was injected around the shield to reduce the frictional forces between the ground and TBM. We gradually improved the settlement performance with boring parameter tweaks and settled on face pressure at 0.9 bar, cutterhead rpm at 0.8 and thrust pressures around 22,000- 26,000kN.
"We kept an eye on settlement with real time monitoring and managed to achieve values of plus/minus 2mm which was a big improvement from 25mm. Because it was a short startup shaft and we were working on umbilicals we could only use one muck car rather than the four cars required to complete a 1.2m ring. So, we bored 400 mm, and then took readings on the surface along the alignment. This information would was then fed back to us and we’d tweak our operating parameters accordingly" Cooper adds: "It was an extensive investigation into the ground and gate foundations and a couple of additional consultants got involved. The engineers got the parameters that were needed, and then went through."
The second machine will pass under the gate at 14.6m (centre line to centre line) from the existing bore so interaction is not expected.
Progress
As Tunnels and Tunnelling went to press, the first TBM had produced 200 rings and was safely beyond the gate. The first permanent ring was formed on 28 May of this year. The second machine was due to launch in mid-October. According to Clark, with a bit of luck tunnel excavation will be finished on the first bore by May 2016, and the second three months later.
With excavation now moving beyond the main challenge on the first bore, and the second bore expected to proceed in much the same way, a successful completion seems likely to the teams.
Logistics are still a laborious challenge in the busy city, and site supply and muck removal can only be undertaken at night, but as Jaipur was the first planned development layout for a city in India, this is not as onerous as it might be. The city was built in 1724 with an American style grid system, and the site footprint rests in one of these crossroads.
The teams are currently still boring with one skip and best performance has been 10 rings in a day. Some 300m per month average is expected.
Operation
According to Tunnels and Tunnelling sister publication Railway Technology, Bharat Earth Movers Limited (BEML) was awarded a INR3.18bn (USD 57M) contract in December 2011 to deliver ten four-car train-sets for the Phase 1 of Jaipur Metro. The first train-set was rolled out at BEML’s factory in Bangalore in April 2013. It is the first indigenously designed and manufactured electric multiple unit by an Indian company.
Alstom was awarded a contract by DMRC in April 2012, to supply an Urbalis train control system to EW-A section of the new Jaipur metro Phase I. Alstom is responsible for the design, production, installation, testing and commissioning of entire train control, traffic management and signalling systems under the contract. Urbalis is an automated train operation and traffic management system allowing safe and energy-efficient train services on the metro rail network.
Thales received a contract from DMRC in April 2013, to supply an advanced passenger information system for the Jaipur Metro. The Thales passenger information system can be integrated with the traffic management system and will deliver accurate visual and audio information to passengers.
