Getting Bangalore Metro north-south line back on track 20 Jan 2016

Breakthrough into the cut and cover station at Majestic on Bangalore’s troubled north-south UG1 line marks one of a number of key milestones that indicate construction of the critical underground section of the Indian city’s new metro system is finally getting back on track.

In the end it was CEC, the contractor engaged by project owner Bangalore Metro Rail Corporation Limited (BMRCL) to complete the total 5,100m of running tunnels for the east–west UG2 line, that achieved the breakthrough, using one of two 6.5m diameter Hitachi Zosen slurry machines (TBM Margarita) procured in joint venture with Soma (India) to successfully complete the UG2 running tunnels contract. After dealing with extremely tough geological conditions, similar to those previously encountered on the UG2 project, breakthrough on the 970m long downline (south tunnel) was achieved in early November (2015).After dealing with extremely tough geological conditions, similar to those previously encountered on the UG2 project, breakthrough on the 970m-long downline (south tunnel) was achieved in early November (2015).

It is now nearly five years since BMRCL engaged the Coastal/Transtonnelstroy (TTS) joint venture to excavate the total 5,000m x 5.6m i.d. running tunnels, two cut and cover stations (Chickpet and City Market) and the north and south cut and cover ramps that together comprise Contract UG1. The 707.9 crore rupee (about US$160 million) design-build contract, signed in April 2011, should have been delivered in three years (April 2014), but the Coastal/TTS JV is now struggling to complete tunnel excavation by a much revised delivery date of March this year (2016).

Bangalore Metro Phase I features elevated sections to the north, south, east and west, of two short, but critical, underground sections, both of them centred around the system’s hub: the cut and cover Majestic Station (Fig 1). The UG1 east–west Purple Line completed a full track test in November last year (2015) of its full 18.1km length. Two elevated sections along the alignment, have begun passenger services, but delays to the underground works on the 24.2km north–south Green Line have hindered overall progress.

Unlike the slurry TBMs selected by CEC/Soma JV on the east–west underground works contract (see associated TunnelTECH article link in the References for discussion about the selection process), the Coastal/TTS JV opted for EPB technology for excavation of the north–south alignment, which is broken into four short parallel drives of 400m (South Ramp–City Market Station), 432m (City Market Station–Chickpet Station), 750m (Chickpet Station–Majestic Station) and 970m (North Ramp–Majestic Station) (Fig 2).

Two 6.45m diameter Herrenknecht EPBMs (TBM Krishna and TBM Kaveri) were procured to tackle three parallel drives, totalling approximately 1,500m each, from the South Ramp just outside City Market underground station, and into the south shaft at Majestic Station; and a Seli EPBM (TBM Godvari) was procured for excavation of both the upline and downline of the 950m-long stretch between the North Ramp just south of Sampige elevated station and the north shaft at Majestic Station.

A difficult mixed ground geology comprising hard granite and soil, high groundwater levels, and problems associated with tunnelling near the weak and shallow foundations of historic buildings in the historic Chickpet area of the city, have all contributed to slow progress.

Excavations at Chickpet Station were delayed as a result of a legal battle that ended up going all the way to the Indian High Court, and construction of the critical North Ramp, from which the Seli EPBM was scheduled to launch, was delayed until mid-2012, almost a year after contract signature, while agreement was reached with the Indian Railways Board over land use.

At this stage it was envisaged that the two Herrenknecht machines, which completed factory testing at the German manufacturer’s facilities in China in February 2012, ahead of an expected 45-day journey to Chennai by sea, would be in the ground by June of the same year. The Seli machine was expected to launch by October.

In the event, the Herrenknecht machines arrived on site in Bangalore in May and August (2012), and it was not until October that the first of them finally launched out of the South Ramp for the short 400m drive to the southern end of the cut and cover City Market Station. The second machine launched a month later, in November.

Launch of the Seli machine out of the North Ramp on the first of its scheduled two drives was even further delayed, to June 2013, some 26 months into the 36 month underground works contract.

It is now 39 months since launch of the Herrenknecht machines, and to date neither one has made final breakthrough into Majestic Station. TBM Kaveri began its final drive in February last year (2015), while TBM Krishna launched last month (23 December 2015) out of Chickpet Station for its third and final 747m-long drive.

Time spent excavating the 432m-long City Market–Chickpet drive account for much of the delay, with TBM Kaveri taking 12 months from launch in October 2013 to achieve breakthrough, and TBM Krishna taking 10 months longer from launch in November 2013 for completion of the parallel drive in August 2015.

Sharad Kumar, Coastal Director of Operations, told TunnelTalk: “In this stretch, both TBMs encountered huge boulders, and in one particular location concrete reinforced blocks. This seriously damaged the TBM cutterhead and resulted in a stoppage to carry out repairs under hyperbaric conditions.

“After these repairs were completed the TBM then encountered a stretch of mixed geology and here there was very slow progress through the boulder zone, mainly due to the need for regular hyperbaric interventions. Each of these interventions was preceded by grouting in the work area. We had hoped to use grout blocks along the alignment, but due to the congested nature of the residential area above, this option was not possible. We therefore had to operate the TBM with less thrust and at a reduced RPM, which in turn resulted in a reduced advance rate.”

In February last year (2015), Robbins was called in by the Coastal/TTS JV to assist with operation and maintenance of the two Herrenknecht EPBMs. Since its engagement, Robbins has had a permanent field service team of more than 70 personnel on site to oversee the maintenance, repair and relaunch of both machines out of Chickpet. The Robbins team continues to supervise all aspects of TBM excavation, segmental lining, mucking out and grout plant operations for the final pair of drives.

A Robbins spokeswoman confirmed to TunnelTalk that TBM Kaveri is currently 575.5m into its final 744m drive, since launching in March (2015). Sister machine, TBM Krishna was relaunched last month (22 December 2015) after undergoing 112 days of repairs and testing. It has currently advanced 25m.

Kumar of Coastal explained: “In order to cope with the very difficult geological conditions in Bangalore, it was felt necessary to bring in the best available expertise. Herrenknecht was unable to offer an operational contract due to their own policy issues, so we approached Robbins to take up the subcontracting work – which they agreed following discussions with the client. Herrenknecht has been instrumental in providing us with the complete support that was needed to refurbish the machines prior to launch on the final drives from Chickpet to Majestic, and during the very difficult drives through the boulder zone [between City Market and Chickpet].”

Meanwhile, the Seli machine, procured by the Coastal/TTS JV for excavation of both the upline and downline of the 950m-long North Ramp–Majestic Station section, found itself in difficult ground right from the start, advancing 352m through a granite/soil mixed face geology in the 13 months following launch in June 2013. In July 2014 it suffered a fatally damaged cutterhead after encountering large boulders and highly fractured rock at about 350m into its drive.

The recovery plan required excavation of a 20m deep access shaft ahead of the cutterhead, but even before this setback occurred a decision had been taken by the client, BMRCL, to try and minimise project slippage by mobilising CEC to complete the downline using one of the Hitachi Zosen slurry TBMs that had successfully completed excavation of the east–west UG2 contract in March (2014).

“The original plan was for the Seli machine to drive from North Ramp to Majestic and then turn around for the parallel drive back to the North Ramp, but due to delays in handing over land [at North Ramp] it became clear that TBM excavation would be delayed,” explained Kumar.

“At that point in time one of the slurry TBMs became available [after finishing its drives for CEC on the east–west UG2 contract] and this machine was subsequently engaged for the North Ramp–Majestic parallel drive. The decision to engage CEC was made well before Seli TBM became stuck.”

“For the record, it should be noted that the slurry TBM also struggled for 5–6 months in the same stretch in which the Seli EPBM had suffered damage.”

After 12 months of recovery and repair work, including manufacture in Italy, by Palmieri, of a new cutterhead, and subsequent transportation by sea to India, the stricken Seli machine finally resumed tunnelling in July last year (2015). Coastal reports that the machine has since advanced a further 300m from its breakdown position and is now only 300m from breakthrough into Majestic Station for completion of tunnelling on the northern section of Contract UG1.

Phase I of the Bangalore Metro is currently two years behind schedule, and 2,200 crore rupees (US$325 million) over its original 11,600 crore rupees budget (about US$2.64 billion in 2011 prices). The latest completion date given by BMRCL is March 2016. While some parts of the elevated sections are already in service, and others are ready for testing, delays in the construction of the critical north–south UG1 contract will inevitably mean further delays.

But in spite of the challenges its EPBMs have faced, Coastal sticks by its choice of EPB technology, even though the geological conditions actually encountered have differed from what the client’s pre-excavation geotechnical studies had predicted.

“Many aspects were considered [prior to machine selection], and discussions were held with TBM manufacturers, experts, designers and general consultants before a final decision was made on the type of machine we would use to execute the UG1 contract,” said Kumar.

“Studies were carried out by our team, taking all the available data concerning geographical and geological conditions into consideration. Technical enquiries were floated to the TBM manufacturers and [on the basis of all of this] it was decided to work with EPBMs. Ultimately it was the decision of Coastal to select EPB technology, and we would not take back that decision. All the machines supplied to us are world class: the stoppages were mainly due to financial challenges rather than technical problems encountered by the EPBMs.”

Kumar added: “The ground conditions along the alignment have thrown up surprises. For example, we encountered soil and boulder areas along the alignment that were unexpected – this feature was not brought out in the preliminary investigations. Floating boulders of up to 2m continued along the tunnel alignment, and we have faced hard granitic rock/soil mixed face conditions. All three TBMs engaged for the project, and which were designed according to the preliminary geological data made available to us, have encountered a tough geology.

“Coastal has made its name in the tunnelling business and is involved in the execution of tunnels across a range of geological and geographical profiles, but this is the first use of EPBMs in Bangalore. TunnelTalk readers should be aware that TBM performance increases with time as the operational team gain experience. We believe that we have learned very important lessons on the Bangalore Metro north–south drives that will help us in reducing delivery time for future underground works of Phase II of the Bangalore Metro.”

UG2 east–west tunnel contract

Excavation of the UG2 east–west running tunnels by the CEC/Soma JV began in March 2010 and was completed in March 2014, inside a revised schedule of December 2014 that was granted as a result of a number of factors including delayed access to site.

A separate contract for excavation and construction of the interchange station at Majestic was awarded to the GYT/Coastal JV, with work now entering the final phases ahead of fit out.

Phase 2, 2A and 3

In the meantime, Government approval has been granted for construction of the 26,405 crore rupees (about US$5.28 billion) Phase 2 of Bangalore’s Metro system. This will add new elevated extensions to all four sections of Phase I, plus two new lines of 18.8km for the Yellow Line and 21.2km for the Red Line for a total addition of 72.1km to the existing 42km of Phase I. An initial sum of 468 crore rupees (about US$69 million) has been released to advance the project towards construction procurement.

The planned north–south Red Line between Gottigere and Nagawara includes 14.2km of twin running underground alignment, and 12 underground stations, between Bangalore Dairy and Nagawara (Fig 3). The underground section is likely to be divided into three, broadly equal-sized, lots on either a build only or design-build basis. Coastal told TunnelTalk: “We are contemplating tendering for Phase II of the Bangalore Metro, and we believe that our execution experience [on Phase I] will put us in a strategically better position.  Our understanding is that BMRCL may begin the tendering process for Phase II by September this year.”

Phase 2A, comprising a 23.4km extension from Nagawara underground station at the northern terminus of the Red Line, for connection with Bangalore International Airport, is currently at the feasibility stage, but the minutes of the 2015 BMRCL Board’s Annual Meeting confirm that a team of design consultants engaged by the owner has narrowed an initial choice of five possible alignments down to two. It is expected that at least the section leading to the airport will require tunnelling.

Phase 3, again at the feasibility stage, involves construction of a new circular line, though details on this are yet to emerge.