Amid the hustle and bustle of India’s fourth largest city, a pattern emerges: traffic gridlocks the city’s arterials. During peak hours nearly half of all roads in the city are at or over 100 per cent capacity. Aging rail lines, built in 1985, are crammed with people. Chennai’s eight million inhabitants take, on average, 11 million trips per day, and six million of those are by car.

Despite those challenges, Chennai was named by travel giant Lonely Planet as one of the top 10 cities to visit in the world. The multi-cultural and progressive melting pot on the coast has consistently ranked in news outlets like the New York Times and the BBC as one of the most desirable cities to live in.

Because of these factors, project owner Chennai Metro Rail Limited (CMRL) is faced with a daunting challenge: revamp the city’s rail transport system, using underground space where possible.

According to CMRL, the situation is critical: "Chennai Metropolis has been growing rapidly and the traffic volumes on the roads have also been increasing enormously. Hence the need for a new rail-based rapid transport system has been felt and towards this objective the Government of Tamil Nadu have decided to implement the Chennai Metro Rail Project.

"This project aims at providing the people of Chennai with a fast, reliable, convenient, efficient, modern and economical mode of public transport."

To achieve that end, CMRL developed a plan for two new rail routes: a 23.1km-long line from Washermenpet to the International Airport, and a 22kmlong line from Chennai Central to St. Thomas Mount. Of those new lines, a total of 24km will be underground, a little more than half of the 45.1km being built. The lines travel through heavily populated zones and through the city’s varied foundation of dense clay, shale, and sandstone.

Multiple TBMs designed for mixed ground conditions were sourced for underground stretches between stations.

Getting started and TBM launch
Line 1 is the priority for CMRL, extending from Washermanpet to the Airport, with a goal completion date set at the end of 2015. Contractor Afcons was named to build two parallel 1km tunnels along the route, comprising Lot UAA-01. The contractor chose a 6.65m diameter Robbins mixed ground EPB for the two tunnels.

The machine was designed to excavate weathered granite, sand, silt, and clay with boulders up to 300mm in diameter. The specialised design uses a combination of 17-inch diameter disc cutters as well as soft ground tools. Small grippers located around the circumference of the machine’s shield allow for cutterhead stabilization in harder ground, and additionally react the forces needed to pull the cutterhead back from the face in difficult conditions.

Launch of the machine took place in January 2012 from a 28m deep starting pit, with the machine encountering challenges from the outset including unexpectedly hard rock. "The TBM bored mixed face conditions consisting of varying strengths of granite ranging from completely weathered and highly fractured through to hard granite with strengths estimated at 150MPa. This resulted in high cutter consumption due to impact damage as the cutter discs rotated through relatively soft rock and encountered the much harder rock," explains Jim Clark, Robbins projects manager – India.

Robbins field service personnel and engineers assisted Afcons in remedying the problem. "Robbins India provided a geologist who carried out face mapping for the whole of the first drive, in both hyperbaric and open mode conditions on a daily basis," Clark says. The facemapping process determined that much more rock was present than expected, and with high abrasivity.

Changing ground conditions
In a report, the geologist describes face-mapping at chainage 5,100: "Face mapping of the tunnel face in this area has revealed high grade fresh granite rock on the invert area (180 – 200MPa ball park value), and weathered schist rock on the crown area of the tunnel face, which was unexpected."

At chainage 5050, even harder rock was found: "Fresh and hard granite was found and estimated as around 300 MPa UCS. There were no fracture sets on it, no zones of weakness observed on it." Face-mapping continued to find fresh, hard granite in key sections of the tunnel route.

The data not only assisted the crew in operating the TBM, but also provided a comprehensive geological record for the second drive.

With the data gleaned from the geological investigation, Robbins was able to advise Afcons on the optimal operating parameters to get through the difficult conditions, including cutterhead RPM, thrust pressure, penetration rate, and cutterhead torque.

"Once these parameters were implemented, the cutter consumption was drastically reduced," said Clark. As an example, he describes the changes in cutterhead RPM and Rate of Penetration (ROP). "In the residual soil at the start of boring, cutterhead RPM was set to around 2.0.

In the mixed face conditions of hard rock and soft ground, the cutterhead RPM and ROP was reduced to prevent impact damage to the cutter discs. A baseline speed of 15m per minute for the outer diameter of the cutterhead was used as a guideline.

This equates to an RPM of approximately 0.75 for the sections of difficult ground. The baseline ROP was approximately 12mm. These parameters were increased as the machine entered a full face of rock and the risk of impact damage lessened."

Robbins field service continued to assist throughout the drive, training the crew and acting as mechanical and electrical/PLC supervisors on the project. Ultimately, the bore was a success, with the TBM achieving rates of up to 44.8m per week.

On 8 July 2014, the TBM broke through at Chennai Metro, capping a challenging drive that saw a full spectrum of difficult conditions. The 1,063m long drive ended in a receiving shaft so the machine could be removed and readied for its second parallel drive. This time, many parameters were known.

Second set
After refurbishment to the screw conveyor and cutterhead, the machine was launched to bore its second tunnel in February. As of late April, the TBM was at ring 90 and encountering similar conditions to the first drive. "From the machine launch to its current position the machine has gone in a lot faster on the second drive," says Chennai project manager Paul Imri.

"We hope we have solved many problems from our experience at the previous drive."

Imri estimates the machine will take about eight months to complete its second tunnel, however even though the geology is known, that doesn’t mean that tunnelling activities are easy.

"The parameters are constantly changing, just like the first drive.

"The ground condition now is sand at the top, compact sand in the middle and soft rock at the bottom [of the excavation face].

"We try to run at less than 3,000kN torque and 18,000kN thrust in such conditions."

The new tunnel route, traveling just 9.5 to 26m below houses, buildings, and a river, also brings the TBM closer to sensitive structures.

"The machine will pass below an old historical church and many houses. There are many bore holes being taken to check for water.

The drive is also being monitored from the surface by surveyors who are checking for settlement. If they detect any problems at all, action will be taken," Imri says.

Despite the areas of difficult work ahead, Imri is confident that the new operational parameters and previous knowledge will see the complex job through to success.

"We have face mapped the entirety of the first drive so we can anticipate any problems.

"We will adjust and be watchful in particular zones of concern. We believe we now know the geology and can act accordingly"