Kolkata TBM failure causes extensive damage 05 Sep 2019

Tailseal failure on the EPBM operating on a running tunnel drive for the Kolkata Metro is reported to be the cause of an uncontrollable inrush of water and material that led to major damage and collapse of buildings. Having started in the evening of Saturday 2 September, the inrush took until end of Sunday 3 September to control, with residents evacuated from dangerously damaged and collapsing homes and businesses in the central Bowbazar area.

Uncontrollable ground-loss settlement caused wide spread damage and collapse of old buildings
Photos by local photographers

The incident occurred 1.6km into the lead heading of the 2.45km running tunnel drives for the new East-West Metro line, which will run from Esplanade to Sealdah Stations (Fig 1). According to senior executives of the Kolkata Metro Rail Corporation (KMRC), the machine had just taken a tight radius turn into an alignment under Ganguly Street, placing severe pressure on the three-row wire brush tailseal. The turn occurred as the machine entered a reportedly unexpected geological deposit of highly water-charged sand. The inrush of water and sand through the damaged tailseal – described as a ferocious flow – quickly became uncontrollable, flooding the tunnel and the TBM, and causing collapse damage on the surface.

Fig 1. Incident occurred on the drive between Esplanade Station, at the Central railway station, and Sealdah Station on the new East-West metro line for Kolkata

The Esplanade to Sealdah drives are part of the eastern 4.5km section of the new metro line that runs between Esplanade to the transition ramp of an elevated guideway to Salt Lake Sector V and are being advanced by the ITD/ITD Cementation Ltd JV. The JV is using two refurbished EPBMs that ITD procured from Herrenknecht some 10 years ago for work on the Delhi Metro. The lead machine launched from the Esplanade Station box in February 2019. The twin machine launched about a month later and is currently about 300m behind the troubled TBM.

Both TBMs are at an indefinite standstill awaiting work to stabilise the area of collapse, assess the condition of damaged buildings, and secure the tunnel drive. More than 450m³ of concrete was injected from the surface and from within the tunnel to stop the inflow of sand and water and continued for the following days to fill the ground-loss void.

A senior management spokesman for KMRC told TunnelTalk in a telephone interview today (Thursday, 5 September 2019) that “not all is yet stable but the settlement increase has stopped.” In addition to the volume concrete pumped into the collapsed area, “a bulkhead of poly-fibre reinforced shotcrete was installed at the end of the 75m TBM to prevent water flowing back into the working shaft. Another bulkhead of cast concrete is now being installed about 5m behind the first.” The spokesman confirmed that, “the 6.35m diameter TBM is completely flooded” and that “it is not known the state of the 5.8m i.d. segmental lining ahead of the shotcrete bulkhead.”

Managing Director of KMRC, Manas Sarkar, explained to local media that, “in normal situations [when encountering poor ground conditions] we inject a mixture of concrete and certain chemicals to plug the holes. But this time, no matter how much we pushed the mixture in, it was getting washed out. After we failed to bring the situation under control, we began evacuating the homes.” Officials told local media reporters that the aquifer of sand and water could not be identified during pre-excavation ground surveys beneath the streets of one of the oldest and most dense urban areas of the city.

The Esplanade to Sealdah drives are the last section of the East-West Metro Line 2 to be completed. The running tunnels from the elevated section transition ramp, through the Phoolbagan underground station and on to Sealdah Station were completed earlier by the ITD/ITD Cementation JV using the same EPBMs.

Specialists called in by KMRC to assist control and recovery of the incident include supervisors who worked with the Afcons-Transtonnelstroy JV to successfully complete 3km of twin running tunnels for the western contract of the East-West Metro Line, including the 520m drives under Hooghly River (Fig 2). The Afcons-Transtonnelstroy JV used two new Herrenknecht EPBMs to connect three underground stations from Howrah Maidan Station in the west to Esplanade Stations in the east.

The twin under river drives, at up to 15m below the river bed and 30m beneath the water surface, started in April 2017 and the last ring of the second drive was erected 66 days later in June 2017. The Afcons-Transtonnelstroy JV is now progressing construction and fit out of the underground stations at Howrah Maidan, Howrah, and Mahakaran.