At least two unique features describe the much awaited underground railway connection between Calcutta (Kolkata) and Howrah, two large cities situated on either side of River Hooghly.
The idea was put forward by a Bengal-born British engineer Sir Harley Hugh Dalrymple-Hay in 1921 and its twin tunnels under River Hooghly are the first of its kind in India.
It has taken almost a century to realise Sir Dalrymple-Hay concept as the East West Metro Railway will have a significant and far reaching impact on Kolkata's transportation system in years to come and the way people commute between these two congested city centres.
Around 30 members from the ICE Kolkata Chapter recently visited the tunnels under construction.
The East-West Metro Corridor Project connects the East of Kolkata (Salt Lake Sector-V, Kolkata's IT Hub) with the west end i.e, Howrah (Kolkata's twin city).
The whole project is being executed by Kolkata Metro Railway Corporation Limited (KMRCL) with partial financial support from JICA. The entire stretch is divided into three parts: UG-1 (underground portion from Howrah Maidan to New Mahakaran), UG-2 (underground portion from Esplanade to Subhash Sarobar) and elevated portion from Subhash Sarobar to Salt Lake Sector-V.
This field trip was for the tunnel portion under UG-1 contract package. This UG-1 contract package is being executed by TransTonnelstroy-AFCONS Joint Venture (TTAJV). A consortium (MYCEL) led by AECOM is the General Consultants (GC) to KMRCL.
The works include 2-tier Howrah Maidan Station, one cross-over near Howrah Maidan Station, a 4-tier Howrah Station adjacent to present Howrah Railway Station (Kolkata's Railway Hub to rest of India) to provide interface between underground and surface railway transportation for commuter traffic coming to the cities from suburbs.
Under the project, there are twin 5.5m tunnels that are 3 km long and a vent shaft/emergency exit on Kolkata's side by Hooghly River. The tunnels under the river pass through mostly clay silts at a depth of 11-15m below the river bed and under submergence of 13m average tide water in the river.
There are EPBM type two Tunnel Boring Machines (TBM) at work from Herrenknecht, Germany (named "Rachana" meaning "Composition" and "Prerana" meaning "Encouragement"). The tunnel is being constructed with pre-cast segments (5 regular segments and 1 key) of M50 grade concrete along with all necessary arrangement to ensure waterproofing, resistance to earthquake and floatation for a design life of 120 years.
During tunnelling (other than utilities and land issues), the biggest challenge is to ensure the safety of buildings and structures above ground (some of them are almost a century old and remnants of the colonial era) with suitable preventive measures. Tunnels have successfully crossed Howrah DRM building with nominal damage and it is considered to be one of the key achievements in this project.