Land Subsidence due to Leakage of Aquitard-aquifer Pore Water in an Under-construction Tunnel of East-West Metro Railway
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Land Subsidence due to Leakage of Aquitard-aquifer Pore Water in an Under-construction Tunnel of East-West Metro Railway Project, Kolkata S. Banerjee* and P. K. Sikdar Department of Environment Management, Indian Institute of Social Welfare and Business Management, Kolkata - 700 073, India *E-mail:[email protected]; [email protected]
ABSTRACT Palaeo-geographical conditions control the sedimentological and hydrological heterogeneity within the upper aquitard and late Pleistocene aquifers beneath Kolkata. Such complexity at Bowbazar was unmapped prior to the Tunnel Boring Machine (TBM) of East-West metro project which hit the local aquifer at 14 m depth. This led to development of cracks in tens of buildings due to land subsidence; few of which collapsed immediately, and some stood in such a precarious position to be eventually razed. Mass evacuation was undertaken from the area. This paper assesses the local hydrogeological conditions and hydrostatic pressure of the aquitard-aquifer system that caused the disastrous land subsidence. INTRODUCTION Presently, about half of the world’s 7 billion population lives in urban areas having 36 megacities supporting more than 10 million inhabitants. By 2025, the world’s population is expected to exceed 8 billion; 58% of which are predicted to be urban dwellers. Therefore, infrastructure development including the communication system of urban areas needs to be enhanced which, in recent years, has come in a big way in the developmental effort for many areas in India. Having understood the population growth, traffic congestion and to ease the mode of communication, Kolkata Metro Rail Corporation Limited (KMRC), had started constructing an East-West (E-W) metro track from Howrah Maidan to Sector-V of Salt Lake city by a combination of underground tunneling and elevated corridor (Fig.1). Other Indian cities that have metro connectivity are Delhi, Mumbai, Bengaluru, Hyderabad, Chennai, Kochi, Jaipur, Lucknow, Noida, Gurgaon and Nagpur. Besides 15 other cities in India have over 500 km of metro lines under different stages of implementation. They are: Ahmedabad, Pune, Indore, Bhopal, Varanasi, Kozhikode, Vijayawada, Meerut, Visakhapatnam, Kanpur, Agra, Coimbatore, Patna, Navi Mumbai and Guwahati. The underground part of the present E-W Metro is constructed by tunneling using an Earth Pressure Balance Tunnel Boring Machine (TBM) along the alignment and underground stations constructed by Top-Down Cut and Cover Method with diaphragm wall. The depth of the underground stations ranges from 17.5 to 31.3 m below ground level (bgl); therefore, the stations and viaducts are connected cutting the upper confining clay/silty clay bed having an average thickness of 40 m (Sikdar et al., 2001). Groundwater in the Quaternary aquifer of Kolkata occurs in a confined to semi-confined condition. Coarse clastic deposits consisting of fine to coarse sand, at places mixed with gravels, form the aquifer, above and below which lie clay sequences (Sikdar et al., 2001). The aquifer continues down
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