The late Professor Sunil K Munshi, formerly professor of geography, University of Calcutta, commenting on construction-driven disasters and catastrophes, was known to have been acerbic about “an engineers’ racket” calling the shots with earth scientists rarely involved with pre-construction investigations or analysis. The recent subsidence in the Bowbazar area courtesy of the ongoing construction of the 16.6 km-plus East-West Corridor project of the Kolkata Metro Rail Corporation (KMRC) is a near repeat of the cave-in less than two years ago at the same Durga Pithuri Lane.
The congestion in the area increases the trauma and, of course, the cost. The earlier misadventure on the part of the KMRC added Rs 900 crore to the cost. The fresh escalations are yet to be estimated. The machine boring the tunnel through which trains bound for Howrah Maid- an would run, apparently hit a sand aquifer, an underground layer of water-bearing sand, resulting in a massive settlement and causing damage to 18 adjacent residential buildings, according to published reports.
As in the past episode, no accountability has been determined and the only assurance is that the KMRC is investigating whether the private company, constructing the 2.45-km stretch between Esplanade and Sealdah, is responsible and how the recurrence of further cave-ins can be avoided in the already vulnerable area. The recent rains have not helped proceedings and cracks developed in buildings above the excavations, where the Tunnel Boring Machines (TBM) have been at work. Nevertheless, the rains should have been factored in and operations conducted in a manner that would obvi- ate the reported 40mm soil subsidence around a 10m radius beyond the site.
Also, it is time that questions are asked if the KMRC clearance for the project was granted despite the reservations of experts or if the opinion of the Geological Survey of India (GSI) was sought. When the first subsidence took place in 2019, Dr Mamata Desai, former head of the department of environment, Netaji Subhas Chandra Bose Open University, told a news channel that the inclusion of the strip where the cave-in happened would continue to be a risk factor and warned about repeated cave-ins or the like and rued the fact that the GSI’s or other earth scientists were not consulted, even though the GSI, in its routine study, had inferred that the East-West Corridor project is environmentally risky, indicating a high probability of cave-ins in future.
The arguments are in the public domain. In a paper published in the leading earth sciences journal of the Geological Society of India (Vol. 96, November 2020, pp. 467-474) titled 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 of the Department of Environment Management, Indian Institute of Social Welfare and Business Management, explained: “The lithofacies (facies is a body of rock with specified characteristics) maps based on grain size ratio reveal that a sand facies occurs below the sub- sided area from 15 m to at least 25 m depth and thus forms an aquifer at a relatively shallow depth. This aquifer is possibly part of a palaeo-channel, which existed in the late Pleistocene Pleistocene and later filled up”.
Their opinion was that the “aquifer continues at depth and is hydraulically connected to the main aquifer… This palaeo-channel was possibly not identified during the site investigation and geotechnical study carried out by the concerned agency. During the construction phase of the metro tunnelling project, the TBM hit this aquifer at its upper-most part leading to the dewatering of the aquifer and filling up of the tunnel. The dewatering of the aquifer resulted in a sharp drop of the piezometric surface with consequent decrease of the hydrostatic pressure and increase of lithostatic pressure in both the aquitard (clay/sandy clay) and aquifer (sand)”. An aquitard is any geological formation of a rather semi previous nature that transmits water at slower rates than an aquifer.
Arguably, speed is of the essence with KMRC. The project is to be ready by January 2023 and the latest setback will mean an eight-to-nine-month delay for the project that is to connect the Howrah Station with the IT hub at Sector, Salt Lake. While IIT Roorkee experts investigate the cause of the accident, one is reminded of the assurance in 2019 that the private contractor would be penalised if found guilty, while the KMRC would bear the cost of building houses, repairs and the delay. The contracting firm was found responsible for the 2019 subsidence and asked to pay for the additional costs, but matters have moved to the arbitrators and the issue remains unresolved. Explaining the soil conditions, Dr Sunando Bandyopadhyay, former head of the department of geography, University of Calcutta, said in a detailed note via email to this writer: “The top few hundred meters of a delta-like ours are formed of unconsolidated layers of sands, silts, and clays, with some occurrences of lateritic nodules and small pebbles. These were deposited by the oscillating rivers and ancient seas for many thousands of years. Coarser segments of these deposits ~ especially the sands ~ are good at holding groundwater and are called ‘aquifers’. The close to the surface saturated sands (not dry sands!) of today behave like viscous liquid if shaken repeatedly ~ a process called ‘liquefaction’. This makes them very unstable during earthquakes and may cause huge property loss. Vibrations at construction sites may also trigger such an event, therefore adequate precautions are always taken.” Bandyopadhyay confirms what Baner- jee and Sikdar had assessed.
The September 2019 Bowbazar cave-in was caused by puncturing a sand aquifer that occurs between 15 and 25 m from the ground and is connected to the principal aquifer hold- ing groundwater below Kolkata. Bandopadhyay explains: “Sands are usually deposited by rivers and the aquifer below Bowbazar mussel thousands of years ago. The tunnel boring machine had intercepted the sand aquifer at its uppermost layer and resulted in its dewatering and flooding of the tunnel. This led to a chain of events and resulted in volumetric land subsidence of just a few centimetres in the surface sediments ~ clays ~ lying above the sands due to compaction but that was enough for cracking and collapsing of the buildings. It seems the 2019 accident repeated itself this month as the groundwater level in the sand aquifer rose following a couple of days’ heavy downpour and the engineers was caught unawares”.
The question to be addressed is whether the subsidence was on account of natural causes as the KMRC suggests that it might be. The point is that the subsidence at the Durga Pithuri Lane area of Bowbazar has no connection with an 18th-century channel that used to flow eastward along or by the side of the modern Greek Row and Creek Lane. “The shortest distance between the subsided area and the erstwhile creek at Subodh Mallick Square is about 570 m but the vicinity of the creek had nothing to do with the cave-in”, that much, a former deputy director of Geological Survey of India, preferring to remain unnamed, admitted.
“Similarly, any movement along any fault line that may be present in the basement rocks many hundreds of metres below Kolkata cannot be held responsible for the localised cave-in at Bowbazar. However, the reactivation of such a fault would cause an earthquake and would affect the entire Kolkata Metropolitan District and the region around it.”
The KMRC is bringing Leonard John Endicott, the British engineer who had supervised the post subsidence work in 2019 again to take stock of the damage and suggest a way forward. Endicott has expressed “sur- prise” about the latest episode because readings of the progress have been sent to him regularly and he believed that the designers’ requirements were being met… though he did not know what had transpired in recent days.
The question survives about the original decision to choose a route that was fraught with such vulnerabilities and if a consultation with scientists and geologists who understand the regional hydrogeology better would have prevented the repeated human trauma and humongous cost overruns.