Over the last month more than 10 buildings in Kolkata have been found to be tilting, that is, leaning one way or the other from the perpendicular. And there is no doubt there are more, including buildings whose tilt may have gone unnoticed in the recent past. What is happening? Several experts told The Indian Express that the leaning buildings represent the combined manifestation of technical failures, administrative negligence, and socio-economic pressures in the city. This explainer unpacks key issues in this extraordinary problem, and explores what can be done to ensure safer, more sustainable development. Soil story: Impact of soil quality on construction Soil movement and settlement is a critical concern in Kolkata, several experts said. Kolkata’s primarily clayey soil poses a significant challenge for construction. The soil should ideally be allowed to settle before building; when that does not happen, the risk of subsidence (sinking) increases. Large parts of the city, including Salt Lake, sit on alluvial and lacustrine soil (river deposits). This type of soil, when mixed with silty sand (a combination of fine and coarse grains with high water content), stabilises. While Salt Lake's development involved a period of pre-construction soil settlement according to regulations, the underlying soil at greater depths remains weak, Debasish Bandyopadhyay, a professor of structural engineering, said. Mixed soils too, can pose a problem, Bandyopadhyay, who has carried out research on leaning buildings and soil-structure interactions, said. "Different soil types within a single plot can lead to differential settlement, where one part of the building settles more than another, causing it to tilt," Bandopadhyay said. Testing failures: Soil tests overlooked, poorly handled Understanding the soil is of paramount importance for proper foundation design. However, soil testing is often skipped, or compromises are made with it, experts said. Bandopadhyay said the cost of testing the soil is a deterrent in some cases. "Economical, low-quality tests fail to provide an accurate estimation, but thorough testing is sometimes seen as expensive. The lack of reliable soil information leads to inadequate foundations, setting the stage for future problems," he said. Poor foundations: Saving costs, increasing the risk When a building comes up in close proximity to an older building, the cutting of the soil and its lateral movement can impact the base of the older building if proper protections are not taken. Piling – a construction technique that transfers the load of the building deeper into the earth – is critical when constructing a tall or high-rise building on a comparatively small plot. But piling adds to the cost, and many developers avoid this to keep the project profitable, experts said. No single cause: A multi-front failure An expert likened the leaning or tilting of a building to a symptom of "multi-organ failure", with responsibility lying on multiple parties. They are often the result of lapses on the part of the administration, engineers, and developers, apart from factors arising out of compulsions of costs and the housing market. “There are economic aspects, technical aspects, and administrative lapses that combine together, leading to weak foundations that result in buildings leaning or collapsing. Many buildings are erected with almost zero gap between them," a structural engineer said. Another engineer said merely testing the soil may not help if other concerns remain. “[The building could be compromised] due to a lack of understanding of soil conditions or a desire to cut costs. Because of compromised structural designs, even with soil test results, engineers may not design structures appropriate for the specific soil type,” the engineer said. "Builders often prioritise profit over safety, cut corners by avoiding essential measures like proper piling or foundation reinforcement, or build only a few columns… They capitalise on the demand for housing, often neglecting the structural integrity of their projects,” he said. “The focus”, he added, “is often on aesthetics and superficial features such as size, colour, or interior design, rather than the crucial structural elements of a building.” Demolitions: A temporary fix, not a solution Pulling down a building that is leaning dangerously is a reactive measure that addresses the crisis at hand, but it is not a solution to the larger problem. "While it might appease public concerns temporarily, it doesn't address the root causes. The focus should be on preventing such situations in the first place. The Garden Reach area (in southwestern Kolkata by the bank of the Hooghly) and even parts of Newtown (a planned satellite city that has come up in an area that once comprised cultivated lands and water bodies) illustrate the dangers,” said another structural engineer. “Newly developed areas, often built on former agricultural land, present unique challenges. Previous pathways, now indistinguishable from the surrounding fields after leveling, can lead to uneven settling if foundations are placed across them,” the expert explained. Earthquake awareness: An important flag According to the Seismic Zoning Map of India published by the Bureau of Indian Standards (BIS) in 2002, Kolkata sits at the margin of seismic zones III and IV. In a written reply provided to Lok Sabha on July 30, 2021, the government said Kolkata is placed in zone IV, after the seismically-most-active zone V. The proliferation of poorly constructed buildings exacerbates the risk of earthquake damage, especially in densely packed areas, where buildings are erected with minimal spaces in between. “In terms of hazard zones, we (Kolkata) were in zone III, and now we are in zone IV in the proposed draft code, which means we are at higher risk from earthquakes. It is all the more important that we construct technically and structurally strong buildings," Bandopadhyay said. Way forward: How to ensuring a safer future MULTI-PRONGED APPROACH: A retired municipal official said, "Municipal regulations must be rigorously enforced. Thorough inspections are crucial at every stage of construction, not just checking credentials. Improved soil testing is important, including mandating and subsidising comprehensive soil testing to ensure accurate data for foundation design.” It is crucial to hold all stakeholders – administrators, engineers, and developers – accountable for adhering to building codes and prioritising safety, the retired official said. AWARENESS: Educating the public about the importance of structural integrity over superficial aesthetics is key, experts said. ROLE OF EXPERTS: Structural engineers play a crucial role in ensuring the safety of buildings. Their expertise must be utilised to understand soil conditions, design appropriate foundations, and create earthquake-resistant structures, several officials and experts agreed.
