BENGALURU: Will the proposed Tunnel Road between Hebbal and Silk Board Junction spell an environmental catastrophe for Bengaluru?
Experts warn so, especially if the road would be dug using Slurry Tunnel Boring Machines (TBMs).
Now the state government sources have confirmed the plan to deploy TBMs. The TBM method will flood excavation sites with bentonite slurry, a type of clay primarily known for its exceptional water absorption and swelling properties, forming a gel-like substance when hydrated.
In Bengaluru’s hydrology, this dense clay can seep into aquifers, choke stormwater drains, and coat lake beds with an impermeable layer, blocking natural groundwater recharge. Urban planners warn that once bentonite contamination occurs, the damage is nearly irreversible and will lead to locking the city into decades of waterlogging and water scarcity.
Bentonite, a naturally occurring clay that is mostly harmless in its raw form, can still cause significant environmental harm during tunnelling projects, not because of its basic composition, but due to how it is handled, disposed of, and the additives mixed with it.
Experts point out that one of the most immediate risks of using bentonite is water pollution. An expert associated with IISc explained that when bentonite slurry escapes into rivers, lakes, or groundwater, its fine particles remain suspended for long periods, turning the water turbid and blocking sunlight from reaching aquatic plants. As the sediment eventually settles, it can smother benthic (bottom-dwelling) organisms, leading to oxygen depletion and disrupting entire ecosystems. In some cases, bentonite mixtures can also slightly alter the pH and mineral balance of water, affecting sensitive aquatic species.
Founder and Convenor of Citizens for Citizens (C4C), Rajkumar Dugar pointed that groundwater contamination is another concern. “Bentonite slurry can seep into rock fractures or aquifers, changing permeability and potentially contaminating drinking water sources. This risk is heightened when tunnel boring projects mix bentonite with polymers, surfactants, anti-foaming agents, or biocides, substances that are often more toxic than the clay itself,” he said.
The problems don’t end with water, Dugar said, further explaining that on land, improperly disposed bentonite slurry can leave behind thick, impermeable layers that suffocate plant roots and prevent natural water infiltration into the soil. Over time, chemical additives may accumulate in these soils, altering microbial communities and reducing soil health.
Officials close to the tunnel project said that the slurry method, which uses bentonite, helps regulate pressure inside the tunnel and stabilise loose or water-bearing ground.
Bentonite’s swelling nature makes it particularly effective at sealing gaps, preventing water from seeping into the tunnel during construction. Beyond its performance, its widespread availability, low cost, and the fact that it is a well-understood material in the engineering community make it a convenient choice. Replacing it with alternative materials would likely drive up costs, introduce untested risks, and slow down project timelines - a trade-off that many infrastructure planners are reluctant to make.
They added that instead of bentonite, hard rock tunnel boring machines (TBMs) had been used in a Mumbai tunnelling project and that technical issues there prompted the choice of slurry TBMs for the current work.