Bengaluru faces ‘tunnel vision’ dilemma

Underground realities

In a bid to decongest Bengaluru, D K Shivakumar, the Bengaluru Development Minister, held many meetings with experts and infrastructure specialists. In one such meeting sprang the idea of a tunnel road network for the city.

While reports say it would be along the now-junked six elevated corridors, there is nothing on record other than the minister’s statement that tunnel roads may be one of the solutions, and the feasibility will be studied along with other solutions. 

Incidentally, the Bruhat Bengaluru Mahanagara Palike (BBMP) called a tender seeking expression of interest to prepare a ‘Comprehensive Bengaluru City infrastructure plan to decongest and to prepare comprehensive traffic management plan’. The tender mentions the ‘feasibility of vehicular tunnel’, though the details of length and alignment will only be known after the proposals are evaluated and accepted.

Public transport vs tunnels

Meanwhile, the National Highway Authority of India had planned to introduce tunnel stretches to ensure signal-free traffic between Mysore Road and Hosur Road, which did not materialise until now. With tunnels in the news, the question is what kind of maintenance tunnel needs and how it will be used.

“If you see what happened with the loss of life in a small underpass near K R Circle, which got flooded, the BBMP blamed it on the drainage maintenance. Now imagine the challenge of maintaining drainage in a long network of underground roads,” says Satya Arikutharam, a mobility activist who previously worked with the Directorate of Urban Land Transport (DULT). “Does BBMP have the capacity to do it?” he questions.

Flooding was a problem even in the 1.3 km-long Pragati Maidan Tunnel in Delhi, inaugurated in June 2022. Since its opening, water leakage has been reported many times. The tunnel was temporarily closed to traffic due to waterlogging whenever it rained. 

Moreover, the tunnel also saw armed robberies, even amid CCTVs and helplines installed inside the tunnel. The fact remains that water drainage is a major challenge, even in small stretches. For more extensive stretches, the maintenance challenge will be bigger.

“More importantly, do we need an underground facility? There is no case for it. I think we are trying to treat the symptoms by adding more and more road capacity. We should treat the root cause—the land use mismatch. If we get the master plan right and follow it, we will have a scope for active mobility and public transport to thrive,” says Arikutharam.

Policymakers in Bengaluru are said to have been inspired by Singapore’s tunnel network, where surface-level planning promotes active mobility and public transport while the tunnels carry high-speed private traffic. Will Singapore’s model fit Bengaluru? There are a few things and differences to consider.

‘Aquifers will be disrupted’

“Shallow aquifers (20-100 ft) and aquifers (beyond 100 feet, recharged by shallow aquifers) in Bengaluru provide upto 600 million litres of water daily. Without this water, the city will find it extremely difficult to manage the water demand. Extensive tunnel projects will disrupt the shallow aquifers tremendously. The overall impact on groundwater in tunnel belts will be negative,” says Vishwanath S, founder of Rainwater Club and a water activist based in the city.

He adds that even the small underground bypasses and double basement parking built in the city impact the aquifers. “Now we imagine that below the ground, everything is fine. We don’t consider the importance of aquifers. Bengaluru has about 4 to 5 lakh borewells,” he explains. He adds that the cheapest water source is groundwater, which can be recharged if lakes are developed right.

When undertaking such projects, the government should realise the seriousness, carefully study the impact on water availability and minimise it, he says, adding how even Bangalore Metropolitan Rail Corporation Limited (BMRCL) had to study the impact of tunnelling on aquifers.

In 2010, BMRCL commissioned a study to the Indian Institute of Science (IISc), Bengaluru, asking the scientists to assess the impact of tunnels on groundwater facilities like borewells in the Chikpet area while tunnelling for Green Line. Since there was no major disclosed impact of metro tunnels on groundwater, BMRCL stopped commissioning such studies for future phases.

However, when the tunnelling happened under Shivajinagar, a house in the area reported a sudden borewell burst. The owner of the borewell, Khasim Qureshi, says they got a compensation of Rs 50,000 and now manage with water supply from Bangalore Water Supply and Sewerage Board.

Delhi model vs Bengaluru model

Delhi and Mumbai are the two cities that have gone for tunnel roads. NHAI is developing one more tunnel road along the Delhi-Gurgaon Expressway. The other important road tunnel is the 2-km-long coastal road in Mumbai. As per media reports, the tunnel road network in Bengaluru will be massive comparatively, but there is no clarity yet.

From a geological point of view, the terrain of Bengaluru is stable. Delhi is in the Gangetic plain and has alluvial (clay) soil. Drilling the tunnels is easy in such a terrain. Boring tunnels in Bengaluru that has hard granite rock is costly, difficult and a painful exercise that takes a lot of time, says a hydro-geology expert, preferring anonymity. However, as the city sits on a rock, Bengaluru’s terrain will be suitable for tunnels, he adds.

The expert adds that from a hydrological point of view, an assessment should be done along the planned corridors to locate the water table. In some sections, the water table could be above the tunnel route and below it in other places. Bengaluru heavily depends on groundwater. If the tunnel intersects with someone’s groundwater supply, it will cause externalities, he cautions.

The water structure and hydrology should be studied thoroughly and underground should be checked for water pumping and storage facilities for huge apartment complexes and housing societies. In cases where the water table is above the tunnel, the boring will impact fractures through which water flows, says the expert. Ultimately, care should be taken so that the water sources of locals are not affected, he adds.

From a geo-engineering point of view

Jyant Kumar, professor of Geotechnical Engineering, IISc, Bengaluru, suggests the following steps to make the tunnelling exercises foolproof: 

Avoid laying tunnels where the topsoil contains loose sand, silt deposits and soft clays. It may unsettle the overlying structures if enough precaution is not taken. 

If the tunnels are planned under lakes, rivers, canals or open drains, ensure that the bottom ground strata comprise of good quality competent rock mass. 

The faults (fractures or zones of fractures between two blocks of rock) need to be properly located, and to the extent possible, the proposed lines of the tunnels should avoid them. Tunnels should also be avoided along the bottom of the existing high-rise buildings since the foundations of such buildings are usually deep, and the zone of stress influence is much greater if it incidentally interferes with the stressed region of the tunnels, which may lead to troubles.  

Before the tunnels are planned, enough geophysical explorations must be carried out based on multi-channel analysis of surface wave (MASW) techniques, cross-borehole shear wave tests and electrical resistivity surveys. Confirmatory in-situ and laboratory tests on soil and rock samples must be performed by drilling boreholes well below the base of the planned tunnels and collecting the undisturbed soil/rock samples for strength, deformability, porosity, permeability, and also to ascertain the thickness and orientation of joints in rock media. Consulting firms with sound geotechnical engineering and engineering geology expertise should only be asked to do such ground exploration works. 

While laying the tunnels, advanced tunnel construction methods with modern tunnel boring machines need to be employed to avoid the occurrence of high values of ground volume loss (GVL) during tunnel formation. 

He says that if substantial care is taken, the existing buildings and other infrastructure lying over and around the tunnels may not face any serious issues.

Lessons from London

The plan to build highways crisscrossing London, including elevated and underground roads in London, was chalked out in the 1960s.

Some elevated highways were built. But, “The tunnels were never built, except for oddities like the tunnel under Hyde Park Corner. It became clear it was very expensive and not worthwhile. The built highways are part of the road network and help mostly private vehicles. By and large, public transport vehicles do not use them as they are not connected well to the rest of the street network for pedestrians,” says Shashi Verma, Chief Technology Officer, Transport for London.

“London and Bengaluru have roughly the same population, although Bengaluru has a land area of only about 60% of that of London and is much denser. This means carefully using the limited road space and other infrastructure is important. You cannot sustain a city like Bengaluru on private vehicles. Investment needs to go into better public transport,” he adds.

He explains the logic behind his argument. “If you build a one-lane tunnel road, that cost is comparable to a tunnel for a railway. A road will only carry perhaps 2,000 people per hour, but a railway can carry up to 80,000 people.” So the choice is simple: Spend the available money on the choices that benefit more people.

“Tunnels are usually constructed only for short distances to get around obstructions, such as to provide grade-separators, to go underwater or cross mountains where the alternatives would be a long diversion,” explains Verma. 

Tunnelling is a complicated activity that disrupts other activities during construction. He says that if it is not well-connected to the rest of the transport infrastructure, there can be long-term problems.

“Across the world, people don’t construct tunnelled roads for anything other than these reasons. Because the cost is prohibitive and the efficiency of infrastructure use is limited. What exactly has Bengaluru discovered to be pursuing this idea that other cities have not?” he questions.