Two days following the launch of the ambitious Rs 6,695-crore project by the Devendra Fadnavis administration, aimed at alleviating congestion on the Mumbai-Pune Expressway, a significant landslide disrupted operations at the expressway’s most critical congestion point. This incident has prompted widespread inquiries into whether the situation could have been averted.

The landslide, which took place on July 6, at the entrance of Tunnel 2 of the Mumbai-Pune Expressway, was triggered by relentless monsoon rains that inundated Maharashtra, transforming roads into torrents. This closure lasted nearly 18 hours, forcing traffic back onto the older Mumbai-Pune Highway and causing severe disruption for thousands of commuters.

The Maharashtra State Road Development Corporation (MSRDC) has attributed the landslide to an unprecedented volume of rainfall. However, experts in tunneling and geotechnical engineering suggest that, despite the unusual weather conditions, better engineering practices could have mitigated the event’s impact.

The intense rainfall led to the saturation of slopes surrounding the tunnel, resulting in a significant collapse that released a cascade of rocks, mud, and boulders onto Tunnel 2’s entrance.

Anilkumar Gaikwad, vice chairman of MSRDC, noted, “The heavy rainfall caused boulders to fall at the entrance of Tunnel 2. Although the tunnel structure itself remains intact, the false frame—an external structure at the tunnel’s edges—has sustained damage.”

Despite stabilization efforts using rock bolts, steel mesh, and other protective measures, the boulders that fell originated from approximately 150 meters above the secured area of the slope.

An official stated, “The protective measures are effective up to about 15 meters above the tunnel portal and have remained undamaged. However, the rocks that fell were from much higher elevations. During such extreme rainfall, engineering solutions have their limits,” as reported by The Indian Express.

Additionally, accumulated water on the hillside found a new path, flowing through the tunnel portal. Debris, including a concrete beam from the ornamental structure of the tunnel portal, collapsed onto the road, necessitating a halt in traffic until the area was cleared.

The tunnel portal, where the underground section meets the hillside, is particularly susceptible to weather-related issues, including rainfall and rockfalls. Engineers reinforce this area with concrete and steel while stabilizing the surrounding slope to minimize the risk of landslides.

Arnold Dix, a former president of the International Tunnelling and Underground Space Association (ITA-AITES), explained, “The outer layer of the hard-rock basalt mountains in the Western Ghats is more susceptible to weathering, which makes it softer. Rainwater permeates this layer, creating slippery conditions that can lead to rock slides.”

According to Ashish Juneja, a professor in the Department of Civil Engineering at IIT Bombay, the excavation necessary for tunneling disrupts the natural slope profile, which underscores the importance of effective protection measures at the tunnel entrance.

Experts ponder whether the damage could have been less severe. The MSRDC stated that rock bolts and steel mesh had been installed up to 15 meters around the tunnel. Juneja noted that rock bolts enhance stability by anchoring the weathered surface to a more robust rock layer.

Another commonly used protective system is the flexible rockfall barrier, which consists of steel mesh fences designed to catch falling rocks before they reach roads or railways. “These barriers are increasingly utilized in the Konkan region and can halt even large boulders,” Juneja remarked.

The experts observed that the debris landing at the tunnel’s entrance despite the barriers indicates that existing protections were insufficient for the conditions experienced on that day.

Water streams above the tunnel had been redirected to prevent them from affecting the portals, but this drainage system was inadequate for the extreme rainfall, leading to the emergence of a new stream that flowed through the tunnel’s entrance.

Dix raised a pertinent issue regarding the design of tunnels, stating, “It is feasible to engineer tunnels to withstand such heavy rainfall. If the tunnel was designed for rainfall exceeding 600 mm and still failed, it indicates an engineering shortcoming.”

Juneja expressed a more cautious perspective, stating, “Rainfall of this intensity is still a rarity. There is always a trade-off between preparing for the worst-case scenario and what is economically feasible.”

Both experts concurred that extreme rainfall events are becoming more frequent. As temperatures increase, the atmosphere retains more moisture, leading to a higher probability of intense rainfalls, particularly in the Western Ghats.