Mumbai-Pune Expressway landslide exposed Missing Link’s weakest spot; authorities race to secure it

Weeks after a landslide shut down the Mumbai-bound lane on the Mumbai-Pune Expressway ‘Missing Link’, the Maharashtra State Road Development Corporation (MSRDC) is undertaking a multi-layered rockfall protection exercise above the expressway tunnel mouths, with the work scheduled to be completed by the end of October.

The measures, recommended by IIT-Bombay after the landslide and carried out by tunnel contractor Navayuga Engineering Company Ltd, include diverting newly formed water channels, removing loose and weathered rock, anchoring vulnerable rock faces with steel bolts, covering sections with high-tensile steel mesh, and installing steel barriers to arrest falling boulders.

It covers about 1.38 hectares of hill surface above the tunnel mouths, and nearly 50-60 per cent of the work has been completed, according to the MSRDC. The first intervention has been to tackle the water itself. New streams that formed on the hillside during the extreme rainfall are being channelised and diverted away from the tunnel mouths.

A hydrologist was consulted to determine the drainage intervention, aimed at preventing water from flowing through vulnerable portions of the slope and further loosening the rock. The next layer involves stabilising the rock face.

Loose and weathered rocks are being chipped away and removed from identified vulnerable patches. Steel anchors are being drilled deep into the hillside where rock needs to be retained, to tie the outer rock layer to more stable rock underneath.

High-tensile steel wire mesh is then being fixed over selected sections to contain smaller loose rocks and prevent them from falling onto the road. Steel barriers are also being installed at different levels on the slope to intercept larger rocks that may break away despite the stabilisation measures, or from higher up the hill.

The interventions are not being applied uniformly across the hillside — Relatively flat and stable sections require less intervention, while steeper and more vulnerable stretches receive rock bolts, mesh or barriers depending on the risk.

“Work on implementing these rockfall mitigation measures began immediately after the incident, but we’ve had to wait for dry spells in the monsoon to carry them out properly. By now, around 50 to 60 per cent of the work has been completed,” said Rajesh Patil, Vice Chairman and Joint Managing Director, MSRDC.

By October, the corporation expects to complete mitigation measures across about 1.4 hectares above the tunnel mouth. The stretch immediately above the landslide spot remains the priority. Similar protection measures are also planned above the other tunnel mouths of the Missing Link.

After the landslide, a team led by Professor D N Singh, a geotechnical engineer from IIT-Bombay’s Department of Civil Engineering, studied the slopes and recommended interventions to prevent such accidents in the future.

The Missing Link had already incorporated rockfall protection measures before its opening in May, covering the area up to around 15 metres above the tunnel exits. The July landslide, however, highlighted the vulnerability of higher portions of the hillside during extreme rainfall.

The MSRDC has consequently raised the design benchmark for the additional protection work, with the strengthened sections being designed to withstand a weather event with a 1-in-1,000-year probability, compared with the original design benchmark of a 1-in-100-year event.

The 13.3-km Missing Link, built for Rs 6,695 crore, was constructed as an alternative to the accident-prone Khandala ghat section of the Mumbai-Pune Expressway. The cost of the additional measures is being calculated separately.

MISSING LINK: WHAT’S BEING DONE TO SECURE THE SLOPE AFTER THE JULY 6 LANDSLIDE, FIVE KEY INTERVENTIONS New streams formed on the hillside are being channelised and diverted away from the tunnel mouth. Loose and weathered rocks are being chipped away from vulnerable portions of the slope.