Mumbai : The Geological Survey of India has submitted its final report on the causes and measures to be adopted to avoid landslides on the Mumbai-Pune Expressway in the future. Water percolation and rock fracturing were identified as key reasons for the landslides that occurred on the Expressway.
According to the report, there is an immediate need to look into the surface water management from the hilltops, ensuring that rainwater does not percolate into cracks and affect the cohesiveness of the rocks. A measure suggested in the report, which will have a decade long, or even more effect, is surface water management. “The rocks have lot of fractures and joints, making the monolith rock into an assemblage of rocks. Because of lot of cohesion across the joints and the cohesion has being decreased with years due to successive percolation of water from the catchment and going into the rock and coming out from the joints.
“The water has gone into the sub surface on the hills in the catchment area. So if the catchment area is properly drained—if you do not allow any water to standstill—then the natural process, which would take 10 years, will take 100 years. Hence, the rock blocks that happens will be delayed exponentially,” explained Dr Sekhar Sarkar, Deputy Director General of GSI (landslide), Nagpur division. The government should construct artificial drainage systems so that water does not accumulate at the top, Dr Sarkar added.
Meanwhile, the short term measure, which the Maharashtra State Road Development Corporation has already undertaken, is removal of loose rocks from the ghat sections of the Expressway. Dr Sarkar added that with chipping off of the rocks which are about to fall would take away the uncertainty. “Uncertainty is which landslides occur, now with rock chipping we know the timeframe of rock,” he said. The GSI report has categorically advised against using explosives to remove loose rocks as it would have adverse effects.
Currently, the MSRDC is also in process of rock-bolting or anchoring blocks which would bolt the smaller rocks to the main rock behind thus preventing them to separate and cause a landslide. “When we studied rocks near Khandala, we found that rocks had deep fractures. And rock blocks had fallen along with their anchors,” Dr Sarkar added. According to the report, the GSI has suggested anchoring of the rocks should not be at a uniform depth of 2 meters.
The report also cautioned the State government against use of any explosives to remove loose rocks, which would have adverse effects. The report also asked the State government to rope in specialised agencies to study the vibrations creative by vehicles. “Length of the anchor blocks has to be based on the local joint pattern of the rocks. At certain places its deeper, so deeper rock-bolting is required—more than two meters. Anchoring of the rocks should not be uniform and has to be specific to the location,” he suggested.
MSRDC is going to appoint an agency soon to carry out anti-landslide work. One measure that the agency would adopt is shotcreating. In engineering parlance shotcreting or shotcret is a semi-fluid mixture of cement and related materials that is sprayed pneumatically at a high velocity onto a surface, be it vertical or overhead, over reinforced steel rods or mesh. GSI in its report has suggested that shotcreting should be done later after identifying rock patterns and the length of the anchoring needed to bolt the rock. “Shotcreting will not have much purpose other than holding smaller loose rocks, he said.
The report has asked the government to rope in specialised agencies to study the vibrations so that remedial measures related to vehicles. “It is a long term research. The vibration is created by the vehicles and also earthquakes due to which coercion loosens,” he added.
Swapnil Rawal
