The Mumbai Pune Expressway is India’s first six-lane concrete, high-speed, access controlled expressway. It spans a distance of 94.5km connecting Mumbai to Pune. It cleaves through the scenic Sahyadri mountain ranges through passes and tunnels.
From June to August 2015, during monsoon season, a series of landslide/rockfall events caused casualties and severe traffic disruptions near the Khandala and Adoshi tunnel portals. Following those life-threatening events, the immediate installation of rockfall protection systems was identified as crucial.
Maccaferri conducted a thorough investigation, taking into account also the views of expert geologists and geotechnical engineers, to identify the most appropriate mitigation systems based on site specific conditions.
As a result, the solutions proposed for the Khandala and Adoshi tunnel locations were a customized combination of rockfall mitigation measures. A combination of rock scaling, sub-surface drainage system, rock slope stabilization systems such as rock nailing and Maccaferri’s secured drapery systems Steelgrid MO and/or high energy absorption (HEA) panels was used at the Khandala tunnel, Pune side portal, while the addition of Maccaferri’s dynamic rockfall barriers was also necessary at the Adoshi tunnel, Pune side location.
Rock scaling was performed by removing the unstable boulders which could be manually dislodged using a crow bar. This operation required a block of the traffic in coordination with the authorities. This rockfall prevention measure should be carried out before any other mitigation measure.
Secured drapery systems
The installation of secured drapery systems aims to retain or stabilize the rocks which are on the verge of movement. The slope surface is draped with one or a combination of meshes with the help of surface, top and bottom anchors.
In this project, where the depth of unstable rock mass was lower than or equal to 2m, surface anchors were installed over the rock slope at specified intervals along with one or a combination of Maccaferri’s high strength meshes: Steelgrid® MO (mono-oriented) geocomposite, combining high tensile steel cables integrated longitudinally into the double twist mesh, and/or High Energy Absorption (HEA) panels, which are woven from a single continuous length of high tensile strength steel wire rope, joined at each crossing point with the patented HEA ‘double knot’ connection.
Rock slope stabilisation system
Where unstable rock masses with depth greater than 2m were identified or deep-seated rock mass failures could be anticipated, slope stabilization measures were proposed employing long nails of varying depth, spacing and yield strength in association with appropriate mesh facia system (Steelgrid® MO and HEA Panel).
Dynamic rockfall barriers
Barriers are designed to arrest and catch falling rocks and boulders before they fall on the road below. The detachment of boulders from upslope could be anticipated along some stretches near the Adoshi tunnel. Anticipated sizes of rockmass along with slope angles were modelled to calculate the impact energy generated by such masses and the most suitable location for the barriers. Maccaferri’s rockfall barrier with energy absorption capacity of 1,500kJ were installed at a height of 4m above the road surface along these stretches.
Water is one of the main triggers of a landslide/rockfall event. Drainage pipes (50mm diameter PVC pipes wrapped in non-woven geotextile) were installed in upward drilled holes near the toe of the rock slopes. This facilitates dipping the phreatic surface and helps decreasing the pore water pressure in rock/soil slopes.
These mitigation works were successfully completed in December 2016 and survived the following monsoon season without any reported rockfall accidents.