Rockfalls are considered one of the major risks in rock cuts for highways and railways in mountainous terrains or unstable slopes. The provision of rockfall mitigation systems ensure that either the rockfall can be prevented to certain extent or in cases where rockfall cannot be totally prevented, system will minimise the impacts of the rockfall. In India there are large ranges of mountainous terrains with different varieties of rocks and even the most stable rock may fail under aggressive situations such as excessive rainfall, snowfall and steep cutting etc. These situations demand for the provision of rockfall mitigation systems as a key element in the design and maintenance of infrastructure networks and have a direct impact on safety and preservation of the infrastructure (IRC-HRB-Special Report-23). One such case study is presented in this paper.
The Mumbai Pune Expressway is Indias first six-lane concrete, high-speed, access controlled tolled expressway. A major section of Mumbai-Pune Expressway passes through the Sahyadhri mountainous range. Rockfall events were reported along the Mumbai –Pune Expressway because of the unplanned cutting done for the construction of the expressway and the continuous weathering action happening throughout the year. The monsoon period of Year 2015 witnessed some rockfall events near Khandala tunnel and Adoshi tunnel which are the two major tunnels along the expressway. Being a major connection route between Mumbai and Pune (two major cities of Maharashtra), stoppage or deceleration of traffic was a huge problem. The requirement of installation of required rockfall mitigation measures on emergency basis on the most vulnerable stretches along the expressway was of the highest priority. This paper gives an insight of the various engineering investigations that are conducted at site, the major observations, and interpretation of the same, which were then used for understanding and analyzing the causes and designing suitable mitigation measures. In this paper, only one critical section is selected and the design and construction of rockfall mitigation works are explained in detail. With large variations in Geologic formations, topography, sub surface water flows, extent of weathering and surface disintegration, limitations on extent of observations and qualitative geotechnical investigations, the design and construction details are aimed at reducing the risk of rockfall in the areas under consideration. All the designs and drawings are prepared based on observations, possible investigations done at site and on best possible engineering judgment of the designer. It needs to be noted that a complete prevention or avoidance of rockfall incidences is not viable. The installation of rockfall mitigation works are completed along Mumbai –Pune Expressway in the major critical stretches and has not reported any damage after one monsoon season.