The Government of India has entrusted the National Highway Authority of India (NHAI) with the responsibility of four laning of Chenani to Nashri Section of NH-1A from km 89.00 to km 130.00 including 9km long bi-directional traffic tunnel with a parallel escape tunnel of 9km on BOT (annuity) basis.
The project area lies in western Himalayan region. The rock masses along the project of the Chenani-Nashri tunnel, belong to the Lower Murree formation that includes a sequence of interbedded sandstones, siltstones and claystones layers.
The tunnel behaviour classification of the rock mass has been performed by Geodata, Italy. These rock behaviour classes has been correlated with the rock mass behavior type of NATM. The supports for various behavior classes have also been designed using the numerical methods as per the requirements.
The tunnel construction work was started in August 2011 from South end and in September 2011 from North end. As expected the experience of tunneling in Himalayan mixed geology having bands of sandstone, siltstone, claystone, intermingled siltstone and claystone and sheared siltstone and claystone is not very encouraging. There have been instances of overbreaks and cracks in shotcrete support in the tunnel. Accordingly the designs have been modified. The benefits of displacements/convergence monitoring have been highlighted at the end.