Planning and design of two highway tunnels through weak rocks of Bilaspur-Sundernagar area

Construction of two tunnels, (1) 1100 m long and (2) 510 m longapproximately in the complex geological hilly terrain on the right bank at Sutlej river is envisaged for an alternative alignment of NH-21 between Bilaspur and Sundernagar in Himachal Pradesh. The alignment of the tunnel passes through Tertiary rocks comprising moisture sensitive and steeply dipping sequence of intricately folded and faulted shales, siltstone, sandstone, mudstone, variegated shales etc. The rocks are affected by major tectonic activity in the area like Main Boundary Fault and Shali Thrust. The paper presents influence of complex geological features in the design of tunnels in weak rocks. Based on the field data, rock mass classification parameters and the geotechnical design and anticipated construction problems are also outlined in the paper. Thorough geotechnical investigation and adjoining cut and cover sections at the tunnel portal site are deemed necessary for evolving most appropriate design and construction methodology.