Excavation Technique and Support in Squeezing Ground Conditions for a Highway Tunnel in India

Squeezing can occur both in hard rock and in jointed rock mass as a result of over stressing. It is related to time dependent shearing, ie shear creep and is associated with the volumetric expansion (dilation) and radial inward displacement of the tunnel surface. While planning a tunnel in hard rock under a high cover of the order of 2 km it is desirable to identify the potential squeezing ground conditions and to select appropriate excavation technique and the support measures. There are several solutions to counter balance the squeezing ground viz over excavation of tunnel profile, assessing appropriate excavation technique, provision of a ductile primary support and time for providing tunnel lining. Low drivage rates and high cost due to support failures requiring rectification arising out of poor anticipation of tunnel closures and support pressures are major problems associated with tunnelling under squeezing ground conditions. Under squeezing ground conditions, the deformation will be gradual, therefore, in order to absorb deformations yielding supports are quite useful as primary support which provide a reliable low initial support pressure and permit uniform stress relief of the surrounding rock along with preventing rock from ravelling. The paper discusses the various factors which influence the selection of appropriate excavation technique and support measures in hard rock under squeezing ground conditions.