LAUSR

Stresses on ground depend upon regional as well as local geological factors. However, the local geological factors are considered more important in controlling the stresses during tunneling activities. These stresses sometimes get amplified (overstressed condition) due to several factors (geological and/or non-geological), known as stress amplification factor (SAF), and if the same has been plotted along the longitudinal geological profile, it will be variable throughout. Here, an attempt has been made to quantify the SAF and correlate the same with the encountered geology in Kohli Adit Tunnel (KAT) located at lesser Himalayan range of Jammu and Kashmir, India. KAT is at the close vicinity of the Murree Thrust (equivalent to the Main Boundary Thrust). Several collapses and significant deformations due to overstressing occurred during the construction phase. Even there were expected very poor ground condition ahead as emancipated from probing. Therefore, heavier supports have been used than the envisaged support systems. The SAF has been calculated on the basis of differences between the actual and envisaged support systems pressure and other variables. 830 m long KAT has been subdivided into 31 SAF zones, where the values range from 1.05 to 1.25. It has been found that the amplification of stresses was due to factors like, thrust, faults and shear zones, different geomechanical behavior around lithological contacts, low overburden zones and high pore-water pressure zones. Further, it has also been inferred that the combinations of these factors have higher influence on overstressing.