The twin transport tunnels of Ghat-ki-Guni project (Jaipur), lying between Latitude 26°53’38.36” and Longitude 75°51’26.20”, were constructed as a bypass route to existing National Highway very close to the old historical forts/temples of Jaipur in Rajasthan, India (Fig. 1). The rock exposed at the site was quartzite belonging to Delhi group. It showed a wide variation in quality and color. The barrier between them was 12 m. The total length of the each tunnel was 960 m with a finished width of 11.9 m and a height of 8.2 m. The left tube had a
The excavation of tunnels using drill & blast sussystem requires an optimal set of design parameters so as to get higher advance ensuring minimal over break (Mandal & Singh, 2009). In case of tunnel excavation the major point of worry is the economics related to the cost of concreting to generate a smooth profile. The above fact leaves important consideration about the blast pattern being used (Singh et al., 2008 & 2011). The extent of overbreak using drilling & blasting is mainly controlled by lookout angle (except use of TBM) and several other parameters like geological disturbance, deviation of drill hole governed by rock properties, changes in structural alignment, blast induced overbreaks (Singh & Bhagat 2011). The unwanted overbreak extending the blast induced cracks towards the sidewall as well as crown sufficiently dilute the overall quality of immediate roof. This becomes significant when we have very low cover above the tunnel. A successful drivage of tunnel requires complete elimination of such blast induced cracks in a way that the required height is maintained free from any cracks. At a number of sites in such cases particularly under low cover zone and poor rock condition, drilling and blasting was prohibited along the periphery of the crown. In a few cases, mechanical rock breaker was used to clear off of the crown to erect the recommended support at their specified height & location (Singh et al., 2008 & 2012). Two tunnels having low cover zone, (cover varying between 0.6 to 0.95 times the width of the tunnel) were studied in detail.