Controlled blasting in proximity to urban residential structures

摘要

This paper deals with the excavation of 19500 m~2 area in granitic hard rock to a depth of 10 m by controlled blasting to accommodate foundation for 75 m tall, 16 floor commercial building in the city of Bengaluru, India. The major task was to excavate granitic hard rock close to constructed residential structures (10 m) and under construction (60 m). To start with, trail blasts were carried out at a distance of 50 m from the existing residential structures. In order to restrict the maximum charge per delay and the air overpressure from trunk lines, blasts were initiated with shock tubes. In the blasting zone, excavation was carried out in five different locations at varying distances from the structures. Controlled blasting in urban areas does not end with controlling the flyrock, vibrations or air overpressure levels within the permissible limits, but the most contributing factor happens to be the human perception. The human response with regard to vibrations and the structural response too was studied. Blast designs were altered site specifically and in addition to that continuous monitoring of ground vibration was under taken to ascertain that the intensities were within the limits. Sufficient number of rubber blasting mats of 1.1t each were used for muffling the blasting area and the flyrock was restricted within 10 m from the blasting area. It was observed that the rubber mats were not only effective in controlling the flyrock but they have reduced the air overpressure levels too considerably. In total around 80000 m~3 of hard rock was successfully excavated without any grievances as the vibrations were limited to below the perception level of the residents rather than the transmissible level of the structures from the neighbours/residents.