Drilled Shaft Construction with Blasting

Construction of a transmission line for a new power plant presented the opportunity to utilize a relatively new construction technique involving blasting for drilled shafts. Drilled shafts are often used as foundations for transmission line pole foundations, where they are commonly constructed by auger and rock coring techniques. However, rock coring, especially for large diameter shafts in hard metamorphic and igneous rock, is slow and expensive. This paper describes how a contractor utilized blasting techniques to assist the construction of drilled shafts for transmission line pole foundations. Controlled blasting techniques generally improved the production rate of drilled shaft excavation in hard schist and quartzite rock, without adverse impacts. However, the resulting rock sockets tended to be irregular in shape and a few had significant amounts of overbreak. Recommendations are provided for improved controlled blasting techniques for drilled shafts.