Criteria for Controlled Close Proximity Blasting at the Oroville Dam Spillways and Flood Control Structure

Emergency work to restore the Oroville Dam spillways required excavation of hard rock and concrete near critical structures, including the Radial Gate Structure and the Emergency Spillway Monolith Structure. Because of the large volume of demolition involved, and the emergency pace of construction required, mechanical excavation methods were deemed impractical for this project. It was also realized that traditional blasting criteria for close-in blasting would lead to unacceptable schedule delays. These challenges led the designers, consultants, and regulators toward innovative changes in the blasting specifications, based on principles presented in this paper. Case history data characterizing the behavior of high frequency blast vibrations and resulting transient strain in mass concrete structures is limited. Historically, regulatory criteria have been focused primarily on Peak Particle Velocity (PPV) developed decades ago to prevent the creation of cosmetic hairline cracks in one and two-story wood-frame homes with gypsum and plaster walls; materials very dissimilar from reinforced concrete structures. This paper presents monitoring results for close-in controlled blasts performed at the Oroville Dam main spillway. Results from this study illustrated that (a) measured values of PPV up to 55 inches/second (ips) at high dominant frequencies (300 to 1250 Hz) resulted in no visual signs of damage to the concrete slab, (b) peak strain decreased exponentially with distance away from the blast sites with negligible strain beyond a distance of 100 feet, and (c) the relationship between peak strain and PPV shows promise but additional data is needed to improve its characterization, particularly at strain levels greater than 100 microstrain. Monitoring peak strain provides a direct way to calculate stresses and to compare to a single allowable stress parameter.