Evolving a safe blasting regime near an earthen dam through geophysical assessment

Earthen dams are vulnerable to damage through nearby construction, particularly vibrations generated by blasting used for the excavation of hard rock. As such, there are restrictions by dam authorities that prohibit excavation beyond a certain depth near a dam. A similar case was observed near an earthen dam in India, where blasting was proposed for construction of a new railway line. To restrict vibrations due to blasting, it was imperative to develop a methodology and define the maximum allowable limits of vibrations near the dam toe. This meant defining the attenuation characteristics of the vibrations and assessing the natural frequency of the dam. A comprehensive geophysical investigation of the dam was conducted, which revealed a natural frequency of 2 to 4 Hz along with subsurface s-wave velocities in the range of 166 to 981 m/s. An empirical method was used to predict natural frequencies ranging from 1.82 to 4.59 Hz, thus matching those measured via geophysical methods. Finally, a vibration criterion based on the frequency and type of structure was used to obtain a threshold vibration level of 5 mm/s. Few blasts were conducted and the actual measurements of the vibrations at approximately 480 m proved that the vibration levels at the dam toe were less than 2 mm/s, thereby validating the predictions of vibrations.