Damage Due to Repeated Blast Vibrations at a Powerhouse Foundation in Jointed Rock Mass

摘要

The detonation of explosive charges releases large quantities of energy that can produce rock and soil deformations in the vicinity of blasting site. Extensive data are available on blasting in general and on the behavior of surface structures subjected to blast vibrations. However, only limited information is available on the effect of blast induced dynamic forces on foundations of heavy structures like dams, bridges. The limited data indicate that, the blast induced dynamic stress, one or several cycles of repeated strains at dam foundation may cause deterioration in the rock mass or create damage to the dam foundation. This paper deals with the research work carried out at Priyadarshini Jurala Hydroelectric Power Project (PJHPP) on the effect of repeated blast vibrations on powerhouse foundation in a jointed rock mass. The damage caused by blast induced vibrations can be categorized into two types: i) near-field damage due to high frequency vibrations when the blast is occurring in the close proximity and ii) far-field damage due to low frequency vibrations when the blast is occurring relatively farther distances. The near-field damage was assessed by monitoring blast vibrations, borehole camera survey and ultrasonic testing of core samples taken from the test site. The far-field damage was assessed by continuous monitoring of vibrations, borehole camera survey and P-wave velocity of the rock mass before and after the repeated blasting. Borehole camera was used to examine the extent of damage in the rock mass. The study reveals that repeated dynamic loading imparted on the jointed rock mass from subsequent blasts, in the vicinity, resulted in damage even at 20% of critical peak particle velocity (V_(max). The far-field damage due to the repeated blast loading of 50-75 cycles was more than 50% of the near-field damage. The results of the study indicate that vibration levels, even at less than critical V_(max), can cuase safety and stability problems to the structures in/on jointed rock mass, when exposed to the repeated blast loading. The paper stresses the need for consideration of the effect of repeated blast loading in fixing the threshold limits of V_(max) to avoid both near-field and far-field damage.