Seismically safe parameters of confined blasting in levelling dry dock bottom

Abstract

During construction of a dry dock, it became necessary to level the bottom of gneissic granite rock mass to 7 m by blasting. The blasting site appeared to be spaced at 8.5–20.0 m from a reinforced concrete caisson and a sand cofferdam reinforced with soil–cement piles. Aiming to ensure the required levelling at 16.2 m, the preset fragmentation quality and the reduced seismic safety, the parameters associate with the drilling-and-blasting pattern were calculated. The values of PPV caused by blasting-induced seismic waves in the caisson and cofferdam were found from the constructed formulas. The experimental measurement of ground vibrations induced by blasting used seismic recorder Vibracord DX. Al in all, 20 large-scale blasts were performed, and the blasted rock volume totaled 9 Km3. The comparison of the actual and calculated values of PPV in the caisson and cofferdam proves reliability of the presented formulas. It is calculated that PPV caused by the blasting-induced seismic waves at the farther wall of the caisson and at the nearest wall of the soil–cement piles are 2.0–27.0 mm/s and 2.0–22.0 mm/s, respectively, which is much less than the allowable PPV of 100–500 mm/s for the hydraulic engineering structures. It has been determined during blasting that the caisson preserves its integrity and no increase in the water inflow from the gulf is observed.