Abstract
Underwater drilling and blasting used for rock dredging is associated with several unwanted effects having potential to cause damage to surrounding structures and environment. Though all these ill effects can’t be completely eliminated, using controlled blasting they can be minimized to acceptable levels. Underwater drilling and blasting was used for about 25,000 m3 of hard rock dredging during construction of a Second Liquid Chemical Berth by Mumbai Port Trust (MbPT), Mumbai. The rock formation at the site mainly consists of basalt with compressive strengths varying between 16.04 MPa and 37.96 MPa. The average depth of required excavation was about 2 m. The Elephanta Caves, a World Heritage Site is located about 2.6 km from the proposed site. It was apprehended that ground vibrations resulted from rock blasting may endanger the safety of Elephanta Caves as well as other structures located nearby. Controlled blasting was used to ensure safety of various structures and its efficacy was established by monitoring of blast vibrations on different structures during actual blasting. The use of small quantity of explosives confined in blast holes, non-electrical delay detonators and initiating each hole with a separate delay, helps in minimizing the ground vibration effects on surrounding structures and complete the rock dredging safely in time.
6. Conclusions
The two important environmental effects of blasting; viz., ground vibration and water borne shock wave pressure are of major concern while using underwater drilling and blasting for hard rock dredging. Use of explosives charge confined in boreholes substantially reduces the effects of shock waves on submerged structures and objects in water. Safety criteria recommended in terms of PPV for residential structures or engineered structures cannot be applied for world heritage structures. Due to the closeness of the operating chemical berth and world heritage sites, highly conservative vibration limits were adopted and all the blasts were monitored during the entire period of rock blasting. Blasts were carried out using much lower charge weight per delay than allowable to minimize the impact of ground vibrations on various structures around construction sites. The use of required quantity of explosives confined in blast holes, non-electrical delay detonators and initiating each hole with a separate delay, helps in minimizing the unwanted effects of underwater rock blasting on surrounding environment and complete the work safely in time.
