Controlled Underwater Blasting near Sensitive Structures at Mithi River Extension Project, Mumbai, India

摘要

Controlled underwater blasting was conducted near sensitive structures in Mumbai city in India for river widening at the mouth of the Arabian Sea.After heavy floods in Mumbai due to narrow passage of Mithi River in 2005,Mumbai Metropolitan Region Development Authority (MMRDA) has unader taken deepening and widening of River by underwater blasting.This lead to bed rock blasting by specialised underwater blasting techniques near sensitive structures like flyover,sewerage plant and old Mahim causeway.Control of vibrations and fly rock were important objectives of the project along with desired fragmentation.Underwater drilling was carried out by drill rigs placed on platoons,specially fabricated for the purpose.After the drilling is carried out to the required depth,drill rods with hammer is removed from the hole keeping the casing intact in the hole to avoid hole collapsing and identification.Couplable Plastic Tube (CPT) explosives used for charging in blast holes from the platform called barge.The blasthole diameter was 75 mm and the depth was restricted to 1.75 m to reduce blast induced ground vibrations.Hole-to-hole delay initiation was carried out to get better fragmentation and to reduce maximum charge per delay for reducing intensity of vibrations.Specially manufactured high density seismic explosives were used to break the rock under water.As the water head was just 0.4 m,blasting was carried out during high tide times with a water head of above 4.5 m to reduce the flyrock projectile and avoiding risk to the habitats in the vicinity.The specific charge used was 0.25～0.35 kg/m3 and specific drilling was 1 ～ 1.17 m/m3 to ensure optimum fragmentation.The vibration intensity in all the blasts was restricted to 5mm/s,which was the threshold limit for the adjacent sensitive structures.The desired optimum fragment size of 0.1 m3 was achieved in all the blasts was without compromising safety parameters.The specially designed underwater blast patterns could yield desired production to complete the project in stipulated time.