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“Facts About Underwater Drilling & Blasting” describes when explosives are necessary to dredge rock and the techniques used to drill and blast effectively.
Rivers, estuaries, coastal and open waters often have a rock seabed which can obstruct various dredging works. Sometimes dredging with a cutter suction dredger can be done to remove the rock, but other times the rock is too hard to be dredged directly and has first to be shattered with explosives. Underwater drilling and blasting is used to fragment the rock so it can subsequently be dredged and removed. After a site investigation of the area to determine the depth of overburden and type of material, level of rockhead (the surface between overlying unconsolidated material and solid bedrock below) and the type of rock, an investigation of the surrounding land areas should also be conducted as drilling and blasting can also impact buildings and embankments with ground vibration and underwater shockwaves. Underwater drilling is the first part of the process during which drilling is done to make boreholes in the rock to place charges or explosives for blasting. The rock needs to be initially drilled in a pre-determined pattern to place the explosives. Drilling work is often done from floating pontoons or from a drilling pontoon on spuds, otherwise referred to as a self-elevating platform. From the pontoons various systems are used for drilling:
the top hammer drilling system,
the down-the-hole (DTH) hammer system, and
the rotary drilling system.
Once drilling has taken place the process of blasting begins. Choosing the right type of explosives is crucial in underwater blasting and a few important factors to consider are: the velocity of detonation, the density, detonation stability, and the water-resistance and shelf life of an explosive. Two types of explosives are mainly used in underwater drilling and blasting operation, nitroglycerine-based explosives (NG) and ammonium nitrate-based explosives (AN). Whatever system is used, the process of drilling and blasting requires a great deal of care to be executed safely, both for the people performing the work as well as for the environment. Obviously working with explosives is not an easy task. “Facts About Underwater Drilling & Blasting” answers essential questions such as:
