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Underwater Blasting in Algeciras, Spain used to help move the Largest Floating Dock in the World

The largest floating dock in the world, constructed in the shipyards of Algeciras, Spain, by DRAGADOS, FCC, and BEC (of France), had to be moved to La Condamine, Monaco.

The project began by removing land between the water and the dock to 14 meters below sea level, allowing water to enter the shipyard so the dock could be floated out to sea. At approximately 8 meters they hit consolidated marl making it too hard to dig, so Blasting became the most logical option.

The closest Blast occurred a mere 39 meters from the floating dock. The hydrodynamic pressure and vibrations had to be controlled and monitored to prevent any damage to the multi-million dollar dock.

A hydrophone was placed underwater adjacent to the floating dock, one geophone was attached to the deck, while others were attached to the inside of the dock below the watermark.

Excavating was completed with a dredger. The ladle movements were mapped using GPS, so the unexcavated consolidated marl surface was very well defined. The Blast hole drilling and loading was done from a floating platform with one hydraulic and two pneumatic drills.

The 90mm diameter holes were drilled using the ODEX method. This allows the placement of the casing before the removal of the drill string, which keeps the drill hole walls from collapsing.

GOMA 2 ECO (UEE’s gelatin explosive) with a diameter of 55mm, a length of 390mm and a weight of 1250grams per cartridge were used. The charges were prepared in PVC tube cases of varying length. The firing sequence was executed with Union Explosivos-Ensign Bickford (UEB) PRIMADET EZ DET non-electrical detonators. The Blast holes were initiated one at a time with 25 ms micro delays.

An air bubble curtain was created between the floating dock and the Blasting zone to reduce the hydraulic wave pressure by ten times.

By the end of the project, 26 underwater Blast measurements had been collected by the Union Espanola de Explosivos (UEE), Servicio Plus’s Mining Engineer, Hernando Espinosa, of Monteros Schurr, with the Spanish South Zone’s Engineer, Ignacio Navarro Fernandez. The Engineers used Instantel® manufactured vibration monitoring equipment and completed the data analysis using the Instantel® Blastware® Advanced Module. The measurements were monitored continuously, making it possible to inform the drilling and blasting crews of results, and aid in the planning process of subsequent blasts.

Analyzing the monitored results in the Instantel Blastware Advance Module, it was possible to see that the event always triggered on the hydrophone channel, concluding that the pressure shock was the source of the vibration.

The detonations generated a shockwave composed of individual waves from each blast hole. The shockwave reflections on the water surface and the sea bottom and sides generated different wave packs, resulting in a complex wave. The shock wave propagation was also influenced by the Blasting geometry, the depth of the charges, the water temperature and salinity.


The detonation gases generate a bubble that expands by the gas pressure and the mass speed of the mass-gas interface, until the moment in which the speed is annulated and the hydrostatic pressure is higher than the gas pressure. At this moment, the bubble starts to contract, compressing the gases. As during the expansion, the water inertia makes the contraction to go on and pass the stable equilibrium which induces a movement inversion for the bubble. This bubble effect rises to the water surface generating potentially damaging low frequency vibrations. By using Fourier analysis, UEE Engineers were able to control the generated frequencies, and by reducing energy in the frequency band limit, any damage to the floating dock.

With constant feedback of information, it was possible to maintain the vibration levels and the shockwave within allowable limits. It was also confirmed that the bubble curtain was effective in attenuating the blast pressures.