Residents learn about proposed confined blasting for Jacksonville Harbor

Published April 5, 2013
Confined blasting method, using 3,000 pounds of explosives to crack rock beneath the water’s surface in Miami Harbor.

Confined blasting method, using 3,000 pounds of explosives to crack rock beneath the water’s surface in Miami Harbor.

Unconfined blasting method, using seven pounds of explosives.

Unconfined blasting method, using seven pounds of explosives.

When most people hear the term “blasting,” they imagine a cosmic explosion of material that shoots into the earth’s atmosphere and shakes foundations. However, for the U.S. Army Corps of Engineers, Jacksonville Harbor Deepening Study, the proposed confined blasting technique to remove rock obstacles will sound more like a bullet as it is fired from a gun and will barely cause a blip on the radar.

The confined blasting technique is a potential option that could be used to deepen Jacksonville Harbor from its current depth of 40 feet to a proposed depth of 47 feet. Depending on the hardness of the rock, confined blasting could occur in multiple areas throughout the navigation channel.  Jacksonville residents learned about the technique during a recent public meeting for the Jacksonville Harbor Deepening Study.

Confined blasting has been successfully used on other Corps projects across the country. In fact, one of the most successful uses of the technique was during the Miami Harbor Deepening project in 2005.

The Port of Miami is situated in Biscayne Bay, home to many protected, threatened and endangered species.  Parts of Biscayne Bay are designated as a National Park, a Florida Aquatic Preserve, Outstanding Florida Waters, and a state Critical Wildlife Area.

Confined underwater blasting has been used as a successful and efficient construction technique to pre-treat and crack hard rock with minimal impacts. In fact, confined blasting limits impacts to the environment by 60 to 90 percent.  It keeps the pressure in the rock, preventing it from migrating and harming the environment.

A technique called “stemming” is used to reduce the pressure. Stemming gets its name from the material placed in the mouth of the drill hole to restrict the gas from escaping during the detonation. The hole in which the explosive material is placed is capped with an inert material, such as crushed rock. This is referred to as “stemming the hole.”  Studies have shown that stemmed blasts have up to a 90 percent decrease in the strength of the pressure wave released, compared to unconfined blasts of the same charge weight.

Protecting the environment during its work is one of the Corps’ top priorities. For the Miami project, the Corps conducted extensive plan formulation, revision and refinement to avoid impacts to the environment. This is the same model that is being followed for the Jacksonville Harbor project.

Successful implementation of the technique in Miami provided a large database of information to facilitate successful implementation of future confined blasting projects in sensitive areas.

Jacksonville District has been asked to assist Alaska District in their confined blasting program at Kodiak Harbor, home to the threatened sea otter.  Jacksonville District continues to be the leading expert in confined blasting techniques and is setting the standard for the program throughout the United States.