Underwater munitions pose a risk to human health and safety as well as a potential threat to the environment. Current management options are limited to removal, in-place detonation, or leave-in-place. Furthermore, munitions subjected to ocean currents may be mobile, confounding monitoring, removal, or detonation after initial detection. Capping technology used in the control of contaminated sediments has the potential to immobilize and isolate munitions from the overlying water column, thereby safeguarding human exposure and the environment.

The objective of this limited-scope research effort was to evaluate the viability of geotextile containers (geobags) for immobilizing submerged munitions. Specific objectives were to fabricate and place geobags on submerged munitions under laboratory conditions and evaluate geobag effectiveness in a dynamic environment with moving water. Of interest were the effectiveness of geobag coverage (encapsulation) of the munitions, geobag stability in flowing water, and an assessment of whether geobags were able to immobilize munitions under flow conditions.

This study was divided into two phases. Phase 1 involved designing geobags, developing techniques for handling geobags, and testing geobag performance in still water. Of primary interest was the geobags’ ability to completely cover or encapsulate a submerged munition. Phase 2 involved evaluating geobag performance under flowing water conditions. These tests were conducted in a large flume with geobags placed on inert 155-mm munitions on hard, smooth surfaces and packed gravel beds. Unrestrained inert 155-mm munitions were also placed in the vicinity of the geobags. The flume was turned on, and the behavior of the geobags and unrestrained munitions was observed. Tests were run using different water levels, which resulted in geobags and unrestrained munitions experiencing a variety of flow conditions.

Geobags used in this study were made out of geosynthetic liner materials with sand as filler. Geobags were rectangular and sized to accommodate the limitations of the tanks available for Phase 1 testing, which translated into filled lengths of 5 ft and widths of 2.5 ft. Depending on the degree of filling, geobag weight varied from 500 to 1200 lbs when dry. Wet bags weighed more due to pore water in the sand. Once filled, the open end of the geobag was securely closed to prevent any filler spillage. Geobags were fitted with lifting straps on the sides to facilitate handling. Geobag sides, lifting straps, and the thread used to stitch the bag together were polyester, which resulted in an estimated design life of 114 years.

Phase 1 results indicated that geobags could be placed on submerged munitions. Multiple munition orientations were evaluated, and it was evident that geobags could be placed regardless of the orientation. Further, it was demonstrated that geobags, if large enough, could completely encapsulate a munition, i.e., the geobag is in contact with the sediment surface at all points around the munition. This results in the geobag acting as a “cap” in the event that there was any leakage or spills from the munition. For example, a partially filled geobag, 500 lbs total weight, was able to completely encapsulate a horizontal 155-mm munition on a hard surface, thereby providing a cap with a minimum of 6 inches of sand over and around the munition.

Phase 2 results indicated that munitions covered by geobags remained in place under conditions that resulted in erosion of the packed gravel bed and movement of uncovered munitions. Complete encapsulation was maintained. Flow generated during these tests matched or exceeded those predicted by modeling for Ordnance Reef, Hawaii, which had been selected as a reference location.

This study developed and demonstrated a low-cost, easily implementable technology for immobilizing submerged munitions. The technology can be used short term to hold a munition in place until removal or detonation. It is also suitable for long-term placement or encapsulation of munitions as an alternative to removal. The filler in the geobag acts as a physical cap that isolates the munition and its components from the surrounding waters. With the design life estimated to exceed 100 years, geobags provide an inexpensive, long-lasting alternative for addressing issues of mobile submerged munitions.