mines and obstacles, without contact, to beyond the mine zone on the beach. The task group proposes that this wave of overwhelming force be followed with guinea pig barges as a second layer of mine countermeasures. The barges would be sunk or, if floating, stopped at the end of the channel to form a causeway for the landing force. The two tactics should result in a more robust system and increase confidence in the effectiveness of this bold approach.

To effectively excavate a channel of sufficient width and depth, the bombs have to be big enough to have a large crater radius and to penetrate sufficient depth. There is much more information on cratering on land than underwater. However, tests, e.g., by Davis and Rooke (1968)2 and analyzed by O’Keeffe and Young (1984)3 indicate that burial of a conventional explosive under a sand or mud bottom in shallow water can significantly increase the crater diameter compared to that from an explosion on the bottom. O’Keeffe and Young indicate that an explosive of W pounds (equivalent of TNT) should be buried to a depth of about W0.33 ft below the bottom for maximum cratering, which for 10,000 lb of TNT equivalent explosive would be 21 ft. Young and O’Keeffe’s data plots and other work on cratering,4 done on land, indicate that the crater radius near the maximum may not be very sensitive to the exact depth of burial. Also, the crater diameter appears to be weakly dependent on the seabed material (except for rocky bottom) and on water depths, for this size of explosive, in a range between 10 and about 3 ft. O’Keeffe and Young suggest that the radius (Rc) of the crater at optimum depth of explosive burial in a soft bottom (about W0.33, in this case 21 ft) would be given by Rc=4.4 W0.33. Thus, a 10,000-lb explosive would produce a crater 95 ft in radius or 64 yd wide in the section of the lane from 10 ft to perhaps 3 ft of water. For lesser water depths and up to the (assumed sand) beach edge, the cratering phenomenology changes, leading to a gradual decrease of crater radius for a 10,000-lb TNT-equivalent explosive to about 65 ft on a wet sand beach, with a corresponding optimum explosive burial depth of about 40 ft, and to about 55 ft in completely dry sand for about the same depth of burial. The crater depths are greater up the beach than underwater.5

Work done at the Army’s Waterways Experiment Station and the Atomic Energy Commission’s project PLOWSHARE included use of row charges, buried in the bottom underwater, ranging from pounds to tons TNT equivalent, and detonated nearly simultaneously to approximate a line charge, to excavate boat channels.6 Thus, PLOWSHARE’s subproject TUGBOAT7 excavated a boat channel and harbor in a coral bottom at

2  

Davis, L.K., and A.D.Rooke. 1968. “High-Explosive Cratering Experiments in Shallow Water,” Miscellaneous Paper No. 1–946, U.S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss.

3  

O’Keeffe, David J., and George A.Young. 1984. Handbook on the Environmental Effects of Underwater Explosions, NSWC TR83–240, Naval Surface Weapons Center, Dahlgren, Va. and Silver Spring, Md., September 13.

4  

Vortman, Luke J. 1969. “Ten Years of High Explosive Cratering Research at Sandia National Laboratory,” presented at the Special Session on Nuclear Excavation, Washington, D.C., November 10–15, 1968, Nuclear Applications and Technology, Vol. 7, No. 3, September, pp. 269–304.

5  

Footnote 4 gives a discussion and formulas for different depths of water table.

6  

The work cited in Footnote 3 discusses excavation of a boat channel in a lake using a row of explosives on the bottom underwater.

7  

Day, Walter C. 1992. “Project TUGBOAT, Explosive Excavation of a Harbor in Coral,” Technical Report E72–23, U.S. Army Waterways Experimental Station, Explosive Excavation Research Laboratory, Livermore, February; LaFrenz, R.L. 1980. “Coral Cratering Phenomenology,” DNA Report 5813T, Defense Nuclear Agency, Washington, D.C., October 31.



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