The influence of a shallow water table on cratering

The influence of a shallow water table on crater shapes and sizes in various types of soils was investigated. Tests using high-explosive charges ranging from 1 to 322 pounds were conducted at four separate sites. Site 1 was a specially constructed sand basin with a variable water table level. Site 2 was a natural sandbar of the Mississippi River near Vicksburg. Site 3 was on a natural sand site in central Florida, and Site 4 was a layered clay/sand site at Camp Shelby, Mississippi. Results showed that craters of drastically different shapes can be formed in the presence of a near-surface water table if the water table is in a highly saturated, loose, granular material. Wide, shallow craters were caused by soil liquefaction in loose, saturated, fine medium-loose sands; however, in a relatively dense sand that was not fully saturated, craters were 5 to 10 percent smaller in radius and depth than normal (in homogeneous soil), but still regular and bowl-shaped. In a clay/sand environment, craters tended to be about 10 to 15 percent wider than normal because of sloughing crater slopes of soft clay caused by the shallow water table in the sand. (Author)